Guidelines for the Secure 

Deployment of IPv6 (Draft)

Recommendations of the National Institute 

of Standards and Technology

Sheila Frankel

Richard Graveman

John Pearce

Special Publication 800-119 

(Draft)
NIST Special Publication 800-119 

(Draft)

Guidelines for the 

Secure Deployment of IPv6 (Draft)

Recommendations of the National 

Institute of Standards and Technology

Sheila Frankel

Richard Graveman

John Pearce 

C O M P U T E R S E C U R I T Y

Computer Security Division

Information Technology Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-8930

February 2010

U.S. Department of Commerce

Gary Locke, Secretary

National Institute of Standards and Technology

Dr. Patrick D. Gallagher, Director
GUIDELINES FOR THE SECURE DEPLOYMENT OF IPV6 (DRAFT)

iii

Reports on Computer Systems Technology

The Information Technology Laboratory (ITL) at the National Institute of Standards and Technology 

(NIST) promotes the U.S. economy and public welfare by providing technical leadership for the nation’s 

measurement and standards infrastructure. ITL develops tests, test methods, reference data, proof of 

concept implementations, and technical analysis to advance the development and productive use of 

information technology. ITL’s responsibilities include the development of technical, physical, 

administrative, and management standards and guidelines for the cost-effective security and privacy of 

sensitive unclassified information in Federal computer systems. This Special Publication 800-series 

reports on ITL’s research, guidance, and outreach efforts in computer security and its collaborative 

activities with industry, government, and academic organizations.

Certain commercial entities, equipment, or materials may be identified in this 

document in order to describe an experimental procedure or concept adequately. 

Such identification is not intended to imply recommendation or endorsement by the 

National Institute of Standards and Technology, nor is it intended to imply that the 

entities, materials, or equipment are necessarily the best available for the purpose.

National Institute of Standards and Technology Special Publication 800-119 (Draft)

Natl. Inst. Stand. Technol. Spec. Publ. 800-119, 175 pages (Feb. 2010)
GUIDELINES FOR THE SECURE DEPLOYMENT OF IPV6 (DRAFT)

iv

Acknowledgments

The authors, Sheila Frankel of the National Institute of Standards and Technology (NIST), Richard 

Graveman of RFG Security, and John Pearce of Booz Allen Hamilton wish to thank their colleagues who 

reviewed drafts of this document, including TBD.
GUIDELINES FOR THE SECURE DEPLOYMENT OF IPV6 (DRAFT)

v

Table of Contents

Executive Summary............................................................................................................ES-1

1. Introduction ................................................................................................................... 1-1

1.1 Authority.................................................................................................................1-1

1.2 Purpose and Scope................................................................................................1-1

1.3 Audience ................................................................................................................1-1

1.4 Document Structure ...............................................................................................1-1

2. Introduction to IPv6....................................................................................................... 2-1

2.1 Early History of IPv6...............................................................................................2-1

2.2 Limitations of IPv4 ..................................................................................................2-1

2.3 Major Features of the IPv6 Specification ................................................................2-3

2.3.1 Extended Address Space........................................................................... 2-3

2.3.2 Autoconfiguration ....................................................................................... 2-3

2.3.3 Header Structure........................................................................................ 2-3

2.3.4 Extension Headers..................................................................................... 2-4

2.3.5 Mandatory Internet Protocol Security (IPsec) Support................................ 2-4

2.3.6 Mobility....................................................................................................... 2-4

2.3.7 Quality of Service (QoS)............................................................................. 2-5

2.3.8 Route Aggregation ..................................................................................... 2-5

2.3.9 Efficient Transmission ................................................................................ 2-5

2.4 IPv4 and IPv6 Threat Comparison..........................................................................2-6

2.5 Motivations for Deploying IPv6 ...............................................................................2-7

3. IPv6 Overview................................................................................................................ 3-1

3.1 IPv6 Addressing .....................................................................................................3-2

3.1.1 Shorthand for Writing IPv6 Addresses........................................................ 3-5

3.1.2 IPv6 Address Space Usage ....................................................................... 3-6

3.1.3 IPv6 Address Types ................................................................................... 3-7

3.1.4 IPv6 Address Scope................................................................................... 3-8

3.1.5 IPv4 Addressing......................................................................................... 3-9

3.1.6 IPv4 Classless Inter-Domain Routing (CIDR) Addressing ........................ 3-10

3.1.7 Comparing IPv6 and IPv4 Addressing...................................................... 3-11

3.2 IPv6 Address Allocations......................................................................................3-12

3.2.1 IPv6 Address Assignments ...................................................................... 3-13

3.2.2 Obtaining Globally Routable IPv6 Address Space.................................... 3-15

3.3 IPv6 Header Types, Formats, and Fields..............................................................3-16

3.4 IPv6 Extension Headers .......................................................................................3-18

3.5 Internet Control Message Protocol version 6 (ICMPv6) ........................................3-23

3.5.1 ICMPv6 Specification Overview ............................................................... 3-23

3.5.2 Differences between IPv6 and IPv4 ICMP................................................ 3-25

3.5.3 Neighbor Discovery.................................................................................. 3-26

3.5.4 Autoconfiguration ..................................................................................... 3-29

3.5.5 Path Maximum Transmission Unit (PMTU) Discovery .............................. 3-30

3.5.6 Security Ramifications.............................................................................. 3-31

3.6 IPv6 and Routing..................................................................................................3-34

3.6.1 Specification Overview............................................................................. 3-34

3.6.2 Security for Routing Protocols .................................................................. 3-35
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3.6.3 Unknown Aspects .................................................................................... 3-37

3.7 IPv6 and the Domain Name System (DNS)..........................................................3-37

3.7.1 DNS Transport Protocol ........................................................................... 3-38

3.7.2 DNS Specification Overview .................................................................... 3-38

3.7.3 Security Impact and Recommendations ................................................... 3-39

4. IPv6 Advanced Topics .................................................................................................. 4-1

4.1 Multihoming............................................................................................................4-1

4.1.1 Differences between IPv4 and IPv6 Multihoming........................................ 4-1

4.1.2 SHIM6 Specification Overview ................................................................... 4-2

4.1.3 Security Ramifications for Multihoming....................................................... 4-4

4.2 IPv6 Multicast.........................................................................................................4-5

4.2.1 IPv6 Multicast Specifications...................................................................... 4-6

4.2.2 Differences between IPv4 and IPv6 Multicast............................................. 4-9

4.2.3 Multicast Security Ramifications................................................................. 4-9

4.2.4 Unresolved Aspects of IPv6 Multicast ...................................................... 4-10

4.3 IPv6 Quality of Service (QoS)...............................................................................4-10

4.3.1 IPv6 QoS Specifications........................................................................... 4-11

4.3.2 Differences between IPv4 and IPv6 QoS ................................................. 4-12

4.3.3 Security Ramifications.............................................................................. 4-12

4.3.4 Unresolved Aspects of IPv6 QoS ............................................................. 4-12

4.4 Mobile IPv6 (MIPv6) .............................................................................................4-13

4.4.1 MIPv6 Specification Overview.................................................................. 4-13

4.4.2 Differences from IPv4 Standards.............................................................. 4-16

4.4.3 Security Ramifications.............................................................................. 4-16

4.4.4 Unknown Aspects .................................................................................... 4-26

4.5 Jumbograms ........................................................................................................4-27

4.5.1 Specification Overview............................................................................. 4-27

4.5.2 Security Ramifications.............................................................................. 4-28

4.6 Address Selection ................................................................................................4-28

4.6.1 Specification Overview............................................................................. 4-28

4.6.2 Differences from IPv4 Standards.............................................................. 4-30

4.6.3 Security Ramifications.............................................................................. 4-30

4.6.4 Unknown Aspects .................................................................................... 4-31

4.7 Dynamic Host Configuration Protocol (DHCP) for IPv6.........................................4-31

4.7.1 Specification Overview............................................................................. 4-32

4.7.2 Differences from IPv4 Standards.............................................................. 4-34

4.7.3 Security Ramifications.............................................................................. 4-34

4.7.4 Unknown Aspects .................................................................................... 4-35

4.8 IPv6 Prefix Renumbering......................................................................................4-36

4.8.1 Specification Overview............................................................................. 4-36

4.8.2 Differences from IPv4 Standards.............................................................. 4-38

4.8.3 Security Ramifications.............................................................................. 4-38

4.8.4 Unknown Aspects .................................................................................... 4-39

5. IPv6 Security Advanced Topics ................................................................................... 5-1

5.1 Privacy Addresses..................................................................................................5-1

5.2 Cryptographically Generated Addresses ................................................................5-3

5.3 IPsec in IPv6 ..........................................................................................................5-4

5.3.1 Specification Overview............................................................................... 5-6

5.3.2 Differences from IPv4 Standards................................................................ 5-8
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5.3.3 Support for Multicast .................................................................................. 5-8

5.3.4 Status of IPsec and On-Going Work........................................................... 5-9

5.3.5 Security Ramifications.............................................................................. 5-15

5.3.6 Unknown Aspects .................................................................................... 5-16

5.4 Secure Stateless Autoconfiguration and Neighbor Discovery ...............................5-17

5.4.1 Using IPsec to Secure Autoconfiguration and ND .................................... 5-18

5.4.2 Using SEND to Secure Autoconfiguration and ND ................................... 5-19

5.4.3 Unknown Aspects .................................................................................... 5-20

6. IPv6 Deployment ........................................................................................................... 6-1

6.1 Security Risks ........................................................................................................6-1

6.1.1 Attacker Community................................................................................... 6-1

6.1.2 Unauthorized IPv6 Clients.......................................................................... 6-2

6.1.3 Vulnerabilities in IPv6................................................................................. 6-2

6.1.4 Dual Operations ......................................................................................... 6-4

6.1.5 Perceived Risk ........................................................................................... 6-4

6.1.6 Vendor Support.......................................................................................... 6-4

6.2 Addressing Security ...............................................................................................6-5

6.2.1 Numbering Plan ......................................................................................... 6-5

6.2.2 Hierarchical Addressing to Support Security Segmentation........................ 6-6

6.2.3 Problems with EUI-64 Addresses............................................................... 6-7

6.2.4 Address Management ................................................................................ 6-7

6.2.5 Privacy Extensions..................................................................................... 6-8

6.3 Transition Mechanisms...........................................................................................6-9

6.4 Dual Stack IPv4/IPv6 Environments .......................................................................6-9

6.4.1 Deployment of a Dual Stack Environment ................................................ 6-10

6.4.2 Addressing in a Dual Stack Environment ................................................. 6-11

6.4.3 Security Implications of a Dual Stack Environment................................... 6-11

6.5 Tunneling .............................................................................................................6-12

6.5.1 General Security Considerations for Tunneling ........................................ 6-13

6.5.2 Configured Tunneling............................................................................... 6-15

6.5.3 Automatic Tunneling ................................................................................ 6-16

6.5.4 6over4 Protocol........................................................................................ 6-16

6.5.5 6to4 and 6rd Protocols ............................................................................. 6-17

6.5.6 Automatic Intra-Site Tunnel Addressing Protocol (ISATAP) ..................... 6-19

6.5.7 Teredo Protocol........................................................................................ 6-22

6.5.8 Tunnel Brokers......................................................................................... 6-27

6.5.9 Automatic Tunneling of IPv4 over IPv6 (Dual Stack Transition Mechanism 

(DSTM))............................................................................................................... 6-28

6.5.10 Carrier-Grade NAT and Dual-Stack Lite ................................................... 6-29

6.6 Translation ...........................................................................................................6-31

6.6.1 SIIT .......................................................................................................... 6-32

6.6.2 NAT-PT.................................................................................................... 6-32

6.6.3 Replacing NAT-PT ................................................................................... 6-34

6.6.4 TRT.......................................................................................................... 6-35

6.6.5 Application Layer Translation ................................................................... 6-35

6.7 Other Transition Mechanisms...............................................................................6-36

6.8 The IPv6 Deployment Planning Process for Security............................................6-36

6.9 IPv6 Deployment ..................................................................................................6-37

6.9.1 Initiation Phase ........................................................................................ 6-38

6.9.2 Acquisition / Development Phase............................................................. 6-40
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6.9.3 Implementation Phase.............................................................................. 6-43

6.9.4 Operations / Maintenance Phase ............................................................. 6-45

6.9.5 Disposition Phase .................................................................................... 6-45

6.10 Summary..............................................................................................................6-46

List of Appendices

Appendix A— Acronyms and Abbreviations ......................................................................A-1

Appendix B— Resources .....................................................................................................B-1

List of Figures

Figure 2-1. The IPv6 Packet Header Format (Field Sizes in Bits)............................................ 2-4

Figure 3-1. IPv6 Address Format ........................................................................................... 3-3

Figure 3-2. 32-Bit Network Prefix ........................................................................................... 3-4

Figure 3-3. 48-Bit Network Prefix ........................................................................................... 3-4

Figure 3-4. 64-Bit Network Prefix ........................................................................................... 3-5

Figure 3-5. A Comparison of IPv4 and IPv6 Addressing....................................................... 3-12

Figure 3-6. The IPv6 Packet Header Format (Field Sizes in Bits)......................................... 3-17

Figure 3-7. Example IPv6 Packet Header ............................................................................ 3-18

Figure 3-8. Next Header Fields in IPv6 and Extension Headers........................................... 3-19

Figure 3-9. IPv6 Extension Header Chaining ....................................................................... 3-20

Figure 3-10. ICMPv6 Message Format................................................................................. 3-24

Figure 3-11. Example of Neighbor Discovery ....................................................................... 3-28

Figure 3-12. Example of Stateless Autoconfiguration........................................................... 3-30

Figure 3-13. Significance of MTU under IPv6....................................................................... 3-31

Figure 4-1. SHIM6 Protocol Stack .......................................................................................... 4-4

Figure 4-2. The Main MIPv6 Components............................................................................ 4-14

Figure 4-3. IKEv1 Identifiers used between a MN and its HA ............................................... 4-20

Figure 4-4. IKEv2 identifiers used between a MN and its HA ............................................... 4-21

Figure 4-5. Return Routability—Init Messages ..................................................................... 4-22

Figure 4-6. Return Routability—Keygen Replies .................................................................. 4-23

Figure 4-7. Reverse Routability—BU and BUA Protected with Kbm..................................... 4-24

Figure 5-1. Example of IPv6 Privacy Addressing.................................................................... 5-2

Figure 5-2. Generating Cryptographic Addresses from Public-Private Key Pairs.................... 5-3
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Figure 5-3. IPsec in the TCP/IP Protocol Stack ...................................................................... 5-5

Figure 5-4. Encryption and Authentication Algorithms for the IPsec Protocol ....................... 5-10

Figure 5-5. Cryptographic Algorithms for Use in IKEv2 ........................................................ 5-11

Figure 6-1. Example of Tunneling IPv6 over IPv4 Networks.................................................. 6-12

Figure 6-2. IPv6 over IPv4 Tunnels Transparent to the IPv4 Infrastructure .......................... 6-14

Figure 6-3. 6over4 Interface Addresses ................................................................................ 6-17

Figure 6-4. Example - Tunneling IPv6 over IPv4 Networks with ISATAP.............................. 6-21

Figure 6-5. Example - Tunneling IPv6 over IPv4 Networks with Teredo ............................... 6-23

Figure 6-6. Teredo Address .................................................................................................. 6-24

List of Tables

Table 3-1. Differences between IPv4 and IPv6....................................................................... 3-1

Table 3-2. IPv6 Address Types .............................................................................................. 3-6

Table 3-3. Assignment of Leftmost, Centermost, and Rightmost Bits ................................... 3-14

Table 3-4. IPv6 Extension Headers and Upper Layer Protocols........................................... 3-22

Table 3-5. ICMPv6 Error Messages and Code Type ............................................................ 3-25

Table 3-6. ICMPv6 Informational Messages......................................................................... 3-25

Table 3-7. ICMPv6 Recommended Filtering Actions – Must Not Drop & Should Not Drop ... 3-33

Table 3-8. ICMPv6 Recommended Filtering Actions – Should Define Policy & Should Drop 3-34

Table 4-1. IPv6 Scoped Multicast Values (from RFC 4291).................................................... 4-7
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Executive Summary

Due to the exhaustion of IPv4 address space, and the Office of Management and Budget (OMB) mandate 

that U.S. federal agencies begin to use the IPv6 protocol, NIST undertook the development of a guide to 

help educate federal agencies about the possible security risks during their initial IPv6 deployment. This 

document provides guidelines for organizations to aid in securely deploying IPv6. Since the majority of 

organizations will most likely run both IPv6 and IPv4 on their networks for the foreseeable future, this 

document speaks about the deployment of IPv6 rather than the transition to IPv6.

1

The deployment of IPv6 can lead to new challenges and types of threats facing an organization. The goals 

of this document are:

 To educate the reader about IPv6 features and the security impacts of those features

 To provide a comprehensive survey of mechanisms that can be used for the deployment of IPv6

 To provide a suggested deployment strategy for moving to an IPv6 environment 

The migration to IPv6 services is inevitable as the IPv4 address space is almost exhausted. IPv6 is not 

backwards compatible with IPv4, which means organizations will have to change their network 

infrastructure and systems to deploy IPv6. Organizations should begin now to understand the risks of 

deploying IPv6, as well as strategies to mitigate such risks. Detailed planning will enable an organization 

to navigate the process smoothly and securely. 

Federal agencies will most likely face security challenges throughout the deployment process, including:

 An attacker community that most likely has more experience and comfort with IPv6 than an 

organization in the early stages of deployment

 Lack of visibility to unknown or unauthorized IPv6 assets on existing IPv4 production networks

 Added complexity while operating IPv4 and IPv6 in parallel

 Lack of IPv6 maturity in security products when compared to IPv4 capabilities

Organizations planning the deployment of IPv6 should consider the following during the planning 

process:

 IPv6 is a new protocol that is not backward compatible with IPv4

 In most cases IPv4 will still be a component of IT infrastructure. As such, even after the deployment

of IPv6, organizations will require mechanisms for IPv6 and IPv4 co-existence.

 IPv6 can be deployed just as securely as IPv4, although it should be expected that vulnerabilities 

within the protocol, as well as with implementation errors, will lead to an initial increase in IPv6-

based vulnerabilities. As a successor to IPv4, IPv6 does incorporate many of the lessons learned by 

the Internet Engineering Task Force (IETF) for IPv4. 

 IPv6 has already been deployed and is currently in operation in large networks globally.

 

1

 Since many of the IPv6-related protocols, tools and mechanisms are typically referred to as transition mechanisms, this 

document does use the word transition in that context.
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To overcome possible obstacles associated with deploying IPv6, organizations should consider the 

following recommendations:

 Encourage staff to increase their level of knowledge of IPv6 to be parallel with their current 

understanding of IPv4

 Plan a phased IPv6 deployment utilizing a wide range of transition mechanisms to support business 

needs

 Plan for a long transition period with dual IPv4/IPv6 co-existence

Organizations that are not yet deploying IPv6 should implement the following recommendations:

 Block all IPv6 traffic, native and tunneled, at the organization's firewall. Both incoming and outgoing 

traffic should be blocked.

 Begin to acquire familiarity and expertise with IPv6, through laboratory experimentation and/or 

limited pilot deployments.

 Make organization web servers, located outside of the organizational firewall, accessible via IPv6 

connections. This will enable IPv6-only users to access the web server and aid the organization in 

acquiring familiarity with some aspects of IPv6 deployment.

Organizations that are deploying IPv6 should implement the following recommendations to mitigate IPv6 

threats:

 Apply different types of IPv6 addressing (privacy addressing, unique local addressing, sparse 

allocation, etc) to limit access and knowledge of IPv6-addressed environments.

 Manual entry of IPv6 addresses is prone to error because of their length and use of hexadecimal 

notation. Thus, the use of an automated address management tool is highly recommended.

 Develop a granular ICMPv6 filtering policy for the enterprise. Ensure that ICMPv6 messages that are 

essential to IPv6 operation are allowed, but others are blocked.

 Use IPsec to authenticate and provide confidentiality to assets that can be tied to a scalable trust 

model (an example is access to Human Resources assets by internal employees that make use of an 

organization’s Public Key Infrastructure (PKI) to establish trust).

 Identify capabilities and weaknesses of network protection devices in an IPv6 environment.

 Enable controls that might not have been used in IPv4 due to a lower threat level during initial 

deployment (implementing default deny access control policies, implementing routing protocol 

security, etc).

 Pay close attention to the security aspects of transition mechanisms such as tunneling protocols.

 IPv6 routers, packet filters, firewalls, and tunnel endpoints should enforce multicast scope boundaries 

and make sure that MLD packets are not routable.

 Be aware that switching from a NATted address environment to unique global IPv6 addresses will 

trigger a change in the FISMA system boundaries.
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After reviewing this document, the reader should have a reasonable understanding of IPv6 and how it 

compares to IPv4, security impacts of IPv6 features and capabilities, as-yet unknown impacts of IPv6 

deployment, and increased knowledge and awareness about the range of IPv4 to IPv6 transition 

mechanisms.
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1. Introduction

1.1 Authority

The National Institute of Standards and Technology (NIST) developed this document in furtherance of its 

statutory responsibilities under the Federal Information Security Management Act (FISMA) of 2002, 

Public Law 107-347.

NIST is responsible for developing standards and guidelines, including minimum requirements, for 

providing adequate information security for all agency operations and assets; but such standards and 

guidelines shall not apply to national security systems. This guideline is consistent with the requirements 

of the Office of Management and Budget (OMB) Circular A-130, Section 8b(3), ―Securing Agency 

Information Systems,‖ as analyzed in A-130, Appendix IV: Analysis of Key Sections. Supplemental 

information is provided in A-130, Appendix III.

This guideline has been prepared for use by Federal agencies. It may be used by nongovernmental 

organizations on a voluntary basis and is not subject to copyright, though attribution is desired. 

Nothing in this document should be taken to contradict standards and guidelines made mandatory and 

binding on Federal agencies by the Secretary of Commerce under statutory authority, nor should these 

guidelines be interpreted as altering or superseding the existing authorities of the Secretary of Commerce, 

Director of the OMB, or any other Federal official.

1.2 Purpose and Scope

The purpose of Guidelines for the Secure Deployment of IPv6 is to provide information security guidance 

to organizations that are planning to deploy IPv6 technologies or are simply seeking a better 

understanding of IPv6. The scope of this document encompasses the IPv6 protocol and related protocol 

specifications. IPv6-related security considerations are discussed with emphasis on deployment-related 

security concerns. The document also includes general guidance on secure IPv6 deployment and 

integration planning. 

1.3 Audience

This document is intended primarily for network engineers and administrators who are responsible for 

planning, building, and operating IP networks, as well as security engineers and administrators who are 

responsible for providing Information Assurance support. Anyone interested in deploying IPv6 

technologies and related security implications may also find the document useful. It is assumed that 

readers are already familiar with basic IPv4, data networking, and network security concepts.

1.4 Document Structure

The remainder of this document is composed of the following sections and appendices:

 Section 2 provides an introduction to IPv6, including its history, features, and comparisons with IPv4.

 Section 3 discusses in more detail IPv6 addressing, allocation, packet organization, and ICMPv6.

 Section 4 examines some of the more advanced features of IPv6 and the security implications like 

multihoming, multicast, QoS, Mobile IPv6, Jumbo grams and address selection.
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 Section 5 provides an introduction to some of the advanced security features included in IPv6 such as 

privacy address, IPsec, and secure stateless autoconfiguration and neighbor discovery.

 Section 6 covers the process of securely moving from IPv4 to IPv6 and discusses the risks, addressing 

security, various transition mechanisms and the transition process.

Appendix A provides a list of acronyms and abbreviations used in this document.

Appendix B lists references and other resources related to IPv6.
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2. Introduction to IPv6

Internet Protocol version 6 (IPv6) is a new network layer protocol. It is an enhancement to Internet 

Protocol version 4 (IPv4), the protocol in use since the 1970s. There are numerous upgrades in IPv6. 

Most significantly, in comparison with IPv4, IPv6 has increased its network address size from 32 bits to 

128. This provides more than enough addresses to satisfy the global demand for unique IP addresses.

This chapter provides an overview of IPv6 as a foundation for later sections. The section starts with the 

early history of IPv6 and the limitations of IPv4, followed by descriptions of the major features of the 

IPv6 specifications. This is followed by a threat comparison between IPv4 and IPv6 and concludes with 

motivations for deploying to IPv6.

2.1 Early History of IPv6

IPv4 was developed in the early 1970s for use in government and academic communities in the United 

States to facilitate communication and information sharing. Today’s networking demand, in particular 

web pages, email, peer-to-peer services, and the use of mobile devices, has grown well beyond its 

originators’ expectations. Widespread deployment and growth of networking technologies and mobile 

communications have surpassed IPv4’s ability to provide adequate globally unique address space2.

Efforts to develop a successor to IPv4 started in the early 1990s within the Internet Engineering Task 

Force (IETF)3. The objective was to solve the address space limitations as well as provide additional

functionality. The IETF started the Internet Protocol Next Generation (IPng) work in 1993 to investigate 

different proposals and to make recommendations for further actions. The IETF recommended IPv6 in 

1994. (The name IPv5 had previously been allocated to the experimental stream protocol.) Their 

recommendation is specified in RFC 17524, The Recommendation for IP Next Generation Protocol. 

Several proposals followed; the Internet Engineering Steering Group approved the IPv6 recommendation 

and drafted a Proposed Standard on November 17, 1994. RFC 18835, Internet Protocol, Version 6 (IPv6) 

Specification, was published in 1995. The core set of IPv6 protocols became an IETF Draft Standard on 

August 10, 1998. This included RFC 24606, which replaced RFC 1883.

IPv6 is a protocol designed to handle the growth rate of the Internet and to cope with the demanding 

requirements of services, mobility, and end-to-end security. The following sections describe the 

limitations of IPv4, the major features of IPv6, and motivations for deploying IPv6.

2.2 Limitations of IPv4

IPv47 was designed over 25 years ago for a relatively small number of users. At that time, it seemed 

unlikely that personal computing technology would become as widespread as it is today in the United 

States and worldwide. The rapid, universal adoption and growth of personal computing technologies, 

including IP networking, were unforeseen in 1981. At that time, the Internet was used almost exclusively 

 

2

See Hagen, Silvia. IPv6 Essentials 2nd Edition. O’Reilly Media, Inc., Sebastopol, CA 2006.

3 The IETF is an open international community charged with the evolution of the Internet architectures and standards. An 

Internet standard begins as an Internet Draft, which may then be published as a Request for Comments (RFC) memorandum. 

RFCs that are intended to become Internet standards evolve through a process known as the standards track. More 

information is available at http://www.ietf.org/overview.html.

4

IETF RFC 1752, The Recommendation for the IP Next Generation Protocol, is available at 

http://www.ietf.org/rfc/rfc1752.txt. 

5

IETF RFC 1883, Internet Protocol, Version 6 (IPv6) Specification, is available at http://www.ietf.org/rfc/rfc1883.txt. 

6

IETF RFC 2460, Internet Protocol, Version 6 (IPv6) Specification, is available at http://www.ietf.org/rfc/rfc2460.txt.

7

IETF RFC 791, Internet Protocol, is available at http://www.ietf.org/rfc/rfc791.txt. 
GUIDELINES FOR THE SECURE DEPLOYMENT OF IPV6 (DRAFT)

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by scholars and researchers, and IPv4’s 4.3 billion theoretically available addresses were considered to be 

more than sufficient.

As a result of growing Internet use, IPv4’s IP address capacity could not meet the demand. In practice, 

the supply of available IPv4 addresses has been limited since the early 1990s. Previously, an organization 

could apply for and receive an order of magnitude more IPv4 addresses than it could actually justify. 

However, as a result of regulatory advances, IP address allocations are now bound by strict policies that 

include formal justification to a Regional Internet Registry (RIR). During the 1990s, address allocation 

policies, along with address reuse and restriction technologies, were put into place to conserve IPv4 

addresses.

Technologies widely adopted in response to the constrained supply of IPv4 addresses are network address 

translation (NAT)8and classless inter-domain routing (CIDR)9; both are discussed in detail in Chapter 3. 

NAT essentially makes private IPv4 addresses (also known as non-routable addresses) at least partially 

functional on the global Internet. Despite their adaptation to other uses, private IPv4 addresses were 

designed for testing and other non-production purposes and never intended to be usable on the Internet. 

Nevertheless, a NAT-capable router positioned at an organization’s boundary has the ability to connect an 

entire network of privately addressed nodes within the organization to the Internet via a single routable IP 

address.

This technology saves IPv4 address space because nodes bearing private addresses are essentially ―on‖ 

the Internet but do not have globally unique IP addresses. Nevertheless, this address conservation 

technology can actually defeat certain aspects of the design intent of IPv4: network layer end-to-end 

security, peer-to-peer (host-to-host connectivity), and interoperability. A host using private addressing 

behind a NAT device cannot have a full peer-to-peer relationship with another host via the Internet or 

backbone enterprise network using globally unique addressing. This is because NAT does not allow 

communication sessions to be initiated from globally addressed nodes to the privately addressed nodes. 

NAT traversal technologies are available to work around some of these barriers. They typically work in 

one of two ways: (1) by maintaining stateful address lookup tables and redirecting inbound traffic to 

appropriate private addresses; (2) by employing application layer gateways that listen for specific port 

numbers and redirect traffic according to pre-configured parameters. Neither of these approaches to NAT 

traversal lends itself to scalability or guarantees compatibility with all forms of NAT, not to mention the 

efforts put into each of these work-arounds. In addition, neither approach lends itself to dynamic 

configuration when, for example, hosts move or networks are renumbered.

Another limitation of IPv4 is that its design favored interoperability over security and did not contain 

features that protected the confidentiality, integrity, or availability of communications. For example, IPv4 

could not cryptographically protect data from eavesdropping or manipulation, and IPv4 did not provide a 

method for endpoints to authenticate each other. Over time, the open nature of IPv4 was increasingly a 

target of exploitation. The multi-path nature of the Internet, which was designed for high availability, 

also allows multiple attack vectors for a variety of threats. As a response, new technologies were added 

to IPv4 to provide needed security functionality. With IPv6, these features were designed into the new 

protocol as mandatory components. 

 

8

IETF RFC 3022, Traditional IP Network Address Translator (Traditional NAT), is available at 

http://www.ietf.org/rfc/rfc3022.txt. 

9

IETF RFC 4632, Classless Inter-Domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan, is 

available at http://www.ietf.org/rfc/rfc4632.txt.
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2.3 Major Features of the IPv6 Specification

IPv6 has many features that make it significantly more powerful than its predecessor. These features 

include extended address space, autoconfiguration, header structure, extension headers, IPsec, mobility, 

quality of service, route aggregation, and efficient transmission. This section discusses these features and 

compares specific aspects of IPv4 and IPv6 to help establish an understanding of the protocols’ 

similarities and differences.

2.3.1 Extended Address Space

Each IPv4 address is 32 bits long and is written as four decimal numbers representing 8-bit octets and 

separated by decimal points or periods. An example address is 172.30.128.97. Each IPv6 address is 128 

bits long (as defined in RFC 4291) and is written as eight 16-bit fields in colon-delimited hexadecimal 

notation (an example is fe80:43e3:9095:02e5:0216:cbff:feb2:7474). This new 128-bit address space 

provides an enormous number of unique addresses, 2128 (or 3.4 x 1038) addresses, compared with IPv4’s 

2

32 (or 4.3 x 109

) addresses. That is enough for many trillions of addresses to be assigned to every human 

being on the planet. Moreover, these address bits are divided between the network prefix and the host 

identifier portions of the address. The network prefix designates the network upon which the host bearing 

the address resides. The host identifier identifies the node or interface within the network upon which it 

resides. The network prefix may change while the host identifier can remain static. The static host

identifier allows a device to maintain a consistent identity despite its location in a network. This 

enormous number of addresses allows for end-to-end communication between devices with globally 

unique IP addresses and can better support the delivery of peer-to-peer services with data-rich content 

such as voice and video. Chapter 3 describes IPv6 addressing in detail.

2.3.2 Autoconfiguration

Essentially plug-and-play networking, autoconfiguration, defined in RFC 4862, IPv6 Stateless Address 

Autoconfiguration10, is one of the most interesting and potentially valuable addressing features in IPv6. 

This feature allows devices on an IPv6 network to configure themselves independently using a stateless 

protocol. In IPv4, hosts are configured manually or with host configuration protocols like Dynamic Host 

Configuration Protocol (DHCP); with IPv6, autoconfiguration takes this a step further by defining a 

method for some devices to configure their IP addresses and other parameters without the need for a 

server. Moreover, it also defines a method, renumbering, whereby the time and effort required to 

renumber a network by replacing an old prefix with a new prefix are vastly reduced. Section 3.5.3 

describes autoconfiguration in detail.

2.3.3 Header Structure

The IPv6 header is much simpler than the IPv4 header and has a fixed length of 40 bytes (as defined in 

RFC 246011). 

Even though this header is almost twice as long as the minimum IPv4 header, much of the header is taken 

up by two 16-byte IPv6 addresses, leaving only 8 bytes for other header information. This allows for 

improved fast processing of packets and protocol flexibility. IPv6 datagrams use a structure that always 

includes a 40-byte base header and, optionally, one or more extension headers. This base header is like 

the header of IPv4 datagrams, though it has a different format. Five IPv4 header fields have been 

removed: IP header length, identification, flags, fragment offset, and header checksum. The IPv6 header 

 

10 IETF RFC 4862, IPv6 Stateless Address Autoconfiguration, is available at http://www.ietf.org/rfc/rfc4862.txt.

11 IETF RFC 2460, Internet Protocol, Version 6 (IPv6) Specification, is available at http://www.ietf.org/rfc/rfc2460.txt.
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fields are as follows: version (IP version 6), traffic class (replacing IPv4’s type of service field), flow 

label (a new field for Quality of Service (QoS) management), payload length (length of data following the 

fixed part of the IPv6 header), next header (replacing IPv4’s protocol field), hop limit (number of hops, 

replacing IPv4’s time to live field), and source and destination addresses. The IPv6 header format is

illustrated in Figure 2-1. The payload can be up to 64KB in size in standard mode, or larger with a jumbo 

payload option. Section 3.3 describes these headers in detail.

Version (4) Traffic Class (8) Flow Label (20 bits)

Payload length (16) Next Header (8) Hop Limit (8)

Source Address (128 bits)

Destination Address (128 bits)

Figure 2-1. The IPv6 Packet Header Format (Field Sizes in Bits)12

2.3.4 Extension Headers

An IPv4 header can be extended from 20 bytes to a maximum of 60 bytes, but this option is rarely used 

because it impedes performance and is often administratively prohibited for security reasons. IPv6 has a 

new method to handle options, which allows substantially improved processing and avoids some of the 

security problems that IPv4 options generated. IPv6 RFC 246013 defines six extension headers: hop-byhop option header, routing header, fragment header, destination options header, authentication header 

(AH), and encapsulating security payload (ESP) header. Each extension header is identified by the Next 

Header field in the preceding header. Section 3.4 describes extension headers in detail.

2.3.5 Mandatory Internet Protocol Security (IPsec) Support

IP security (IPsec) is a suite of protocols for securing Internet Protocol (IP) communications by 

authenticating the sender and providing integrity protection plus optionally confidentiality for the 

transmitted data. This is accomplished through the use of two extension headers: the Encapsulating 

Security Payload (ESP) and the Authentication Header (AH). The negotiation and management of IPsec 

security protections and the associated secret keys is handled by the Internet Key Establishment (IKE) 

protocol. IPsec is a mandatory part of an IPv6 implementation; however, its use is not required. IPsec is 

also specified for securing particular IPv6 protocols (e.g., Mobile IPv6 and OSPFv3). Section 5.3 

describes IPsec in detail.

2.3.6 Mobility

Mobile IPv6 (MIPv6) is an enhanced protocol supporting roaming for a mobile node, so that it can move 

from one network to another without losing IP-layer connectivity (as defined in RFC 377514). RFC 3344, 

 

12 Additional illustration and explanation of the major differences between the IPv6 and IPv4 headers is available from GAO’s 

document titled: Internet Protocol Version 6: Federal Agencies Need to Plan for Transition and Manage Security Risks. 

http://www.gao.gov/new.items/d05471.pdf.

13 IETF RFC 2460, Internet Protocol, Version 6 (IPv6) Specification, is available at http://www.ietf.org/rfc/rfc2460.txt.

14 IETF RFC 3775, Mobility Support in IPv6, is available at http://www.ietf.org/rfc/rfc3775.txt.
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IP Mobility Support for IPv415, describes Mobile IP concepts and specifications for IPv4. Nevertheless, 

using Mobile IP with IPv4 has various limitations, such as limited address space, dependence on address 

resolution protocol (ARP), and challenges with handover when a device moves from one access point to 

another. Mobile IPv6 uses IPv6’s vast address space and Neighbor Discovery16 to solve the handover 

problem at the network layer and maintain connections to applications and services if a device changes its 

temporary IP address. Mobile IPv6 also introduces new security concerns such as route optimization17

where data flow between the home agent and mobile node will need to be appropriately secured. 

Section 4.4 describes Mobile IPv6 in detail.

2.3.7 Quality of Service (QoS)

IP (for the most part) treats all packets alike, as they are forwarded with best effort treatment and no 

guarantee for delivery through the network. TCP adds delivery confirmations but has no options to 

control parameters such as delay or bandwidth allocation. QoS offers enhanced policy-based networking 

options to prioritize the delivery of information. Existing IPv4 and IPv6 implementations use similar 

QoS capabilities, such as Differentiated Services and Integrated Services, to identify and prioritize IPbased communications during periods of network congestion. Within the IPv6 header two fields can be 

used for QoS, the Traffic Class and Flow Label fields. The new Flow Label field and enlarged Traffic 

Class field in the main IPv6 header allow more efficient and finer grained differentiation of various types 

of traffic. The new Flow Label field can contain a label identifying or prioritizing a certain packet flow 

such as voice over IP (VoIP) or videoconferencing, both of which are sensitive to timely delivery. IPv6 

QoS is still a work in progress and security should be given increased consideration in this stage of 

development. Section 4.3 describes QoS in detail.

2.3.8 Route Aggregation

IPv6 incorporates a hierarchal addressing structure and has a simplified header allowing for improved 

routing of information from a source to a destination. The large amount of address space allows 

organizations with large numbers of connections to obtain blocks of contiguous address space. 

Contiguous address space allows organizations to aggregate addresses under one prefix for identification 

on the Internet. This structured approach to addressing reduces the amount of information Internet routers 

must maintain and store and promotes faster routing of data12. Additionally, it is envisioned that IPv6 

addresses will primarily be allocated only from Internet Service Providers (ISPs) to customers. This will 

allow for ISPs to summarize route advertisements to minimize the size of the IPv6 Internet routing tables. 

This is covered in more detail in Section 3.1.

2.3.9 Efficient Transmission

IPv6 packet fragmentation control occurs at the IPv6 source host, not at an intermediate IPv6 router. 

With IPv4, a router can fragment a packet when the Maximum Transmission Unit (MTU) of the next link 

is smaller than the packet it has to send. The router does this by slicing a packet to fit into the smaller 

MTU and sends it out as a set of fragments. The destination host collects the fragments and reassembles 

them. All fragments must arrive for the higher level protocol to get the packet. Therefore, when one 

fragment is missing or an error occurs, the entire transmission has to be redone. In IPv6, a host uses a 

procedure called Path Maximum Transmission Unit (PMTU) Discovery to learn the path MTU size and 

 

15 IETF RFC 3344, IP Mobility Support for IPv4, is available at http://www.ietf.org/rfc/rfc3344.txt.

16 IETF RFC 4861, Neighbor Discovery for IP Version 6 (IPv6), is available at http://www.ietf.org/rfc/rfc4861.txt.

17 IETF RFC 4449, Securing Mobile IPv6 Route Optimization Using a Static Shared Key, is available at 

http://www.ietf.org/rfc/rfc4449.txt.
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eliminate the need for routers to perform fragmentation. The IPv6 Fragment Extension Header is used 

when an IPv6 host wants to fragment a packet, so fragmentation occurs at the source host, not the router, 

which allows efficient transmission. PMTU is discussed in Section 3.5.5, and Section 4.5 describes 

efficient transmission in detail.

2.4 IPv4 and IPv6 Threat Comparison

The deployment of IPv6 can lead to new challenges with respect to the types of threats facing an 

organization. This section provides a high-level overview as to how threats differ from an IPv4 

environment to an IPv6 environment and combined IPv4-IPv6 environment. Following chapters provide 

additional details to these threats as required. It should be noted that many IPv6 threat discussions rely on 

IPsec to provide protection against attack. Due to issues with key management and overall configuration 

complexity (including applications), it is possible that IPsec will not be deployed much more than it is 

with IPv4 today for initial IPv6 use. IPsec is covered in detail in Section 5.

Network reconnaissance is typically the first step taken by an attacker to identify assets for exploitation. 

Reconnaissance attacks in an IPv6 environment differ dramatically from current IPv4 environments. Due 

to the size of IPv6 subnets (264 in a typical IPv6 environment compared to 28in a typical IPv4 

environment), traditional IPv4 scanning techniques that would normally take seconds could take years on 

a properly designed IPv6 network. This does not mean that reconnaissance attacks will go away in an 

IPv6 environment; it is more likely that the tactics used for network reconnaissance will be modified. 

Attackers will still be able to use passive techniques, such as DNS name server resolution, to identify 

victim networks for more targeted exploitation. Additionally, if an attacker is able to obtain access to one 

system on an IPv6 subnet, the attacker will be able to leverage IPv6 neighbor discovery to identify hosts 

on the local subnet for exploitation. Neighbor discovery-based attacks will also replace counterparts on 

IPv4 such as ARP spoofing.

Prevention of unauthorized access to IPv6 networks will likely be more difficult in the early years of IPv6 

deployments. IPv6 adds more components to be filtered than IPv4, such as extension headers, multicast 

addressing, and increased use of ICMP. These extended capabilities of IPv6, as well as the possibility of 

an IPv6 host having a number of global IPv6 addresses, potentially provides an environment that will 

make network-level access easier for attackers due to improper deployment of IPv6 access controls. 

Moreover, security related tools and accepted best practices have been slow to accommodate IPv6. Either 

these items do not exist or have not been stress tested in an IPv6 environment. Nevertheless, global 

aggregation of IPv6 addresses by ISPs should allow enhanced anti-spoofing filtering across the Internet 

where implemented.

Attacks that focus on exploitation above the IP layer, such as application-based attacks and viruses, will 

not see a difference in the types of threats faced in an IPv6 environment. Most likely, some worms will 

use modified IPv6 reconnaissance techniques for exploitation. Additionally, because many IPv4 

broadcast capabilities have been replaced with IPv6 multicast functionality, broadcast amplification 

attacks will no longer exist in an IPv6 environment.

From this comparison of IPv4 and IPv6 threats, one can surmise that IPv6 will not inherently be either 

more or less secure than IPv4. While organizations are in the process of deploying IPv6, the lack of 

robust IPv6 security controls (described in Section 6) and a lack of overall understanding of IPv6 by 

security staff may allow attackers to exploit IPv6 assets or leverage IPv6 access to further exploit IPv4 

assets. There is a very likely possibility that many IPv6 services will rely on tunneling IPv6 traffic in 

IPv4 for infrastructures that do support the protocol, which will also increase the complexity for security 

staff. Additionally, since IPv6 systems and capabilities are not yet widely used in production 

environments, there is a distinct possibility that the number of vulnerabilities in software from 
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2-7

implementing IPv6 capabilities could rise, as IPv6 networks are increasingly deployed.

Based on of the threat comparison between IPv4 and IPv6, the following actions are recommended to 

mitigate IPv6 threats during the deployment process:

 Apply different types of IPv6 addressing (privacy addressing, unique local addressing, sparse 

allocation, etc) to limit access and knowledge of IPv6-addressed environments.

 Develop a granular ICMPv6 filtering policy for the enterprise.

 Use IPsec to authenticate and provide confidentiality to assets that can be tied to a scalable trust 

model (an example is access to Human Resources assets by internal employees that make use of an 

organization’s Public Key Infrastructure (PKI) to establish trust).

 Identify capabilities and weaknesses of network protection devices in an IPv6 environment.

 Enable controls that might not have been used in IPv4 due to a lower threat level during initial 

deployment (implementing default deny access control policies, implementing routing protocol 

security, etc).

 Pay close attention to the security aspects of transition mechanisms such as tunneling protocols.

 On networks that are IPv4-only, block all IPv6 traffic.

2.5 Motivations for Deploying IPv6

IP technologies were invented in the United States, and the early adoption of those technologies occurred 

predominantly in the United States. As mentioned in Section 2.2, early address allocation policies were 

relatively relaxed and large quantities of IPv4 addresses were assigned upon request, even when those 

allocations were not thoroughly justified. This resulted in a high concentration of IPv4 address 

allocations in the United States, with more than half of all routable IPv4 addresses assigned to U.S.-based 

organizations. Some large U.S.-based Internet backbone service providers have more IPv4 addresses than 

all of the nations that comprise the Asian region of the world. 

These circumstances have left most of the world, especially Asia, with little choice other than to adopt the 

IPv6 specification if they are to become pervasive participants in IP technologies or the global Internet at 

large. Nations such as Japan have built IPv6-capable Internet infrastructures to support their growing 

demand for Internet connectivity. Further, the advanced state of wireless telecommunications in Asia 

produced an environment where globally unique IP addresses are required to enable the features of Third 

Generation (3G) wireless technologies. In essence, every mobile 3G device becomes a mobile personal 

computing platform, and each of those devices requires true end-to-end connectivity to realize its full 

potential. Because the United States did not have the same motivation to implement IPv6 as many other 

countries did, it is noticeably far behind them in adopting IPv6.

All organizations making use of IP networking should study and consider IPv6’s feature set when 

designing and managing their networks. Even with no intent to replace IPv4, the IPv6 security controls 

discussed later in this document should be planned and deployed to detect unauthorized use of IPv6. 

Fundamental knowledge of IPv6—what it is, what its attributes are, and how it operates—is critical to 

any organization. 

As the IPv6 protocol becomes increasingly ubiquitous, all enterprise and Internet-connected networks 

need to be prepared for specific threats and vulnerabilities that the new protocol will bring. For example, 

an IPv4-only network segment may contain several newly installed hosts that are both IPv4 and IPv6-
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capable, as well as hosts that have IPv6 enabled by default. This circumstance can come about simply as 

a result of the normal systems life cycles. Additionally, IPv6 could be enabled on a host by an attacker to 

circumvent security controls that may not be IPv6-aware. Taken further, IPv6 traffic could be 

encapsulated within IPv4 packets using readily available tools and services and exchanged with malicious 

hosts via the Internet.

Interoperability of geographically dispersed Internet-connected nodes may become a profit motivation for 

some organizations to deploy IPv6. For instance, content providers are making more multimedia features 

available via a diverse set of customer platforms. Mobile phones, handheld personal computers, notebook 

computers, desktop PCs, and home multimedia and gaming centers are all IPv4-capable today. 

Delivering multimedia content to those platforms is increasingly viable given the broadband network 

bandwidths available. Nevertheless, IPv4 clearly cannot address all of these devices without using an 

address conservation technology like NAT, and NAT by its nature denies true end-to-end IP connectivity. 

Multimedia service offerings and ultimately the market for those offerings are likely always to be 

constrained by IPv4, while IPv6 may prove to be an enabling technology. 

If an organization is not constrained by IPv4 address availability or the disruption that NAT causes to true 

end-to-end connectivity between nodes, it should still plan for a world in which IPv6 will eventually be 

ubiquitous. All major vendors of IT products are shipping IPv6-capable products. Wholesale 

replacement of computing platforms and network infrastructure as a deployment requirement is less likely 

now than only five years ago, since many operating systems and networking products contain a native 

IPv6 protocol stack. Also, tunneling IPv6 over the existing IPv4 Internet is possible today by using free, 

readily available tunnel clients. An end user may download client software, obtain a routable IPv6 

address, and begin tunneling IPv6 over IPv4 networks with few technical or administrative barriers. 

Many open source IP networking tools are IPv6-capable, as are many consumer-oriented wireless access 

points. Many consumers of personal computing and home networking equipment are IPv6-capable, even 

if they do not use the features.

Because of the increasing availability and use of IPv6, as well as many years of coexistence between IPv6 

and IPv4, management and technical experts within any organization should understand IPv6 

technology—its background, basis, and capabilities, and how they can mitigate risks associated with 

running dual stack IPv4 and IPv6 networks. In the context of this document, dual stack means that nodes 

are running both IPv4 and IPv6 protocols concurrently. The remainder of this document examines certain 

aspects of the IPv6 specification in detail, and discusses threats, vulnerabilities, and the mitigation of 

risks, in detail.
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3. IPv6 Overview

IPv6 is both simpler and more flexible than its IPv4 predecessor. Section 2 introduced a number of 

enhancements and features in IPv6. Most significant is the vast amount of address space, along with 

support for orderly address assignment and efficient network address aggregation on the Internet. 

Illustrated in Table 3-1 are some of the major differences between IPv4 and IPv6 followed by basic IPv6 

terminology used later in this guide. These differences can have implications for IPv6 security and are 

discussed throughout this and subsequent sections. 

Table 3-1. Differences between IPv4 and IPv618

Property IPv4 IPv6

Address size and 

network size

32 bits,

network size 8-30 bits

128 bits,

network size 64 bits

Packet header size 20-60 bytes 40 bytes

Header-level extension limited number of small IP options unlimited number of IPv6 

extension headers

Fragmentation sender or any intermediate router 

allowed to fragment

only sender may fragment

Control protocols mixture of non-IP (ARP), ICMP, 

and other protocols

all control protocols based on 

ICMPv6

Minimum allowed MTU 576 bytes 1280 bytes

Path MTU discovery optional, not widely used strongly recommended

Address assignment usually one address per host usually multiple addresses per 

interface

Address types use of unicast, multicast, and 

broadcast address types

broadcast addressing no longer 

used, use of unicast, multicast 

and anycast address types

Address configuration devices configured manually or 

with host configuration 

protocols like DHCP

devices configure themselves 

independently using stateless 

autoconfiguration or use DHCP

Basic Terms19

The following basic IPv6 definitions are important for any IPv6 discussion.

 Address. An IPv6-layer identifier for an interface or a set of interfaces.

 Node. A device on the network that sends and receives IPv6 packets

 

18 The National Security Agency document, Router Security Configuration Guide Supplement – Security for IPv6 Routers, is 

available at http://www.nsa.gov/snac/routers/I33-002R-06.pdf.

19 IETF RFC 2460, Internet Protocol, Version 6 (IPv6) Specification, is available at http://www.ietf.org/rfc/rfc2460.txt. 

Additional terminology is included in RFC 4862, IPv6 Stateless Address Autoconfiguration, is available at 

http://www.ietf.org/rfc/rfc4862.txt
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 Deprecated address. An address, assigned to an interface, whose use is discouraged, but not 

forbidden. A deprecated address should no longer be used as a source address in new 

communications, but packets sent from or to deprecated addresses are delivered as expected.

 Router. A node that sends and receives packets, and also accepts packets and forwards them on 

behalf of other nodes.

 Host. A node that may send and receive packets but does not forward packets for other nodes.

 Link. A communication facility or medium over which nodes can communicate at the link layer, i.e., 

the layer immediately below IPv6. Examples are Ethernets (simple or bridged); Point-to-Point 

Protocol (PPP); X.25, Frame Relay, or Asynchronous Transfer Mode (ATM) networks; and layer 

three (or higher) tunnels, such as tunnels over IPv4 or IPv6 itself.

 Link MTU. The maximum transmission unit (MTU), i.e., maximum packet size in octets, which can 

be conveyed over a link.

 Path MTU. The minimum link MTU of all the links in a path between a source node and a 

destination node.

 Upper Layer. A protocol layer immediately above IPv6. Examples are transport protocols such as 

Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), control protocols such as 

Internet Message Control Protocol (ICMP), routing protocols such as Open Shortest Path First 

(OSPF), and internet or lower-layer protocols being tunneled over (i.e., encapsulated in) IPv6 such as 

Internetwork Packet Exchange (IPX), AppleTalk, or IPv6 itself.

 Interface. The point at which a node connects to a link. Unicast IPv6 addresses are always 

associated with interfaces.

 Packet. An IPv6 header plus payload.

 Neighbors. Nodes attached to the same link.

This section provides general information about IPv6 as a foundation for later sections. The rest of this 

section is a resource for understanding the similarities and differences between IPv4 and IPv6, with a 

focus on addressing.20 Section 3.1 discusses IPv6 addresses, how the IPv6 address space is used, and 

IPv6 address types and scope. This is followed by a review of IPv4 addressing and IPv4 Classless InterDomain Routing (CIDR) addressing. Then IPv4 and IPv6 addressing are summarized and compared. 

Section 3.2 covers IPv6 address allocation. IPv6 headers, their formats, and fields are discussed in 

Section 3.3. Sections 3.4 through 3.7 cover extension headers, ICMPv6, IPv6 routing, and IPv6 Domain 

Name System (DNS) respectively. 

3.1 IPv6 Addressing

Described in RFC 4291, IPv6 addresses are 128 bits long and are written in what is called colon-delimited 

hexadecimal notation. An IPv6 address is comprised of eight distinct numbers representing 16 bits each 

and written in base-16 (hexadecimal or hex) notation. The valid hex digits are 0 through 9 and A through 

F and together with the colon separator are the only characters that can be used for writing an IPv6 

address. A comparison of IPv4 and IPv6 addressing conventions is illustrated in Figure 3-5 and discussed 

 

20 IETF RFC 4291, IP Version 6 Addressing Architecture, is available at http://www.ietf.org/rfc/rfc4291.txt. 
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in more detail in section 3.1.7.

An example of an IPv6 address is:

7f87:43e3:9095:02e5:0216:cbff:feb2:7474

Note that the address contains eight distinct four-place hex values, separated by colons. Each of these 

values represents 16 bits, for a total of 128 bits in the entire address.

IPv6 addresses are divided among the network prefix, the subnet identifier and the host identifier portions

of the address. The network prefix is the high-order bits of an IP address, used to identify a specific 

network and, in some cases, a specific type of address (refer to Table 3-2). The subnet identifier

identifies a link within a site. The subnet ID is assigned by the local administrator of the site; a single site 

can have multiple subnet IDs. This is used as a designator for the network upon which the host bearing 

the address is resident. The host identifier (host ID) of the address is a unique identifier for the node 

within the network upon which it resides. It is identified with a specific interface of the host. Figure 3-1

depicts the IPv6 address format with the network prefix, subnet identifier and host identifier.

Figure 3-1. IPv6 Address Format

RFC 4291 also describes the notation for prefixes. The network prefix is analogous, but not equivalent, 

to the subnet mask in IPv4. IPv4 addresses are written in Classless Inter-domain Routing (CIDR) 

notation, with a subnet mask that contains ―1‖s in the bit positions that identify the network ID (refer to 

Section 3.1.6). There is no subnet mask in IPv6, although the slash notation used to identify the network 

address bits is similar to IPv4’s subnet mask notation. The IPv6 notation appends the prefix length and is 

written as a number of bits with a slash, which leads to the following format:

IPv6 address/prefix length

The prefix length specifies how many of the address’s left-most bits comprise the network prefix. An 

example address with a 32-bit network prefix is:

7f87:43e3:9095:02e5:0216:cbff:feb2:7474/32

Quantities of IPv6 addresses are assigned by the international registry services and Internet service 

Network Prefix Subnet ID Host ID

n bits 64 bits

Identifies a link Interface ID, 64 bits

within a site

Identifies the address 

range assigned to a 

site

64 - n bits

128 bits
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providers (ISP) (refer to section 3.2.2) based in part upon the size of the entity receiving the addresses. 

Large, top-tier networks may receive address allocations with a network prefix of 32 bits as long as the 

need is justified. In this case, the first two groupings of hex values, separated by colons, comprise the 

network prefix for the assignee of the addresses. The remaining 96 bits are available to the local 

administrator primarily for reallocation of the subnet ID and the host ID. The subnet ID identifies a link 

within a site, which can have multiple subnet IDs. The host ID within a network must be unique and 

identifies an interface on a subnet for the organization, similar to an assigned IPv4 address. Figure 3-2

depicts an IPv6 address with 32 bits allocated to the network prefix.

Figure 3-2. 32-Bit Network Prefix

Government, educational, commercial, and other networks typically receive address allocations from toptier providers (ISPs) with a network prefix of 48 bits (/48), leaving 80 bits for the subnet identifier and 

host identifier. Figure 3-3 depicts an IPv6 address with 48 bits allocated to the network prefix.

Figure 3-3. 48-Bit Network Prefix

Subnets within an organization often have network prefixes of 64 bits (/64), leaving 64 bits for allocation 

to hosts’ interfaces. The host ID should use a 64-bit interface identifier that follows EUI-64 (Extended 

Unique Identifier) format when a global network prefix is used (001 to 111), except in the case when 

multicast addresses (1111 1111) are used20 21. Figure 3-4 depicts an IPv6 address with 64 bits allocated to 

the network prefix.

 

21 See also IEEE EUI-64, Guidelines for 64-Bit Global Identifier (EUI-64) Registration Authority, is available at 

http://standards.ieee.org/regauth/oui/tutorials/EUI64.html.

Network Prefix Subnet ID Host ID

48 bits 80 bits

7f87:43e3: 9095: 02e5: 0216:cbff:feb2:7474

Network Prefix Subnet ID Host ID

32 bits 96 bits

7f87:43e3: 9095:02e5: 0216:cbff:feb2:7474
GUIDELINES FOR THE SECURE DEPLOYMENT OF IPV6 (DRAFT)

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Figure 3-4. 64-Bit Network Prefix

3.1.1 Shorthand for Writing IPv6 Addresses

Due to their length, IPv6 addresses do not lend themselves to human memorization. Administrators of 

IPv4 networks typically can recall multiple IPv4 network and host addresses; remembering multiple IPv6 

network and host addresses is more challenging. The notation for IPv6 addresses may be compressed and 

simplified under specific circumstances.

One to three zeroes that appear as the leading digits in any colon-delimited hexadecimal grouping may be 

dropped. This simplifies the address and makes it easier to read and to write. For example:

7f22:065f:0aba:02e5:0000:0ee9:0000:0444/48 becomes

7f22:65f:aba:2e5:0:ee9:0:444/48 or

7f22:65f:aba:2e5:0:ee9:0:444/48

It is important to note that trailing zeroes may not be dropped, because they have intrinsic place value in 

the address format. 

Further efficiency is gained by combining all-zero portions of the address. Any colon-delimited portion 

of an address containing all zeros may be compressed so that nothing appears between the leading and 

trailing colons. For example:

7f22:065f:0055:0000:cd23:0000:0000:0205/48 becomes

7f22:65f:55:0:cd23::205/48

In this example, the sixth and seventh 16-bit groupings contain all zeroes; they were compressed by 

eliminating the zeroes completely, as well as the colon that divided the two groupings. Nevertheless, 

compressing an address by removing one or more consecutive colons between groups of zeroes may only 

be done once per address. The fourth 16 bit-grouping in the example also contains all zeroes, but in the 

condensed form of the address, it is represented with a single zero. A choice had to be made as to which 

group of zeroes was to be compressed. The example address could be written:

7f22:65f:55::cd23:0:0:205/48, but this is not as efficient as 7f22:65f:55:0:cd23::205/48.

It is important to note that both of the addresses in the preceding paragraph are properly formatted, but the 

latter address is shorter. Compression is just a convention for writing addresses, it does not affect how an 

address is used, and it makes no difference whether compression falls within the network prefix, host 

identifier, or across both portions of the address.

Network Prefix Subnet ID Host ID

7f87:43e3: 9095: 02e5: 0216:cbff:feb2:7474

64 bits 64 bits
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3.1.2 IPv6 Address Space Usage

This section introduces the different types of IPv6 addresses, their scope, and use. It introduces IPv6 

addressing as basic information needed for secure adoption and deployment of the protocol. RFC 4291,

20

IP Version 6 Addressing Architecture, is the authoritative source for information on IPv6 addressing, and 

it should be referenced for comprehensive details. Mechanisms for generating and assigning IPv6 

addresses are discussed in detail in subsequent sections of this document.

Table 3-2. IPv6 Address Types

Address Type Binary Prefix IPv6 notation Uses

Embedded IPv4 

address

00...1111 1111 

1111 1111

(96 bits)

::FFFF/96 Prefix for embedding IPv4 address in an 

IPv6 address

Loopback 00...1

(128 bits)

::1/128 Loopback address on every interface [RFC 

246022]

Global unicast 001 2000::3 Global unicast and anycast (allocated) [RFC 

429120]

Global unicast 01 – 1111 1000 0 4000::/2 –

FC00::/9

Global unicast and anycast (unallocated)

Teredo 0010 0000 0000 

0001 0000 

0000 0000 

0000

2001:0000::/32 Teredo [RFC 438023]

Nonroutable 0010 0000 0000 

0001 1101 

1000 1000 

0000

2001:D88::/32 Nonroutable. Documentation purposes only 

[RFC 384924]

6to4 0010 0000 0000 

0010

2002::/16 6to4 [RFC 3056]

6Bone 0011 1111 1111 

1110

3FFE::/16 Deprecated. 6Bone testing assignment, 

1996 through mid-2006 [RFC 370125]

Link-local unicast 1111 1110 10 FE80::/10 Link local unicast

Reserved 1111 1110 11 FEC0::/10 Deprecated. Formerly Site-local address 

space, unicast and anycast 

[RFC 387926]

Local IPv6 

address

1111 110 FC00::/7 Unicast Unique local address space, unicast 

and anycast

[RFC 419327]

Multicast 1111 1111 FF00::/8 Multicast address space [RFC 4291]

IPv6 addressing differs from IPv4 in several ways aside from the address size. First, addresses 

specifically belong to interfaces, not to nodes. Because addresses are not in short supply, interfaces often

 

22 IETF RFC 2460, Internet Protocol, Version 6 (IPv6) Specification, is available at http://www.ietf.org/rfc/rfc2460.txt.

23 IETF RFC 4380, Teredo: Tunneling IPv6 over UDP through Network Address Translations (NATs), is available at 

http://www.ietf.org/rfc/rfc4380.txt.

24 IETF RFC 3849, IPv6 Address Prefix Reserved for Documentation, is available at http://www.ietf.org/rfc/rfc3849.txt.

25 IETF RFC 3701, 6bone (IPv6 Testing Address Allocation), is available at http://www.ietf.org/rfc/rfc3701.txt.

26 IETF RFC 3879, Deprecating Site Local Addresses, is available at http://www.ietf.org/rfc/rfc3879.txt.

27 IETF RFC 4193, Unique Local IPv6 Unicast Addresses, is available at http://www.ietf.org/rfc/rfc4193.txt.
GUIDELINES FOR THE SECURE DEPLOYMENT OF IPV6 (DRAFT)

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have multiple addresses. As discussed in 3.1, IPv6 addresses consist of a network prefix in the higher 

order bits and an interface identifier in the lower order bits. Moreover, the prefix indicates a subnet or 

link within a site, and a link can be assigned multiple subnet IDs. 

Many IPv6 address ranges are reserved or defined for special purposes by the IETF’s IPv6 standards and 

by the Internet Assigned Number Authority (IANA). Table 3-2 lists the major assignments and how to 

identify the different types of IPv6 address from the high-order bits. 

All address ranges not listed in Table 3-2 are reserved or unassigned. IANA currently assigns only out of 

the binary range starting with 001.

28

3.1.3 IPv6 Address Types

IPv6 uses the notion of address types for different situations. These different address types are defined 

below: 

 Unicast Addresses. Addresses that identify one interface on a single node; a packet with a unicast 

destination address is delivered to that interface.

 Multicast Addresses. RFC 429120 defines a multicast address as, ―An identifier for a set of 

interfaces (typically belonging to different nodes). A packet sent to a multicast address is delivered to 

all interfaces identified by that address.‖ Although multicast addresses are common in both IPv4 and 

IPv6, in IPv6 multicasting has new applications. The single most important aspect of multicast 

addressing under IPv6 is that it enables fundamental IPv6 functionality, including neighbor discovery

(ND) and router discovery. Multicast addresses begin with FF00::/8. They are intended for efficient 

one-to-many and many-to-many communication. The IPv6 standards prohibit sending packets from a 

multicast address; multicast addresses are valid only as destinations. Multicast Addressing is 

discussed in Section 4.2. 

 Anycast Addresses. Addresses that can identify several interfaces on one or more nodes; a packet 

with an anycast destination address is delivered to one of the interfaces bearing the address, usually 

the closest one as determined by routing protocols. Anycast addressing was introduced as an add-on 

for IPv4, but it was designed as a basic component of IPv6.

The format of anycast addresses is indistinguishable from unicast addresses.

n bits 128 n bits

subnet prefix 00000000000000

The subnet prefix in an anycast address is the prefix that identifies a specific link. Anycast addresses 

are intended for efficiently providing services that any one of a number of nodes can perform (e.g., a 

Home Agent for a Mobile IP node). Anycast addresses may not be used as source addresses and, as 

of the writing of this guide, may only be assigned to routers. It should be noted that there are no 

defined mechanisms for security or registration for anycast, nor is there a way to verify that a 

response to a packet sent to an anycast address was sent by an interface authorized to do so. This 

leaves open the possibility of impersonating anycast servers.

 

28 IANA Internet Protocol Version 6 Address Space, is available at http://www.iana.org/assignments/ipv6-address-space
GUIDELINES FOR THE SECURE DEPLOYMENT OF IPV6 (DRAFT)

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 Broadcast Addresses. Broadcast addressing is a common attribute of IPv4, but is not defined or 

implemented in IPv6. Multicast addressing in IPv6 meets the requirements that broadcast addressing 

formerly fulfilled.

3.1.4 IPv6 Address Scope

The shortage of IPv4 addresses led to the designation of non-routable addresses in RFC 191829 and the 

widespread use of Network Address Translation (NAT) to share globally routable addresses (with certain 

limits placed on the hosts using so-called RFC 1918 addresses). IPv6 has no such shortage, so the use of 

NAT is unnecessary; nevertheless, the usefulness of addresses with limited scope was identified and 

maintained in IPv6. IPv6 addresses with different scopes were defined. In the original design for IPv6, 

link local, site local, and global addresses were defined; later, it was realized that site local addresses were 

not well enough defined to be useful. Site local addresses were abandoned and replaced with unique local 

addresses. Older implementations of IPv6 may still use site local addresses, so IPv6 firewalls need to 

recognize and handle site local addresses correctly.

The IPv6 standards define several scopes for meaningful IPv6 addresses:

 Interface-local. This applies only to a single interface; the loopback address has this scope.

 Link-local. This applies to a particular LAN or network link; every IPv6 interface on a LAN must 

have an address with this scope. Link-local addresses start with FE80::/10. Packets with link-local 

destination addresses are not routable and must not be forwarded off the local link.

Link-local address:

10 bits 54 bits 64 bits

1111 1110 10 0000..................0000 Interface ID

FE80/10 0000..................0000 Interface ID

Link-local addresses are used for administrative purposes such as neighbor and router discovery. 

 Site-local. This scope was intended to apply to all IPv6 networks or a single logical entity such as the 

network within an organization. Addresses with this scope start with FEC0::/10. They were 

intended not to be globally routable but potentially routed between subnets within an organization. 

Site local addresses have been deprecated and replaced with unique local addresses. 

 Unique local unicast. This scope is meant for a site, campus, or enterprise’s internal addressing. It 

replaces the deprecated site-local concept. Unique local addresses (ULAs) may be routable within an 

enterprise. Use of unique local addresses is not yet widespread; see RFC 4193,

27 Unique Local IPv6 

Unicast Addresses, for more information. 

 Global. The global scope applies to the entire Internet. These are globally unique addresses that are 

routable across all publicly connected networks.

 

29 IETF RFC 1918, Address Allocation for Private Internets, is available at http://www.ietf.org/rfc/rfc1918.txt.
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 Embedded IPv4 Unicast. The IPv6 specification has the ability to leverage existing IPv4 addressing 

schemes. The transition to IPv6 will be gradual, so two special types of addresses have been defined 

for backward compatibility with IPv4: IPv4-compatible IPv6 addresses (rarely used and deprecated in 

RFC 4291) and IPv4-mapped IPv6 addresses. Both allow the protocol to derive addresses by 

embedding IPv4 addresses in the body of an IPv6 address.20 An IPv4-mapped IPv6 address is used to 

represent the addresses of IPv4-only nodes as an IPv6 address, which allows an IPv6 node to use this 

address to send a packet to an IPv4-only node.

IPv4-compatible IPv6 address:

80 bits 16 bits 32 bits

0000.......................................0000 0000 IPv4 address

IPv4-mapped IPv6 address:

80 bits 16 bits 32 bits

0000.......................................0000 FFFF IPv4 address

The two IPv4 embedded address types are similar. The only difference is the sixth group of 16 bits. 

IPv4-compatible addresses set these to 0; IPv4-mapped addresses set these to 1. 

 Other address or Special Address types. IPv6 makes use of addresses other than those shown 

above. The unspecified address consists of all zeros (0:0:0:0:0:0:0:0 or simply ::) and may be the 

source address of a node soliciting its own IP address from an address assignment authority (such as a 

DHCPv6 server). IPv6-compliant routers never forward a packet with an unspecified address. The 

loopback address is used by a node to send a packet to itself. The loopback address, 0:0:0:0:0:0:0:1 

(or simply ::1), is defined as being interface-local. IPv6-compliant hosts and routers never forward 

packets with a loopback destination.

An essential design consideration for IPv6 is to simplify routing in enterprise and global networks. One 

of the intents of the IPv6 address schema is to facilitate hierarchical routing. Hierarchical routing in turn 

accelerates the end-to-end routing function, and routing table convergence and maintenance are vastly 

simplified. 

A typical IPv6 interface is configured to receive packets sent to several addresses. In addition to its link 

local and global unicast addresses, it may have a unique local address. It can also receive multicast 

messages sent to the all hosts and solicited node multicast addresses, as well as possibly to other multicast 

addresses. Finally, because of renumbering, multiple instances of some of these addresses may be active 

at once. How these addresses are selected is covered in the Sections 4.6, Address Selection, and 4.2, 

Multicast.

3.1.5 IPv4 Addressing

Each IPv4 address is 32 bits long and is written as four decimal numbers (0-255) representing eight bits 

each and separated by decimal points or periods. This is called dotted decimal. An example of an IPv4 

address is 172.30.128.97. Each IPv4 address is associated with an additional component called a subnet 
GUIDELINES FOR THE SECURE DEPLOYMENT OF IPV6 (DRAFT)

3-10

mask, which denotes how many high-order bits of the address are assigned to the network address.30 The 

remaining lower-order bits are used to identify the node.

Three primary subnet types or network classifications were designed for IPv4: Class A, Class B, and 

Class C.7 Typically, Class A networks were assigned to the early pioneers of the Internet. Class B 

networks typically were assigned to larger enterprises and service providers, and Class C network 

addresses usually were allocated to smaller organizations and treated as subnets of larger networks. The 

following are examples of IPv4 network addresses and their related subnet masks:

 Class A: 10.0.0.0 netmask 255.0.0.0 The first octet denotes the network and the remaining three 

octets (24 bits) are available to identify a node on that network. This means that over 16 million host 

addresses are available on this single Class A network. Class A allocations were often made to 

organizations that could never put 16 million distinct host addresses to use.

 Class B: 172.30.0.0 netmask 255.255.0.0 The first two octets denote the network and the remaining 

two octets (16 bits) are available to identify a node on that network. More than 65,000 distinct

addresses are available to network nodes in each Class B network. As with Class A allocations, this 

also produced a wasteful situation, because many recipients of Class B address allocations did not 

need to employ more than a small fraction of the addresses.

 Class C: 192.168.1.0 netmask 255.255.255.0 The first three octets denote the network and the final 

octet (8 bits) is available to identify a node on that network. This provides 254 addresses for 

allocation to network nodes (the all ones and all zeros addresses are reserved for other uses). More 

than two million Class C networks were available. Class C was the smallest, most granular network 

and host address allocation possible until the introduction of CIDR in 1993.

3.1.6 IPv4 Classless Inter-Domain Routing (CIDR) Addressing

CIDR addresses do not follow the Class A/B/C model. Netmasks in CIDR addresses are not confined to 

the octet boundaries of an IPv4 address. For example, the CIDR address 192.168.1.1/27 indicates that the 

IP address is 192.168.1.1 and the netmask splits the address after the 27th bit.31 The first 27 bits are 

designated for the network address, and the final five bits are available to provide 30 node or host 

addresses within that network. This allows for a much more granular approach to address allocation 

because ranges of addresses can be sized appropriately to the organization receiving them. Of equal 

importance to address conservation is the related mechanism for routing efficiency that CIDR brings. 

CIDR addressing allows multiple subnets, defined by common netmasks and having adjacent addresses, 

to be supernetted together. This means that multiple networks are aggregated and reachable under one 

routing table entry.

The Internet and many large enterprise networks are comprised of core routers (also known as backbone 

routers) that move vast amounts of data between networks. These routers connect disparate networks and 

thus make the Internet what it truly is: a network of networks. This same concept applies to large, 

geographically dispersed enterprise networks. Core routers maintain large, complex routing tables that 

contain accurate and timely information about how to reach nearly every network that is a part of the 

global Internet. 

The number of entries in these backbone routing tables has increased dramatically since CIDR addressing 

 

30 IETF RFC 950, Internet Standard Subnetting Procedure, is available at http://www.ietf.org/rfc/rfc0950.txt

31 If written in the classful notation described previously, it would be represented as 192.168.1.1 netmask 255.255.255.224.
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was introduced in 1993,32 despite the best intentions of supernetting CIDR address space together. As a 

result, core routers are burdened with ever increasing demands on their memory and processing 

capacities. In short, IPv4 does not lend itself to a highly scalable and efficient Internet backbone 

infrastructure.

Routing prefix aggregation allows contiguous groupings of CIDR addresses to be advertised to the global 

Internet as a single network rather than as multiple, distinct networks. Separate routing table entries no 

longer need to be made for each allocation of address space. Much like the concept of supernetting, this 

means that two distinct organizations sharing only one common attribute, their Internet Service Provider 

(ISP), can be attached to the Internet with unique IP addresses from an appropriately sized allocation. Yet 

those two distinct entities are reachable through the global Internet using only one globally unique 

network route. The two concepts discussed here, scalability of address allocations and routing efficiency 

through prefix aggregation, are integral aspects of the design of IPv6. 

3.1.7 Comparing IPv6 and IPv4 Addressing

IPv6 was designed to provide sufficient numbers of globally unique IP addresses to enable true peer-topeer communication between nodes on interconnected networks. It was also designed to provide a 

simplified hierarchical routing architecture across the Internet backbone—one that does not suffer from 

inefficiencies and increasing demands for memory and processing capacities on backbone Internet 

routers. Several accommodations have been made to retrofit these concepts onto IPv4, while these same 

concepts are native to the IPv6 specification.

IPv6 provides an enormous volume of unique addresses, about 3.4 x 1038 compared with IPv4’s roughly 

4.3 x 109 addresses. The number of possible IPv6 addresses is so large that many analogies and 

metaphors have been created that attempt to convey its magnitude. For example, if each IPv6 address 

weighed one gram, the sum total weight of all IPv6 addresses would be greater than the weight of 56 

Earths. The available address space under IPv6 is generally considered to be sufficient for the foreseeable 

future, even considering the historical growth of the Internet and the devices expected to connect to it in 

the future. See Figure 3-5 for a comparison of IPv4 and IPv6 addressing conventions.

 

32 IETF RFC 4632, Classless Inter-Domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan, is 

available at http://www.ietf.org/rfc/rfc4632.txt.
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Figure 3-5. A Comparison of IPv4 and IPv6 Addressing12

The constraints of IPv4 addressing were major considerations when IPv6 addressing was designed. The 

IPv6 addressing architecture is different not only in terms of address length, but also in terms of address 

types, address notation, and address aggregation. As discussed in Section 2, as well as later in Sections 3 

and 4, each of these differences enables new features in IPv6.

In both IPv4 and IPv6, Dynamic Host Configuration Protocol (DHCP) and the Domain Name System 

(DNS) can be used to assign, monitor, administer, and change IP addresses. IPv6 also includes an 

autoconfiguration capability for assigning IP addresses to hosts. Due to the smaller amount of address 

space available with IPv4, address management was often not complex, with some organizations 

manually tracking address assignments. The longer, more complex IPv6 addresses, as well as the much 

larger amount of address space, will most likely require the use of address management tools to avoid 

errors. 

3.2 IPv6 Address Allocations

IPv6 addresses have a flexible structure for address assignments. This enables registries, ISPs, network 

designers, and others to assign address ranges to organizations and networks based on different criteria, 

such as size of networks and estimated growth rate. Often, an initial assignment does not scale well if a 

small network becomes larger than expected and hence needs more addresses. The assignment authority 

may not be able to allocate contiguous addresses if they were already assigned to another network.

Section 3.2.1 describes address assignments using leftmost, rightmost, and centermost strategies. With 

these methods, organizations have the flexibility to aggregate their IPv6 address allocations efficiently. 

Section 3.2.2 explains how organizations can obtain IPv6 addresses allocations globally through several 

regional registry services.
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3.2.1 IPv6 Address Assignments

IPv6 network prefix assignment is the first step in network deployment. Understanding several methods 

such as leftmost, rightmost, and centermost helps provide for flexibility and efficient aggregation of an 

assigned IPv6 block, as described in RFC 3531,33 A Flexible Method for Managing the Assignment of Bits 

of an IPv6 Address Block. If done without foresight, boundaries between sub-allocations become difficult 

to move, and future increases in the use of address space cannot be kept contiguous.

The easiest but least flexible solution is to make block address assignment in order from the beginning of 

the organization’s allocated IPv6 block. For example, if an organization is assigned the prefix 

7f87:43e3:9095::/48, prefixes can be distributed in simple sequential order: 

7f87:43e3:9095:0001::/64

7f87:43e3:9095:0002::/64

7f87:43e3:9095:0003::/64

This is the simplest way to distribute address assignments, but it lacks consideration for future needs and 

does not take into account grouping networks by site for clean routing aggregation. Additionally, this 

method makes it impossible to make an existing network assignment larger and keep its address space 

contiguous.

RFC 3531 proposes a method to manage the assignment of bits of an IPv6 address block or range. First, 

the scheme defines parts of the IP address as p1, p2, p3, ....pN in order, so that an IP address is composed 

of these parts contiguously. Boundaries between each part are based on the prefix assigned by the next 

level authority. Part (p1) is the leftmost part probably assigned to a registry, Part (p2) can be allocated to 

a large ISP or national registry. Part (p3) can be allocated to a large customer or a smaller provider, etc. 

Each part can be of different length. 

p1 p2 p3 p4 .... pN

 IPv6 addresses 

The algorithm for allocating addresses is as follows: (p1) for the left-most part, assign addresses using the 

leftmost bits first; (pN) for the rightmost part, assign addresses using the rightmost bits first; and for all 

other parts (center parts), predefine an arbitrary boundary (prefix) and then assign addresses using center 

bits of the part being assigned first.

This algorithm increases the assigned bits in such way that it keeps unassigned bits near the boundaries 

between the parts. This means that the boundary between any two parts can be changed forward or 

backward, later on, up to the assigned bits. See Table 3-3 for the assignment of leftmost, centermost, and 

rightmost bits.

 

33 IETF RFC 3531, A Flexible Method for Managing the Assignment of Bits of an IPv6 Address Block, is available at 

http://www.ietf.org/rfc/rfc3531.txt.
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Table 3-3. Assignment of Leftmost, Centermost, and Rightmost Bits

– Leftmost – Centermost – Rightmost

Binary Hex Binary Hex Binary Hex

0000 0000 00 0000 0000 00 0000 0000 00

1000 0000 80 0000 1000 08 0000 0001 01

0100 0000 40 0001 0000 10 0000 0010 02

1100 0000 C0 0001 1000 18 0000 0011 03

0010 0000 20 0000 0100 04 0000 0100 04

1010 0000 A0 0000 1100 0C 0000 0101 05

0110 0000 60 0001 0100 14 0000 0110 06

1110 0000 E0 0001 1100 1C 0000 0111 07

0001 0000 10 0010 0000 20 0000 1000 08

A brief example based on RFC 3531 uses a provider called P1. This provider has been assigned the 

3ffe:0b00/24 prefix and wants to assign prefixes to its connected networks. It expects in the foreseeable 

future a maximum of 256 customers consuming 8 bits. One of these customers, named C2, expects a 

maximum of 1024 customers’ assignments under it, consuming 10 other bits (see RFC 3531 for greater 

detail). The assignment will be as follows, not showing the first 24 leftmost bits (3ffe:0b00/24 or 0011 

1111 1111 1110 0000 1011):

P1 assigns address space to its customers using leftmost bits:

1000 0000 : assigned to customer 1 (C1)

0100 0000 : assigned to customer 2 (C2)

1100 0000 : assigned to customer 3 (C3)

0010 0000 : assigned to customer 4 (C4)

C2 assigns address space to its customers (C2C1, C2C2, ....) using centermost bits:

0000 10000 : assigned to C2C1

0001 00000 : assigned to C2C2

0001 10000 : assigned to C2C3

Customer of C2 uses centermost bits for maximum flexibility and then the last aggregators (which should 

be networks within a site) will be assigned using rightmost bits.

Putting all bits together for C2C3:

P1 C2 C2C3

Hex 3ffe:0b00 40 0C

Binary 0011 1111 1111 1110 0000 1011 0100 0000 0000 1100 00

    

 growing bits
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By using this method, P1 will be able to expand the number of customers, and the customers will be able 

to modify their first assumptions about the size of their own customers, until the reserved bits are 

assigned.

Predicting future network requirements will always be a challenge with ever changing business needs and 

unforeseen technological advances. Nonetheless, a strategy to account for organizational needs, possible 

growth areas, and consideration to address assignment will provide as much downstream flexibility as 

possible. 

3.2.2 Obtaining Globally Routable IPv6 Address Space

The Internet Corporation for Assigned Names and Numbers (ICANN)34 and IANA35 have delegated most 

IPv6 address allocation to five Regional Internet Registries (RIR):

 Africa and the Indian Ocean (AfriNIC), http://www.afrinic.net/ or 

http://www.afrinic.net/registrationServices.htm

 Australia, Oceania, and most of Asia (APNIC), http://www.apnic.net/ or 

http://www.apnic.net/services/ipv6_guide.html

 Europe, parts of Asia, and the Middle East (RIPE NCC), http://www.ripe.net/ or 

http://www.ripe.net/rs/index.html

 Latin America and the Caribbean (LACNIC), http://www.lacnic.net/ or http://lacnic.net/en/btIPv6.html

 North America (ARIN), http://www.arin.net/ or http://www.arin.net/registration/ipv6/index.html. 

ISPs find information about their regional registries at these Web sites. Organizations and end users get 

their address allocations from their ISPs. Normally, a RIR allocates a /32 address to qualified ISPs, 

which are called Local Internet Registries (LIR), and the ISP allocates /48 addresses to its customers36.

American Registry for Internet Numbers (ARIN) is allowing some large government agencies to get 

provider independent (PI) IPv6 address assignments, defined in ARIN’s Number Resource Policy Manual 

(NRPM) Section 6.5.8, Direct Assignments from ARIN to end-user organizations37. To qualify for a 

direct assignment, an organization must not be an IPv6 local Internet registry and must qualify for an IPv4 

assignment or allocation from ARIN under the IPv4 policy currently in effect. If these criteria are met, an 

organization is eligible to receive a minimum assignment of /48, and with justification can request 

additional subnets. These assignments will be made from a distinctly identified prefix with a reservation 

of growth of at least /44. 

 

34 ICANN Regional Internet Registries is available at http://aso.icann.org/rirs/. 

35 IANA IP Address Services is available at http://www.iana.org/ipaddress/ip-addresses.htm. 

36 ICANN Global Policy for Allocation of IPv6 Space is available at http://aso.icann.org/docs/aso-global-ipv6.pdf and 

http://aso.icann.org/docs/rir-policy-matrix.html#3. 

37 ARIN IPv6 Policies section 6.5.8 Direct Assignments from ARIN to end-user organizations is available at 

http://www.arin.net/policy/nrpm.html#six58. Additional information ARIN’s Policy Proposal 2005-1: Provider-independent 

IPv6 Assignments for End Sites is available at http://www.arin.net/policy/proposals/2005_1.html. 
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From one point of view, the case for PI assignments can allow for a small number of large organizations 

to avoid a significant expense due to address renumbering. In addition, organizations may not want to be 

locked in to a specific Internet provider. On the other hand, the main concerns regarding PI assignment 

include two major issues. The first is the possibility of a large increase in the size of the IPv6 default-free 

routing table; these tables generally point only to top-level domains of aggregated routes. PI assignments 

do not fit into the normal aggregation and will increase the size of these tables. Secondly, the fear is that 

early adopters, similarly to IPv4, would have an unfair advantage vis à vis those who adopted later. 

IPv6 address allocation is designed to allow routing prefix aggregation. IPv6 network addresses may be 

aggregated in the same sense that IPv4 CIDR addresses are. IPv6 address allocation is based on the 

hierarchy mentioned previously, and allocated blocks of addresses are widely dispersed with top-tier 

allocations having network prefixes of 32 bits. This leaves 96 bits’ worth of addresses that can all be 

aggregated through a single route advertisement on the Internet backbone. 

Consider routing prefix aggregation for a large backbone service provider. The service provider, 

hypothetically, receives a block of address space with a 32-bit network prefix. In turn, the provider 

allocates this address space to customers. Those customers could be multiple regional network service 

providers or large enterprises that receive blocks of addresses with 48-bit network prefixes from that 

single large backbone service provider. Subnets within those enterprises and smaller regional service 

providers may have address space with 64-bit network prefixes. 

This arrangement may easily result in tens of millions of nodes attached to millions of subnets, all of 

which are aggregated and reachable via the global Internet through one route on the Internet’s backbone 

routers.

IPv6 address allocation is a work in progress. RFC 3177,

38 IAB/IESG Recommendations on IPv6 Address 

Allocations to Sites, documents an ongoing effort to provide the latest information for the Internet 

community regarding current practices, status, and clarifications for IPv6 address allocations. ARIN39

will document its policy as well.

3.3 IPv6 Header Types, Formats, and Fields

The design of the IPv6 header is the culmination of lessons learned from more than 20 years of 

experience with IPv4. Two primary design goals for the new header were efficiency and extensibility. 

The IPv6 header is always 40 bytes long and contains only eight fields, whereas IPv4 headers may be as 

short as 20 bytes or as long as 60 bytes and contain at least 12 different fields (some of which may be 

unused). When comparing these attributes, it becomes apparent that the IPv6 header is simpler and more 

efficient to process. Three examples are:

 The checksum has been removed, because error checking is usually performed in link layer and 

transport layer protocols.

 Fragmentation has been relegated to an extension header, the minimum MTU has been increased to 

1280 bytes, and fragmentation and reassembly are only performed by endpoints.

 Routers have to examine more than the 40-byte header only when the Next Header (NH) field is zero.

 

38 IETF RFC 3177, IAB/IESG Recommendations on IPv6 Address Allocations to Sites, is available at 

http://www.ietf.org/rfc/rfc3177.txt.

39 ARIN IPv6 Policies is available at http://www.arin.net/policy/nrpm.html#ipv6. 
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The design also pays careful attention to alignment for 64-bit processors; for example, the addresses are 

aligned on 64-bit boundaries.

The constant size of IPv6 headers makes the header length field found in IPv4 unnecessary. Routers and 

intermediate nodes handling the packets are not required to accommodate variability in the length of the 

headers, which expedites packet handling. The IPv6 header format is illustrated in Figure 3-6.

Version (4) Traffic Class (8) Flow Label (20 bits)

Payload length (16) Next Header (8) Hop Limit (8)

Source Address (128 bits)

Destination Address (128 bits)

Figure 3-6. The IPv6 Packet Header Format (Field Sizes in Bits)40

The fixed length of the IPv6 header does not preclude flexibility in favor of function. Options are 

handled with extension headers, which are described in detail in the next section. The following are the 

eight fields in the fixed IPv6 header:

 Version. This is the version of the protocol. This is a 4-bit value and must equal 6 (in binary, 0110).

 Traffic Class. Traffic class indicates the type of traffic or service. This eight-bit value is the same as 

type of service (ToS) in IPv4 and tags packets for special treatment during transmission. RFC 2474,

41

Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers, describes 

how the Traffic Class field in IPv6 can be used. The Traffic Class field in the IPv6 header is referred 

to as the DS field in RFC 2474, as well as the ToS field in the IPv4 header.

 Flow Label. This is a 20-bit value used to identify packets belonging to the same flow or stream of 

data. It plays an important role in Quality of Service (QoS) differentiation under IPv6 (discussed in 

Section 4.3).

 Payload Length. Payload length is the length of data carried after the fixed IPv6 header. This 16-bit 

field identifies the length of the payload to which the 40-byte header is attached. The Payload Length 

field represents the payload length as an unsigned integer with a maximum value of 65535, so the 

maximum length is 40 + 65535 = 65575. (This limit can be extended with the Jumbogram Hop-byHop Option discussed in Section 4.5.)

 Next Header. Next Header (NH) contains a protocol number for an extension header or upper layer 

protocol. Called the Protocol Type field in IPv4, the NH field is part of a chain of headers. See 

Section 3.4 for a complete description.

 Hop Limit. Hop Limit defines the maximum number of hops a packet can transit, the same as the 

Time to Live (TTL) field in IPv4. (The IPv4 TTL was originally defined as a number of seconds, but 

it almost always means hop count.) Intermediate nodes decrement this unsigned eight-bit value by 

 

40 IETF RFC 2460, Internet Protocol, Version 6 (IPv6) Specification, is available at http://www.ietf.org/rfc/rfc2460.txt.

41 IETF RFC 2474, Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers, is available at 

http://www.ietf.org/rfc/rfc2474.txt.
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one for each node the packet traverses. For example, if a packet source sets the Hop Limit to four and 

there are four routers and five hops in the path between source and destination, the packet is discarded 

by the fourth router. That packet never reaches the destination, because the packet has a hop limit 

value of zero after being processed by the fourth router. Generalized TTL Security Mechanism 

(GTSM) is designed to protect a router or host’s TCP/IP based control plane from various attacks

originating off of the local link. It is a simple security mechanism that avoids these remote attacks by 

requiring a maximum TTL or Hop Limit for incoming packets. Any packets from a remote attacker 

would have to travel through at least one intervening router, would have a lower-than-maximum TTL 

or Hop Limit (255 hops), and would be dropped on receipt. This mechanism based on an expected 

TTL value can provide a simple and reasonable defense against infrastructure attacks based on forged 

protocol packets from external sources. This is further described in RFC 3682.

42

 Source Address. This is a 128-bit value representing the unicast IPv6 address of the packet’s source. 

IPv6 addressing is discussed in Section 3.1.

 Destination Address. This is a 128-bit value representing the IPv6 address of the packet’s 

destination. It may be a unicast, multicast, or anycast address.

A packet capture of an IPv6 header is illustrated in Figure 3-7.

Figure 3-7. Example IPv6 Packet Header

3.4 IPv6 Extension Headers

Extension headers provide major services and functions for the IPv6 protocol. As discussed, the IPv6 

header is much simpler with its eight fields and 40-byte header allowing faster processing. Moreover, 

IPv6 has a new way to deal with options that has substantially improved processing: it handles options in 

additional headers called extension headers.

2

 Extension headers are inserted into a packet only if the 

options are needed.

Within the IPv6 packet header and its eight fields, the first extension header is identified in the NH field. 

In IPv6, optional Internet-layer information is encoded in separate headers that may be placed between 

the IPv6 header and the upper-layer header in a packet.

43

 There are a small number of such extension 

headers, each identified by a distinct NH value (see Table 3-4, IPv6 Extension Headers and Upper Layer 

Protocols). An IPv6 packet may carry zero, one, or more extension headers, each identified by the NH 

field of the preceding header and thus forming a chain illustrated below in Figure 3-8. The NH fields

indicate the Routing extension header; next, the Fragment extension header; the ESP extension header;

and, finally, the TCP header.

 

42 IETF RFC 5082, The Generalized TTL Security Mechanism (GTSM), is available at http://www.ietf.org/rfc/rfc5082.txt.

43 IETF RFC 2460, Internet Protocol, Version 6 (IPv6) Specification, is available at http://www.ietf.org/rfc/rfc2460.txt.
GUIDELINES FOR THE SECURE DEPLOYMENT OF IPV6 (DRAFT)

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Figure 3-8. Next Header Fields in IPv6 and Extension Headers

Except for Hop-by-Hop Options, extension headers are examined or processed only by the node identified 

in the Destination address field of the IPv6 header (or a set of nodes, in the case of multicast)2and must 

be processed strictly in the order in which they appear in the packet. The Hop-by-Hop Option Header is 

indicated by the value zero (0) in the NH field and requires that information within the packet be 

examined and processed by every node along the path of the packet.

TCP Header 

+ Data IPv6 Header

Next Header =

Routing

Fragmentation

Header

Next Header =

Security (ESP)

Security Header 

(ESP)

Next Header =

TCP 

Routing

Header

Next Header =

Fragment
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Figure 3-9. IPv6 Extension Header Chaining18

Figure 3-9 illustrates further how the NH field in the IPv6 packet header points to the following header in 

a chain of NHs, which defines the different parts of the payload in the packet. In this example, the 

payload of the last extension header in the packet contains the type of the upper-layer protocol, e.g., TCP

(value 6) or UDP (value 17). See Table 3-4 for the most common NH values. The IPv6 specification 

(RFC 2460) defines six NH values or extension headers. Other extension headers such as the Mobility 

Header are defined elsewhere.

 Hop-by-Hop Option header. NH value of 0 in the IPv6 base header. This header must occur first 

and is used to carry optional information that must be examined by every node along a packet’s 

delivery path. When the NH value is zero, then the node knows to examine the contents of one or 

more options contained in the extension header. An example is the Jumbogram IPv6 option, which 

allows IPv6 to transport packets larger than normal. Jumbograms are discussed in Section 4.5.

 Routing Header. NH value of 43 in the immediately preceding header. The Routing Header is used 

by an IPv6 source (sending host) to list one or more intermediate nodes to be traversed on the way to 
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a packet’s destination. The sending host sets the NH extension to direct a packet through a certain 

path. In IPv4, this is called the Loose Source and Record Route option. The Type 0 Routing Header 

was identified as a security risk, because it may allow attackers to bypass firewalls and carry out 

denial of service or other attacks. For these reasons the IETF deprecated the use of Type 0 Routing 

Headers. The Mobile IPv6 specification defines a Type 2 Routing Header which allows the data 

exchange between the care-of address of a mobile node and a correspondent node without being 

routed through the home agent. Mobile IPv6 is discussed in Section 4.4.

 Fragment header NH value of 44 in the immediately preceding header. This header is used by an 

IPv6 source to send a packet larger than the Path Maximum Transmission Unit (PMTU) to its 

destination. Each fragment of a packet must have the same identification value as well as identical 

source and destination addresses. Fragmentation in IPv6 is performed only by source nodes, not by 

routers along a packet’s delivery path as allowed in IPv4.

 Authentication header. NH value of 50 in the immediately preceding header. The Authentication 

header is used to provide connectionless integrity and data origin authentication for IP datagrams and 

to provide protection against replays. Also known as IPsec Authentication Header (AH), this 

capability is used in IPv4 as well as IPv6. IPsec is covered in Section 5.3.

 Encapsulating Security Payload header. NH value of 51 in the immediately preceding header. 

This header is used to provide confidentiality, data origin authentication, connectionless integrity, an 

anti-replay service (a form of partial sequence integrity), and limited traffic flow confidentiality. 

Also known as the IPsec Encapsulating Security Payload header (ESP), this capability is used in IPv4 

as well as IPv6 to provide the same functions as AH and also, optionally, confidentiality. IPsec as 

well as AH and ESP is covered in Section 5.3.

 Destination Options header. NH value of 60 in the immediately preceding header. This header is 

used to carry optional information that needs to be examined only by a packet’s destination node(s). 

Intermediate nodes (for which the IPv6 base header destination address is not the address of the 

current node or more Routing Header processing is required) do not examine the Destination 

Options header. The Destination Options header works similarly to the Hop-by-Hop Option 

header, as it may carry one or more options, where each option is encoded in type-length-value 

format. A node receiving a packet that matches the IPv6 destination address examines the NH field, 

notes the presence of a Destination Options, and processes the Destination Options before forwarding 

the payload to the upper layer protocol. A Destinations Options header could appear before or after 

an ESP header; however, it should be placed after an ESP header for increased protection.

 Mobility header. NH value of 135 in the immediately preceding header. This header is used by a 

mobile node, corresponding node, and home agent to manage mobile IPv6 bindings. Mobile IPv6 

(MIPv6) provides enhanced security, streamlined administrative protocols, and greater efficiency

compared with Mobile IPv4. MIPv6 is covered in Section 4.4.

Extension headers are established at the source before transmission of the packet. Their order and 

contents are not altered by intermediary nodes. 

Extension headers provide important services and functions for the IPv6 protocol. They may direct 

intermediary nodes about how packet payloads are to be handled prior to reaching their ultimate 

destination. Defined extension headers for the IPv6 protocol should occur in a recommended order. RFC 

2460, Internet Protocol, version 6 (IPv6) Specification, indicates that future extension header 

specifications may be more precise when it comes to ordering. To accommodate the definition and 

deployment of additional extension headers, each extension header includes information that instructs the 
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receiver how to behave if it does not recognize a new extension header type. The two basic types of 

behavior are: skip the unrecognized extension header but continue processing the packet or discard the 

packet.

The following table illustrates the recommended order for extension headers. Only the Hop-by-Hop 

Options are required by the specification to be placed immediately after the IPv6 header (literally the 

IPv6 ―next header‖). Note that more than one extension header may be used in a packet. In those cases, 

RFC 2460 recommends the first four extension headers be prioritized according to the first four entries in 

the table below.

Table 3-4. IPv6 Extension Headers and Upper Layer Protocols44

Extension Header Type Remarks

Hop-by-hop Options 0 used for options that apply to intermediate routers

Routing 43 used for source routing

Fragment 44 processed only by the final recipient

Destination Options 60 used for options that apply only for the final recipient

Authentication header (AH) 50 used for IPsec integrity protection

Encapsulating Security Payload 

(ESP)

51 used for IPsec integrity and confidentiality protection

Mobility 135 used for managing mobile IPv6 bindings 

Protocol Type Remarks

TCP 6 protocol type, same as IPv4

UDP 17 protocol type, same as IPv4

IPv6-in-IPv6 41 protocol type for IPv6 in IPv6 tunnels

GRE 47 protocol type for Generic Routing Encapsulation tunnels

ICMPv6 58 protocol type, Internet Control Message Protocol for IPv6

No next header 59 dummy packet, often used with ESP

OSPF 89 protocol type, Open Shortest Path First version 3 routing 

protocol

PIM 103 protocol type, Protocol Independent Multicast routing

SCTP 132 protocol type, Stream Control Transmission Protocol

A NH value of 43 causes the node specified in the Destination Address to examine the Routing Header

for further routing instructions. A Routing Header initially contains (1) an ordered list of intermediate 

Destination Addresses that the packet traverses on its way to its final destination at the end of the list and 

(2) a pointer to the first one of these. Each time the Destination Address in the IPv6 header is reached, 

the next Destination Address from the Routing Header is swapped with the Destination Address in the 

IPv6 header, and the pointer in the Routing Header is advanced until the final destination is reached. 

Note that, from a security point of view, simply examining the Destination Address in the IPv6 header 

may not provide all of the information needed to make packet filtering decisions.

The NH field and the related extension headers provide IPv6 with flexibility and extensibility, while 

taking advantage of fixed-length headers with a reduced number of fields as compared with IPv4. NH

 

44 A full list of Extension Headers is available at IANA, Protocol Numbers, at http://www.iana.org/assignments/protocolnumbers. 
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functionality in IPv6 provides the foundation for enhanced services such as IPv6 security and mobility. 

They should be kept in mind when securing IPv6 networks, to be discussed later in this document.

Using extension headers can have a number of security implications. Extension headers incorporate 

additional complexity for the purpose of traffic filtering. An example is enforcing a policy that blocks

IPv6 traffic with mobility headers if IPv6 mobility is not being used by an organization. Incorporating 

extension header filtering policies may also impact a device’s overall performance. Filtering is discussed 

in greater detail in Section 6, and ICMPv6 recommended filtering recommendations are provided in 

Tables 3-7 and 3-8. Extension headers can also be used as a ―covert channel‖ to hide communications 

between two systems, e.g., in Destination Options. 

3.5 Internet Control Message Protocol version 6 (ICMPv6)

The IPv6 specifications redefine the Internet Control Message Protocol (ICMP) of IPv4 with a number of 

additions and changes. The resulting protocol is documented in RFC 444345 and called ICMPv6. 

Specific details regarding ICMPv6 are provided here, along with examples of how the protocol differs 

from its counterpart under IPv4.

ICMPv6 is an integral aspect of the IPv6 specification. It reports errors if packets cannot be processed 

properly and sends informational messages about the status of the network. An operational IPv6 network 

depends upon proper implementation and functionality of ICMPv6. To achieve secure IPv6 operations, it 

is crucial that network administrators and managers understand the design of ICMPv6 and how it 

functions. Managers of IPv4-only networks should consider adding the capability of detecting ICMPv6 

traffic to enhance security on their networks.

ICMPv6 provides IPv6 with administrative and network diagnostic functions. ICMPv6 provides familiar 

capabilities like ping and destination unreachable. In IPv6, as in IPv4, ping can be used by network 

administrators as a diagnostic tool to confirm that a node’s address is properly configured and responsive 

to specific ICMPv6 requests, called echo requests. ICMPv6 also makes new features like Neighbor

Discovery (ND) and path MTU discovery possible within IPv6. ND, described in RFC 486146, is the 

process by which an IPv6 node may learn important information such as link layer addresses of interfaces 

on its own link. 

ND effectively replaces the Address Resolution Protocol (ARP) used with IPv4. ND’s multicast 

messages eliminate the need for the link-level broadcast messages associated with ARP. 

3.5.1 ICMPv6 Specification Overview

This section provides an overview of the ICMPv6 specification. The intent of this section is to introduce 

the reader to fundamentals of the ICMPv6 specification: message format, error handling, and diagnostics. 

As mentioned previously, specific applications of ICMPv6 (MTU discovery, ND, etc.) are defined in their 

respective RFCs. 

A standard IPv6 header precedes every ICMPv6 message. The IPv6 header may or may not contain 

extension headers. The IPv6 header identifies the ICMPv6 header with a NH value of 58.

ICMPv6 messages have the following general format, illustrated in Figure 3-10:

 

45 IETF RFC 4443, Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification, is 

available at http://www.ietf.org/rfc/rfc4443.txt.

46 IETF RFC 4861, Neighbor Discovery for IP Version 6 (IPv6), is available at http://www.ietf.org/rfc/rfc4861.txt.
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Figure 3-10. ICMPv6 Message Format

 Type. Indicates the message type. This eight-bit value determines the format of the remaining data;

see Tables 3-5 and 3-6, below.

 Code. Depends on the message type (see below). This eight-bit value is used to create an additional 

level of message granularity.

 Checksum. This 16-bit field is used to detect data corruption in the ICMPv6 message and parts of 

the IPv6 header. To calculate the checksum, a node must determine the Source and Destination 

address in the IPv6 header. RFC 4443 describes rules for choosing the address if a node has more 

than one unicast address. Moreover and new with ICMPv6, there is a pseudo-header included in the 

checksum calculation. The reason for the change is to protect ICMP from misdelivery or corruption

of those fields of the IPv6 header on which it depends, which, unlike IPv4, are not covered by an 

Internet-layer checksum. The Next Header field is included in the pseudo-header for ICMP and 

contains the value 58, which identifies the IPv6 version of ICMP.

 Message Body. Field length varies depending on the type and code of the message. ICMPv6 

messages are grouped into two classes: error messages and informational messages. 

The ICMPv6 specification defines two classes of ICMP messages: error and informational. Other 

protocols such as Mobile IPv6 define additional messages within each class. Tables 3-5 and 3-6 provide 

an overview of the different message types, along with the additional code information, which depends on 

the message type.

Error messages are used, for example, if a transmitting node sends a packet of 1500 bytes, but an 

intermediary hop has an MTU of only 1300. The intermediary node then sends an ICMPv6 Packet Too 

Big (Type = 2) error message to the sender with information about the problem. For ICMPv6 error 

messages, the sender includes, at least, the start of the packet causing the error to allow the originator of 

the packet to identify the upper-layer protocol and perhaps the process that sent the packet. Informational 

messages may contain configuration information such as a local router’s address in a Router 

Advertisement (RA) message.
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Table 3-5. ICMPv6 Error Messages and Code Type

Message 

Number

Message Type Code Field

1 Destination Unreachable 0 = No route to destination

1 = Communication with destination administratively 

prohibited

2 = Beyond scope of source address

3 = Address unreachable

4 = Port unreachable

5 = Source address failed ingress/egress policy

6 = Reject route to destination

2 Packet Too Big Set to 0 (zero) by the originator and ignored by the receiver

3 Time Exceeded 0 = Hop limit exceeded in transit

1 = Fragment reassembly time exceeded

4 Parameter Problem 0 = Erroneous header field encountered

1 = Unrecognized Next Header type encountered

100 and 

101

Private Experimentation RFC 4443

127 Reserved for expansion of 

ICMPv6 error messages

RFC 4443

Table 3-6. ICMPv6 Informational Messages

Message 

Number

Message Type Code Field

128 Echo Request RFC 4443. Used for the ping command

129 Echo Reply

130 Multicast Listener Query RFC 2710. Used for multicast group management

131 Multicast Listener Report

132 Multicast Listener Done

133 Router Solicitation RFC 4861. Used for neighbor discovery and autoconfiguration

134 Router Advertisement

135 Neighbor Solicitation

136 Neighbor Advertisement

137 Redirect Message

200 and 

201

Private Experimentation RFC 4443

255 Reserved for expansion of 

ICMPv6 informational 

messages

RFC 4443

3.5.2 Differences between IPv6 and IPv4 ICMP

Several differences exist between the ICMP specifications for IPv4 and IPv6. These include using ND to 
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replace ARP, dynamic PMTU discovery, and several automated administrative functions unique to IPv6. 

In particular, some of these differences are:

 Next Header Value. IPv6 identifies ICMPv6 messages with a NH value of 58. In IPv4, the 

corresponding next protocol value is 1.

 Neighbor Discovery (ND) replaces ARP. The ICMPv6 ND function serves to locate link-local 

neighbors and is similar to the function of ARP with IPv4. However, IPv4 has no means to detect 

whether a neighbor is reachable. With IPv6, ND locates link-local routers, identifies duplicate IPv6 

addresses, and eliminates the link-local broadcast traffic generated by ARP. This substantially 

improves packet delivery in case of failed routers or link interfaces that changed their link-layer 

address, which solves the problem of outdated ARP caches.

 Increased PMTU. The minimum MTU that nodes are required to handle under IPv4 is 576 bytes. In 

IPv6, all links must handle a datagram size of at least 1280 bytes, and the minimum recommended 

MTU is 1500 bytes. This is a dramatic increase in the minimum payload each packet must be able to 

carry, and it results in higher efficiency because fewer headers may need to be processed for a given

amount of data. 

 Elimination of in-transit packet fragmentation through the use of PMTU discovery. In IPv4, 

packets may be fragmented at any point: at the source or in transit by routers forwarding those 

packets. In IPv6, only the source may fragment packets. The result of this requirement is that the 

packet source must use ICMPv6 to determine the PMTU prior to sending traffic and perform 

fragmentation where needed. The destination node performs reassembly of fragmented packets under 

both IPv4 and IPv6.

 Multicast Listener Discovery (MLD). This is a set of three ICMPv6 messages equivalent to version 

2 of the Internet Group Management Protocol (IGMP) for IPv4 used to manage subnet multicast 

membership. Instead of using IGMP, IPv6 uses ICMPv6 messages for the same functionality, now 

called MLD. MLD is the protocol that allows multicast listeners to register for multicast addresses 

they want to receive. Unlike IPv4, IPv6 does not have broadcast addresses. In IPv6, multicast is used 

with ICMPv6 for infrastructure applications like neighbour discovery and autoconfiguration on local 

links. IPv6 multicast addresses have new capabilities such as scope, which limits the network realm 

in which a multicast address is applicable; and embedded unicast prefixes, which limit the scope of 

the address to the portion of the network that is addressed by that prefix.

ICMPv6 specifies a framework for control messages to provide IPv6 with error handling and parameter 

establishment functions. Several of ICMPv6’s functions are new or different from ICMP under IPv4. 

Furthermore, ICMPv6 is a fundamental and essential component of any IPv6 implementation. For 

example, no IPv6 or dual stack IPv4/IPv6 network can function properly without ICMPv6. The next 

sections contain more detail about ND, Autoconfiguration, and the security ramifications of ICMPv6.

3.5.3 Neighbor Discovery

ND, described in RFC 4861,46 is the process by which an IPv6 node may learn important information 

such as link layer addresses of interfaces on the same local segment. ND effectively replaces the ARP 

found in IPv4. Additionally, it combines this with ICMP Router Discovery and Redirect capabilities. 

This subsection describes ND in brief, and improvements over the IPv4 set of protocols are noted. 

IPv4 is limited in determining whether a neighbor is reachable. The Neighbor Discovery Protocol (NDP) 

for IPv6 specification is used by all nodes, hosts and routers. IPv6 nodes use ND for the following 
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purposes:

2

 For autoconfiguration of IPv6 addresses

 To determine network prefixes and other configuration information

 For Duplicate IP Address Detection (DAD)

 To determine layer two addresses of nodes on the same link

 To find neighboring routers that can forward their packets

 To keep track of which neighbors are reachable and which are not (Neighbor Unreachability 

Detection, or NUD)

 To detect changed link-layer addresses.

As described in RFC 4861, nodes use ND to determine the link-layer addresses for neighbors known to 

reside on attached links and to purge cached values that become invalid. Hosts also use ND to find 

neighboring routers that are willing to forward packets on their behalf. Finally, nodes use the protocol to 

keep track of which neighbors are reachable and which are not, and to detect changed link-layer 

addresses. When a router or the path to a router fails, a host actively searches for functioning alternates.

ND plays an important role in addressing because it provides address resolution and address 

autoconfiguration. These are accomplished through the different processes in the ND protocol, which 

consists of five different ICMP packet types: a pair of Router Solicitation and Router Advertisement

messages, a pair of Neighbor Solicitation and Neighbor Advertisement messages, and a Redirect message 

(listed above under ICMPv6 informational messages). RFC 4861 defines the purpose of these messages, 

which are all sent via multicast ICMPv6:

 Router Solicitation (RS). When an interface becomes enabled, hosts may send RSs that request 

routers to generate RAs immediately rather than at their next scheduled time.

 Router Advertisement (RA). Routers advertise their presence together with various link and 

Internet parameters either periodically or in response to a RS message. RAs contain prefixes used 

for on-link determination and address configuration, a suggested hop limit value, the Maximum 

Transmission Unit (MTU) for the link, etc.

 Neighbor Solicitation (NS). Nodes send NSs to determine the link-layer address of a neighbor 

or to verify that a neighbor is still reachable via a cached link-layer address. NSs are also used 

for Duplicate Address Detection (DAD).

 Neighbor Advertisement (NA). A response to a NS message. A node may also send unsolicited 

NAs to announce a link-layer address change.

 Redirect Message. Used by routers to inform hosts of a better first hop for a destination.

A common use of the NS and NA messages is to resolve IP addresses by discovering the MAC addresses 

of nodes on the same link. A sender must discover the recipient’s MAC address to send a data packet. 

Figure 3-11 illustrates this example of the ND process.
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Figure 3-11. Example of Neighbor Discovery

For NS, the sending node (Host A) knows the unicast address of the destination node (Host B) is local 

because of the network prefix. Knowing this, it uses a link-local solicited-node multicast address to send 

its NS message. The process that allows the sending node to resolve this link-local MAC address is as 

follows:

Destination Node Unicast Address:

2001:DB8::1234:5678:BBBB

NS solicited node multicast destination address (last 24-bits of the unicast address):

FF02::1:FFxx:xxxx final form  FF02::1:FF78:BBBB

The sending node takes the low-order 24-bits of the unicast address of the destination node and uses them

to fill in the link-local solicited-node multicast address with format ―FF02::1:FFxx:xxxx‖. The NS

message includes both the IPv6 and MAC address of the sender, so the recipient can answer directly and 

supply its own MAC address. The destination node, knowing its unicast address, will listen for the 

corresponding solicited node multicast address. When the multicast message arrives at the destination, 

the destination node analyses the packet. Seeing the FF02::1 multicast with the ICMPv6 MAC request, 

the destination checks that it is the intended recipient, updates its neighbor cache, and replies with a NA

sending its true MAC address.

ND is essential to the correct operation of an IPv6 network, as well as associated procedures like NUD 

and DAD. Routers must provide multicast services correctly and obey scope rules, and interfaces must be 

able to receive solicited node multicast messages. Secure Neighbor Discovery is discussed in Section 5.
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3.5.4 Autoconfiguration

Autoconfiguration, described in RFC 4862,47 is essentially plug-and-play networking. 

One of the most interesting and potentially valuable addressing features implemented in IPv6, this new 

feature allows devices on an IPv6 network to configure addresses independently using stateless 

autoconfiguration. Whereas in IPv4, hosts were originally configured manually or with host 

configuration protocols like DHCP, IPv6 autoconfiguration goes a step further by defining a method for 

some devices to configure their IP address and other parameters automatically without the need for a 

server. Moreover, it also defines a method, renumbering, whereby large numbers of IP addresses on a 

network can be renumbered. This subsection describes autoconfiguration in brief as well as several 

improvements over the IPv4 set of protocols. 

IPv6 defines both Stateful and Stateless address autoconfiguration. Stateless autoconfiguration, 

illustrated in Figure 3-12, requires no manual configuration of hosts, minimal (if any) configuration of 

routers, and no additional servers. This allows a host to generate its own addresses using a combination 

of locally available information and information advertised by routers. Locally available information is 

delivered to a host when routers advertise prefixes that identify the subnets associated with a link. In turn, 

a host generates an interface identifier that uniquely identifies an interface on a subnet. As previously 

discussed, an address is formed by combining the two. If a router is not available to advertise subnet 

prefixes, a host can only generate link-local addresses, which are sufficient for allowing communication 

among nodes attached to the same link.

Stateful autoconfiguration for IPv4 is known as DHCP. In this case, hosts obtain interface addresses or 

configuration information and parameters from a server. DHCP servers maintain a database to keep track 

of which addresses have been assigned to each host. The IPv6 version, DHCPv6, is described in Section 

4.7.

As defined in RFC 4862, an IPv6 address can have different states:

 Tentative Address. An address whose uniqueness on a link is being verified, prior to its assignment 

to an interface. A tentative address is not considered assigned to an interface in the usual sense. An 

interface discards packets addressed to a tentative address except for ND packets related to DAD.

 Preferred Address. An address assigned to an interface whose use by upper layer protocols is 

unrestricted. Preferred addresses may be used as the source (or destination) address of packets sent 

from (or to) the interface.

 Valid Address. A preferred or deprecated address. A valid address may appear as the source or 

destination address of a packet, and the internet routing system is expected to deliver packets sent to a 

valid address to their intended recipients.

 Invalid Address. An address that is not assigned to any interface. A valid address becomes invalid 

when its valid lifetime expires. Invalid addresses should not appear as the destination or source 

address of a packet. In the former case, the Internet routing system will be unable to deliver the 

packet; in the latter case the recipient of the packet will be unable to respond to it.

 

47 IETF RFC 4862, IPv6 Stateless Address Autoconfiguration, is available at http://www.ietf.org/rfc/rfc4862.txt.
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ICMPv6 Router Solicitation to FF02::2 (all routers link local multicast) ICMPv6 Router Advertisement Returned: 2001:DB8:53AA:64C:: Figure 3-12. Example of Stateless Autoconfiguration

3.5.5 Path Maximum Transmission Unit (PMTU) Discovery

With IPv4, a router can fragment a packet when the Maximum Transmission Unit (MTU) of the next link 

is smaller than the packet it has to send. The router does this by slicing the packet to fit into the smaller 

MTU and sending it out as a series of fragments. The packet is then reassembled at the final destination, 

which can be very inefficient. It can also introduce additional traffic into the network, in the form of an 

increased number of smaller-than-necessary packets, as well as necessitating re-transmission of packets if 

all of the fragments do not arrive within a specific time interval. With IPv6, routers do not fragment 

packets; instead, the sender discovers the maximum packet size by using Path Maximum Transmission 

Unit (or PMTU) for the entire path. PMTU discovery is the process by which each node on the network 

establishes an important IPv6 parameter for a given communication session. The PMTU establishes the 

maximum packet size, measured in bytes, which may be carried across a sequence of network nodes. 

PMTU for a local network segment can be determined directly from the MTU in RA messages. 

The IPv6 specification calls for a MTU of at least 1280 bytes. In other words, all links must accept 

packets of any size up through 1280 bytes. Note that the minimum recommended MTU for IPv6 is 1500 

bytes. The IPv6 specification does not allow intermediate nodes (such as routers) to fragment packets in 

transit. Instead, PMTU is determined dynamically and maintained throughout each communication 

session. By disallowing fragmentation by intermediary devices, IPv6 achieves a level of efficiency that is 

generally not available under IPv4. Intermediate nodes operate faster and with less processing overhead 

by not having to fragment packets, and destinations receive fewer packets requiring reassembly.

The PMTU of any given network path is as large as the smallest MTU along that path. For example, 

Figure 3-13 depicts two nodes establishing a communications session across three intermediate nodes. 

The links to the two end nodes (A and B) have MTUs of 1500, whereas the links connecting the three 

intermediary nodes (1 and 2; 2 and 3) have MTUs of 1300 and 1800, respectively. The PMTU of the 

network path between nodes A and B is 1300 bytes. This is because the MTU from node 1 to node 2 is 

only 1300 bytes, the smallest of the four hops. 
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In this example, ICMPv6 is employed by all nodes to recognize and configure the PMTU automatically. 

The two endpoints of the path set the PMTU to 1300 without administrative intervention. The size of an 

IPv6 packet traversing this path for this session needs to conform to the PMTU for the path; otherwise it 

is dropped and the sender receives an ICMPv6 error message requesting it to retransmit using a smaller 

packet size.

Figure 3-13. Significance of MTU under IPv6

The PMTU protocol is detailed in RFC 198148.

3.5.6 Security Ramifications

This section describes security considerations for ICMPv6. Examples include denial of ICMPv6 traffic 

that can effectively result in a denial of service condition for a given network. The functions of ND and 

PMTU discovery are entirely dependent upon the proper functioning of ICMPv6. IPv6 nodes have no 

default route. Nodes learn their preferred routes via RS and RA messages, so proper ND message 

handling is required. Not only is proper ND configuration essential to a functional IPv6 network, but 

misconfiguration of RA and RS parameters can compromise the security of the network.

Rogue routers may be inserted on a local network segment and configured to propagate false RA 

messages. Network nodes on this segment might then learn false routing information that would result in 

network traffic being sent through the rogue router. Further, forged RA messages may be sent from 

legitimate hosts on an IPv6 network segment, possibly causing other nodes to forward packets to nonexistent routers and resulting in a denial of service or man-in-the-middle attack. Additionally, malicious 

responses to DAD messages can cause denial of service conditions on a local network segment. Secure 

Neighbor Discovery (SEND) was developed to help mitigate some of the security issues caused by rogue 

devices. SEND is discussed in more detail in Section 5.

Router access control lists (ACL), firewalls, and other security components must be carefully managed to 

retain ICMPv6 functionality. Any security measures on a network segment must allow IPv6 nodes to use 

ICMPv6 to accomplish ND, PMTU discovery, and other essential tasks. If an IPv6 default router on a 

 

48 IETF RFC 1981, Path MTU Discovery for IP version 6, is available at http://www.ietf.org/rfc/rfc1981.txt.
GUIDELINES FOR THE SECURE DEPLOYMENT OF IPV6 (DRAFT)

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network segment is unable to receive and reply to legitimate RS messages, nodes sending those messages 

may experience a denial of service condition.

Some ICMPv6 traffic (RS, RA, and ND) is only useful on the local segment, uses link-local addresses, 

and should never be routed. ACLs implemented on common routers can be configured to allow other 

appropriate ICMPv6 traffic to pass despite the presence of the default deny all parameter. Network 

administrators should confirm the inherent ICMPv6 capabilities of their router’s operating system and 

confirm the configuration parameters that govern the handling of ICMPv6.

Any IP network, whether it is IPv4-only or a dual stack IPv4/IPv6 network, must have the capability to 

detect and examine ICMPv6 and IPv6 packets. Without this capability, rogue IPv6 nodes may be 

operating on a network that is intended to handle only IPv4. ICMPv6 is fundamental to the operation of 

IPv6 networks, and even malicious IPv6 nodes will depend upon ICMPv6 to operate. Network 

administrators and managers should evaluate the capability of their existing tools used to monitor ICMP 

traffic for similar support for ICMPv6. 

Use of IPsec to authenticate the sender and validate the contents of ICMPv6 messages is often not 

possible. Additionally, establishing security associations with all possible sources of ICMPv6 messages 

is generally not possible. Furthermore, some ICMP messages (e.g., PMTU) may be returned from 

intermediate routers, not from the message’s ultimate destination. A further discussion about the use of 

IPsec with ICMPv6 is included in Section 5. Due to this inability to establish security associations, 

alternatives should be used to reduce the vulnerability to ICMPv6-based attacks. It is essential to 

establish strict filtering policies in site firewalls to limit ICMPv6 messages that can pass between the site 

and the Internet. Tables 3-7 and 3-8 list recommendations for ICMPv6 firewall filtering based on RFC 

4890, Recommendation for Filtering ICMPv6 Messages in Firewalls. These recommendations49 allow 

propagation of ICMPv6 messages needed to maintain functionality of the network but drop messages 

posing potential security risks. Many ICMPv6 messages should only be used in a link-local context, 

rather than end-to-end, and filters need to be concerned with the types of addresses in ICMPv6 packets as 

well as the specific source address, destination addresses, and ICMPv6 Type. RFC 4890 classifies 

ICMPv6 messages according to whether they are designed for end-to-end communications (traffic to 

transit a firewall) or local communications within a link (local traffic addressed to an interface on a 

firewall).

 

49 IETF RFC 4890, Recommendation for Filtering ICMPv6 Messages in Firewalls, is available at http://www.rfceditor.org/rfc/rfc4890.txt.
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Table 3-7. ICMPv6 Recommended Filtering Actions – Must Not Drop & Should Not Drop

Must Not Drop Should Not Drop

Message (Type) Transit Local Transit Local

Maintenance of Communication

Destination Unreachable (1) – All codes ● ●

Packet Too Big (2) ● ●

Time Exceeded (3) – Code 0 only ● ●

Parameter Problem (4) – Codes 1 and 2 only ● ●

Connectivity Checking

Echo Request (128) ● ●

Echo Response (129) ● ●

Address Configuration and Router Selection

Router Solicitation (133) ●

Router Advertisement (134) ●

Neighbor Solicitation (135) ●

Neighbor Advertisement (136) ●

Inverse Neighbor Discovery Solicitation (141) ●

Inverse Neighbor Discovery Advertisement (142) ●

Link-Local Multicast Receiver Notification

Listener Query (130) ●

Listener Report (131) ●

Listener Done (132) ●

Listener Report v2 (143) ●

SEND Certification Path Notification

Certification Path Solicitation (148) ●

Certification Path Advertisement (149) ●

Multicast Router Discovery

Multicast Router Advertisement (151) ●

Multicast Router Solicitation (152) ●

Multicast Router Termination (153) ●

Error Messages

Time Exceeded (3) – Code 1 ● ●

Parameter Problem (4) – Code 0 ● ●

Mobile IPv6

Home Agent Address Discovery Request (144) ●

Home Agent Address Discovery Reply (145) ●

Mobile Prefix Solicitation (146) ●

Mobile Prefix Advertisement (147) ●
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Table 3-8. ICMPv6 Recommended Filtering Actions – Should Define Policy & Should Drop

3.6 IPv6 and Routing

Routing protocols fall into two general types. Interior Gateway Protocols (IGP) are designed for use 

within an autonomous system (AS), that is, among routers that are all controlled by the same enterprise or 

organization. Exterior Gateway Protocols (EGP) are designed for exchanging routes between 

autonomous systems, such as between network carriers or between a large enterprise and its network 

service providers. To support IPv6, routing protocols, such as RIP, OSPF, IS-IS, EIGRP, and BGP, had 

to be updated. This section covers changes to routing protocols, as well as security support for such 

protocols.

3.6.1 Specification Overview

Open Shortest Path First (OSPF) is a link-state hierarchical Interior Gateway Protocol (IGP). Dijkstra’s 

algorithm is used to calculate the shortest path tree. It uses path cost as its routing metric. Path cost is 

determined generally by the speed (i.e., bandwidth) of a given route. IPv4 networks run OSPF version 2 

Should Define Policy Should Drop

Message (Type) Transit Local Transit Local

Experimental & Other Types

Seamoby Experimental (150) ●

Redirect (137) ●

Router Renumbering (138) ●

Experimental (100) ● ●

Experimental (101) ● ●

Experimental (200) ● ●

Experimental (201) ● ●

Unallocated Error Message Types

Unallocated Error messages – (5-99 inclusive) ● ●

Unallocated Error messages – (102-126 inclusive) ● ●

Reserved Extension (127) ● ●

Informational Message Types

Unallocated Informational messages 

– (154-199 inclusive)

● ●

Unallocated Informational messages 

– (202-254 inclusive)

● ●

Node Information Query (139) ● ●

Node Information Response (140) ● ●

Reserved Extension (255) ● ●
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(OSPFv2) as specified in RFC 2328.

50

OSPF version 3 (OSPFv3) is designed specifically for IPv6 and is specified in RFC 5340.51 OSPFv3 for 

IPv6 is a completely independent routing protocol from OSPFv2 for IPv4. OSPFv3 expands on OSPFv2 

to provide support for IPv6 routing prefixes and the larger size of IPv6 addresses.

The Routing Information Protocol (RIP) is a distance-vector routing protocol that employs hop count as a 

routing metric. IPv4 uses RIP version 2 (RIPv2) as specified in RFC 2453.

52

 The IPv6 version of RIP is a 

simple distance vector routing protocol, standardized in RFC 2080.

53

 It is easy to configure, but offers 

limited flexibility and scalability. RIP enhancements for IPv6, detailed in RFC 2080, also known as 

RIPng, include support for IPv6 addresses and prefixes and the use of the all-RIP-routers multicast group 

address FF02::9 as the destination address for RIP update messages. Each RIPng update contains a copy 

of the entire routing table. RIPng is suited for networks of modest size only.

IS-IS is an IGP that advertises link-state information throughout the network to create a picture of the 

network topology. IS-IS is an Open Systems Interconnection (OSI) hierarchical routing protocol that 

designates an intermediate system as a Level 1 or Level 2 device. Level 2 devices route between Level 1 

areas to create an intradomain routing backbone. Integrated IS-IS uses a single routing algorithm to 

support several network address families, such as IPv6, IPv4, and OSI. It runs point to point over the link 

layer protocol; it does not use IPv4 or IPv6. Each IS-IS update contains only changes to the network 

topology. IS-IS is flexible, efficient, and suitable for large IPv4/IPv6 networks.

Enhanced Interior Gateway Routing Protocol (EIGRP) is Cisco’s proprietary routing protocol loosely 

based on their original IGRP. EIGRP is an advanced distance-vector routing protocol, with optimizations 

to minimize both the routing instability incurred after topology changes, as well as the use of bandwidth 

and processing power in the router. EIGRP and IGRP are compatible with each other. EIGRP for IPv4 

and EIGRP for IPv6 are configured and managed separately.

BGP4 is the Exterior Gateway Protocol (EGP) used mainly to connect separate routing domains that 

contain independent routing policies (autonomous systems). Connecting to a service provider for access 

to the Internet is a common use for Border Gateway Protocol (BGP). BGP version 4 with multi-protocol 

extensions supports both IPv4 and IPv6. Each BGP update contains only changes to the network 

topology. BGP is efficient and flexible. Multi-protocol BGP is standardized in RFC 4760.54

3.6.2 Security for Routing Protocols

Routing protocols can be subject to threats such as unauthorized updates for either IPv4 or IPv6 routes. 

Security capabilities have been designed for routing protocols to mitigate unauthorized update threats. 

Some IPv6 routing protocols rely on similar mechanisms to those in IPv4 for protection, while others 

have incorporated IPsec for protection. These security mechanisms do not provide end-to-end security 

for routing protocols across multiple hops, because they provide integrity assurance for routing protocol 

messages between nodes, but do not verify the integrity of messages received from other nodes that are 

not part of a security association. This is a major security concern for EGPs and a somewhat lesser one 

for IGPs.

 

50 IETF RFC 2328, OSPF Version 2, is available at http://www.ietf.org/rfc/rfc2328.txt.

51 IETF RFC 5340, OSPF for IPv6, is available at http://www.ietf.org/rfc/rfc5340.txt.

52 IETF RFC 2453, RIP Version 2, is available at http://www.ietf.org/rfc/rfc2453.txt.

53 IETF RFC 2080, RIPng for IPv6, is available at http://www.ietf.org/rfc/rfc2080.txt.

54 IETF RFC 4760, Multiprotocol Extensions for BGP-4, is available at http://www.ietf.org/rfc/rfc4760.txt.
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RIPng

RIP for IPv4 uses an MD5-based integrity mechanism; this was removed from RIPng. RIPng offers no 

integrity assurance features. Per RFC 2080,

53 RIPng leverages IPsec for security. It should be noted that 

hardware vendors have not incorporated IPsec features as a configuration option, instead relying on native 

IPv6 IPsec support from the operating platform for protection. RIPng is suitable only for small, private 

networks where the threat of routing attacks is substantially reduced.

OSPFv3

Securing OSPFv2 in a dual stack environment will protect neither the OSPFv3 protocol nor the OSPFv3 

routing table. OSPFv2 allows null, password-based, or cryptographic authentication using MD5-based 

integrity for routing updates. The authentication fields found in OSPFv2 have been removed from the 

OSPFv3 packet for IPv6, so MD5 is not an authentication option. OSPFv3 offers no integrity assurance 

features itself and relies on IPsec AH or ESP for authentication, integrity, and confidentiality. Note that 

OSPFv3 uses unicast and multicast, and IKE does not work with multicast, so the default method is to use 

manual keying. IPsec for OSPFv3 is detailed in RFC 4552.

55

 

With routing protocols, routing integrity is usually a greater concern than confidentiality. The ESP 

parameter NULL indicating no encryption is generally regarded to be an acceptable choice for OSPF 

security. 

IS-IS and EIGRP

Both IS-IS and EIGRP support simple MD5-based integrity for protecting IPv6 routing updates, similar to 

protecting routing updates for IS-IS and EIGRP for IPv4.

BGP

The use of BGP as an inter-AS routing protocol means that it can be subject to serious threats. Three 

mechanisms exist to mitigate threats to BGP. The first is the use of MD5-based integrity to protect 

routing updates. The second mechanism to mitigate threats to BGP is GTSM [RFC3682]). GTSM is a 

simple security mechanism for rejecting spoofed BGP messages based on their IP TTL or Hop Limit. 

The sending BGP router always uses a TTL=255, and the receiving BGP router checks that the TTL has 

the expected value of 255. Any packets from a remote attacker would have to travel via intervening 

routers, would have a smaller-than-maximum TTL, and would be dropped on receipt. Note that a router 

operating as the endpoint of a tunneling protocol may not decrement the hop count upon receiving packets 

through the tunnel, so these could conceivably come from anywhere with TTL=255. The third 

mechanism to mitigate threats to BGP is IPsec. IPsec key management can use shared secrets or public 

key certificates, which allow IPsec to offer scalability. GTSM has the lowest overhead of the three 

mechanisms, and is the easiest to configure. It also offers the least effective protection. The MD5 

Signature mechanism offers low overhead and effective protection, but it forces administrators to disrupt 

their BGP sessions at each key update, and it does not scale well. IPsec offers the most effective 

protection, least disruption, and best scalability. It also imposes the highest overhead (although the 

overhead is still small), and it is the most complex mechanism to configure. In summary, using an MD5 

checksum is certainly better than nothing, but MD5 itself can be attacked successfully, and most of these 

methods have no easy ways to change hash functions or even change keys. IPsec is preferable for routing 

protocols that support its use. All of the above security mechanisms protect against unauthorized 

 

55 IETF RFC 4552, Authentication/Confidentiality for OSPFv3, is available at http://www.ietf.org/rfc/rfc4552.txt.
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insertion or manipulation of routing protocol messages.56 They do not protect against a corrupted or 

malfunctioning router that may construct and pass along incorrect routing information. Many approaches 

to providing better end-to-end security for BGP have been proposed, and work continues to reach 

consensus on this subject.

3.6.3 Unknown Aspects 

As IPv6 deployment grows globally so will the Internet’s IPv6 global routing tables. It is very well 

possible that this expanded growth can impact service providers that already are faced with large IPv4-

only routing tables. By assigning IPv6 addresses through service providers, in a hierarchical fashion, 

there is a greater possibility that efficient aggregation for IPv6 addressing will occur. If more 

organizations succeed in obtaining PI addressing, then there is a higher risk that global routing tables 

could explode in size, and hence cause possibly costly hardware upgrades to deal with extremely large 

routing tables. To assist with developing a scalable Internet architecture, use of techniques that separate 

end-systems’ addressing space and routing locators’ space (locator-ID split) are also being investigated.

3.7 IPv6 and the Domain Name System (DNS)

The Domain Name System (DNS) is essential for almost all use of the Internet. It must be available, and 

it must provide accurate information. Threats such as denial of service against the top-level servers are 

taken extremely seriously. Perhaps the biggest network security story of 2008 was the demonstration of a 

new attack that can insert false information into a DNS server’s cache. Upgrading DNS security is one of 

the major current challenges for ISPs57.

The Secure Domain Name System (DNS) Deployment Guide58 contains background information on DNS 

as it is used with IPv4, the possible attacks against DNS, and appropriate security measures. The Domain 

Name System – Security Technical Implementation Guide version 4r159 covers similar topics for DOD 

networks and also has advice on using DNS on particular computing platforms and operating systems. 

Both of these documents emphasize extensive experience with IPv4. This section reviews the main 

aspects of DNS briefly to help understand the changes needed to make DNS work with IPv6 and how 

these affect the secure operation of DNS.

DNS is a hierarchical, distributed database that translates logical, human readable names such as 

www.example.com into binary IP addresses used by applications like email and web browsers. DNS 

database entries are called resource records60. IPv4 and IPv6 addresses are different types of resource 

records, but the DNS handles both essentially the same. What is important to remember is that there is 

only one DNS: a name may have IPv4 addresses, IPv6 addresses, both, or neither.

DNS services are fully defined for IPv6 in RFC 359661. The root and common top-level domain (TLD) 

 

56 See also NIST SP 800-54, Border Gateway Protocol Security, http://csrc.nist.gov/publications/nistpubs/800-54/SP800-

54.pdf

57 See http://www.isoc.org/isoc/conferences/dnspanel/ for details of a July 2009 Internet Society Workshop on this topic. 

58 See NIST Special Publication 800-81, Secure Domain Name System (DNS) Deployment Guide, 2006, available from 

http://csrc.nist.gov/publications/nistpubs/800-81/SP800-81.pdf. The current revised draft is available from 

http://csrc.nist.gov/publications/drafts/800-81-rev1/nist_draft_sp800-81r1-round2.pdf.

59 The Defense Information Systems Agency document, Domain Name System – Security Technical Implementation Guide 

version 4r1, is available at http://iase.disa.mil/stigs/stig/dns_stig_v4r1_20071017.pdf.

60 Technically, we are discussing the IN (Internet) CLASS of the DNS. A complete list of resource record types can be found 

at http://www.iana.org/assignments/dns-parameters.

61 IETF RFC 3596, DNS Extensions to Support IP Version 6, is available at http://www.ietf.org/rfc/rfc3596.txt. Additional 

information can be found in RFC 4472, Operational Considerations and Issues with IPv6 DNS, 

http://www.ietf.org/rfc/rfc4472.txt.
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servers are all IPv6 capable today. Nevertheless, IPv6 addresses for production services usually cannot 

be found in the DNS, particularly for services based in the United States or North America. Most of these 

services do not have IPv6 access enabled, and most users would not have easy IPv6 connectivity to them 

if they were. Getting IPv6 DNS entries close to parity with IPv4 DNS on the Internet is unlikely to 

happen without demand for IPv6 access to popular, high-volume web sites and other services. 

Most DNS implementations have been upgraded to support IPv6, but older software applications may 

assume that DNS address queries return only 32-bit IPv4 addresses. Fully IPv6-capable DNS 

implementations not only need to handle 128-bit IPv6 addresses but also need to run over IPv6 with the 

same UDP and TCP port number, 53, as used by IPv462. 

The primary components of DNS are the DNS root and TLD servers, authoritative DNS servers, local 

caching servers, and clients called resolvers. A resolver requests resource records from a local caching 

server. If the local caching server does not have a requested record, it uses the information it does have to 

start querying authoritative servers. 

3.7.1 DNS Transport Protocol

It is important to note that while IPv6 address queries may be made over an IPv4 or IPv6 network, IPv6 

transport of DNS messages is not required for looking up IPv6 addresses in DNS. The query, not the 

transport protocol, should always determine what information is returned in the Answer, Authority, and 

Additional sections of a response. A host can request an IPv6 address even if the network on which that 

host resides is IPv4-only. To obtain both the IPv4 and IPv6 addresses, two separate requests should be 

used. Hosts and DNS servers running both IPv4 and IPv6 should have no problem with this aspect of 

DNS, but all zones should be set up so that they have at least one IPv4-enabled authoritative server, and 

IPv6-only systems should follow or exceed the minimum configuration guidelines in RFC 3109 to ensure 

that they do not get cut off from the rest of the DNS tree because they cannot communicate with any 

IPv4-only DNS servers. 

3.7.2 DNS Specification Overview

RFC 3596, DNS Extensions to Support IP Version 6, defines the changes needed to DNS to support IPv6. 

A new resource record type, AAAA (pronounced quad-A), is defined to store a host’s IPv6 address. A 

128-bit IPv6 address is encoded in the data portion of a AAAA resource record in network byte order 

(high-order byte first). A AAAA resource record stores a single IPv6 address, so a host with more than 

one IPv6 address may have more than one such record. AAAA queries for a specified domain name 

return all associated AAAA resource records in the answer section of a response. 

A special domain is defined to look up names corresponding to an IPv6 address. The intent of this 

domain is to provide a reverse mapping of an IPv6 address to a host name (stored as a DNS PTR resource 

record). The domain is rooted at IP6.ARPA. An IPv6 address is represented as a name in the IP6.ARPA 

domain with a sequence of four-bit nibbles written as hexadecimal digits and separated by dots with the 

suffix IP6.ARPA. The sequence of 32 nibbles is encoded in reverse order, i.e., the low-order nibble is 

encoded first, followed by the next low-order nibble and so on. For example, the reverse lookup domain 

name corresponding to the address 4321:1:2:3:4:5:678:90ab is: 

b.a.0.9.8.7.6.0.5.0.0.0.4.0.0.0.3.0.0.0.2.0.0.0.1.0.0.0.1.2.3.4.IP6.ARPA

 

62 But see RFC 3901, DNS IPv6 Transport Operational Guidelines, available at http://www.ietf.org/rfc/rfc3901.txt. It is 

incumbent upon authoritative name servers accessible with IPv6 transport to maintain connectivity with today’s 

predominant part of the DNS accessible with IPv4 transport to avoid splitting the namespace. 
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Note that zero suppression and double-colon compression cannot be used in reverse DNS names. 

All existing query types that perform type A (IPv4 address) additional section processing, i.e., name 

server (NS), location of services (SRV), and mail exchange (MX) query types, must be modified or 

redefined to perform both type A and type AAAA additional section processing. This means that a name 

server must add any relevant IPv4 addresses and any relevant IPv6 addresses available locally to the 

Additional Section of a response when processing any one of the above queries. 

Several DNS implementations have been observed handling queries for AAAA records incorrectly, and 

these have been documented in RFC 407463. They may return the wrong data, wrong error codes, or 

nothing at all. Most of these errors result in unreachable services, delays, timeouts, or faulty assumptions 

by caching servers, but the RFC indicates places where these errors can also be exploited in denial-ofservice attacks. 

DNS is responsible only for resolving a domain name to a set of IP addresses. Applications and operating 

systems are responsible for choosing how to use the IPv6 AAAA or IPv4 A records that may be returned. 

This topic is called address selection, and it is an important part of using IPv6 and dual IPv4-IPv6 

networks correctly. As a result, receiving unexpected AAAA records may cause an application that is not 

IPv6 aware to fail. Combining both IPv6 and IPv4 records into the same domain can lead to application 

problems that are beyond the scope of the DNS administrator. The Domain Name System – Security 

Technical Implementation Guide version 4r1 mentioned above recommends using different DNS names 

for IPv6-enabled hosts until all such problems (or at least the critical ones) are fixed. It also may be useful 

during pilot tests or early deployment to use names in separate domains for IPv6 servers, e.g., 

imap.IPv6.example.com, although one would like such names to handle IPv4 and IPv6 completely 

transparently in the long term. 

A dual stack host needs a mechanism for choosing between IPv4 and IPv6. When DNS returns a set of 

different addresses, resolvers need to be configured either to choose which addresses to pass to an 

application or to forward all of the addresses and leave the choice to the application. 

IPv6 link-local addresses should never be put into the DNS (and site-local addresses should not be used at 

all). Temporary (RFC 304164) addresses are usually meant to be anonymous, so putting them into DNS 

would be an unusual choice and would require frequent updates. Putting 6to4 addresses into DNS may be 

sensible, but one may need cooperation from a local or regional registry to set up the IP6.ARPA PTR 

records for 6to4 addresses and for other address formats with specific prefixes and embedded IPv4 

addresses. 

For additional details about adding DNS records for new services, handling time-to-live values in caches, 

obtaining a list of DNS servers when DHCPv6 is not used, updating forward (AAAA) and reverse (PTR) 

entries, handling dynamic DNS, and renumbering, see RFC 4472. 

3.7.3 Security Impact and Recommendations

3.7.3.1 General DNS Security Recommendations

All of the general accounts of threats against DNS65 and advice for securing DNS, independent of IPv6, 

 

63 IETF RFC 4074, Common Misbehavior Against DNS Queries, is available at http://www.ietf.org/rfc/rfc4074.txt.

64

 IETF RFC 3041, Privacy Extensions for Stateless Address Autoconfiguration in IPv6, http://www.ietf.org/rfc/rfc3041.txt

65 See RFC 3833, Threat Analysis of the Domain Name System (DNS), available from available at 

http://www.ietf.org/rfc/rfc3833.txt.
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apply. This begins with host security and software security for DNS servers. It includes keeping up with 

new software releases, vulnerability alerts, and patches. The security of DNS servers can be improved 

with isolation, redundancy, geographic diversity, network path diversity, and potentially platform 

diversity. Administrative access should be controlled and secured. Tools for checking that a zone file is 

well formed or that other configuration variables are correctly set should be used. DNS servers should be 

included in any penetration testing exercises.

It is good practice to implement certain well-know ―security by obscurity‖ measures. The IP address of a 

hidden master server should not be advertised, nor should the DNS software version number, lest an 

attacker easily exploit bugs known to be in a certain release. Common implementations of DNS support 

access control lists based on IP addresses, and at least some of these support access control lists based on 

both IPv4 and IPv6 addresses. Address-based security is regarded as a rather weak form of authentication, 

particularly for important actions like DNS dynamic update, but nevertheless it is an efficient way to 

provide some protection if it is used together with ingress and egress address filtering. The widely-used 

technique of ―split DNS,‖ whereby an enterprise’s DNS servers provide different answers to internal and 

external queries, can be used the same way with either A records or AAAA records. 

Based on recent experiences, two attack scenarios are particularly likely. One is denial of service. Excess 

capacity and diversity help. Firewalls and intrusion detection systems can help protect a server located in 

a ―demilitarized zone.‖ Administrators should be prepared to contact the appropriate emergency response 

teams and law enforcement agencies. RFC 535866 contains advice for configuring DNS servers to make 

using them as amplifiers in a denial-of-service attack against a third party more difficult. 

The second scenario is so-called cache poisoning—inserting false information into a server’s cache, so 

that, for example, users may be misdirected to a bogus web site. This is an old idea, but a much more 

effective method for accomplishing it became well known in 2008. With or without cryptographic 

protection, the recommended remedy is to force the attacker to guess two randomly chosen 16-bit values 

simultaneously instead of just one. See RFC 545267.

3.7.3.2 Cryptographic Protection of DNS

Two standard cryptographic protocols are available for securing DNS. They can be used equally with 

IPv4, IPv6, and combined IPv4-IPv6 implementations. DNSSEC, the DNS security extensions, is 

defined in three RFCs,68,69,70 and TSIG, the Secret Key Transaction Authentication protocol is described 

in a fourth RFC.

71

The TSIG protocol provides data origin authentication and message integrity for DNS transactions by 

adding message authentication codes based on shared secrets. Originally, only the HMAC-MD5 

algorithm was specified, but TSIG now requires HMAC-SHA-1 and HMAC-SHA-256 as well72. It can be 

used to authenticate dynamic updates as coming from an approved client, responses as coming from an 

approved recursive name server, or zone transfers as coming from an authoritative server. Its most 

 

66 IETF RFC 5358, Preventing Use of Recursive Nameservers in Reflector Attacks, is available from available at 

http://www.ietf.org/rfc/rfc5358.txt.

67 IETF RFC 5452, Measures for Making DNS More Resilient against Forged Answers, is available from available at 

http://www.ietf.org/rfc/rfc5452.txt.

68 IETF RFC 4033, DNS Security Introduction and Requirements, is available at http://www.ietf.org/rfc/rfc4033.txt. 

69 IETF RFC 4034, Resource Records for the DNS Security Extensions, is available at http://www.ietf.org/rfc/rfc4034.txt

70 IETF RFC 4035, Protocol Modifications for the DNS Security Extensions, is available at http://www.ietf.org/rfc/rfc4035.txt. 

71 IETF RFC 2845, Secret Key Transaction Authentication for DNS (TSIG), is available at http://www.ietf.org/rfc/rfc2845.txt. 

72 IETF RFC 4635, HMAC SHA TSIG Algorithm Identifiers, specifies additional mandatory and optional TSIG algorithms and 

how to handle truncation of the message digest. It is available at http://www.ietf.org/rfc/rfc4635.txt.
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common use is to protect zone transfers, but protecting dynamic updates is an important application as 

well. 

TSIG is widely deployed and strongly recommended. Normally, network administrators use an out-ofband mechanism to configure name servers and resolvers with shared secrets. However, a secure, 

automated mechanism for key distribution and key update is a much more desirable solution. It simplifies 

operations and enhances security. The TKEY protocol73 describes offers several options—the DiffieHellman method is a practical choice. One limitation with TSIG is that there are no levels of authority, so 

any host with the secret key may update any record.

An alternative to TSIG called SIG(0) and described in RFC 293174 uses public keys and digital signatures 

instead of message authentication codes based on shared secrets. It is far less widely used than TSIG, but 

it may be a practical alternative especially for securing dynamic updates.

DNSSEC provides an entirely different set of cryptographic security mechanisms. Its objective is to 

secure the DNS database itself by deploying a hierarchical infrastructure of signed resource records and 

its own built-in public key infrastructure. It accomplishes this by defining four new types of resource 

record. The RRSIG resource resord contains a digital signature of another resource record. The DNSKEY 

resource record contains a signature verification key; it, in turn, is signed with an RRSIG resource record. 

The DS (delegation signer) RR names the signer of a delegation. If DNSSEC is fully deployed, the DS 

records can form a chain from any zone to the root. The NSEC (next secure) resource record specifies the 

name of the next secured entry in a zone (in lexicographic order), so that the non-existence of a resource 

record can be verified cryptographically. Finally, DNSSEC also defines new header bits.

Besides its added complexity, especially for signing large zones, DNSSEC also increases the size of files 

and messages substantially. Also, administrators need to protect the secrecy of their private signing keys

carefully. DNSSEC has not been deployed widely yet, but this is gradually changing. Newer releases of 

DNS software support DNSSEC, and plans are underway to sign the root and many major top-level 

domains. The US Government’s .gov root was signed in Feb. 2009, and OMB mandated75 that the 

Government’s second-level domains should be signed by Dec. 2009. It is hoped that this will provide the 

impetus and experience necessary to get DNSSEC deployed for commonly used services and large 

enterprises throughout the Internet. 

3.7.3.3 IPv6-Specific DNS Security Recommendations

During any IPv6 deployment, DNS services may have to support both IPv4 and IPv6. In fact, DNS 

services should be among the first to be fully dual stack capable in any transition effort. Software 

applications need to be modified to query for both forms of addresses and to choose between them. IPv4 

may need to remain the network protocol between a caching server and authoritative DNS servers to 

ensure continuity of service. Many factors can affect network performance and availability during this 

conversion process, and these can impact both IPv4 and IPv6 access. DNS may supply AAAA records 

before services are fully turned on and reachable, and timeouts may occur. Applications may be 

unprepared to handle them. Caching resolvers may have to deal with A and AAAA records having 

different time-to-live values. Also, the DNS servers may function improperly. 

It is important to verify that client resolvers are receiving the correct responses and resource records. An 

 

73 See IETF RFC 2930, Secret Key Establishment for DNS (TKEY RR), available at http://www.ietf.org/rfc/rfc4035.txt.

74 IETF RFC 2931, DNS Request and Transaction Signatures ( SIG(0)s ), is available at http://www.ietf.org/rfc/rfc2931.txt.

75

 See OMB Memorandum M-08-23, issued in Aug. 2008, at http://www.whitehouse.gov/omb/memoranda/fy2008/m08-

23.pdf.
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ongoing testing phase for IPv6-capable DNS services is a necessary step during any organization’s IPv6 

deployment and transition effort. When this process includes testing IPv6 application software, separate 

DNS servers and domain names should be used to support simultaneous IPv6 testing and operational IPv4 

name resolution.

DNS responses with AAAA records are longer than similar IPv4 responses, because the records are 

simply larger than comparable A records and interfaces may have more than one IPv6 address in the 

DNS. The added overhead is, however, much less than that resulting from use of DNSSEC. 

Some authoritative servers ignore queries for an AAAA record and cause a resolver first to wait and 

timeout and then to fall back to a query for an A record, which may cause a fatal timeout at the 

application that called the resolver. Even if the resolver and application eventually succeed, the result can 

be an unacceptable delay for the application’s user, especially with interactive applications like web 

browsing.

Advertising IPv6 addresses in DNS during the transition requires additional care. Availability problems

can easily arise if AAAA records are inserted into the DNS zone before IPv6 services are working. The 

recommendation is that AAAA records for a service should not be added to a DNS zone until the address 

is assigned to an interface on a host, the address is configured and enabled on the host’s interface, and 

finally the interface is on a link connected to the IPv6 infrastructure. 
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4. IPv6 Advanced Topics

This chapter provides specific details about the status, requirements, capabilities, and security impacts of

more advanced IPv6 topics such as multihoming, multicast, quality of service, mobile IPv6, jumbograms, 

address selection, DHCPv6, and IPv6 renumbering. As of the writing of this guide, some of these topics 

have not yet been fully specified or implemented and are not ready to deploy. In these cases, this is noted 

and interim methods are recommended where appropriate.

4.1 Multihoming

Multihoming means having the ability to utilize more than one connection to the Internet. A host, for 

example, may have more than one network connection (e.g., 100baseT and WiFi); a connection to a LAN 

with more than one router to the Internet on it; or a connection to a single router that has more than one 

Internet connection. Frequently, one refers to multihoming for an entire site, which may be a home, small 

office, or campus location within a large enterprise.

Multihoming is extremely useful but has a potentially large impact on the global Internet architecture. 

This section describes the motivations for multihoming, the problems it creates, the requirements for a 

good solution, potential solutions, and their security implications.

Users have several strong motivations for multihoming. First and foremost, having more than one 

Internet connection provides greater reliability and resiliency should one link fail or one ISP have a 

prolonged outage. Other reasons for multihoming include lower cost, better performance (load balancing 

or QoS differentiation), and policy enforcement (including security policy).

While multihoming offers obvious advantages, it is complicated by another issue: the use of Internet 

addresses for two purposes. On the one hand, addresses are used for forwarding packets to the right 

location. On the other hand, they are used for identifying an endpoint, e.g., a transport protocol (TCP or 

UDP addresses plus port numbers) or IPsec security association (destination address plus security 

parameters index). For multihoming to work, either the entire Internet has to know multiple paths to a 

multihomed site, or the nodes at a site have to be able to use multiple addresses seamlessly and 

transparently. The former implies enormous growth in the Internet’s core forwarding tables. The latter 

implies that nodes have to cope with TCP connections, UDP responses, IPsec security associations, and 

other upper layer protocols (ULPs) tied to addresses.

Multihoming solutions also need to satisfy other requirements. They need to avoid causing problems 

with fragmentation, renumbering, and domain names. They need to scale to the size of the global Internet 

and not affect performance too greatly. They also need to work with firewalls and ingress filtering.

4.1.1 Differences between IPv4 and IPv6 Multihoming

Any multihoming solution must satisfy two main goals. The first is to make multihoming transparent to 

upper layer protocols. Otherwise, multihoming is no better than changing service providers and 

renumbering one’s network manually. The second is to avoid causing explosive growth in the global 

routing and forwarding tables. Dealing with this growth is the number one challenge Internet engineers 

face today, with IPv4 or IPv6, with or without multihoming.

Compared with IPv6, IPv4 interfaces are normally limited to a single address, the entire supply of IPv4 

network prefixes is much more limited, and the addresses themselves are four times shorter, all of which 

constrains the problem somewhat. IPv4 users with provider independent (PI) addresses can achieve fully 

transparent multihoming with resiliency and load balancing at the expense of global router table growth. 
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If NAT is used, resiliency is not transparent, and load balancing within a connection is impossible, so 

IPv6 multihoming without NAT potentially offers more powerful capabilities.

On the one hand, obtaining PI addresses is the most practical way to achieve IPv6 multihoming, whereas, 

on the other hand, the router table growth caused by IPv4 multihoming (frequently called CIDR address 

prefix de-aggregation) is problematical for the Internet, and it is a potentially overwhelming problem with 

IPv6. Therefore, provisioning IPv6 PI addresses has been a vigorously debated topic. Policies differ 

among the Regional Internet Registries (RIRs). The ARIN Number Resource Policy Manual76 requires 

that an organization must be an end site and not a local registry and ―qualify for an IPv4 assignment or 

allocation from ARIN under the IPv4 policy currently in effect, or demonstrate efficient utilization of all 

direct IPv4 assignments and allocations, each of which must be covered by any current ARIN RSA‖. 

Qualifying organizations may obtain a /48 PI IPv6 assignment. Until a solution both satisfying the global 

forwarding problem and providing host transparency is available, IPv6 sites needing multihoming and not 

qualifying for a PI assignment should attempt to get their primary ISP to accept /48 prefixes from 

secondary ISPs and thus achieve partial multihoming, although such requests may or may not be honored.

4.1.2 SHIM6 Specification Overview 

Several architectural approaches to IPv6 multihoming have been considered. These are described in 

detail in RFC 417777:

Use the global routing infrastructure, as is done with IPv4.

Base the solution on Mobile IPv6.

Modify protocols in hosts to accommodate dynamic changes of locators.

Design the intelligence, including rewriting addresses, into site exit-routers.

Add a network layer protocol element to split addresses into locators and identifiers.

The most forward looking solutions to the multihoming problem introduce the notion of splitting an IP 

address into an identifier and locator. The idea is that upper layer protocols use the identifier, and core 

network routing and forwarding use the locator. Multihoming is accomplished by dynamically managing 

the bindings between the two. To make this work, protocols for establishing and maintaining these 

relationships must be provided. This requires protocol elements that update locator lists, switch locators 

in use, and so forth.

More than one proposal has been made along these lines, but the active standards track work on 

specifying split IPv6 identifiers and locators is called SHIM6 and is being done in the IETF’s SHIM6 

Working Group. The name is derived from the way the additional address is specified in a shim header 

inserted into the packet. 

SHIM6 is a network layer, host-based protocol to establish identifier-locator bindings. Its goals are78:

To preserve established communications in the presence of certain classes of failures, for example, 

TCP connections and UDP streams

 

76 ARIN, ARIN Number Resource Policy Manual, is available at http://www.arin.net/policy/nrpm_20080805.pdf.

77 IETF RFC 4177, Architectural Approaches to Multi-homing for IPv6, is available at http://www.ietf.org/rfc/rfc4177.txt.

78 IETF SHIM6, Site Multihoming by IPv6 Intermediation (shim6), is available at http://www.ietf.org/html.charters/shim6-

charter.html.
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To have minimal impact on upper layer protocols in general and on transport protocols and 

applications in particular

To address security threats identified in RFC 421879 through the combination of hash-based or 

cryptographically-generated addresses and additional measures.

Not to require extra roundtrip up front to set up shim-specific state

To take advantage of multiple locators for load spreading so that different sets of communication to a 

host (e.g., different connections) may use different locators of the host.

The general idea is to use IPv6 normally but, through SHIM6 signaling, set up alternative locators that 

can be used when needed. The steps, roughly, are:

An AAAA DNS query provides a (possibly incomplete) set of locator addresses.

The source chooses a locator address to establish a conversation (choose a different Locator ID if the 

first attempt is not successful).

Once upper layer communication begins, end hosts can signal SHIM6 capabilities and exchange a 

complete set of locators.

The current source and destination locators are used as source and destination Upper Layer IDs.

In case of an outage, the source or destination can detect path failure in the forwarding plane and 

change source or destination locator to any in the locator set.

Existing sessions continue uninterrupted using the unchanged Upper Layer ID.

An important design paradigm is that no new name space is needed. The SHIM6 protocol uses four IPv6 

extension header messages called I1, I2, R1, and R2 to accomplish all of this. Figure 4-1 shows where 

the shim header fits into the IPv6 protocol stack.

Although the most promising work on IPv6 multihoming is SHIM6, this is likely to get much more 

discussion and revision. Some concerns about this approach have been expressed. In particular, it may 

be difficult to provide adequate traffic engineering with a host-based solution like SHIM6. For large 

enterprises with complex routed networks, site-based multihoming may be more useful. On large servers 

with many simultaneous connections, the overhead of maintaining SHIM6 state information may impact 

performance significantly.

 

79 IETF RFC 4218, Threats Relating to IPv6 Multihoming Solutions, is available at http://www.ietf.org/rfc/rfc4218.txt.
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Figure 4-1. SHIM6 Protocol Stack78

4.1.3 Security Ramifications for Multihoming

RFC 4218 presents an overview of security vulnerabilities inherent in multihoming. In summary, one 

must be concerned about denial of service, re-routing packets to unintended destinations or black holes, 

and multicast issues. It advises, in general, that connectionless transport protocols like UDP present more 

security problems than connection-oriented transport protocols. Also, if identifiers and locators are split, 

security should be tied to the identifiers to lessen the impact of attacks on the identifier-locator binding. 

Securing other parts of the infrastructure such as the DNS and routing protocols helps minimize the 

potential attacks on multihoming. Multihoming solutions also need to account for and work with ingress 

filtering so that spoofed addresses can not be used to attack systems. (RFC 282780 and RFC 370481).

Networks need to ensure that they do not announce their prefixes in a way that generates asymmetric 

traffic flows. Traffic following asymmetrical paths might get blocked by strict Reverse Path Forwarding 

(RPF) checks or stateful packet filters. It may also make it impossible to implement IPsec at site border 

routers.

The SHIM6 protocol contains several security measures:

Hash-based addresses allow one to prove address ownership and to prevent redirection attacks.

Reachability probes allow one to identify third party flooding attacks.

Two-way communications are required before the responder creates any state. This means that a 

state-based DoS attack (trying to use up all available memory on the responder) at least reveals an

IPv6 address that the attacker was using.

Context establishment messages use nonces to prevent replay attacks and to prevent off-path attackers 

from interfering with the establishment.

 

80 IETF RFC 2827, Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing,

is available at http://www.ietf.org/rfc/rfc2827.txt.

81 IETF RFC 3704, Ingress Filtering for Multihomed Networks, is available at http://www.ietf.org/rfc/rfc3704.txt.
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After context establishment, every SHIM6 control message contains the context tag assigned to the 

particular context. This implies that an attacker needs to discover a valid context tag before being 

able to spoof any SHIM6 control message. This also helps protect the SHIM6 protocol from off-path 

attackers.

Packet filters may need to be aware of SHIM6 and modify their actions accordingly. First, along with 

upper layer protocol and port numbers, they may need to match on the upper layer identifiers as well as or 

instead of the IPv6 locator addresses. Second, they may need to understand when established sessions 

begin using shim headers or change locators.

4.2 IPv6 Multicast

Multicast refers to sending a packet to an IP address designated as a multicast address; one or more hosts 

specifically interested in the communication then receive a copy of that single packet. This differs from 

broadcast, which delivers packets to all hosts on a subnet, because multicast traffic is only sent to hosts 

subscribed to the multicast group. Multicasting is often used, for example, to stream audio and video 

more efficiently. Senders achieve two primary advantages by using multicast. First, the sender only 

needs to create and send one packet, instead of creating and sending a separate packet to each recipient. 

Second, the sender does not need to keep track of who the actual recipients are. Multicasting can also be 

advantageous from a network perspective, because it reduces network bandwidth consumption.

This section describes how multicast works in an IPv6 environment. In IPv6, broadcast has been 

eliminated and multicast takes on a much larger role. In addition to replacing broadcast, it also works 

with ICMPv6 neighbor discovery and router discovery on the local link to perform stateless 

autoconfiguration and address resolution. 

First, consider an example of how IPv6 multicast makes Neighbor Discovery in IPv6 more efficient than 

using ARP with IPv4. Running IPv4, when a host has an IP address on its own subnet and needs to know 

the corresponding link layer address, it broadcasts an ARP request containing the IPv4 address. Every 

host on the subnet gets a copy.

Suppose, running IPv6, an interface wants to find the link layer address for the link local IP address 

FE80::4DF2:54C8:B8C7:113A. It takes the low-order 24 bits of this address (C7:113A) and appends 

them to the well-known solicited node multicast prefix, FF02:0:0:0:0:1:FF00::/104 to form the solicited 

node multicast address FF02::1:FFC7:113A. Then it sends an ICMPv6 Neighbor Solicitation message to 

this multicast address. The message gets delivered to the interface at FE80::4DF2:54C8:B8C7:113A 

because it belongs to the multicast group FF02::1:FFC7:113A. Interfaces using any unicast or anycast 

address must join the solicited node multicast group corresponding to the above prefix and low-order 24 

bits of their address. They use ICMPv6 Multicast Listener Discovery (MLD) to join a multicast group. 

The result is a more efficient address resolution procedure, in which a smaller number of hosts are queried 

to determine the address.

The solicited node multicast addresses range from FF02::1:FF00:0 to FF02::1:FFFF:FFFF, so the 

Neighbor Solicitation traffic is partitioned into 224 solicited node multicast groups. This makes receiving a 

Neighbor Solicitation intended for a different address highly unlikely. 

IPv6 multicast addresses are easy to recognize. They always begin with eight 1 bits: FF. The next eight 

bits, 02 in this case, specify that this is a well-known multicast address with link local scope. Other 

examples of well-known multicast addresses with link-local scope are:

FF02::1 All Nodes
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FF02::2 All Routers

FF02::1:2 All DHCP Agents

In addition to scoped multicast addresses, other features such as source-specific multicast have been 

added to IPv6. These different features and their applications are described below, and areas still needing 

work are noted (e.g., multicast with SHIM6 and IPsec).

4.2.1 IPv6 Multicast Specifications 

In addition to providing an essential part of the IPv6 infrastructure, multicast applications include 

groupware, multimedia distribution, searching, routing, database replication, grid computing, and realtime information delivery.

With IPv6, multicast addresses have scope ranging from a single interface or link to the global Internet. 

They can be permanently assigned and well-known, as in the examples above, or they can be used 

transiently for specific purposes. 

RFC 429120, IP Version 6 Addressing Architecture, defines a multicast address as: ―An identifier for a set 

of interfaces (typically belonging to different nodes). A packet sent to a multicast address is delivered to 

all interfaces identified by that address.‖ Normally, these are the addresses that have previously joined a 

given multicast group. The Multicast Listener Discovery (MLD) Protocol is the method interfaces use to 

join and leave multicast groups, and routers keep track of these groups for each interface on which they 

forward packets. Version 2 (MLDv2) (RFC 381082) manages multicast group membership with two 

ICMPv6 message types:

Multicast Listener Query (Type = 130)

Version 2 Multicast Listener Report (Type = 143).

MLDv2 is backward compatible with MLDv1 (RFC 271083), so MLDv2 also supports:

Version 1 Multicast Listener Report (Type = 131)

Version 1 Multicast Listener Done (Type = 132).

All of these messages are sent with a link-local IPv6 source address (or the unspecified source address if 

necessary), an IPv6 Hop Limit of 1, and an IPv6 Router Alert option (RFC 271184) in a Hop-by-Hop 

Options header. (The Router Alert option forces routers to examine MLD messages sent to IPv6 

multicast addresses in which the routers themselves previously had no interest.)

How routers actually implement multicast depends on the Layer 2 networking technology. It is trivial on 

point-to-point links; it may be implemented by using promiscuous mode on link layers with a natural 

broadcast capability; or it may require Layer 2 protocol logic on non-broadcast multi-access link layers.

The biggest change introduced in MLDv2 is Source Specific Multicast. MLDv2 allows an interface to 

specify, for each multicast address, from which source addresses it does or does not want to receive 

packets.

 

82 IETF RFC 3810, Multicast Listener Discovery Version 2 (MLDv2) for IPv6, is available at 

http://www.ietf.org/rfc/rfc3810.txt.

83 IETF RFC 2710, Multicast Listener Discovery (MLD) for IPv6, is available at http://www.ietf.org/rfc/rfc2710.txt.

84 IETF RFC 2711, IPv6 Router Alert Option, is available at http://www.ietf.org/rfc/rfc2711.txt.
GUIDELINES FOR THE SECURE DEPLOYMENT OF IPV6 (DRAFT)

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RFC 460485 updates MLDv2 and describes source-specific multicast for IPv4 (IGMPv3) and IPv6 

(MLDv2) in a single document.

As stated above, IPv6 multicast addresses are easy to recognize. They are exactly the addresses 

beginning with eight 1 bits or hexadecimal FF:

 | 8 | 4 | 4 | 112 bits 

 +--------+----+----+--------------------------------/.../---------+

 |11111111|flgs|scop| group ID |

 +--------+----+----+--------------------------------/.../---------+

The next eight bits in a multicast address specify the flags and scope. The final 112 bits in the multicast 

address are called the Group ID and are used to specify the set of nodes that are members of a multicast 

group. In a previous version, the Group ID was only 32 bits, and some implementations still stick to 32 

bits.

In the IPv6 specification, multicast addresses always have a scope that limits the set of receiving nodes. 

The scope is the group of nodes for which the packet is intended relative to its source. Its values are 

shown in Table 4-1.

Table 4-1. IPv6 Scoped Multicast Values (from RFC 429120)

Value Scope

1 Interface Local

2 Link Local

4 Admin. Local

5 Site Local

8 Organization Local

E Global

Some well-known multicast Group IDs are defined for variable scopes. A good example is the ―All NTP 

Servers‖ address:

FF02::101 All NTP Servers Link Local

FF04::101 All NTP Servers Admin Local

FF05::101 All NTP Servers Site Local

FF08::101 All NTP Servers Organization Local

FF0E::101 All NTP Servers Global

RFC 237586 contains a list of well-known IPv6 multicast addresses categorized by scope, and that list has 

been extended by newer specifications. The complete and current version is at 

http://www.iana.org/assignments/ipv6-multicast-addresses. 

The flags specify, first, whether a multicast address is a well-known, pre-defined address, or whether it is 

a transient address not permanently defined. Second, the flags specify whether a transient multicast 

address has an authorized and properly scoped unicast prefix embedded in it, and, if so, whether it also 

 

85 IETF RFC 4604, Using Internet Group Management Protocol Version 3 (IGMPv3) and Multicast Listener Discovery 

Protocol Version 2 (MLDv2) for Source-Specific Multicast, is available at http://www.ietf.org/rfc/rfc4604.txt.

86 IETF RFC 2375, IPv6 Multicast Address Assignments, is available at http://www.ietf.org/rfc/rfc2375.txt.
GUIDELINES FOR THE SECURE DEPLOYMENT OF IPV6 (DRAFT)

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specifies a rendezvous point. (See RFC 395687.) The unicast prefix eliminates the need for an additional 

protocol to allocate unique multicast addresses.

A rendezvous point for a multicast group is the root of a tree used with Protocol Independent Multicast—

Sparse Mode (PIM-SM) (RFC 460188), a sophisticated multicast routing protocol. PIM-SM builds a tree 

of senders and group members. Then, traffic from senders is first routed upstream towards the 

rendezvous point and then downstream to group members (receivers).

For well-known multicast addresses, the four flag bits are always all zeros. The first flag bit is reserved 

and must be 0. The remaining three flag bits are called R, P, and T (for rendezvous, prefix, and transient) 

and are non-zero for transient multicast addresses:

 flgs

+-+-+-+-+

|0|R|P|T|

+-+-+-+-+

The flags are set as follows (see RFC 330689 and RFC 395687):

0 0 0 0 Well-known, pre-defined multicast address (as in all of the examples above)

0 0 0 1 Transient multicast address without an embedded unicast prefix

0 0 1 1 Transient multicast address with an embedded unicast prefix and no rendezvous

point

0 1 1 1 Transient multicast address with an embedded unicast prefix and rendezvous

point

This provides a simple way to allocate multicast addresses belonging to or allocated by a given network 

prefix in the following format, without adding any new multicast address allocation protocol:

 | 8 | 4 | 4 | 8 | 8 | 64 | 32 |

 +--------+----+----+--------+--------+----------------+----------+

 |11111111|flgs|scop|reserved| p-len | network prefix | group ID |

 +--------+----+----+--------+--------+----------------+----------+

The eight bits after the flags and scope must be zero. The next eight bits specify a prefix length, up to 64, 

followed by the prefix, left justified and zero filled. 

This example comes from RFC 3306. The address FF38:0030:3FFE:FFFF:0001:0:1234:5678 is: 

Multicast (FF)

 

87 IETF RFC 3956, Embedding the Rendezvous Point (RP) Address in an IPv6 Multicast Address, is available at 

http://www.ietf.org/rfc/rfc3956.txt.

88 IETF RFC 4601, Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised), is available at 

http://www.ietf.org/rfc/rfc4601.txt.

89 IETF RFC 3306, Unicast-Prefix-based IPv6 Multicast Addresses, is available at http://www.ietf.org/rfc/rfc3306.txt.
GUIDELINES FOR THE SECURE DEPLOYMENT OF IPV6 (DRAFT)

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Transient with embedded prefix (3)

Organization Local scope (8)

Using a prefix of length 48 (30)

Specifying the prefix as 3FFE:FFFF:0001::/48

Using a 32-bit Group ID of 1234:5678

Note that the scope of such multicast addresses must not be greater than the scope of their embedded 

prefixes.

4.2.2 Differences between IPv4 and IPv6 Multicast

Although multicast addresses are common in both IPv4 and IPv6, several important differences exist:

Unlike IPv4, IPv6 does not have broadcast addresses. Instead, IPv6 uses optimizations like the 

Solicited Node multicast groups and the all routers multicast addresses, which make better use of 

network resources than broadcast.

In IPv6, multicast is used with ICMPv6 for infrastructure applications like neighbor discovery and 

autoconfiguration on local links.

IPv6 multicast addresses have new capabilities such as scope and embedded unicast prefixes. In 

general, IPv6 extensions to multicast have been added to make multicast more useful over internets.

Multicasting is managed with ICMPv6 message types collectively called MLD instead of IGMP.

4.2.3 Multicast Security Ramifications

IPv6 routers, packet filters, firewalls, and tunnel endpoints need to enforce multicast scope boundaries 

and make sure that MLD packets are not routable.

Attackers may take advantage of well-known multicast addresses to find hidden resources such as routers 

or particular servers. These addresses need to be blocked at the appropriate places according to local 

security policy.

Denial of service attacks may use multicast to amplify bandwidth consumption or attempt to exhaust 

other resources. So-called reflector attacks may send packets with a source address of the target of attack 

and a multicast destination address, to try to get all multicast receivers to respond to the target. These 

attacks need to be intercepted and dropped.

IPsec coverage for multicast is incomplete. If a multicast group has more than one sender, the replay 

protection mechanism does not work. More importantly, IKE is a unicast UDP protocol that only works 

between two parties, so automated key management for multicast IPsec is lacking. For additional details 

about using IPsec with IPv6 multicast, see Section 5.3.3.

Attacks on the MLD protocol include denial of service, causing unwanted traffic to be delivered, and 

downgrading capabilities from MLDv2 to MLDv1. By properly enforcing the unicast scope and hop 

count rules for MLD, these attacks can be confined to a local link.
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Forging Protocol Independent Multicast (PIM) messages can cause unwanted traffic to be sent to replace 

the role of designated routers. IPsec can be used for cryptographic protection of these messages. 

Cryptographically protecting PIM messages also stops many denial-of-service attacks. Other precautions 

include limiting the set of neighbors from which Join, Prune, Assert, and Hello messages are accepted.

Routers should check that a valid Hello message was received first and that source addresses are legal for 

the interfaces on which they are received. See also RFC 460990 for a list of suggestions for rate limiting 

PIM messages, in particular, the inter-domain multi-source discovery protocol (MSDP).

Issues concerning how link-local ICMPv6 multicast traffic used for neighbor discovery and 

autoconfiguration can be secured are covered in Section 5. 

4.2.4 Unresolved Aspects of IPv6 Multicast

IPsec and IKE were not designed with multicast security in mind, and three important unresolved aspects 

of multicast security are related to IPsec and key management for IPsec.

The fundamental IPsec security association architecture and protection protocols, ESP and AH, were 

designed mainly for unicast, and, although they work with multicast in principle, they are incompletely 

specified for multicast, and many open issues exist. (For example, what happens to a multicast security 

association when members join or leave the group?)

Key management for IPsec, provided by IKE, is inherently a two-party protocol. Different protocols for 

group key management have been proposed, but an agreed-upon and widely implemented standard for 

IPsec multicast key management has not emerged. Protocol specifications like PIM suggest using IPsec 

with manual keying, but this solution that does not scale well over time or space and has other limitations. 

For example, the IPsec replay detection feature is not supposed to be used with manual keying, 

The security recommendations for PIM call for using IPsec AH, even for unicast messages. Where IPsec 

is used with IPv4, AH has generally fallen out of use in favor of ESP, with NULL encryption when 

authentication-only is desired. Thus the IPsec standards (RFC 430191) no longer require AH, and many 

implementations omit it. See Section 5.3.6 for additional discussion of this topic.

4.3 IPv6 Quality of Service (QoS)

The TCP/IP Network Layer—IPv4 and IPv6—was intentionally designed without any of the features 

normally associated with QoS such as admission controls; resource guarantees; and in-order, lossless 

delivery. QoS on TCP/IP networks or the Internet is a somewhat imprecise concept, which may have 

different meanings varying from ―anything except undifferentiated best effort‖ to specific service level 

contracts between a provider and user. It may mean:

Providing a given user with certain levels of overall availability, throughput, low latency, maximum 

packet loss, or even security

Treating different types of traffic differently, according to content: real-time audio or video requires 

high throughput and consistently low latency but can tolerate small losses, whereas file transfer can 

tolerate delay but no losses whatsoever.

Many aspects of engineering QoS depend on technologies running at Layer 1 and Layer 2, such as MPLS 

 

90 IETF RFC 4609, Protocol Independent Multicast - Sparse Mode (PIM-SM) Multicast Routing Security Issues and 

Enhancements, is available at http://www.ietf.org/rfc/rfc4609.txt.

91 IETF RFC 4301, Security Architecture for the Internet Protocol, is available at http://www.ietf.org/rfc/rfc4301.txt.
GUIDELINES FOR THE SECURE DEPLOYMENT OF IPV6 (DRAFT)

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(MultiProtocol Label Switching) and ATM (Asynchronous Transfer Mode). IETF work on QoS for 

TCP/IP began with Integrated Services (intserv), which was designed to provide QoS guarantees. Intserv 

has been replaced with Differentiated Services (diffserv), which simply recognizes that different types of 

traffic have different QoS requirements and need to be marked accordingly. The signaling protocol (i.e., 

the protocol used for QoS setup and specification) to establish QoS requests is RSVP—Resource 

Reservation Protocol. IPv6 end-to-end addressing allows services that are difficult to deploy with NAT as 

well as end-to-end use of diffserv and RSVP. Many of these services may have real-time and multimedia 

content, so QoS is likely to become a more important topic with the widespread use of IPv6.

The notion of improved QoS has always been linked with IPv6. In fact, IPv6 was designed to support 

certain QoS improvements, but not all of these have been completely specified or implemented.

4.3.1 IPv6 QoS Specifications

Several aspects of IPv6 implicitly or explicitly support QoS. These include:

A streamlined header with fewer fields, no checksum processing, sufficient address space to make 

address translation unnecessary, and a simple test for whether routers need to examine anything past 

the fixed length header promote efficient packet forwarding.

Requiring a larger minimum MTU and PMTU discovery also increase efficiency.

Eliminating in-route fragmentation removes one of the greatest sources of performance degradation in 

IPv4.

Eliminating broadcast and building in better support for multicast and anycast make better use of 

network resources.

A new Flow Label field and larger Traffic Class field in the main IPv6 header allow more efficient 

and finer grained differentiation of various types of traffic.

The IPv6 Traffic Class field replaces the IPv4 Type of Service field. The original intent of the IPv4 Type 

of Service field has been replaced by diffserv (RFC 247441). Although the RFCs are vague on this point, 

this is the way the IPv6 Traffic Class is usually used, and the functionality in IPv6 is equivalent to that in 

IPv4. Because of the way diffserv works, this field may be rewritten in transit. For example, RFC 2474 

describes how packet marking is performed by traffic conditioners at network boundaries, including the 

edges of the network (first-hop router or source host) and administrative boundaries, and RFC 316892 sets 

aside two bits in this field called Explicit Congestion Notification for routers to indicate network 

congestion to end hosts.

When IPv6 QoS is mentioned, most frequently the last of these aspects, especially the Flow Label, is 

cited. Rudimentary use of the Flow Label is defined in RFC 180993 and RFC 3697.

94

 A Flow label value 

is always associated with a source and destination address pair with the same Hop-by-Hop options and 

Routing Header. (A zero value means that the field is not being used.) This has the advantage of

specifying flows completely in the main header. It is not necessary to examine extension headers, upper 

layer protocols, and port numbers to identify the packet. RFC 3697 recommends that each new transport

connection and application data stream be given a new value, and it requires that applications be able to 

specify this value. It stipulates that Flow Labels be delivered intact, and it gives rules for timeouts and 

 

92

 IETF RFC 3168, The Addition of Explicit Congestion Notification (ECN) to IP, http://www.ietf.org/rfc/rfc3168.txt

93 IETF RFC 1809, Using the Flow Label Field in IPv6, is available at http://www.ietf.org/rfc/rfc1809.txt.

94 IETF RFC 3697, IPv6 Flow Label Specification, is available at http://www.ietf.org/rfc/rfc3697.txt.
GUIDELINES FOR THE SECURE DEPLOYMENT OF IPV6 (DRAFT)

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reuse. Finally, it prohibits using specific bits or mathematical interpretations of the value—it is just a 20-

bit label.

Many IPv6 implementations do choose different Flow Label values for each TCP connection, for 

example, but few if any make additional use of the field. Thus, realizing the potential for providing better 

QoS offered by the IPv6 Flow Label lies in finding improved ways to use this feature in the future.

4.3.2 Differences between IPv4 and IPv6 QoS

The specific differences in QoS capabilities between IPv4 and IPv6 are covered point-by-point in the 

preceding section. The overall design of IPv6 is better thought out with respect to QoS; several specific 

improvements in IPv6 allow for more efficient network usage, and room has been left for additional QoS 

capabilities when these are defined.

4.3.3 Security Ramifications

One aspect is securing the QoS mechanisms themselves, to prevent theft of service, traffic analysis, or 

other attacks. For example, the Type of Service and Flow Label in the IPv6 header are not protected, 

even by AH. This is because the Type of Service can be altered while a packet is in transit. Although 

RFC 3697 specifies a nonalterable Flow Label field, when AH was originally designed, the Flow Label 

was also thought to be capable of alteration; the updated version of AH maintained that view for 

backwards compatibility. An application able to forge these fields may be able obtain preferred service 

fraudulently. If this is a major concern, IPsec can be run in tunnel mode, the QoS parameters can be 

copied from the inner header to the outer header, and the protected inner header can be compared with the 

outer header upon delivery.

Because the QoS fields in the outer header are not protected, firewalls cannot blindly trust the Type of 

Service and Flow Label alone for access control decisions. Also, traffic analysis may become simpler, 

because someone monitoring traffic flows can take advantage of the same efficiencies as legitimate 

routers forwarding traffic.

A different aspect is making sure that security does not impede the required QoS. QoS may need to be 

applied to packets secured with IPsec, in which case information about the upper layer protocols may not 

be accessible, but in this case, the methods in RFC 220795 can be used to differentiate IPsec-protected 

traffic. When planning to provide QoS, one must take into account that cryptographic protection, packet 

filtering, and examination by intrusion prevention systems all add some delay.

A third consideration is that securing QoS signaling protocols such as RSVP presents some difficulties, 

because these protocols often do not run strictly end to end but presume that intermediate points examine 

(or, worse, modify) the contents. For an overview of RSVP security, see RFC 4230.96

4.3.4 Unresolved Aspects of IPv6 QoS

General aspects of using Flow Labels have been specified, but details needed to take advantage of them 

are still missing. Because of the ways QoS depends on lower-layer protocols, it is unclear where and how 

progress on this front will be made.

 

95 IETF RFC 2207, RSVP Extensions for IPsec Data Flows, is available at http://www.ietf.org/rfc/rfc2207.txt.

96 IETF RFC 4230, RSVP Security Properties, is available at http://www.ietf.org/rfc/rfc4230.txt.
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4.4 Mobile IPv6 (MIPv6)

IP-layer mobility has long been considered a useful and important feature, but today, with the exploding 

growth of laptop computers, PDAs, and mobile phones connecting to the Internet from more than one 

location, and, tomorrow, with the Internet in every motor vehicle and yet unimagined portable devices, it 

is becoming an essential ingredient in advanced services.

The work on Mobile IPv4 (MIPv4) envisioned many of the features and functions of Mobile IPv6 

(MIPv6), but MIPv4 has never been practical on a large scale. The capabilities built into IPv6 make 

widespread use of MIPv6 practical: plentiful end-to-end addresses, security, optimized routing, increased 

reliability, and more. Thus, it is expected that use of mobility will increase with IPv6.

The central issue with IP mobility is the same as with multihoming: disconnecting from one network and 

reconnecting to another is easy; changing IP addresses while keeping one’s TCP connections, IPsec 

security associations, and streaming protocols running takes more work. Not surprisingly, MIPv4 and 

MIPv6 have common elements, but mobility is inherently a difficult problem, and MIPv6 involves quite a 

bit of sophisticated protocol design.

4.4.1 MIPv6 Specification Overview

MIPv6 allows an IPv6 interface to disconnect and reconnect physically in an internet topology while 

logically retaining its ―home‖ IPv6 address, so MIPv6 nodes can change their point of attachment to the 

network while maintaining seamless connectivity. The primary document describing MIPv6 is RFC 

3775, Mobility Support in IPv6,

97 but many other published RFCs and work still in progress support 

MIPv6. 

As one might expect, MIPv6 has its own terminology, a certain amount of which is essential for 

understanding how it works. The most important terms are:

Mobile Host (MN). A node using MIPv6 to change its point of network attachment

Home Address (HoA). The permanent, routable unicast address of the MN

Home Link. The link on which the MN’s HoA is defined

Foreign Link. Any link except the home link

Care-Of Address (CoA). A routable unicast address used by the MN on a foreign link

Correspondent Node (CN). A peer with which the MN is communicating

Home Agent (HA). A router on the MN’s Home Link with which the MN registers its CoA and 

which forwards traffic to and from the MN at its CoA

Binding. The association of a HoA and CoA for a given amount of time

Binding Cache (on HA or CN). A table of other nodes’ bindings and their lifetimes

Binding Update List (on MN). A MN’s table of HA and CN bindings

 

97 IETF RFC 3775, Mobility Support in IPv6, is available at http://www.ietf.org/rfc/rfc3775.txt. This RFC is currently being 

revised to include items like changes to IKE and IPsec, new work on bootstrapping, and an update to the IPv6 addressing 

architecture. See http://tools.ietf.org/wg/mext/draft-ietf-mext-rfc3775bis/ for the status of this work.
GUIDELINES FOR THE SECURE DEPLOYMENT OF IPV6 (DRAFT)

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Route optimization. Direct communications between a MN and CN without involving a HA

As with SHIM6, MIPv6 solves the problems created by using IP addresses for both identity and location. 

A MN’s identity is its HoA, and its location is its CoA. The goal is to establish and use bindings between 

these securely and efficiently, and the first step is for a MN to set up IPsec-secured communications with 

a HA. This may be done before leaving the home link. Figure 4-2 shows the main components of 

MIPv6.

Figure 4-2. The Main MIPv6 Components

To make this work, several functions must be provided, and these require underlying protocol elements. 

The four new protocol elements added for MIPv6 are:

A new IPv6 Extension Header, the Mobility Header (MH), with eight different message types. The 

first four are used to set up bindings and the last four to run a security protocol called return 

routability, which is described in Section 4.4.3.3, below:

– Binding Update

– Binding Acknowledgement

– Binding Refresh Request

– Binding Error

– Home Test Init

– Home Test

– Care-of Test Init

– Care-of Test
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Four new ICMPv6 message types:

– Home Agent Address Discovery Request

– Home Agent Address Discovery Reply

– Mobile Prefix Solicitation

– Mobile Prefix Advertisement

Two new types of Destination Options Extension Header called the Home Address Option and 

Alternate Care-of-Address Option

A new Routing Header, Type 2.

To get an idea of how these are used, consider the following examples:

A. A MN announces a new CoA:

– A Binding Update (BU) and Binding Acknowledgement (BUA) are exchanged between 

the MN and its HA and between the MN and each of its CNs.

– The BU uses a Binding Update (Type 5) IPv6 Mobility Header and the Home Address 

Option. The BUA uses a Binding Update Acknowledgment (Type 6) IPv6 Mobility 

Header and a Type 2 Routing Header.

B. A MN finds a HA:

– The MN initiates the Dynamic Home Agent Address Discovery (DHAAD) protocol.

– To do this, the MN sends an ICMPv6 Home Agent Address Discovery Request to the

Mobile IPv6 Home-Agent’s anycast address (with which the MN was previously 

configured) for its home subnet prefix. A HA returns an ICMPv6 Home Agent Address 

Discovery Reply.

C. A MN learns about home link renumbering:

– A MN receives an unsolicited ICMPv6 Mobile Prefix Advertisement (MPA) indicating 

that renumbering is occurring.

D. A MN uses route-optimized communications between it and a CN:

– The MN sends normal traffic with a Home Address Option.

– The CN sends normal traffic with an IPv6 Type 2 Routing Header.

E. A MN receives a Binding Refresh request from a CN:

– The CN sends a Binding Refresh Request (Type 0) Mobility Header to the MN’s HoA.

– The MN checks that the CN is in its Binding Update List and starts Return Routability 

and a new BU.

F. A CN sends a Binding Error to a MN:
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– A CN receives an unrecognized HoA and sends the MN a Binding Error with a Binding 

Error (Type 7) MH, error status 1 for ―unknown binding,‖ and the HoA it received in 

error.

– The MN checks that the CN is in its Binding Update List; if it has indication that 

communications with the CN are working, it ignores the message; otherwise it deletes the 

binding and sends subsequent communications with the CN through the HA or 

alternatively starts Return Routability and a new BU.

G. A MN includes an Alternate CoA in a BU: 

– The BU contains a Home Test (Type 3) MH option to indicate a CoA different from the 

source address (because of network topology or security, for example).

– The CN uses the Alternate CoA instead of the original CoA. 

4.4.2 Differences from IPv4 Standards

MIPv4 is standardized in RFC 334498. MIPv4 and MIPv6 share much of the same motivation and have 

somewhat similar designs, but MIPv6 provides enhanced security, streamlined administrative protocols, 

and greater efficiency. This is not an accident: the reason MIPv6 has so many advantages over MIPv4 is 

that is uses the new features and capabilities found in IPv6 but not in IPv4. IPv6 has autoconfiguration, 

globally unique addressing (without NAT), flexible extension headers, and mandatory IPsec. The much 

larger IPv6 address space makes MIPv6 easier to deploy. Some of the biggest differences are:

Route optimization is standard with MIPv6, and it uses a new approach to security called return 

routability.

MIPv6 does not use the MIPv4 last-hop foreign agent. The tunnel endpoint is built directly into the 

MN, which also allows end-to-end security.

MIPv6 does not have any new, special-purpose AAA support. MIPv6 uses standard link layer and IP 

network access methods for AAA along with IPsec. Using standard, already understood methods is 

always preferable. 

MIPv6 uses two-way tunneling, which works better with ingress filtering.

MIPv6 has many security improvements.

MIPv4 and MIPv6 use different protocols. MIPv4 uses ICMP(v4) Router Discovery, Port 434 (UDP or 

TCP), and ―home-grown‖ security, whereas Mobile IPv6 uses ICMPv6, IPv6 Routing and Mobility 

Extension Headers, Destination Options, and IPsec. 

4.4.3 Security Ramifications

For MIPv6, security has always been a primary design concern. A thorough approach to security requires 

looking at all of the potential vulnerabilities and choosing appropriate measures to deal with them. The 

designers of MIPv6 actually used a security threat analysis as the basis for the design. MIPv6 security 

starts with the base specification, Using IPsec to Protect Mobile IPv6 Signaling between Mobile Nodes 

 

98 IETF RFC 3344, IP Mobility Support for IPv4, is available at http://www.ietf.org/rfc/rfc3344.txt.
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and Home Agents (RFC 377699), and extends into several other RFCs, primarily:

RFC 4225,

100 Mobile IP version 6 Route Optimization Security Design Background

RFC 4285101, Authentication Protocol for Mobile IPv6

RFC 4487102, Mobile IPv6 and Firewalls: Problem Statement

RFC 4449103, Securing Mobile IPv6 Route Optimization Using a Static Shared Key

RFC 4877104, Mobile IPv6 Operation with IKEv2 and the revised IPsec Architecture

RFC 4882105, IP Address Location Privacy and Mobile IPv6: Problem Statement.

The goals set by those specifying MIPv6 security have been to address the most serious security 

vulnerabilities first, to use existing security methods where they fit and can be deployed easily, to avoid 

introducing new vulnerabilities, and, as a last resort, to design new security methods where necessary.

Attacks clearly exist if someone can forge or modify any of the main MIPv6 messages:

BU between MN and HA

BU between MN and CN

IPv6 Routing and Mobility Headers

IPv6 Home Address and Alternate CoA options

The most serious new vulnerabilities introduced with MIPv6 involve BUs. Many of these can lead to

Denial of Service (DoS) of one type or another. It is possible to starve the MN or to flood another host. 

Beyond DoS, attacks on these protocol messages may attempt connection hijacking, eavesdropping, or 

other variations of man-in-the-middle or impersonation.

A forged BU between a MN and its HA may be sent by another legitimate MN or any other party. Such 

an attack can be prevented if the MN is forced to show ―ownership‖ of its HoA, and this can usually be 

arranged through the on-going relationship a MN has with its HA.

On the other hand, a bogus BU between a MN and CN presents a more challenging problem, because a 

prior relationship between a MN and CN is less likely to exist, and in any case, protocol designers cannot 

count on their having one. This attack can be used to redirect traffic between any pair of hosts. For 

example, suppose Alice is communicating with Bob. Eve sends Bob a BU that Alice’s new CoA is Eve’s 

address. Alice does not have to be mobile to make this work—Bob does not know. It may be possible to 

 

99 IETF RFC 3776, Using IPsec to Protect Mobile IPv6 Signaling Between Mobile Nodes and Home Agents, is available at 

http://www.ietf.org/rfc/rfc3776.txt.

100 IETF RFC 4225, Mobile IP Version 6 Route Optimization Security Design Background, is available at 

http://www.ietf.org/rfc/rfc4225.txt.

101 IETF RFC 4285, Authentication Protocol for Mobile IPv6, is available at http://www.ietf.org/rfc/rfc4285.txt.

102 IETF RFC 4487, Mobile IPv6 and Firewalls: Problem Statement, is available at http://www.ietf.org/rfc/rfc4487.txt.

103 IETF RFC 4449, Securing Mobile IPv6 Route Optimization Using a Static Shared Key, is available at 

http://www.ietf.org/rfc/rfc4449.txt.

104 IETF RFC 4877, Mobile IPv6 Operation with IKEv2 and the revised IPsec Architecture, is available at 

http://www.ietf.org/rfc/rfc4877.txt.

105 IETF RFC 4882, IP Address Location Privacy and Mobile IPv6: Problem Statement, is available at 

http://www.ietf.org/rfc/rfc4882.txt.
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play this attack ahead of time, so that it takes effect when Alice and Bob communicate later. Conversely, 

this attack can be turned around into a DoS attack against Eve.

BUs, however, are not the only vulnerability in MIPv6. Attacks on prefix propagation and HA discovery

are possible. Spoofing an ICMPv6 Mobile Prefix Solicitation (MPS) or Mobile Prefix Advertisement 

(MPA) can break HA-MN connectivity, and merely eavesdropping on these messages can reveal

addressing and topology information about the Home Link.

The ICMPv6 Home Agent Discovery is sent to the HA anycast address on the Home Link. Both this and 

the ICMPv6 Home Agent Reply are unprotected.

Other attacks include many denial-of-service opportunities, for example:

Inducing extra BUs with bogus CNs. Although no satisfactory defense exists, route optimization is 

optional, and the tradeoff is to risk suboptimal routing. A MN can be selective about route 

optimization.

Preventing a legitimate BU from completing while sending a bogus BU to a CN (where the attacker is 

on the same link as the victim)

Reflection attacks, whereby the victim’s address is forged as the source, so that the victim is flooded 

with replies

Replaying old route optimization BUs, especially if sequence numbers are unreliable because of 

crashes or rollover

Bypassing firewall egress filtering with a forged Home Address Option

The next three sections describe how security is provided between a MN and, first, its HA, and, second, a 

CN. In summary:

Mobile IPv6 security is based on a security goal and a threat analysis.

Mobile IPv6 uses IPsec where clearly practical and has updated its specifications to use IKEv2 and 

RFC 4301106.

Return routability was added as a practical method for securing route optimizations between a MN 

and CN.

4.4.3.1 Securing MN to HA Binding Updates

The top priority for MIPv6 security is stopping a forged BU. Given the goal of using existing security 

systems wherever practical, IPsec was the logical choice. A working relationship between a MN and its 

HA naturally exists, and IPsec has always been a mandatory part of IPv6. The latest version of IPsec 

(RFC 4301) contains several improvements for securing communications between a MN and its HA. In 

addition to more efficient cryptographic transformations and the simplifications in IKEv2, selectors for 

ICMPv6 message types and the Mobility Header are now included, and the Peer Authorization Database 

(PAD) can also be used on the HA. The HA uses the PAD to specify how to authenticate the MN and tie 

the MN's identity to its HoA to prevent attacks that impersonate a MN. IPsec ESP in transport mode is 

used for:

 

106

 IETF RFC 4301, Security Architecture for the Internet Protocol, is available at http://www.ietf.org/rfc/rfc4301.txt.
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BU: MN  HA

BUA: HA  MN

The MN’s security associations must use its HoA in either the source address, the Home Address 

Destination Option, or a Type 2 Routing Header. The integrity transform (ESP-NULL) is required, 

confidentiality is optional, and replay detection is recommended if dynamic keying (IKEv1 or IKEv2) is 

used. The following protocol elements are used:

BU Message:

IPv6 Header: Source = CoA; Destination = HA

Home Address Option: Address = HoA 

ESP header: transport mode, authentication

Mobility header: Alternate CoA Option = CoA 

BU Acknowledgement:

IPv6 Header: Source = HA; Destination = CoA 

Type 2 Routing Header: Address = HA

ESP header: transport mode, authentication

Mobility header: BU Acknowledgement Option

To use IPsec for BUs and BUAs between a MN and its HA, both the MN and HA must have appropriate 

entries in their IPsec Security Policy Database (SPD) and Security Association Database (SAD). The 

following examples show how to do this with Transport Mode. RFC 4877 contains examples using 

Tunnel Mode.

MN SPD:

SPD in: Use SA1 for: Source = HA; Destination = HoA; Protocol = Mobility Header

SPD out: Use SA2 for: Source = HoA; Destination = HA; Protocol = Mobility Header

MN SAD:

SA1 (IN, SPI, ESP, TRANSPORT): Source = HA; Destination = HoA; Protocol = Mobility Header

SA2 (OUT, SPI, ESP, TRANSPORT): Source = HA; Destination = HoA; Protocol = Mobility Header

HA SPD:

SPD in: Use SA1 for: Source = HoA; Destination = HA; Protocol = Mobility Header

SPD out: Use SA2 for: Source = HA; Destination = HoA; Protocol = Mobility Header

HA SAD:

SA1 (IN, SPI, ESP, TRANSPORT): Source = HoA; Destination = HA; Protocol = Mobility Header
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SA2 (OUT, SPI, ESP, TRANSPORT): Source = HoA; Destination = HA; Protocol = Mobility Header

Setting up security associations with IKEv1 or IKEv2 is optional, but necessary for enabling replay 

detection. It may be done with public keys or pre-shared secrets. With IKEv1 and pre-shared secrets, 

aggressive mode must be used in Phase 1. One cannot use the source address of the MN, which is the 

CoA, to select the pre-shared secret (there is no identity hiding in aggressive mode), and, similarly, one 

cannot use ID_IPV6_ADDR in Phase 1, so the recommendation is to use a FQDN in Phase 1 and the 

HoA in Phase 2. The HA must verify the relationship between these. Thus, the HoA cannot be 

dynamically assigned. How the HA does this is not specified, but it could be done with DNSSEC, X.509, 

or Cryptographically Generated Addresses107. Figure 4-3 illustrates the IKEv1 identifiers used between a 

MN and its HA.

Figure 4-3. IKEv1 Identifiers used between a MN and its HA

RFC 4877 specifies the entire IKEv2 exchange. Figure 4-4 shows the IKEv2 Identifiers used between a 

MN and its HA.

 

107 The general problem of configuring MIPv6 is defined in RFC 4640, Problem Statement for Bootstrapping Mobile IPv6 

(MIPv6), available from http://www.ietf.org/rfc/rfc4640.txt. 
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Figure 4-4. IKEv2 identifiers used between a MN and its HA

The MN inserts its identity (e.g., its HoA or FQDN) in the IDi payload in the third message, which is 

encrypted and authenticated. The HoA and traffic selectors for protecting BUs and BUAs are included in 

the TSi (traffic selector—initiator). The MN or HA can then send CREATE_CHILD_SA exchanges to 

protect other traffic. Again, the MN uses its HoA in the TSi.

4.4.3.2 Securing Other MN to HA Traffic

Once an IPsec SA is established between a MN and its HA for BUs and BUAs, it can be used for other 

protocol elements:

ICMPv6 between the HA and MN for MPS and MPA prefix discovery and for DHAAD

The return routability messages Home Test Init and Home Test (see Section 4.4.3.3)

User traffic (everything else)

At this point, the security associations at the MN include four sets of ESP SPD and SAD entries to and 

from the HA. All must use the ESP data authentication (integrity) service. Confidentiality may be used if 

needed. The four sets of entries are:

Transport mode for Mobility Headers for BU and BUA with the HA

Transport mode for ICMPv6 for home network prefix discovery 

Tunnel mode for Mobility Headers for return routability messages to and from a CN

Optionally, tunnel mode for all other traffic

4.4.3.3 Securing MN to CN Communications

A MN needs to exchange a BU and BUA with a CN to establish route optimization. The major threats 
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are that a CN gets a forged BU, or a CN processes a forged Home Address Option. The original drafts 

recommended using IPsec, but it was decided that sufficient infrastructure for IPsec authentication did not 

exist to make this widely deployable, and a new security protocol called return routability was invented 

to authenticate and share a key (called Kbm) between a MN and CN. The idea was to make sure the MN 

can receive messages both directly from the CN over its optimized route and indirectly over its IPsec 

connection with its HA. Return routability works as follows: the MN sends the CN Care-of Test Init and 

Home Test Init over these two paths, respectively. These paths are illustrated in Figure 4-5.

Figure 4-5. Return Routability—Init Messages

Note that route optimization is optional for both parties. A MN always has a tradeoff between optimal 

packet forwarding and location privacy, and a CN may ignore BUs (when it suspects an attack, for 

example).

The CN can now compute and return Keygen Tokens for each path:

Home Keygen Token = First(64, HMAC_SHA1(Kcn, (Home Address, HoA Nonce, 0)))

Care-of Keygen Token = First(64, HMAC_SHA1(Kcn, (Care-of Address, CoA Nonce, 1)))

The CN sends these back and forgets them so as to avoid a denial-of-service attack. Later, when 

addresses and nonce indices are returned, it can re-compute these and:

Kbm = SHA1(Home Keygen Token, Care-of Keygen Token)
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The CN does not allocate Binding Cache storage until authentication completes. Figure 4-6 illustrates the 

Keygen replies.

Figure 4-6. Return Routability—Keygen Replies

The MN now computes the following and sends the BU:

Kbm = SHA1(Home Keygen Token, Care-of Keygen Token)

Binding Update Message Authentication Code (BU MAC) = 

First(96, HMAC-SHA1(Kbm, (Care-of Address | CN address | BU*)))

BU = (Home Address Option, BU MAC, sequence number,

Home Address Nonce Index, Care-of Address Nonce Index).

Figure 4-7 illustrates the BU and BUA path.
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Figure 4-7. Reverse Routability—BU and BUA Protected with Kbm

Now, over the optimized route, the MN sends normal traffic with the Home Address Option, and the CN 

sends normal traffic with a Type 2 Routing Header.

To prevent various abuses, restrictions exist on using the Home Address Option and Type 2 Routing 

Header. For the former, the rules are:

Only one Home Address Option is allowed per packet, it must not be altered en route, it must contain 

a routable, unicast address, and it must not cause changes in the routing or binding cache.

A BU at a CN must be authenticated with a Kbn established with return routability.

A BU at a HA must be authenticated with transport mode ESP.

All other cases must correspond to an entry in the binding cache.

The rules for Type 2 Routing Headers are:

Only one type 2 routing header is allowed per packet, and it may only have one segment remaining.

The HoA in the Type 2 Routing Header cannot have smaller scope than the CoA in the destination, it

must be routable and unicast, and it must be the correct one for the MN.

In summary, an attacker has to intercept two messages sent along different paths to get Kbm. Perhaps the 

greatest danger of this is on the MN’s local link, but here IPsec with encryption protects the Home 

Keygen Token in the Home Test (message 3).

RFC 4449108 describes an efficient alternative to return routability, whereby the MN and CN have pre-

 

108 IETF RFC 4449, Securing Mobile IPv6 Route Optimization Using a Static Shared Key, is available from 
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shared a Kcn and nonces, and both can compute Kbm directly. This may be useful, for example, when the 

MN accesses servers at its home site. The CN needs to trust the MN not to use its pre-shared data to 

launch DoS attacks. Also, the IETF is working on a draft specification, Using IPsec between Mobile and 

Correspondent IPv6 Nodes109, for using IPsec where it is suitable or desirable to secure BUs and other 

communications between a MN and CN. 

4.4.3.4 Other Security Considerations for MIPv6

This section briefly covers three topics that do not fit into the above sections: an alternative to IPsec for 

MN-HA security proposed by the Third Generation Partnership Project (3GPP2), issues with making 

mobility and firewalls work together, and a discussion of location privacy. For a discussion of other 

security issues including attacks by a legitimate but misbehaving MN that creates routing loops or tries to 

bypass ingress filtering, see the Internet Draft on Mobile IPv6 Residual Threats110.

Some designers of 3GPP2 networks consider including IPsec in handsets too difficult. RFC 4285,

Protocol for Mobile IPv6 Authentication,

111 describes a shared-key, lightweight alternative to IPsec 

for securing communications between a MN and its HA designed specifically for 3GPP2 networks. 

The IETF has considered this document informational, not standards track, and it has not been 

recommended for use in other environments. A draft revision to RFC 4285, including timestampbased replay detection, was written but has expired112

.

However, additional questions have been raised about the suitability of IPsec for securing MIPv6, and 

a standards-track alternative has been proposed113. The reasons given for supporting the 

standardization of alternatives to IPsec are: 

1. Software complexity: Because IPsec is implemented in the operating system and no 

convenient application programming interface exists, it is difficult for a third party to 

implement MIPv6 on a system supporting IPsec and IKEv2. 

2. NAT traversal: The need to run MIPv6 on dual stack IPv4-IPv6 systems114 across an IPv4 

network address translation (NAT) component requires UDP encapsulation and makes 

running IKEv2 and IPsec more complicated. 

3. Dynamic HoA assignment: Unless this is built into the IKEv2 processing, securely 

configuring a dynamic address leads to a chicken-and-egg problem. Using this capability 

within IKEv2 requires communicating the home prefix to IKEv2. 

4. Scalability: The number of security associations a HA must support may exceed its capacity.

5. Availability: Some IPv6 platforms may not have IPsec and IKEv2. 

 

http://www.ietf.org/rfc/rfc4449.txt.

109

 The Internet Draft, Using IPsec between Mobile and Correspondent IPv6 Nodes, is available at 

http://tools.ietf.org/html/draft-ietf-mip6-cn-ipsec

110 IETF work in progress Mobile IPv6 Residual Threats, is available at http://tools.ietf.org/html/draft-haddad-mext-mip6-

residual-threats. 

111 IETF RFC 4285, Authentication Protocol for Mobile IPv6, is available at http://www.ietf.org/rfc/rfc4285.txt.

112 See https://datatracker.ietf.org/drafts/draft-ietf-mip6-rfc4285bis/ for the current status of this work.

113 See IETF work in progress Transport Layer Security-based Mobile IPv6 Security Framework for Mobile Node to Home 

Agent Communication, available from http://tools.ietf.org/html/draft-korhonen-mext-mip6-altsec.

114 IETF RFC 5555, Mobile IPv6 Support for Dual Stack Hosts and Routers, is available from 

http://www.ietf.org/rfc/rfc5555.txt.
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6. Communications overhead: The total overhead for IKEv2 and ESP exceeds that of other 

security solutions.

While these opinions, taken in total, may have some merit, they are controversial. Nevertheless, it is 

likely that at least one alternative to IPsec will proceed along the IETF standards track. It is unclear 

what all of the details will look like and what the time frame for completing this work will be.

Many IPv6 firewalls are not compatible with MIPv6. RFC 4487, Mobile IPv6 and Firewalls: 

Problem Statement,

115 considers four cases:

– The MN is in a network protected by firewalls: The MN needs to be able to get IPsec ESP 

packets through the firewall for BUs and Home Test Init messages. The firewall also has to 

understand these protocols and allow appropriate responses (BUA and Home Test). The Care-of 

Test Init and reply also must be allowed through. If the MN moves from a network protected by 

one firewall to a network protected by another, the firewalls need to retain state jointly.

– The CN is in a network protected by firewalls: The firewall needs to understand inbound Home 

Test Init and Care-of Test Init messages, so that these are not dropped. The firewall has no way 

to examine a BU, distinguish legitimate instances from an attack, and update its state accordingly.

– The HA is in a network protected by firewalls: The firewalls need to handle ESP and unsolicited 

incoming connections. Movement by the MN may result in traffic arriving at the HA through a 

different firewall from before, so stateful firewalls need to maintain this state jointly.

– The MN moves into a network protected by firewalls: First, the BU with the HA has to get 

through. Then, existing connections, which have no prior state, need to continue. Finally, return 

routability with CNs needs to work. 

It is unclear how the combined goals of accommodating these cases and still repelling attacks will be 

satisfied, so more work on this topic is needed.

Many privacy issues exist at all protocol layers, from MAC addresses to application-layers. RFC

4882, IP Address Location Privacy and Mobile IPv6: Problem Statement ,

116 considers only location

privacy with MIPv6, and only the IP layer. Two issues are identified: disclosure of a MN’s HoA to 

eavesdroppers and disclosure of its CoA to CNs. A solution to the former is to use confidentiality 

with ESP and not to use route optimization. A solution to the latter is not to use route optimization.

4.4.4 Unknown Aspects

MIPv6 is a flexible yet complex capability. Some aspects of the specification are not yet complete and 

not all implementations support mobility yet. This section highlights some considerations for minimizing 

unknown or unforeseen behaviors on networks using MIPv6. Emerging topics such as bootstrapping,

dual stack operation, and reliability are covered.

A mobile node needs a HoA, a HA address, and a security association with its HA. Statically 

provisioning this information can be administratively expensive, so work has started on obtaining it 

automatically through a process called bootstrapping. Variations exist in types of service providers and 

methods of address assignment and authentication. These issues are discussed in RFC 4640.

117 A 

 

115 IETF RFC 4487, Mobile IPv6 and Firewalls: Problem Statement, is available at http://www.ietf.org/rfc/rfc4487.txt.

116 IETF RFC 4882, IP Address Location Privacy and Mobile IPv6: Problem Statement, is available at 

http://www.ietf.org/rfc/rfc4882.txt.

117 Besides IETF RFC 4640, for additional references outside the IETF documents, see Kempf, J., J. Arrko, and P. Nikander, 
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distinction is made between the case in which the same service provider offers mobility and provides 

authorization and the case in which these functions are split between service providers. Proposals for 

providing bootstrapping information with DHCPv6 and making bootstrapping work with RADIUS 

authentication are under discussion. Solutions have been developed for the split case (different home 

network and mobile providers) in RFC 5026118, Mobile IPv6 Bootstrapping in Split Scenario, and for the 

integrated (single provider) case in IETF work in progress, MIP6-bootstrapping for the Integrated 

Scenario119.

A MN with dual IPv4 and IPv6 protocol stacks could run both MIPv4 and MIPv6, but this would be 

expensive and also awkward if the mobility protocols are not integrated. It also raises security issues, not 

the least of which is the need to include the completely different approaches to security in MIPv4 and 

MIPv6. One immediate question is which versions of IP the foreign link supports. A second is whether 

the same HA supports both MIPv4 and MIPv6. A third is which protocols the CNs are capable of using. 

None of the tunneling mechanisms designed to help IPv4-to-IPv6 transition includes specific support for 

mobility. Because mobility is essentially a tunneling protocol, ways exist, in principle, to use a single HA 

and a single CoA (IPv4 or IPv6) to deliver either IPv4 or IPv6 packets. One such solution, documented in 

RFC 5555120, allows dual stack systems to run both IPv6 and IPv4 over MIPv6. It also handles Network 

Address Translation (NAT) in the IPv4 networks.

The MIPv6 HA is a single point of failure. Switching HAs requires detecting failure, finding an 

alternative HA, transferring state, and reestablishing security. Different solutions are possible: a MN 

could maintain connectivity with a ―hot spare‖ HA, the HA could share state with backup systems, or the 

MN could, in effect, start over when a failure is detected. These involve different tradeoffs, and solutions 

are under discussion121. Another case being considered is when, for load balancing, planned outages, or 

other reasons, a HA can announce that it will no longer be available.

.

4.5 Jumbograms

RFC 2675122 defines an IPv6 Hop-by-Hop Option called jumbograms, which allows an IPv6 packet to 

contain a payload longer than 65,535 octets. Jumbograms are only defined for IPv6 interfaces that may 

be attached to links with a MTU greater than 65,575 octets. Other interfaces can ignore jumbograms. 

Many IPv6 implementations allow jumbograms but do not support any link layers that can handle them 

(except the loopback interface). 

4.5.1 Specification Overview

The 16-bit Payload Length in the IPv6 Header is set to zero and the Hop-by-Hop Jumbo Payload Option, 

Type C2 base 16 (1100 0010), must be included right after the header. It specifies a 32-bit Payload 

Length. Jumbograms must be at least 65,536 bytes and must not be fragmented.

 

―Mobile IPv6 Security,‖ Wireless Personal Communications, Vol. 29, (2004): 389-414; and Soliman, H., Mobile IPv6, 

Addison-Wesley, 2004.

118 IETF RFC 5026, Mobile IPv6 Bootstrapping in Split Scenario, is available at http://www.ietf.org/rfc/rfc5026.txt.

119 IETF work in progress MIP6-bootstrapping for the Integrated Scenario, available at http://tools.ietf.org/html/draft-ietfmip6-bootstrapping-integrated.

120 IETF RFC 5555, Mobile IPv6 Support for Dual Stack Hosts and Routers, is available at http://www.ietf.org/rfc/rfc5555.txt.

121 See IETF work in progress Home Agent Reliability Protocol, available at http://tools.ietf.org/html/draft-ietf-mip6-

hareliability.

122 IETF RFC 2675, IPv6 Jumbograms, is available at http://www.ietf.org/rfc/rfc2675.txt.
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Common transport payloads such as TCP and UDP have packet size limits (in the UDP length field and 

the TCP MSS option and Urgent field), so modifications are needed for them to use jumbograms.

4.5.2 Security Ramifications

RFC 2675 says, ―The Jumbo Payload option and TCP/UDP jumbograms do not introduce any known new 

security concerns,‖ but the BSD and KAMI implementations claim that IPsec AH and jumbograms are 

incompatible due to an unexplained ―quirk in the specs‖ regarding the AH header size. Given the limited 

use of jumbograms, it is not surprising that some (perhaps most) IPsec implementations do not support 

them.

Jumbograms may be used as a vehicle for DoS. They can cause substantial latency, increase the cost of 

retransmission, and interfere with QoS. A maximum length jumbogram will tie up a one gigabit per 

second link for over a half a minute, and if the UDP checksum fails, the receiving interface may request 

retransmission.

Intermediate systems such as application layer firewalls and intrusion detection systems may be 

unprepared to handle huge jumbograms (which can be up to four gigabytes long).

Finally, many IPv6 implementations may contain places in which the Payload Length is stored as an 

unsigned 16-bit integer. Finding and exploiting these may be a method of causing buffer overflows, 

kernel crashes, or other unintended effects.

4.6 Address Selection

An IPv4 interface typically has one IPv4 unicast address, which may or may not be globally routable, plus 

the loopback address (127.0.0.1). An IPv6 interface, in contrast, typically starts with an interface local 

loopback address, a link local address, a unique local address, and a globally routable address. 

Renumbering and multihoming can result in having more than one address of certain types. There is 

nothing to stop one from generating and using additional addresses, and renumbering may cause more 

than one address of a given type to be active. Some addresses should still be accepted or handled by an 

implementation, but their use is no longer recommended. A choice of source addresses exists for every 

packet. Often, a choice of destination addresses exists as well.

In a system running both IPv4 and IPv6, address selection is more complicated, and rules need to be 

given for which of these to prefer.

RFC 3484123 specifies rules for choosing source and destination addresses, but more recent experience has 

shown that additional rules are needed for networks with multiple prefixes. This section examines IPv6 

address selection rules and their consequences.

4.6.1 Specification Overview

This section summarizes the rules for source and destination address selection specified in RFC 3484. 

Criteria for choices and preferences in each case are covered.

Address selection may have to choose among IPv4 versus IPv6, addresses with different scopes, public or 

private address, and so forth. Some of the rules seem obvious, for example, choosing an address that has 

 

123 IETF RFC 3484, Default Address Selection for Internet Protocol version 6 (IPv6), is available at 

http://www.ietf.org/rfc/rfc3484.txt.
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not been deprecated over one that has, but nevertheless, software has to be written to make the right 

choice. In general, source and destination pairs are chosen to match scopes and types (e.g., native IPv6, 

6to4, or IPv4-mapped), to prefer smaller scopes, and to prefer native addresses over transitional 

addresses. If the destination is a multicast address, the multicast scope is used in choosing the scope of 

the unicast source address. One of the principles introduced in address selection is longest prefix match, 

which means, in the absence of other criteria, one chooses source and destination addresses to try to take 

advantage of route aggregation.

Another new concept is the address selection policy table, which allows administrators to add or override 

address selection rules. IPv4 addresses are represented in the table as IPv4-mapped IPv6 addresses, and 

these are assigned scopes corresponding to IPv6 link-local or global addresses. RFC 3484 gives an 

example of a policy table:

 Prefix Precedence Label Use

 ::1/128 50 0 Loopback

 ::/0 40 1 Default (including native IPv6)

 2002::/16 30 2 6to4

 ::/96 20 3 IPv4 Compatible

 ::ffff:0:0/96 10 4 IPv4 Mapped

Given an address, the entire table is scanned to find the entry with the longest common prefix matching 

the address. Then the corresponding Precedence and Label are returned. This has the effect of, first, 

matching labels of the source and destinations, and, second, preferring native IPv6 addresses over IPv4 or 

the different tunneling addresses (6to4 or v4-compatible).

The first step in source address selection is to make a list of candidate choices. In general, the choice 

should match the interface and scope. The next step is to sort the candidates by going through an ordered 

list of rules, starting with rule 1, and proceeding to the next rule only to break ties:

1. Prefer a source address that equals the destination.

2. Prefer the smallest scope that is at least as large as the destination’s scope. (This rule is 

mandatory.)

3. Prefer an address that is not deprecated.

4. Prefer a home address over a care-of address, unless the application chooses to reverse this.

5. Prefer an address on the correct outgoing interface for the given destination.

6. Prefer an address that matches the label of the destination in the policy table.

7. Prefer a public address over a temporary address, unless the application chooses to reverse this.

8. Use the address with the longest matching prefix in common with the destination.

Selecting a destination address follows a similar set of rules. One main difference is that choosing a 

destination involves asking what source would be used for each choice. Candidates are listed and 

compared, starting at rule 1, and continuing down the list of rules to break ties:
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1. Avoid unusable destinations (ones that are unreachable or have no usable source address).

2. Prefer a destination with a source of matching scope.

3. Prefer a destination with a source that is not deprecated.

4. Prefer a destination with a source that is just a home address to one with a source that is just a 

care-of address.

5. Prefer a destination with a source that has a matching label (in the policy table).

6. Prefer a destination with higher precedence (in the policy table).

7. Prefer a destination reached with native transport over one using encapsulation.

8. Prefer a destination with smaller scope.

9. Prefer a destination with a source having the longest matching prefix.

10. Prefer a destination that occurred first in the original list. That is, leave the order unchanged.

4.6.2 Differences from IPv4 Standards

The address selection problem itself is new with IPv6 for several reasons:

Freedom to use more addresses because of the larger address space

Scoped addresses (link local, unique local, etc.)

Explicit rules that allow use of multiple addresses of the same type

Prefix renumbering

Address deprecation.

The address selection rules recognize MIPv6 and distinguish home addresses from care-of addresses. The 

simultaneous presence of IPv4, IPv6, and transition tunneling protocols requires address selection rules 

that integrate the use of both IPv4 and IPv6.

4.6.3 Security Ramifications

The proper use of the address selection rules given here does not have a large impact on security. 

Improper address selection can result in dropping packets inadvertently or inefficient packet forwarding, 

which are not security problems per se. Many attacks, however, use spoofed addresses, which can be 

characterized as systems not following the rules.

Proper source address selection is important for ingress filtering. Note that it may be difficult for ingress 

filtering to distinguish temporary addresses from spoofed ―in-prefix‖ addresses used in DoS attacks.

Address selection works across IPv4 and IPv6 addresses and can determine which is used when both are 

available or which type of tunneling is used. Ensuring that address selection policy is correctly specified 

can affect other assumptions about security.

The address selection rules can help enable some attacks on privacy. By probing a host with requests that 

come from different source addresses and observing what source addresses the target uses for replies, one 
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can extract information about the set of addresses used by the target.

4.6.4 Unknown Aspects

Experience using the rules described above for address selection has unveiled several problematic 

configurations, which require further attention. When a link has more than one router, or a site border 

router is connected to more than one ISP, multiple prefixes can be used. If the prefix of a source address 

does not correspond to the connection used, the packet may be dropped because of ingress filtering, or an 

asymmetrical route may be established, and returned packets may be dropped due to packet filtering. 

This problem is even worse in the case in which one of the routers is not globally connected but only 

connected to a closed network segment. Prefix renumbering increases the complexity of this, because 

choices of appropriate prefixes are not static.

The longest prefix match rule can always distinguish currently allocated global addresses from unique 

local addresses, but this will become an issue when global addresses with a leading one bit begin to be 

allocated.

Applications may need to have better control over address selection in certain cases. One case is 

choosing between temporary and permanent addresses; another is between home and care-of addresses. 

The above selection rules prefer IPv6 over IPv4 addresses, but cases exist in which the IPv4 address is the 

better choice. For example, choosing the IPv6 address may result in use of a less efficient tunnel through 

the same IPv4 network or choosing an IPv6 ULA with only local connectivity instead of an IPv4 address 

with global connectivity.

Work is underway to find ways to remedy all of these situations, but in the mean time, installations need 

to be aware of these potential problems and configure ways to work around them, case by case.

4.7 Dynamic Host Configuration Protocol (DHCP) for IPv6

Dynamic Host Configuration Protocol for IPv6 (DHCPv6) is a client-server protocol that provides IPv6 

interfaces with address assignments and other configuration information. The alternative automated 

protocol, stateless autoconfiguration, is the ―native IPv6‖ way to obtain a dynamic IPv6 address, but it 

does not supply information like DNS and NTP servers’ addresses or perform dynamic DNS updates. 

Also, stateless autoconfiguration does not offer centralized control of address assignments, which some 

network operators may require. DHCPv6 keeps track of address assignments and is called stateful, in 

contrast to stateless. DHCPv6 is not described in the IPv6 standards as an essential component, but as 

more enterprises start to use IPv6, demand for DHCPv6 is growing.

The original DHCP for IPv4 (DHCPv4) and DHCPv6 are two separate protocols. One of the reasons for 

using DHCPv4 with IPv4 is address conservation. Addresses may be in short supply and need to be 

recycled, for example, on a dial-in network. This is not normally a concern with IPv6. Nevertheless, 

IPv6 addresses are more difficult to memorize and type in, so having automated tools to manage them is 

an aid to users and administrators.

DHCPv6 servers assign IPv6 addresses to network interfaces on a lease basis. The client may use the 

assigned IP address for an administratively pre-determined amount of time before the lease expires. This 

means that IPv6 address assignments made by DHCPv6 servers are not permanent, and over time, more 

than one node may use a given IP address, but no more than one node can use an address at one time.
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4.7.1 Specification Overview

DHCPv6 is defined in RFC 3315, Dynamic Host Configuration Protocol for IPv6.124 It is a quite 

sophisticated protocol with many message types, options, status codes, and timers. It performs three 

services:

Address configuration: allocating addresses and providing a network prefix and address of the default 

router

Providing other configuration information: primarily, a DNS server’s address, but potentially many 

other types of information

Allocating prefixes to routers.

In addition to RFC 3315, many other standards supplement or modify DHCPv6:

RFC 3319,125 Dynamic Host Configuration Protocol (DHCPv6) Options for Session Initiation 

Protocol (SIP) Servers, defines an option for DHCPv6 clients to obtain a list of domain names or 

addresses for SIP servers.

RFC 3633,126 IPv6 Prefix Options for Dynamic Host Configuration Protocol (DHCP) version 6, 

describes using DHCPv6 between routers for delegation of address prefixes, which is not discussed 

further this document.

RFC 3646,127 DNS Configuration Options, defines options for configuring a list of DNS name servers 

and domain search lists.

RFC 3736,128 Stateless Dynamic Host Configuration Protocol (DHCP) Service for IPv6, defines the 

subset of RFC 3315 that needs to be implemented for stateless DHCPv6.

RFC 3898,129 Network Information Service (NIS) Configuration Options, defines four options for 

configuring NIS services.

RFC 4014,130 Remote Authentication Dial-In User Service (RADIUS) Attributes Suboption for the 

Dynamic Host Configuration Protocol (DHCP) Relay Agent Information Option, specifies an option 

for a DHCPv6 relay agent to pass information obtained during authentication of the client to the 

DHCP server.

 

124 IETF RFC 3315, Dynamic Host Configuration Protocol for IPv6 (DHCPv6), is available at 

http://www.ietf.org/rfc/rfc3315.txt.

125 IETF RFC 3319, Dynamic Host Configuration Protocol (DHCPv6) Options for Session Initiation Protocol (SIP) Servers, is 

available at http://www.ietf.org/rfc/rfc3319.txt.

126 IETF RFC 3633, IPv6 Prefix Options for Dynamic Host Configuration Protocol (DHCP) version 6, is available at 

http://www.ietf.org/rfc/rfc3633.txt.

127 IETF RFC 3646, DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6), is available at 

http://www.ietf.org/rfc/rfc3646.txt.

128 IETF RFC 3736, Stateless Dynamic Host Configuration Protocol (DHCP) Service for IPv6, is available at 

http://www.ietf.org/rfc/rfc3736.txt.

129 IETF RFC 3898, Network Information Service (NIS) Configuration Options for Dynamic Host Configuration Protocol for 

IPv6 (DHCPv6), is available at http://www.ietf.org/rfc/rfc3898.txt.

130 IETF RFC 4014, Remote Authentication Dial-In User Service (RADIUS) Attributes Suboption for the Dynamic Host 

Configuration Protocol (DHCP) Relay Agent Information Option, is available at http://www.ietf.org/rfc/rfc4014.txt.
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RFC 4075,131 Simple Network Time Protocol (SNTP) Configuration Option for DHCPv6, defines an 

option for obtaining a list of SNTP servers’ addresses.

RFC 4361,132 Node-specific Client Identifiers for Dynamic Host Configuration Protocol Version Four 

(DHCPv4), updates DHCPv4 to be compatible with naming and DNS updates used in DHCPv6, so 

that systems running both IPv4 and IPv6 can work with both versions of DHCP separately but 

compatibly.

Three primary configuration options are available for IPv6 interfaces:

1. Use autoconfiguration and not DHCPv6.

2. Use DHCPv6 and not autoconfiguration.

3. Get an address with autoconfiguration and then use DHCPv6 to retrieve additional information. 

This choice is driven by two flags (called M for managed and O for other) in Router Advertisements 

(RA). When the M flag is set to 1, DHCPv6 should be used for address assignment. When the O flag is 

set to 1, DHCPv6 should be used to obtain other configuration information.

Clients and servers use UDP to exchange DHCPv6 messages; they listen on ports 546 and 547, 

respectively. Both clients and servers include a DHCP Unique Identifier (DUID) to identify themselves. 

A client’s request for an address is called a Solicit message. It is normally sent from a link local source 

address to the link-scoped All_DHCP_Relay_Agents_and_Servers multicast address (FF02::1:2). A 

typical response to a Solicit message is an Advertise message containing a list of allocated addresses, the 

address of the default router, and the network prefix for the interface. DHCPv6 is capable of assigning 

multiple addresses to a single IPv6 interface. Being able to choose a default router allows DHCPv6 to 

perform load balancing. (For load balancing requirements without using DHCPv6, see RFC 4311.133) 

Once the client has determined the address of a DHCPv6 server, it may, in some cases, send unicast

messages directly to the server.

If a client reaches a relay agent, the agent normally queries the All_DHCP_Servers site-scoped multicast 

address (FF05::1:3). This allows a DHCPv6 client to reach a DHCP server not attached to the same link. 

The relay operation is transparent to the client, and the discussion in the remainder of this section omits 

message relaying.

4.7.1.1 Client-server Exchanges Involving Four Messages

To request the assignment of one or more IPv6 addresses, a client first locates a DHCP server and then 

requests the assignment of addresses and other configuration information from the server. The client 

sends a Solicit message to the All_DHCP_Relay_Agents_and_Servers address to find available DHCP 

servers. Any server that can meet the client’s requirements responds with an Advertise message. The 

client then chooses one of the servers and sends a Request message to the server asking for confirmed 

assignment of addresses and other configuration information. The server responds with a Reply message 

that contains the confirmed addresses and configuration.

 

131 IETF RFC 4075, Simple Network Time Protocol (SNTP) Configuration Option for DHCPv6, is available at 

http://www.ietf.org/rfc/rfc4075.txt.

132 IETF RFC 4361, Node-specific Client Identifiers for Dynamic Host Configuration Protocol Version Four (DHCPv4), is 

available at http://www.ietf.org/rfc/rfc4361.txt.

133 IETF RFC 4311, IPv6 Host-to-Router Load Sharing, is available at http://www.ietf.org/rfc/rfc4311.txt.
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4.7.1.2 Client-server Exchanges Involving Two Messages

When a DHCP client does not need to have a DHCP server assign it IP addresses, the client can obtain 

configuration information such as a list of available DNS servers or NTP servers by exchanging a single 

message pair with a DHCP server. The client first sends an Information-Request message to the 

All_DHCP_Relay_Agents_and_Servers multicast address. A server responds with a Reply message 

containing the configuration information for the client.

Even if a client needs address allocations, if a server has IPv6 addresses and other configuration 

information committed to a client, the client and server may be able to complete the exchange using only 

two messages. In this case, the client sends a Solicit message to the multicast 

All_DHCP_Relay_Agents_and_Servers address requesting the assignment of addresses and other 

configuration information. The client indicates it is willing to accept an immediate Reply message from 

the server. A server that is willing to commit the assignment of addresses to the client immediately 

responds with a Reply message. The configuration information and the addresses in the Reply message 

are then immediately available for use by the client.

Each address assigned to the client has preferred and valid lifetimes specified by the server. The valid 

lifetime is always longer. When the preferred lifetime expires, the address is deprecated for new uses but

may be used for existing connections (e.g., TCP or IPsec) until the valid lifetime expires. To request an 

extension of the lifetimes assigned to an address, the client sends a Renew message. The server returns a 

Reply message with the new lifetimes, and the client may continue to use the address without 

interruption.

4.7.2 Differences from IPv4 Standards

DHCPv4 and DHCPv6 perform similar functions, but distinct differences exist. Many of these are due to 

either the underlying differences between IPv4 and IPv6 or the chance to make improvements in DHCPv6 

based on what has been learned running DHCPv4.

One primary difference is that DHCPv4 and DHCPv6, unlike DNS, are two separate protocols and 

services. To date, no single standard for DHCP on dual stack IPv4-IPv6 systems exists. RFC 4477, 

Dynamic Host Configuration Protocol (DHCP) IPv4 and IPv6 Dual Stack Issues,

134 covers potential 

problems for clients receiving both DHCPv4 and DHCPv6 information and considers potential solutions, 

both with a single server and separate servers. In either case, many questions arise about how best to 

merge the information about IPv4 and IPv6 configurations.

A second major difference is due to the presence of stateless autoconfiguration in IPv6. Dynamic IPv6 

addresses can be configured without DHCPv6, and whether DHCPv6 is used is determined by RAs, not 

by hosts.

DHCPv6 generates far less traffic than DHCPv4. The differences include use of broadcast with IPv4 

versus multicast with IPv6. DHCPv6 also contains many optimizations, for example, the ability to assign 

multiple network addresses to a single interface.

4.7.3 Security Ramifications

A network that relies upon DHCPv6 for address assignment is subject to DoS if DHCPv6 services are 

 

134 IETF RFC 4477, Dynamic Host Configuration Protocol (DHCP): IPv4 and IPv6 Dual-Stack Issues, is available at 

http://www.ietf.org/rfc/rfc4477.txt.
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interrupted. Also, if the information obtained from DHCP is unreliable or inaccurate, the network may 

degrade or fail. Even the inability to reach a DNS server makes the Internet essentially unusable. Finally, 

an attack against DHCPv6 may cause packets to be misrouted, which then allows the attacker to 

compromise the confidentiality or integrity of their contents. Examples of attacks include:

Rogue responses from unauthorized servers distributing false information, addresses, or otherwise

Maliciously high volumes of client requests causing denial of service

The potential exhaustion of internal server memory by huge numbers of requests (although address 

exhaustion is not a possibility on, for example, a /64)

Access to a private network or theft of service on a public network by unauthorized devices.

One problem that often occurs is having improperly configured experimental or test servers distribute 

inaccurate information. Many of these problems can be mitigated by:

Blocking DHCPv6 at firewalls (the port numbers are easily recognized)

Using the DHCPv6 authentication option between clients and servers. This is defined in RFC 3118135

and implemented as a shared-secret-based HMAC-MD5 calculation with replay detection. The 

drawback to this, of course, is that it requires pre-configured security parameters and authentication 

credentials. 

Securing connections between relay agents and servers, for example, with ESP in transport mode

Reviewing logs of DHCPv6 operations for unexpected events.

4.7.4 Unknown Aspects

DHCPv6 is not universally supported in all IPv6 implementations, although demand for it is increasing as 

users gain more operational experience with IPv6. One still has to check whether it is included.

It is expected that dual IPv4-IPv6 protocol stacks may be a standard method of operation for many 

networks for a significant time. Running separate servers for DHCPv4 and DHCPv6 is somewhat of an 

inconvenience, but limiting conflicting responses and resolving such conflicts if they arise are more 

difficult problems. More work on this topic is needed.

Work is also in progress on several extensions to DHCPv6:

An option for servers to specify a domain name suffix if a client requests one. This may be useful for 

IPv6 hosts or residential gateways, because IPv6 provides globally reachable addresses.

An option for relay agents to request servers to echo relay agent options in their entirety. This is now 

specified in RFC 4994136. The point is that a relay agent may add options to a request, the server may 

ignore them, and if they are not echoed, they may not get back to the client. With this echo option, 

DHCPv6 works more like DHCPv4 and avoids the problem just described. 

An option for relay agents to use message sequence numbers with servers for replay protection

 

135

 IETF RFC 3118, Authentication for DHCP Messages, http://www.ietf.org/rfc/rfc3118.txt

136

 IETF RFC 4994, DHCPv6 Relay Agent Echo Request Option, http://www.ietf.org/rfc/rfc4994.txt
GUIDELINES FOR THE SECURE DEPLOYMENT OF IPV6 (DRAFT)

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A method to query servers about active leases similar to the method specified in RFC 4388137 for 

DHCPv4. This may be useful for administrators who want to poll their networks for active addresses. 

4.8 IPv6 Prefix Renumbering

This section covers renumbering of IPv6 sites, that is, changing network prefixes. Renumbering hosts 

(interfaces) individually is covered in Section 4.1 on Multihoming, Section 4.4 on Mobile IPv6, and 

Section 4.6 on Address Selection. Site renumbering with IPv4 is regarded as a major chore, although 

NAT, where it is used, simplifies the problem somewhat. IPv6 networks likely will not use NAT, but the 

IPv6 specifications contain provisions for making site renumbering without a network outage work 

smoothly. Nevertheless, one must pay attention to a significant number of details to avoid errors, 

oversights, and unexpected results. 

The most commonly cited reason for renumbering network prefixes is to change ISPs. Renumbering 

sometimes becomes necessary when companies with large intranets merge or reorganize or when an ISP 

itself forces renumbering. Renumbering affects a large number of components: routers, firewalls, filters, 

DNS, DHCPv6, routing protocols, system configuration tables, applications, network management tools, 

log files, etc.—anything that specifies addresses. Renumbering an ISP, although it presents many of the 

same problems discussed here, is outside the scope of this document.

4.8.1 Specification Overview

The underlying basis for network prefix renumbering is specified in RFC 486146 and RFC 486247. 

Renumbering is facilitated because RAs lease multiple addresses to interfaces. This has two 

consequences: old addresses time out, are deprecated, and eventually disappear; new addresses can be 

immediately available. This ―make-before-break‖ switchover is accomplished by having two lifetimes 

for prefixes in RAs, a preferred lifetime and a valid lifetime. This allows individual addresses to be 

categorized as preferred or deprecated. The intention is to allow upper layer protocols ample time to 

complete using old, deprecated addresses and begin using new, preferred ones. At the same time, linklocal addresses of routers do not change, so host-to-router ICMPv6, DHCPv6, and such infrastructure 

communications are not interrupted.

RFC 4861 presents an example in which a prefix is initially advertised as having a long preferred lifetime 

that is later shortened. The point is that nodes unplugged from the network may miss the shortened 

announcement, so even after the prefix has expired, the prefix must continue to be advertised with a zero 

lifetime until the original, long period has expired. This example is accompanied with a warning about 

advertising infinite lifetimes.

Another complication is that RAs themselves have lifetimes, in addition to the lifetimes of the prefixes 

they are announcing. Lacking a valid prefix, no global IPv6 address exists; lacking a valid RA, no default 

route exists.

Just because interfaces can pick addresses with a new prefix does not make renumbering work. RFC 

4192138 contains a description of the steps one needs to take to renumber a network with a fairly normal 

complement of services. The process involves allocating sub-prefixes of the new prefix to links and 

updating all occurrences of addresses from the old prefix to the new one. These include:

 

137 IETF RFC 4388, Dynamic Host Configuration Protocol (DHCP) Leasequery, is available at 

http://www.ietf.org/rfc/rfc4388.txt.

138 IETF RFC 4192, Procedures for Renumbering an IPv6 Network without a Flag Day, is available at 

http://www.ietf.org/rfc/rfc4192.txt.
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Manually assigned addresses for interfaces on routers

Routing information and link prefixes advertised by routers

Addresses on routers, firewalls, and packet filters used for access control or ingress filtering

Addresses assigned to interfaces with stateless autoconfiguration

Addresses and other information provided by DHCPv6

DNS records (primarily AAAA and PTR records, as well as DNSsec)

All other instances of addresses in applications, command sequences, configuration files, and 

elsewhere.

Some parts of a network can be renumbered independently of others, which may allow administrators to 

break the task up and make it more manageable. In other situations, more than one old prefix may be 

combined under a new prefix.

The steps outlined in RFC 429120 can be summarized as follows:

1. Obtain the new prefix and DNS reverse zone. Plan how sub-prefixes will be allocated to links. 

Shorten DNS TTLs and DHCP licenses. List critical services and applications and then test each 

one to discover all of the places where addresses will need to be updated.

2. Update DNS records. This can be done centrally or delegated to individual systems that use 

dynamic DNS. In any event, it needs to be coordinated with updates to addresses on interfaces.

3. Configure routers and network infrastructure services with the new prefix in parallel with 

continued use of the old prefix. DHCPv6 prefix delegation can be used to distribute sub-prefixes 

to links. Check that routing protocols, DNS, DHCPv6, firewalls and packet filters, intrusion 

detection systems, and any other infrastructure components are working properly with the new 

prefix before advertising it outside its network.

4. Allocate addresses with the new prefix to interfaces on hosts with stateless autoconfiguration, 

DHCPv6, or manual procedures. Check that the new prefix is working correctly.

After this step stabilizes, the network should be working correctly with both prefixes. The rest of the 

process consists of removing the old prefix.

5. Begin using the new prefix. Phase the old prefix out of internal and external DNS servers. Allow 

individual systems to switch to the new prefix asynchronously. This spreads the load on systems

like dynamic DNS. Individual systems may need to run certain scripts or tools to complete their 

reconfiguration. Applications may have to be forced to reset their address caches. Utilities can 

check where the old prefix is still being used.

6. RAs then deprecate the old prefix by setting the preferred lifetime of link prefixes and addresses 

with the old prefix to zero.

7. Once sessions using the old prefix complete, it is removed from the routing infrastructure, DNS, 

anything that depends on DNS, and any other configuration files. The old prefix may be 

reclaimed by whoever allocated it. The DNS reverse zone may be deleted and its delegation 

removed. All timers, in particular DNS TTLs, can be reset to their normal values.
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Two of the more difficult cases involve systems controlled by another administrative entity (such as an 

external DNS server) and manually configured devices. 

RFC 4192138 and drafts proposing updates to it continue with advice on various topics from software 

development to routing protocols on how to make renumbering go easier:

Applications should not ignore the DNS’s TTL values.

Routers should not rely on manual configuration but should either use DNS (correctly!) or provide an 

integrated capability for renumbering.

Care must be taken in updating BGP not to cause routing outages or route flaps that propagate 

throughout the network. See RFC 4192, Section 3.4.

Using the Stream Control Transmission Protocol (SCTP) may be a good choice for critical 

applications that need to keep running during renumbering, because it allows dynamic changes in an 

endpoint’s IP addresses.

For MIPv6, renumbering at the visited site can be handled as if the mobile moved to a new location, 

and it can learn of renumbering at the home site with the MPS and MPA ICMPv6 messages.

Multicast addresses with embedded unicast prefixes should be updated after renumbering to avoid 

future address clashes.

Using ULAs for services within a site during renumbering may be a good idea. Renumbering does 

not have to change ULAs.

During renumbering, some traffic will be sent from the old to the new prefix or vice versa. Routers 

and packet filters need to be aware of this.

4.8.2 Differences from IPv4 Standards

RFC 2072139 contains a description of IPv4 renumbering at a comparable level of detail with RFC 4192. 

On the one hand, NAT simplifies renumbering, but renumbering plans that involve different size CIDR 

networks tend to be more complicated than, for example, mapping one /48 to another in IPv6.

Because IPv4 does not provide the basic building blocks for automated renumbering—starting with RAs 

containing preferred and valid timers and continuing with DHCPv6 prefix delegation and other features—

IPv4 networks must be renumbered manually, which usually means continuous operation throughout the 

renumbering process is not possible. The processes described in this section do not correspond to any 

processes typically followed in IPv4 networks.

4.8.3 Security Ramifications

Three main points to keep in mind are:

Errors and omissions during renumbering can result in timeouts or lost connectivity during the 

renumbering. This applies especially to infrastructure components like DNS (including the reverse 

tree) and routing protocols.

 

139 IETF RFC 2072, Router Renumbering Guide, is available at http://www.ietf.org/rfc/rfc2072.txt.
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Renumbering, if not carried out with attention to security, can leave the network in intermediate states 

vulnerable to attack. If one wants to extend the castle metaphor for firewalls, then renumbering is the 

changing of the guard.

Incomplete renumbering of security systems can leave permanent vulnerabilities.

If renumbering is due to changing service providers, and both prefixes are available during an overlap or 

transition period, all of the security considerations for multihoming apply here:

Multiple addresses and prefixes lead to complications, especially for enforcing access control lists. 

Ingress filtering, if implemented at both service providers, will cause some loss of connectivity, 

unless some arrangement can be made to relax the filtering rules temporarily.

Other security considerations include the following:

Renumbering will break long-lived IPsec SAs and SSL-TLS connections.

Log files need to be interpreted correctly with respect to address changes.

Dynamic DNS updates should be secured with the TSIG and SIG(0) mechanisms defined in DNSsec.

4.8.4 Unknown Aspects 

In spite of all that has been written about IPv6 renumbering, little practical experience exists, although 

there are reports covering some controlled experiments. Guidelines, rough lists of steps, and some 

warnings exist, but an agreed upon set of best practices has not yet emerged.

Three topics identified as needing more attention are:

Detailed prefix renumbering procedures for specific products

A stable method to apply renumbering to BGP without causing route flaps or outages

Renumbering of DNS across administrative domains, and, in particular, management of the DNS 

reverse zone. 

Two potential methods of simplifying renumbering are:

Assigning well-known anycast addresses for routers and servers that provide important parts of the 

network infrastructure

Deploying a split identifier-locator technology like SHIM6, in which case renumbering only involves 

the locator part of the address. 
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5. IPv6 Security Advanced Topics

This chapter covers four additional security topics that are new or different with IPv6. The first two 

topics are privacy addresses and cryptographically generated addresses, which become feasible because 

of the enormous size of the IPv6 address space. IPsec is the third security topic; new developments and 

certain aspects specific to IPv6 are covered here. The fourth topic is security for neighbor discovery 

(ND), which uses ICMPv6 on the local link. The two distinct approaches to security for ND are described 

and compared.

5.1 Privacy Addresses 

IPv6 offers a number of different addressing options that can assist security architectures. Privacy 

addresses can be used by client applications to inhibit user tracking, which may be useful for protecting 

external communications. This section defines and discusses privacy addressing in IPv6, differences 

between the IPv6 and IPv4 standards, and their security ramifications.

According to RFC 429120, interfaces using stateless autoconfiguration generate interface identifiers based

on their IEEE EUI-64 identifiers. This provides strong support for uniqueness, but it allows an interface 

to be tracked, even if it moves from one network to another, or the network prefix is changed. 

Consider, for example, a mobile device that connects to different wireless networks from different 

locations. Using IPv4, the device is likely to use DHCP in the different locations and receive completely 

uncorrelated addresses. If it used IPv6 autoconfiguration, its wireless interface’s address would have the 

same interface identifier in each instance. Moreover, the IEEE EUI-64 interface ID, because it is based 

on the hardware MAC address, might reveal what type of device it is. 

An interface that accepts inbound connections and has a DNS name clearly cannot have a private address, 

but it is still possible to use different addresses for outbound connections. RFC 4941140, Privacy 

Extensions for Stateless Address Autoconfiguration in IPv6, defines a way to generate and change such 

temporary addresses. The important requirements are that the sequence of temporary addresses an 

interface chooses must be totally unpredictable and have low probability of colliding with choices made 

by other interfaces. 

The method recommended in RFC 4941 works approximately as follows:

1. Obtain the interface identifier that would be used without this scheme.

2. Apply a cryptographic hash function to this value and either a saved history value or a randomly 

chosen 64-bit number.

3. Use the output of the hash function to select the interface identifier and to update the history 

value.

4. Run Duplicate Address Detection (DAD).

5. Set the appropriate lifetimes and join the solicited node multicast group corresponding to the 

interface identifier.

6. Continue to use prior interface identifiers for ongoing connections but not for new ones.

 

140 IETF RFC 4941, Privacy Extensions for Stateless Address Autoconfiguration in IPv6, is available at 

http://www.ietf.org/rfc/rfc4941.txt.
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7. Repeat this process whenever one connects to a new network or the timers set during the previous 

iteration expire.

Figure 5-1. Example of IPv6 Privacy Addressing

As an example of privacy addressing, Figure 5-1 shows an Ethernet interface with a /24 non-routable 

IPv4 address, a /64 globally routable IPv6 address based on its IEEE address, a second /64 globally 

routable IPv6 address generated with privacy addressing, and link local IPv6 address based on its IEEE 

address.

For several reasons, this privacy extension mechanism should be used with care, if it is used at all:

 How much privacy is actually provided is questionable. Particularly on small networks that do not 

change much, anyone who can observe network traffic can also correlate activity fairly accurately 

regardless of whether or not addresses change periodically. An observer may even be able to 

determine how often each interface is generating such new addresses.

 On some networks, administrators may want to have better control of what is connected and 

correspondingly, which addresses are used. Local security policy may dictate that for audit or 

forensic purposes, all addresses must be centrally assigned and logged. In such cases, it is better not 

to allow either privacy addresses or stateless autoconfiguration but to require using DHCPv6 for 

address assignments.

 Good networking practice is to apply ingress filtering, that is, not allow packets without valid source 

addresses into the core of the network. Some distributed denial-of-service (DDoS) attacks have used 

forged source addresses with valid prefixes. Privacy addresses may be difficult to distinguish from 

addresses used in these attacks without additional measures such as rate limiting or complete reverse 

path checking.

For these reasons, privacy addresses should not be turned on by default, and careful consideration should 

be given case-by-case as to whether it is worth using. In many cases where enterprises are operating their 

own networks, DHCPv6 address assignments may be preferable.
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5.2 Cryptographically Generated Addresses

Cryptographically Generated Addresses (CGAs), also called Hash Based Addresses, provide a method to 

prove ownership of the source address in a packet. The idea is to choose a public and private key pair 

suitable for creating digital signatures with the private key and then to verify them with the public key 

(see Figure 5-2). Then, the public key (along with other parameters) is used to generate an interface 

identifier, the public key is inserted into the packet, and the packet is signed with the private key. Upon 

receipt, the public key can be used to check both the address and signature. An attacker without the 

private key could not sign a forged packet.

Figure 5-2. Generating Cryptographic Addresses from Public-Private Key Pairs

Four processes are needed to make this work:

 The sender must:

1. Generate a key pair and corresponding address.

2. Insert the public key in a packet and sign it with the private key.

The recipient must:

3. Check that the source address corresponds to the public key.

4. Verify the signature with the public key.

Note that using a CGA does not prove one’s actual identity, but it does show that the same entity (the one 

with the private signing key) generated each of the packets, and that the packets were not subsequently 

modified by another entity. The point is that no one without the private key can legitimately use the 

CGA. 

CGAs may be used to enhance security in any IPv6 network, but in many cases they have no discernible 

advantage and more overhead than IPsec or higher-layer security protocols. They may be particularly 

helpful, however, in cases involving ownership of a newly generated address. CGAs have been 

standardized as the main security building block for the IPv6 Secure Neighbor Discovery (SEND) 
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protocol RFC 3971141, and they have been proposed for use with the Site Multihoming for IPv6 (SHIM6) 

protocol. A different construction based on the same ideas is used in the experimental Host Identity 

Protocol (HIP).

In all cases, the hash algorithm currently specified for CGA is SHA-1, the signature algorithm is RSA, 

and the signature format follows RSA’s PKCS #1, version 1.5, described in RFC 3447142.

CGAs are specified in RFC 3972143 and RFC 4581144. Implementations need to generate and store 

cryptographic values securely to use these protocols safely. See in particular RFC 4086145 for a 

discussion of securely generating pseudo-random values.

The strength of the cryptographic security provided by CGAs is limited. The RSA keys are specified to 

be from 384 to 2096 bits long. The smaller end of this range is certainly vulnerable to factoring attacks, 

whereas the larger end is in line with current security recommendations. Also, the CGAs themselves have 

only 59 bits of cryptographic payload, so conceivably one could mount a search for a key that matches a 

known CGA. Mass searches for such matches are made more difficult by adding a randomly chosen 

―salt‖ value to each CGA. 

5.3 IPsec in IPv6

In the early 1990s, the IETF began to view the lack of IP-level security as a serious drawback, and it 

started developing a collection of network-layer security protocols known as IPsec to be used specifically 

to secure IP communications. The IETF has published three versions of IPsec, which now provides 

strong, up-to-date confidentiality and integrity protection, access control, replay detection, key 

management, and strong peer-entity authentication for IPv4. IPsec has proved difficult to deploy with 

IPv4, and its widespread use has been limited to protecting certain virtual private networks (VPNs) and 

for secure remote access to enterprise networks when strong security is a requirement. One of the reasons 

for this has been the constrained availability of IPv4 addresses. Other explanations have included the 

perceived complexity of IPsec, the lack of sufficient infrastructure for authentication, and inability of 

application programs to interface with an IPsec subsystem located in an operating system, networking 

card, or external device. All of these have been or are being addressed to a certain extent. Although 

various workarounds such as UDP encapsulation have been deployed in the above scenarios, full use of 

IPsec depends upon true peer-to-peer, end-to-end interaction at the network layer. However, the use of 

Network Address Translation (NAT) at many edge or customer premise routers breaks the end-to-end 

model by using non-routable addresses at end systems.

IPsec was designed long after IPv4. Today, most operating systems, routers, and security appliances 

bundle or integrate IPsec with their IPv4 protocol stacks, but historically, IPsec was implemented 

separately from IPv4. This is in contrast to IPv6, for which IPsec is an integral part of the specification. 

IPv6 does not have the addressing limitations that inhibit end-to-end use of IPsec with IPv4. Also, IPsec 

has been recommended as the way to secure important features of IPv6 such as OSPFv3 routing, 

mobility, and even neighbor discovery (see Section 5.4).

 

141 IETF RFC 3971, SEcure Neighbor Discovery (SEND), is available at http://www.ietf.org/rfc/rfc3971.txt.

142 IETF RFC 3447, Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1, is 

available at http://www.ietf.org/rfc/rfc3447.txt.

143 IETF RFC 3972, Cryptographically Generated Addresses (CGA), is available at http://www.ietf.org/rfc/rfc3972.txt.

144 IETF RFC 4581, Cryptographically Generated Addresses (CGA) Extension Field Format, is available at 

http://www.ietf.org/rfc/rfc4581.txt.

145 IETF RFC 4086, Randomness Requirements for Security, is available at http://www.ietf.org/rfc/rfc4086.txt.
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When IPv6 was developed146, one view (or hope) was that it could provide end-to-end security for all 

network communications and eliminate the need for intermediate security layers and devices such as 

firewalls. This meant that hosts would simply be able to establish secure tunnels to other hosts as a 

complete security solution. Security has turned out to be more complicated than that. Many 

organizations provide internal applications with weak security and rely on a perimeter defense to exclude 

outsiders. Others wish to exercise tight controls on information entering or leaving their internal 

networks. Additionally, an authenticated and encrypted virus or worm is still a virus or worm. Therefore, 

most organizations are unlikely to rely solely upon IPsec services implemented on individual hosts to 

provide sufficient protection for all IPv6 network communications. The security requirements and 

models that evolve as accepted practices for IPv6 networks are likely to be at least somewhat similar to 

those currently found in IPv4 networks.

On the other hand, because the transition to IPv6 will last a long time, implementations of IPsec on IPv4 

networks are likely to continue to be used indefinitely. It is expected that IPsec will be widely deployed 

in mixed IPv4 and IPv6 environments for many years to come.

Figure 5-3. IPsec in the TCP/IP Protocol Stack

TCP/IP communications are composed of four layers that work together: application (and session), 

transport, network, and data link (see Figure 5-3). Security protocols exist for network communications 

at each of the four layers. Data prepared for transport are passed from the highest to the lowest layer, 

with each layer adding more information. Because of this, a security control at a higher layer cannot 

provide full protection for lower layers, because the lower layers add information to the communications 

after the higher layer security controls have been applied. The primary disadvantages of lower layer 

security controls is that they are less flexible and granular than higher layer controls and they may not be 

 

146 RFC 1883, Internet Protocol, Version 6 (IPv6) Specification, December 1995. RFC 1883 was later replaced by RFC 2460, 

December 1998.
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able to provide end-to-end protection. Accordingly, network layer security, i.e., IPsec, has become a 

popular choice for securing communications. It provides a balanced approach between the highest layer 

and lowest layer security protocols and is capable of providing end-to-end protection for all 

communications between two points.

5.3.1 Specification Overview

IPsec is a framework of open standards for ensuring authenticated and private communications over 

public networks. It has become the most common network layer security protocol, typically used in IPv4 

to create a virtual private network (VPN)147 or to provide secure remote access to a private network. 

IPsec is a complete set of protocol security services at the IP layer. It can provide entity authentication, 

confidentiality, data origin authentication, replay detection, and data integrity. It also provides limited 

traffic flow confidentiality, which masks packet sizes and rates and can make inferential attacks based on 

traffic analysis substantially more difficult.

IPsec offers three primary models for protection, as follows:

 Gateway-to-gateway. This model protects communications between two specific networks, such as 

an organization’s main office network and a branch office network, or two business partners’ 

networks.

 Host-to-gateway. This model protects communications between one or more individual hosts and a 

specific network belonging to an organization. The host-to-gateway model is most often used to 

allow hosts on unsecured networks, such as traveling employees and telecommuters, to gain secure 

remote access to internal organizational services, such as the organization’s email, Web servers, and 

custom applications.

 Host-to-host. A host-to-host architecture protects communication between two specific computers. 

It is most often used when a small number of users need to use or administer a remote system that 

requires network layer security for some or all of its higher layer protocols. OSPFv3 is an example. 

IPv6 offers increased opportunities to use this mode, because IPsec is a mandatory component for 

every IPv6 implementation, and end-to-end connectivity without Network Address Translation 

(NAT) makes using IPsec easier.

IPsec consists of the following components:

 Security Associations (SA). Each instance of IPsec has a security association database (SAD) of 

IPsec connections, called security associations. SAs describe the endpoints of the secure connection, 

exact type of protection provided, cryptographic parameters and keys, expiration time, and label 

(Security Parameters Index or SPI) to identify packets protected with each SA.

 Two security protocols: Authentication Header (AH) and Encapsulating Security Payload 

(ESP). AH can provide integrity and replay protection for packet headers and data, but it cannot 

encrypt them. ESP can provide encryption, integrity, and replay protection for packets, but it cannot 

protect the outermost IP header, as AH can. This protection is not needed in most cases. 

Accordingly, ESP is used much more frequently than AH because of its encryption capabilities, as 

well as other operational advantages described in this document. The latest IPsec specification states 

 

147 See the National Institute of Standards and Technology Guide, Guide to IPsec VPNs, is available at 

http://csrc.nist.gov/publications/nistpubs/800-77/sp800-77.pdf.
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that implementations MAY implement AH, and many choose not to. For a VPN, which requires 

confidential communications, ESP is the natural choice.

 Internet Key Exchange (IKEv1 or IKEv2) protocol. IPsec uses IKEv1 or IKEv2 to authenticate 

endpoints to each other; define the security parameters of IPsec-protected connections; negotiate and 

establish IPsec security associations; negotiate secret keys; and manage, update, and delete IPsec’s 

security associations. 

 Access Control. A necessary component of IPsec is to specify and enforce proper protection. 

Therefore, IPsec implementations contain a security policy database (SPD) and packet filter that 

protects, passes, or drops traffic as specified in a security policy.

 IP Payload Compression Protocol (IPComp). Optionally, IPsec can compress packet payloads 

before encrypting them.

The following points summarize the main ideas behind how IPsec works:

 Both AH and ESP can be used in transport mode, which means that the AH or ESP protocol 

following the IP header encapsulates and protects the upper layer protocol that follows, or they can be 

run in tunnel mode, which means that the AH or ESP protocol encapsulates and protects an entire IP 

datagram. A given SA uses one mode or the other. Typically, transport mode runs end-to-end, and 

tunnel mode runs between intermediate security gateways, but exceptions to this exist. 

 AH provides integrity protection for all packet headers and data, with the exception of a few IP 

header fields that change unpredictably in transit. Because AH includes source and destination IP 

addresses in its calculations, AH is incompatible with NAT. The use of AH has decreased because 

more recent versions of ESP (RFC 2406 in 1998 and RFC 4303 in 2005) can now provide integrity 

protection services with or without confidentiality. ESP does not provide the integrity checks for the 

outermost IP header that AH can.

 In tunnel mode, ESP can provide encryption and integrity protection for an entire encapsulated IP 

packet including its ―inner‖ header. ESP tunnel mode is the most commonly used IPsec protocol and 

mode. Because it can encrypt the original IP header, it can conceal the true source and destination of 

the packet. Also, ESP can add padding to packets and send dummy packets (IPv4 Protocol or IPv6 

Next Header 59) to further complicate traffic analysis. Another advantage of ESP tunnel mode is that 

it can be compatible with NAT.

 In transport mode, ESP can provide encryption and integrity protection for the payload of an IP 

packet and for the ESP header and trailer. Transport mode is not compatible with NAT.

 IPsec can use IKE to create security associations. IKEv1 phase 1 creates an IKE SA; IKEv1 phase 2 

creates an IPsec SA through a channel protected by the IKE SA. IKEv1 phase 1 has two modes: main 

mode and aggressive mode. Main mode negotiates the establishment of the bidirectional IKE SA 

through three pairs of messages, whereas aggressive mode uses only three messages. Although 

aggressive mode is faster, it is also less flexible and secure. IKEv1 phase 2 has one mode: quick 

mode. Quick mode uses three messages to establish a pair of unidirectional IPsec SAs. Quick mode 

communications are encrypted by the method specified in the IKE SA created by phase 1.

 IKEv2 contains many simplifications and improvements listed in Section 5.3.4, and whenever 

possible, it should be used instead of IKEv1.
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 Although ESP does not provide integrity-protection for the outer IP addresses, the ability to use the 

SA correctly implies possession of the message integrity check key for the SA. If the parties at these 

addresses used IKE to negotiate the SA, they should be the only ones with this key, so the recipient 

can infer the source of the packet, regardless of whether the source address is intact. This argument 

does not hold if this key was manually distributed to more than two parties, but IPsec was not 

intended to be used in this way. 

 IPComp can provide lossless compression for IPsec payloads. Because applying compression 

algorithms to certain types of payloads may actually make them larger, IPComp, when used, is not 

applied to such protocols.

5.3.2 Differences from IPv4 Standards

All major aspects of IPsec under IPv6 are the same as they are under IPv4. IPsec is defined for both 

protocols in the same specifications. AH, ESP, tunnel mode, transport mode, SA, and other aspects of the 

protocol are identical to those found within the IPv4 version. Coverage of the IP header in transport 

mode, ICMP selectors, and fragmentation handling are different.

Unlike with IPv4, IPsec has been a mandatory component of IPv6 from the beginning. Neither IKEv1

nor IKEv2 is included in this requirement. Using IPsec without IKE normally implies using manual 

keying, which limits the functionality and scalability of IPsec. Therefore, users should seek 

implementations that include at least one of IKEv1 or IKEv2.

Once IPv6 is widely deployed, it is more likely to be deployed with end-to-end addressing rather than 

with NAT. Thus, end-to-end IPsec is more feasible with IPv6. Work in the IETF on connection latching 

and APIs may prove to be more useful in end-to-end IPv6 environments and is discussed in Section 5.3.4.

5.3.3 Support for Multicast 

The current version of IPsec does not fully support protection for multicast traffic, because IPsec was 

designed specifically for protecting communications between two specific points, not among many points 

at once. Each multicast packet may have many recipients (see Section 4.2), which raises many IPsecrelated issues. For example, many recipients may need to decrypt the same packet, but sharing a secret 

key among them is not a sound security practice.148 Another issue is that many different hosts may be 

sending packets to the multicast address. Because each of these hosts needs to share the same 

authentication mechanism, one source host can spoof the identity of another source host, and the 

recipients may not be able to detect it, eliminating IPsec’s data origin authentication capability. Also, the 

anti-replay protection provided by the sequence number is not available, because multiple senders could 

simultaneously generate legitimate packets that happen to use the same sequence number. These are but a 

few examples of the problems caused by attempting to have IPsec provide support for multicast traffic. In 

addition, IKE is inherently a two-party protocols running over UDP. Entirely different key management 

methods are needed for multicast.

As of the writing of this guide, researchers have been attempting to find viable ways to extend IPsec so it 

can support multicast traffic without losing its methods of protection, particularly source authentication. 

One of the biggest challenges is to find a solution that is not too resource intensive. Because multicast is 

 

148 If the hosts share the same secret key, and one host should no longer have access to the multicast traffic, then the secret key 

needs to be updated on all the remaining hosts in a timely manner. Distributing the new key in a secure manner to all these 

hosts may be extremely challenging.
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typically used for applications such as streaming video that are constantly generating packets, IPsec 

cannot add too much overhead to the processing of each packet or the applications’ functionality may be 

seriously impaired. Researchers expect that multiple multicast solutions may be created, each addressing 

a particular multicast need (e.g., single-sender multicast or multicast groups with a small number of 

members). It is outside the scope of this document to examine the proposed methods. Detailed 

information is available from research efforts that have been seeking solutions for multicast security 

issues, including the concluded Group Security Research Group (GSEC) within the Internet Research 

Task Force (IRTF)149 and the Multicast Security (MSEC) Working Group within the IETF.150

5.3.4 Status of IPsec and On-Going Work

The third version of IPsec was published in December 2005. It is defined in a series of RFCs, all of 

which are Proposed Standards. This section summarizes the capabilities in these specifications; so that 

users can identify features they need and check that vendors supply them. Not all implementations are 

complete. ICSA Labs (www.icsalabs.com) holds periodic IPsec interoperability tests broken down by 

features, and their Web site can be consulted along with vendors’ documentation.

 RFC 430191, Security Architecture for the Internet Protocol. This specifies general requirements for 

IPsec. It contains a new processing model that eliminates SA bundles and instead processes SAs 

iteratively. One important IPv6 feature is support for selectors for the different types of ICMPv6 

messages. It also adds support for combined integrity and confidentiality algorithms and a Peer 

Authorization Database (PAD) to specify acceptable authentication credentials. Finally, it 

downgrades the requirement for implementing AH from MUST to MAY.

 RFC 4302151, IP Authentication Header. This defines AH. The main new feature is extended, 64-bit 

replay detection counters.

 RFC 4303152, IP Encapsulating Security Payload (ESP). This defines ESP. It allows longer padding 

to inhibit traffic analysis, combined mode algorithms, and also the extended sequence numbers used 

in AH.

 RFC 4835153, Cryptographic Algorithm Implementation Requirements for Encapsulating Security 

Payload (ESP) and Authentication Header (AH). This specifies the cryptographic algorithms to be 

used with ESP and AH, shown in Figure 5-4. The encryption algorithms are only applicable to ESP. 

MUST— means an algorithm may be downgraded and SHOULD+ means potentially upgraded in the 

future. 

 

149 The IRTF home page is located at http://www.irtf.org/ . For more information on the GSEC Research Group, visit their 

Web page at http://www.securemulticast.org/gsec-index.htm .

150 The IETF home page is located at http://www.ietf.org/ . For more information on the MSEC Working Group, visit their 

Web page at http://www.securemulticast.org/msec-index.htm .

151 IETF RFC 4302, IP Authentication Header, is available at http://www.ietf.org/rfc/rfc4302.txt.

152 IETF RFC 4303, IP Encapsulating Security Payload (ESP), is available at http://www.ietf.org/rfc/rfc4303.txt.

153 IETF RFC 4835, Cryptographic Algorithm Implementation Requirements for Encapsulating Security Payload (ESP) and 

Authentication Header (AH), is available at http://www.ietf.org/rfc/rfc4835.txt.
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Figure 5-4. Encryption and Authentication Algorithms for the IPsec Protocol

 RFC 4306154, Internet Key Exchange (IKEv2) Protocol. This is the replacement for IKEv1. It 

removes many of the less used features and contains many simplifications and improvements. It 

does away with phases and modes and takes four messages to set up SAs initially. It has 

improved security, diagnostics, documentation, reliability, and performance. It also is integrated 

with items that were added only later to IKE(v1): DHCP(v6), NAT traversal, explicit congestion 

notification (ECN), extended sequence numbers (ESN) and extensible authentication protocol 

(EAP). Clarifications to IKEv2 were later published in RFC 4718 (Informational), and the IETF 

is now revising RFC 4306 to fold these in.

 RFC 4307155, Cryptographic Algorithms for Use in the Internet Key Exchange Version 2 (IKEv2).

This RFC specifies the cryptographic algorithms for IKEv2, simplifying document maintenance. 

Figure 5-5 shows the REQUIRED and RECOMMENDED methods. These are the algorithms that 

IKEv2 uses to protect its own traffic. In addition, IKEv2 has the capability to negotiate the 

algorithms listed in Figure 5-4 for use in ESP and AH.

 

154 IETF RFC 4306, Internet Key Exchange (IKEv2) Protocol, is available at http://www.ietf.org/rfc/rfc4306.txt.

155 IETF RFC 4307, Cryptographic Algorithms for Use in the Internet Key Exchange Version 2 (IKEv2), is available at 

http://www.ietf.org/rfc/rfc4307.txt.
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Figure 5-5. Cryptographic Algorithms for Use in IKEv2

 RFC 4308156, Cryptographic Suites for IPsec. After a long debate about whether individual 

cryptographic algorithms or entire suites should be defined with IKEv2 and IPsec, the decision was to 

use the former but provide specific guidance for the latter. This RFC provides such guidance by 

defining two suites, one oriented towards triply iterated Data Encryption Standard (3DES) and SHA1, a second oriented towards AES in CBC Mode. Additional suites (see RFC 4869157) oriented 

towards elliptic curves and AES in Galois/Counter Mode have emerged more recently. 

Work on IPsec standards has continued in several areas, and the following RFCs, categorized by subject, 

have been published:

Cryptographic methods

 2404 The Use of HMAC_SHA-1-96 within ESP and AH PS

 2410 The NULL Encryption Algorithm and its use with IPsec PS

 2451 The ESP CBC-Mode Cipher Algorithms PS

 3526 More Modular Exponential (MODP) Diffie-Hellman groups for PS

Internet Key Exchange (IKE)

 3566 The AES-XCBC-MAC-96 Algorithm and its use with IPsec PS

 3602 The AES-CBC Cipher Algorithm and its use with IPsec PS

 

156 IETF RFC 4308, Cryptographic Suites for IPsec, is available at http://www.ietf.org/rfc/rfc4308.txt.

157 IETF RFC 4869, Suite B Cryptographic Suites for IPsec, is available at http://www.ietf.org/rfc/rfc4869.txt.
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 3686 Using Advanced Encryption Standard (AES) Counter Mode with PS

IPsec Encapsulating Security Payload (ESP)

 4106 The Use of Galois/Counter Mode (GCM) in IPsec Encapsulating PS

Security Payload (ESP)

 4309 Using Advanced Encryption Standard (AES) CCM Mode with PS

IPsec Encapsulating Security Payload (ESP)

 4359 The Use of RSA/SHA-1 Signatures within Encapsulating Security PS

Payload (ESP) and Authentication Header (AH)

 4434 The AES-XCBC-PRF-128 Algorithm for the Internet Key Exchange (IKE) PS

Protocol

 4543 The Use of Galois Message Authentication Code (GMAC) in IPsec ESP and AH PS

 4753 ECP Groups for IKE and IKEv2 INFO

 4754 IKE and IKEv2 Authentication Using the Elliptic Curve Digital PS

Signature Algorithm (ECDSA)

 4868 Using HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512 with IPsec PS

 4869 Suite B Cryptographic Suites for IPsec INFO

 4894 Use of Hash Algorithms in Internet Key Exchange (IKE) and IPsec INFO

 5114 Additional Diffie-Hellman Groups for Use with IETF Standards INFO

 5282 Using Authenticated Encryption Algorithms with the Encrypted Payload of the PS

Internet Key Exchange version 2 (IKEv2) Protocol

Three main ideas dominate the work on new cryptographic methods for IPsec:

 Strengthen or find alternatives to hash-based MACs

In 2005, vulnerabilities were found to exist in SHA-1. NIST’s resulting policy158 on the use of hash 

functions recommends the use of alternative hash functions for digital signatures and other 

applications that require collision resistance; it requires the use of the SHA-2 family of hash 

functions (SHA-224, SHA-256, SHA-384 and SHA-512) instead of SHA-1 after 2010. 

Since the SHA-1 vulnerabilities do not affect HMAC-SHA-1, its continued use is permissible. IPsec 

and IKE use HMAC-SHA-1, and not SHA-1, for integrity protection.

 Use combined encryption-integrity modes for high-speed implementations

 Provide elliptic curve alternatives to public key cryptography based on modular arithmetic in IKEv1

and IKEv2.

Of these, the following are likely to be the most important:

 

158

 See http://csrc.nist.gov/groups/ST/hash/policy.html.. NIST is also conducting a competition to define SHA-3, the successor 

to SHA-2.
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 RFC 4309 specifies the first combined mode encryption and integrity algorithm based on AES in 

Counter Mode. More recently, a more efficient construction called Galois Counter Mode has become 

an alternative choice. RFC 4543 describes its use for integrity protection in ESP and AH; RFC 4106 

defines its use within ESP as a combined mode algorithm.

 RFC 4868 (HMAC-SHA-256), which is likely to be included in mainstream implementations

 RFCs 4753, 4754, and 4869, which are of interest to those who want to use IKEv2 with elliptic curve 

public key cryptography.

IKE and key management 

 4430 Kerberized Internet Negotiation of Keys (KINK) PS

 4718 IKEv2 Clarifications and Implementation Guidelines INFO

 3706 A Traffic-Based Method of Detecting Dead Internet Key Exchange (IKE) Peers INFO

 3947 Negotiation of NAT-Traversal in the IKE PS

 4304 Extended Sequence Number (ESN) Addendum to IPsec Domain of PS

Interpretation (DOI) for Internet Security Association and Key Management 

Protocol (ISAKMP)

Of these, RFC 4718 is important to anyone using IKv2 until an update to RFC 4306 is published. RFCs 

3706, 3947 and 4304 define additional features for IKEv1; for IKEv2, these features are included in RFC 

4306. 

Applications (routing, mobility, IPv6 over IPv4 tunneling)

 4552 Authentication/Confidentiality for OSPFv3 PS

 4555 IKEv2 Mobility and Multihoming Protocol (MOBIKE) PS

 4621 Design of the IKEv2 Mobility and Multihoming (MOBIKE) Protocol INFO

 4877 Mobile IPv6 Operation with IKEv2 and the Revised IPsec Architecture PS

 4891 Using IPsec to Secure IPv6-in-IPv4 Tunnels INFO

The three applications all bear directly on IPsec with IPv6. For OSPF on IPv6 networks (which is called 

v3), integrity is specified, ESP is REQUIRED, and AH is OPTIONAL. MOBIKE defines an efficient 

way to move IPsec SAs when an address changes, which happens much more often in IPv6 because of 

mobility, multihoming, or renumbering. RFC 4877 brings IPv6 mobility up to date with the RFC4301-

series version of IPsec. Finally, RFC 4891 answers the question of what protocol stack and type of 

tunneling works best when IPv6 is tunneled through an IPv4 network, and IPsec is applied at the tunnel 

endpoints. Using Protocol 41 and ESP in transport mode is the easiest configuration to achieve this goal.

Management of security

 4807 IPsec Security Policy Database Configuration MIB PS
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PKI (Public Key Infrastructure) for IPsec and IKE

 4945 The Internet IP Security PKI Profile of IKEv1/ISAKMP, IKEv2, and PKIX PS

 4809 Requirements for an IPsec Certificate Management Profile INFO

The use of Public Key Certificates, and the interpretation of their contents, has been a source of 

interoperability problems within IPsec and IKE. These RFCs attempt to pin down some of the details and 

enhance interoperability. However, they are not specific enough to eradicate these problems. 

Weaker authentication and trust models

 5387 Problem and Applicability Statement for Better Than Nothing Security (BTNS) INFO

 5386 Better-Than-Nothing-Security: An Unauthenticated Mode of IPsec PS

Recognizing that the lack of a global PKI for peer authentication is a major hurdle to the universal 

deployment of IPsec, the BTNS (Better Than Nothing Security) Working Group defined a method of 

using IPsec to protect communications, but allowing lesser methods of authentication (e.g., self-signed 

certificates or raw public keys). This provides protection to the communications themselves, but does not 

guarantee the identities of the peers. This approach could be used to protect communications for 

applications that provide their own peer authentication.

The Applicability of IPsec

 5406 Guidelines for Specifying the Use of IPsec Version 2 PS

This contains useful advice for thinking through how IPsec can be applied, but the current version is not 

up to date with the most recent IPsec RFCs.

IP Compression (IPcomp)

 3173 IP Payload Compression (IPcomp) PS

 2394 IP Payload Compression Using DEFLATE INFO

 2395 IP Payload Compression Using LZS INFO

APIs (Application Program Interfaces) for IPsec

 5660 IPsec Channels: Connection Latching PS
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A number of different API Internet Drafts have been proposed, but have not progressed to standards. One 

issue that needs to be resolved is how application program security policy and system security policy 

interact and can be aligned with each other.

IPsecme (IPsec maintenance and extensions)

As IPsec and IKE have been deployed, operational issues, feature gaps and additional requirements have 

surfaced. The IPsecme working group was formed to address several of these issues, including:

 Combining the two IKEv2 RFCs (RFCs 4306 and 4718) into a single RFC, which will include other 

clarifications and corrections, without introducing changes to the underlying protocol.

 Extending IKEv2 to allow IPv6 remote clients to negotiate all required parameters needed to conduct 

IPv6-based IPsec communications.

 Adding two new capabilities to IKEv2 that will improve performance for remote clients: allow a 

client to re-authenticate to a gateway in a streamlined manner, and permit a gateway to re-direct a 

remote client to another gateway. 

 Facilitating the ability of middleboxes (firewalls, IDS/IPS systems, etc.) to distinguish unencrypted 

ESP packets (referred to as ESP-NULL) from encrypted ones. For networks whose policy dictates 

that end-to-end encrypted packets must be dropped, this will allow high-speed processing and 

inspection of the ESP-NULL packets. Two approaches have been proposed: a heuristics-based 

approach, requiring no alteration to the communicating peers; and the addition of a new protocol 

number for ESP-NULL, a more straightforward approach, but one that does require changes to the 

base ESP protocol.

 Writing an IPsec roadmap document to help novices navigate the interconnected and ever-increasing 

maze of IPsec- and IKE-related documents.

These capabilities are currently defined in the following documents. Some have already progressed to 

RFCs; the others are also expected to progress to RFCs:

– Internet Key Exchange Protocol: IKEv2, draft-ietf-ipsecme-ikev2bis

– IP Security (IPsec) and Internet Key Exchange (IKE) Document Roadmap, draft-ietf-ipsecmeroadmap

– IPv6 Configuration in IKEv2, draft-ietf-ipsecme-ikev2-ipv6-config

– Re-direct mechanism for IKEv2, RFC 5685, PS

– IKEv2 Session Resumption, RFC 5723, PS

– Wrapped ESP for Traffic Visibility, draft-ietf-ipsecme-traffic-visibility

– Heuristics for Detecting ESP-NULL packets, draft-ietf-ipsecme-esp-null-heuristics

5.3.5 Security Ramifications

IPsec has a potentially significant impact on any network’s performance. Security policy enforcement, 
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security association lookup, cryptographic operations, and key management are all necessary components 

of any network employing IPsec. In some studies, it was shown that a security gateway implementing 

IPsec required more resources for managing and looking up security associations than for performing 

cryptographic operations. The proper implementation and configuration of these components is, 

therefore, critical to network performance.

IPsec can use IKE for entity authentication and key management based on pre-shared secret keys, but as 

the use of IPsec scales upward, this becomes unwieldy to deploy, keep secure, and update with fresh keys. 

To avoid this, public-key-based methods are recommended. A fully compliant X.509 PKI can be used, 

but this again involves expense and overhead—consider, for example, checking certificate revocation 

lists. Therefore, lighter-weight approaches based on some simplifications (i.e., security shortcuts) in the 

trust model have been proposed. 

IPsec is not the antidote for all security concerns with IPv6. It provides specific functionality such as 

cryptographic transforms to mitigate threats and vulnerabilities from the network layer up through the 

application layer. It does not replace other security functionality dealing with malware, spam, access 

controls, intrusion detection, and so forth.

IPsec may introduce a new security consideration insofar as it may thwart deep-packet inspection 

mechanisms between the two IPsec endpoints. Security tools such as intrusion detection systems and 

virus scanners cannot inspect encrypted packets. One viable approach in such cases is to distribute the 

intrusion detection services and virus scanners between network-based and host-based components. 

Denial of service attacks and covert channels exist with IPsec, but it is not clear that these are any worse 

than those using IP without IPsec. Also, IPsec can protect against certain TCP-level denial of service 

attacks like SYN floods, whereas session layer security protocols such as SSH and SSL cannot.

The biggest security problem with IPsec seems to be faulty choices of security services or parameters. 

For example, vendor documentation may recommend using IPsec encryption without authentication in a 

way that is not consistent with up-to-date security guidelines.

5.3.6 Unknown Aspects

Although cryptographic security reduces the risks of networking, it cannot totally eliminate them. For 

example, a VPN implementation may have flaws in algorithms or software, it may be set up with insecure 

configuration settings and values, or it may be subject to denial of service attacks.

One significant concern with IPsec in IPv6 has been incomplete vendor implementations of the protocol, 

which may result in end-to-end incompatibility. This situation is gradually improving.

A debate exists over whether to use AH or ESP with NULL encryption for IPsec packets requiring

integrity protection but not confidentiality. The standards themselves do not answer the question: IPv6 

standards tend to recommend or require AH, whereas IPsec standards have downgraded AH from MUST 

implement to MAY implement. OSPFv3 originally specified AH, but this has been changed to ESPNULL. Some implementations only provide ESP. AH has not been widely deployed. (The common 

IPv4 applications of IPsec—VPNs and secure remote access—usually provide confidentiality, so this 

question does not arise.) AH may also require more processing steps to compute or verify the integrity 

check value. On the other hand, one of the main arguments in favor of AH is that it makes it easier for 

devices such as packet filtering routers, firewalls, and intrusion detection systems to recognize plaintext 

and examine packets (because ESP-NULL transmits plaintext but gives no visible indication that the 

payload is unencrypted). One proposed remedy for this, still under discussion, is to allocate a new and 
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distinct protocol number (i.e., IPv6 Next Header number) to ESP-NULL. As far as the cryptographic 

protection provided by AH versus ESP, it is difficult to discern any tangible difference when IPsec is used 

as intended. In some cases, AH may protect certain vulnerable parts of the header or extension headers, 

but these cases are rare, and in these few cases, ESP-NULL can achieve the same effect by being run in 

tunnel mode.

As IPv6 applications attempt to use IPsec for end-to-end security, more work will be needed on security 

APIs. As of the writing of this guide, a promising approach is being discussed in the IETF’s BTNS 

Working Group.

IPsec also relies on some security infrastructure outside of the IPsec protocols for authentication. This 

can be based on shared secret keys, locally generated certificates, or an external public key infrastructure.

Finally, many have questioned the need for work on failover and redundancy, and some multicast issues, 

as noted above, remain open.

5.4 Secure Stateless Autoconfiguration and Neighbor Discovery

Security for autoconfiguration and neighbor discovery protocols has always been a difficult topic, because 

it is naturally problematical to have pre-existing trust relationships with entities one has not yet 

―discovered‖ or to use security before address configuration.

Attacks against address configuration and address resolution protocols include spoofing or hijacking 

addresses, denial of service, propagating inaccurate information, forging unreachability conditions, 

redirecting traffic, disabling routers, advertising bogus prefixes, misrepresenting network parameters, and 

others. Most are directed at compromising the availability of the local network, but some may also target 

confidentiality or integrity of information.

IPv4 networks have no means of stateless autoconfiguration and perform neighbor discovery with the 

Address Resolution Protocol (ARP). ARP has no cryptographic security mechanisms, and it has always 

been subject to spoofing, cache poisoning, and denial-of-service attacks. Most installations cope with this 

by restricting ARP to local links (it is inherently a non-routable Layer 2 protocol) and policing the nodes 

on such local links. This has become increasingly difficult with wireless LANs. IPv6 neighbor ND is 

implemented with ICMPv6, and it is both possible and desirable to make sure such messages are not 

routed, but it is also possible to apply cryptographic security. 

The security requirements for autoconfiguration and ND vary according to the type of network used. For 

a more detailed discussion and examples of this, see RFC 3756159. For example:

 On a closed network within an enterprise, all users may trust each other to behave properly, and any 

malfunctioning nodes can be removed and corrected administratively. Cryptographically securing 

autoconfiguration and ND in this type of network may not be essential, but it could limit the damage 

that a corrupted or malfunctioning node can do.

 In a college dormitory or on a network offering public access, for example, users depend on a 

trustworthy router to provide service, but they have little knowledge or control over what other users 

are doing. In this case, cryptographic authentication of autoconfiguration and ND represents a 

substantial improvement in robustness and security.

 

159 IETF RFC 3756, IPv6 Neighbor Discovery (ND) Trust Models and Threats, is available at 

http://www.ietf.org/rfc/rfc3756.txt.
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 In ad hoc networks, none of the nodes may start out trusting each other. In this case, it is likely that 

no basis for authentication exists, and there is no pre-existing key management infrastructure. 

Nevertheless, limited cryptographic protection can be provided.

IPv6 Autoconfiguration and ND use ICMPv6 messages to perform several different tasks. The main 

ICMPv6 message types are:

 NS (Neighbor Solicitation)

 NA (Neighbor Advertisement)

 RS (Router Solicitation)

 RA (Router Advertisement)

 Redirect

 Router renumbering

These processes may, in addition, use Multicast Listener Discovery (MLD) or Multicast Router 

Discovery (MRD) ICMPv6 messages. See Section 4.2 for a description of Multicast.

This section defines and discusses two cryptographic security mechanisms for autoconfiguration and 

ND—IPsec and SEND.

5.4.1 Using IPsec to Secure Autoconfiguration and ND

IPsec can, in principle, be used to secure any IP packet, but deciding how IPsec should cover all of the 

different types and uses of ICMPv6 messages and actually specifying this behavior are not simple tasks. 

Certain ICMPv6 messages are used with mobility or multicast; others, such as the following, report error 

conditions or simply provide information:

 Destination unreachable

 Packet too big (also used to determine PMTU)

 Time exceeded

 Parameter problem

 Echo request

 Echo reply.

These are not used in autoconfiguration or ND, and securing these messages is covered in Section 3.5.2 

and 3.5.3. This section covers:

 NS and NA messages used in stateless address autoconfiguration and Duplicate Address Detection 

(DAD)

 RS and RA messages used for router and prefix discovery

 NS and NA messages used for address resolution and reachability detection
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 Redirect and router renumbering messages sent by routers.

These messages use a variety of types of addressing. Most of them use a unicast source address, but 

autoconfiguration RS and NS messages also use the unspecified all-zero address (::). Both unicast and 

multicast destination addresses are used.

RFCs 2461 and 2462 (which have now been replaced by RFC 4861160 and RFC 4862161, respectively) 

suggest that IPsec AH SAs be used to secure these messages. No specific attention was given to what 

SAs are needed, how they are established, or how they work (or do not work) with IPsec’s key 

management protocols (IKEv1 and later IKEv2). Subsequent attempts to use this mechanism ran into 

problems:

 It is impossible to use IKE without getting into an endless spiral. IKE uses UDP, which requires ND, 

which in turn uses the very same ICMPv6 messages IPsec was supposed to protect. Also, IKE does 

not work with multicast addresses.

 A large number of security associations needs to be pre-established.

 AH was downgraded to MAY in RFC 430191 and is not included in many implementations.

Nevertheless, it is possible to use IPsec to protect autoconfiguration and ND if security associations can 

be set up beforehand. One approach that has been suggested is to use pre-established security 

associations from a known set of temporary IPv6 addresses to secure setting up longer-lived addresses 

and then new IPsec security associations for these addresses. This process needs to cover certain 

multicast addresses (All Nodes, Solicited Node for each unicast address, All Routers, etc.) as well as 

unicast. Although this has been described at a high level, little practical experience working out all of the 

details exists. The following limitations appear to be unavoidable:

 Manual keying must be used, and IKE cannot be used.

 All interfaces and their hardware (MAC) addresses must be known ahead of time.

 Replay detection must be turned off.

 A fixed set of IPsec parameters must be used without any negotiation.

5.4.2 Using SEND to Secure Autoconfiguration and ND

SEND was designed by operators of networks who needed to secure IPv6 autoconfiguration and ND but 

found IPsec to be an impractical choice. Many of these were mobile service operators incorporating IPv6 

into third generation mobile handsets.

The core ideas behind SEND141 are:

 Use CGAs. 

 Add an RSA Signature option to ICMPv6.

 Define trust anchors capable of attesting for public keys of routers.

 

160 IETF RFC 4861, Neighbor Discovery for IP Version 6 (IPv6), is available at http://www.ietf.org/rfc/rfc4861.txt.

161 IETF RFC 4862, IPv6 Stateless Address Autoconfiguration), is available at http://www.ietf.org/rfc/rfc4862.txt.
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 Add Nonce and Timestamp options to ICMPv6 for replay detection.

Nodes are configured with public keys of trust anchors, which let them verify signatures on RAs. By 

using CGAs and signatures, addresses cannot be spoofed or hijacked. Also, certain denial-of-service 

attacks are prevented. Replay attacks can be detected by examining timestamps on multicast messages 

and verifying that nonces are returned in two-way exchanges.

In ad hoc networks, using CGAs and signatures ensures that messages from the same address really came 

from the same entity, and no one else has stolen the address.

5.4.3 Unknown Aspects

Cryptographic protection for autoconfiguration and ND has not been widely used, so it remains to be seen 

whether they will become parts of generally accepted practices for IPv6 security.

SEND also has not been widely implemented or deployed. Issues reportedly holding back some vendors 

are intellectual property claims and licensing terms concerning CGAs.
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6. IPv6 Deployment

This section suggests approaches to a successful and secure IPv6 deployment. Although this document 

uses the term deployment, rather than transition, many of the methods and mechanisms described in this 

section are referred to as transition mechanisms; to avoid confusion, this document also uses the word 

transition in that context. The overarching goal during the process of IPv6 deployment should be to 

maintain functional parity with existing networks and services. This section addresses that goal. IPv6 

deployment focuses on integrating IPv6 with an existing IPv4 environment while maintaining, if not 

enhancing, the existing level of security. It includes the following subjects:

 Security risks

 Addressing security

 Transition mechanisms

 Transition planning

6.1 Security Risks

This section provides an overview of risks organizations may face when moving from an IPv4 to 

IPv4/IPv6 and eventually IPv6 environment.

The deployment of IPv6 is inevitable. The IPv4 address space is almost exhausted and the only long-term 

solution is to deploy IPv6. IPv6 is not backwards compatible with IPv4, which means organizations will 

have to change their network infrastructure and systems to deploy IPv6. Organization should begin now 

to understand the risks and the risk mitigation strategies. Planning will enable an organization to make a 

smooth, secure and successful transition. 

Some general risks organization may face include:

 The attacker community’s use of IPv6

 Unauthorized deployment of IPv6 on existing IPv4 production networks

 Vulnerabilities present in IPv6

 Complexity added by dual IPv4/IPv6 operations

 Immaturity of IPv6 security products

 Possible lack of vendor support.

6.1.1 Attacker Community

The attacker community is adapting to the IPv6 environment, posing a serious risk to all organizations 

connected to the Internet. The attacker community leverages IPv6 to infiltrate and attack both IPv4 and 

IPv6 networks. The publication of a United States Computer Emergency Readiness Team (US-CERT) 

whitepaper162 and the public release of IPv6-enabled attack tools

163 in May 2005 demonstrate how the 

attacker community is attacking IPv6 implementations and using IPv6 to compromise systems.

 

162 US CERT, IPv6 Malware Tunneling, is available at http://www.us-cert.gov/reading_room/IPv6Malware-Tunneling.pdf.

163

 One such tool, THC-IPv6, can be found at http://freeworld.thc.org/thc-ipv6.
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The US-CERT whitepaper warned administrators that malware could use IPv6 for covert communications 

and tunnel traffic out of IPv4 networks. The US-CERT cited two causes in the whitepaper:

 Most current operating systems now support IPv6 by default. 

 Firewall and IDS equipment not configured to recognize IPv6 traffic could be bypassed. 

The warning that attackers could use IPv6 to tunnel traffic did not surprise the security community. As 

early as 2002164, security researchers began documenting instances of attackers enabling IPv6 on 

compromised systems to evade IPv4 security controls. 

Also in 2005, a security researcher released an IPv6 attack toolkit at a security conference. This toolkit 

includes tools to attack ICMPv6, detect IPv6 hosts, and hijack autoconfiguration. 

These two events demonstrated that the attacker community is taking advantage of IPv6 and is developing 

techniques and tools for exploitation. If an organization’s security controls are not already monitoring for 

IPv6 traffic, then IPv6 traffic to and from client devices may not be detectable by existing security 

controls. The mitigation for this risk is to reconfigure or deploy security controls to be both IPv4 and 

IPv6 aware. 

6.1.2 Unauthorized IPv6 Clients

The Internet is actively deploying IPv6. IPv6 support is available for most operating systems. The 

commands to enable IPv6 on these operating systems are easily accessible and user-friendly. Using IPv6 

autoconfiguration, a host can configure its own global address if it is able to find a prefix. 

Recent operating systems not only support IPv6 but also often enable IPv6 by default. This exposes 

organizations to vulnerabilities it may not be able to detect or mitigate. The following options are 

available to reduce this risk:

 Locate and disable any IPv6 enabled equipment. 

 Block IPv6 and IPv6 tunnel traffic at the perimeter. 

 Include IPv6 use policies in the organization’s security plan. 

6.1.3 Vulnerabilities in IPv6 

The IETF regards security as an important design constraint when developing standards for IPv6. Work 

to address a lack of confidentiality and integrity in IPv4 has resulted in the development of IPsec. IPv6 

with IPsec resolves these issues, but IPsec has not addressed other weaknesses found in the TCP/IP 

protocol stack. 

IPv6 does not solve many traditional layer 2 attacks like sniffing traffic, traffic flooding, man-in-themiddle attacks, rogue devices or Address Resolution Protocol (ARP) table overflow attacks. Some of 

these attacks are dealt with partially in IPv6 while other attacks, similar in nature, exploit different 

features.

Although most modern operating systems have supported IPv6 since at least 2003, the protocol stacks of 

these operating systems have not been fully proven. A review of the National Vulnerabilities Database

 

164 Spitzner, Lance, Honeypots: IPV6, is available at http://seclists.org/honeypots/2002/q4/0105.html.
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(NVD)165 shows stack vulnerabilities in most major operating systems. As new vulnerabilities are 

exposed, vendors will release updates. With any new code, vendors will need time to stabilize or harden 

their code. When deploying IPv6, an organization should understand that the code used in the IPv6 

protocol stacks could be relatively new.

IPv4 Layer 2 and layer 3 attacks are possible because IPv4 assumes all network nodes will behave in a 

trustworthy manner. IPv4 uses ARP to associate physical addressing to logical addressing. 

This assumption allowed attacks that interfered with IP address resolution and the association of physical 

addresses with logical addresses. As discussed in Section 3, IPv6 does not use ARP to map IP addresses 

to physical interfaces. Instead, IPv6 uses ICMPv6. IPv6 uses ICMP for neighbor discovery and stateless 

address autoconfiguration processes that associate physical and logical addresses. IPv6 is still vulnerable 

to layer 3 attacks. A type of layer 3 attack is the host initialization attack. Three common host 

initialization attacks are: 

 Neighbor Solicitation (NS). For neighbor discovery, a client node sends an NS message. The 

problem is that any node can claim to be any other node on the LAN. An attacker can falsify a NS 

message with their MAC address and the fake IP address. This technique is similar to ARP spoofing 

in IPv4 and only works on link-local.

 Router Solicitation (RS). With stateless autoconfiguration, the client node sends an RS message. 

Like the NS attack, any node can claim to be the default router and subvert the initialization process. 

 Duplicate Address Detection (DAD) Process. When a node attempts to configure its link-local 

address, it runs DAD. An attacker can cause a denial of service attack by responding to a DAD 

request. As a result, the victim’s interface will fail to initialize.

The subnet size in IPv6 can present its own security challenges. These challenges are discussed in RFC 

5157, IPv6 Implications for Network Scanning166. The subnet size is much larger than it was in IPv4; a 

default subnet can have 264 addresses. The issues with scanning an address space this large was discussed 

in section 2.4. Subnet size and resulting lengthy scan times are often presented as security features since 

they will slow down a malicious scanner or the propagation of malicious logic. The subnet size, however, 

makes it difficult for network and system administrators to manage assets. Attackers and administrators 

need to rely on other discovery techniques like layer 2 discovery and DNS to find hosts. The large 

address space makes it more difficult to find rogue hosts, possibly causing administrators to rely on 

passive discovery techniques. Administrators may need to resort to predictable or sequential numbering 

schema for device addressing.

To mitigate most layer 3 vulnerabilities, administrators should consider using fixed addressing or 

DHCPv6 with DHCP rogue detection instead of stateless autoconfiguration. Autoconfiguration and ND 

can be secured with SEND. More information on SEND is available in Section 5.4.2. Layer 2 discovery, 

802.1x based network access control, and good configuration management practices will help to mitigate 

layer 3 vulnerabilities. 

The deployment of IPv6 reinforces the basic security lessons learned with IPv4. These security practices 

include defense in depth, diversity, patching, configuration management, access control, and system and 

network administrator best practices. Good security practices remain unchanged with the deployment of 

IPv6. Good security practices will reduce exposure and recovery time in case of a security event.

 

165

 The NVD can be found at: http://nvd.nist.gov.

166 RFC-5157, IPv6 Implications for Network Scanning, is available online at http://www.ietf.org/rfc/rfc5157.txt.
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6.1.4 Dual Operations

An organization that deploys IPv6 may support IPv4 for legacy applications, services, and clients. This 

will result in a dual protocol environment and increased complexity. The risk is that an organization’s 

security infrastructure may not be aware of both protocols. Dual protocols will increase the complexity of 

the environment. Two protocols mean twice as many things can go wrong, and twice the number of 

configurations are required to install new equipment or change existing equipment. Attacks against upper 

layer protocols could use either the IPv4 or IPv6 stack to reach the client. Administrators will need to 

maintain the same level of coverage for both protocols to mitigate risks. 

6.1.5 Perceived Risk

A misconception about IPv6 is that IPv6 by itself introduces many more security risks than the IPv4 

protocol. Perceived risk can cause an organization to delay deployment despite the fact that IPv6 enabled 

equipment is already on hand. As discussed in Section 2 of this document, IPv6 is no more or less secure 

than IPv4. General security concepts are the same for both IP protocols. However, it will take time to 

acquire the level of operational experience and practical deployment solutions that have been developed 

for IPv4 over the years.

6.1.6 Vendor Support

Many security vendors are waiting for customer demand before implementing support for IPv6, while 

customers are waiting for vendors to support IPv6 before purchasing software and systems. This has 

resulted in a ―chicken and egg‖ problem. While some vendors fully support IPv6, many do not support 

IPv6 at all or only offer limited support. 

Organizations need to analyze their existing IPv4 security infrastructure and develop baseline 

requirements that an IPv6 infrastructure must meet to achieve the same or better security as their IPv4 

environments. New base requirements may be added to address IPv6-only requirements or deficiencies in 

the existing environment. Some existing IPv4 equipment may support both IPv4 and IPv6 requirements, 

but organizations should plan to evaluate products with IPv6 support to supplement and/or replace 

existing equipment or bridge security gaps. Use the developed baseline security requirements to evaluate 

IPv6 security products. Product evaluation, selection and procurement can slow down the deployment 

schedule for IPv6. Organizations planning to deploy IPv6 should begin a dialog with their security 

vendors to support IPv6 as early as possible. 

There are no industry standards that define IPv6 support. Organizations must understand they will be 

gaining and losing capabilities by deploying IPv6 and this will change their security posture. When 

vendors claim IPv6 support, it is up to the organization to evaluate whether they can function with the 

vendors’ level of IPv6 support. Often IPv6 support is not as complete as IPv4 support. An example of 

this is that many IPv6 security devices require IPv4 addressing for management and configuration. 

NIST has issued a special publication to define the capabilities IPv6 devices should support. NIST SP 

500-267, A Profile for IPv6 in the U. S. Government – Version 1.0, 167 will help organizations identify and 

select products that support IPv6. This document has a section dedicated to Network Protection Devices 

that discusses the capabilities firewalls, application firewalls, and intrusion detection systems should 

support. When evaluating these types of security devices, organizations should ensure the equipment 

 

167 See NIST SP 500-267 , A Profile for IPv6 in the U.S. Government – Version 1.0, which is available at 

http://www.antd.nist.gov/usgv6/usgv6-v1.pdf.
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complies with the profile. To facilitate that goal, NIST has launched a USGv6 test program168 to verify 

the IPv6 conformance and interoperability of three classes of IPv6 devices: hosts, routers and Network 

Protection Devices (NPDs). Testing is conducted by independent laboratories; the laboratories are 

accredited to perform the testing by ISO-certified accreditors. Vendors declare products that have 

qualified as USGv6-compliant through the use of a Supplier's Declaration of Conformity (SDOC), which 

enables potential users of the product to identity the capabilities that were tested and the laboratory that 

conducted the testing.

NIST SP 500-267 does not address several classes of security devices including security event/log 

management, vulnerability/patch management, flow (NetFlow, SFlow), forensic tools, and authentication 

systems. Many vendors are able to process IPv6 traffic but lack capabilities to present the information

effectively, require IPv4 enabled equipment for management, or are not as feature-rich. Organizations 

will have to develop capability profiles for their security tools. 

Organizations should accept the possibility that they may not achieve full parity in their IPv4 and IPv6 

environments and take steps to mitigate associated risks.

6.2 Addressing Security

Administrators often see IP addressing as an operational issue. The numbering plan can also affect the 

organization’s security posture. The addressing structure defines the fundamental organization and 

function of a network. Consider the following components in the IPv6 addressing plan to reduce threats 

to security and privacy:

 Numbering plan

 Hierarchical addressing to support security segmentation

 Security implications of EUI-64 addresses

 Address management

 Privacy extension. 

These components are covered individually in the following sections.

6.2.1 Numbering Plan

The numbering plan describes how the organization subnets its IPv6 allocation. Most organizations 

would likely receive a /48 address block. This will allow the organization to support 65,000 subnets. 

Most of the RFCs concerned with numbering plans focus on address allocations above the site level, but 

the numbering plan designer should be familiar with the IETF’s intent and philosophy of IPv6 addressing. 

Some RFCs of interest include:

 RFC 3056 – Connection of IPv6 Domains via IPv4 Clouds 

 RFC 3879 – Deprecating Site Local Addresses

 

168

 More information about the USGv6 test program can be found at http://www.antd.nist.gov/usgv6/testing.html, including 

information for users: USGv6 Testing Program User's Guide, http://www.antd.nist.gov/usgv6/docs/NIST-SP-500-281-

v1.0.pdf and for testing laboratories: USGv6 Test Methods: General Description and Validation, 

http://www.antd.nist.gov/usgv6/docs/NIST-SP-500-273.v2.0.pdf
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 RFC 4007 – IPv6 Scoped Address Architecture

 RFC 4193 – Unique Local IPv6 Unicast Addresses

 RFC 4291 – IP Version 6 Addressing Architecture. 

RFC 5375, IPv6 Unicast Address Assignment Considerations,

169 specifies best practices to follow when 

creating a numbering plan. Site allocations are normally provided as a /48, /56 or /64.

 IPv6 Unicast Address Assignment Considerations addresses exceptions to this rule. It provides the 

following factors as considerations when designing a number plan:

 Prefix aggregation

 Network growth

 ULA (Unique Local Address) usage in large networks

 Sparse subnet numbering.

These design considerations are mostly operational concerns. A good numbering plan will streamline 

security operations by simplifying access control lists and firewall rule sets, identifying ownership of 

sites, links, and interfaces, and allowing for the rapid location of interfaces. IPv6 Unicast Address 

Assignment Considerations states that the lack of a numbering plan could slow down the deployment of 

IPv6.

6.2.2 Hierarchical Addressing to Support Security Segmentation

Network provisioning is simpler with IPv6 than in IPv4. With IPv4, the administrator is concerned with 

creating subnets that can support enough available IP addresses for current and future requirements. The 

more subnets created, the greater the number of lost addresses due to broadcast and subnet addresses. 

With IPv6, the addresses are so large that a subnet can support current and future addressing 

requirements. Because address space availability is not an issue with IPv6, the administrator can 

concentrate on creating a hierarchical address-numbering plan and not on address availability. A 

numbering plan should support network segmentation. Most networks are composed of different 

communities with differing requirements for access and protection. By segmenting a network, 

administrators can meet these various groups’ requirements with access control rules to protect sensitive 

sites, links, and hosts. 

IPv6 distinguishes itself from IPv4 by identifying not just a host by an address but also by identifying the 

regional provider based on the global hierarchy enforced by the RIRs. The network prefix of an address 

identifies an RIR and an ISP providing the address. IPv6 allows 16 subnet bits (/48) for the Site-Level 

Aggregation Identifier (SLA). Organizations can use these 16 bits to create a site hierarchy. Some 

suggested methods of subnetting include:

 Sequentially numbering subnets

 VLAN number

 AS number

 IPv4 subnet number

 

169 RFC 5375, IPv6 Unicast Address Assignment Considerations, is available at http://www.ietf.org/rfc/rfc5375.txt.txt.
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 Physical location (building, city, county, etc.)

 Functional unit (HR, Operations, Finance, etc.).

The goal of address hierarchy design is to give an organization the ability to group assets logically and to 

simplify administration and security.

6.2.3 Problems with EUI-64 Addresses

One of the more common ways to number interfaces is to use EUI-64 addresses. Using EUI-64 

addressing can be a security risk. Autoconfiguration is one method that uses EUI-64 to generate an 

interface identifier (IID). Any machine that is trying to autoconfigure its own IP address will combine the 

network identifier with the EUI-64 address to create a unique address. 

The physical address of the interface (MAC address) is an input to the algorithm that generates EUI-64 

addresses. RFC 4291 documents the methods for extracting the MAC address from the IPv6 address. 

Using a EUI-64 address could potentially reveal the make and model of a remote machine. An attacker 

could use that information to target attacks.

The use of the MAC address in EUI-64 addresses theoretically makes it easier for an attacker to scan a 

network. The MAC address is 48 bits in length, which yields a large number of host addresses (248). 

Address spaces of this size are difficult to scan. The first 24 bits of a MAC address are the 

Organizationally Unique Identifier (OUI), which identifies the hardware manufacturer. Often 

manufacturers allocate a limited range for the second 24 bits to specific models of equipment. Evaluating 

specific OUI codes and bit ranges greatly reduces the number of EUI-64 addresses.

There are several strategies to mitigate the risk. The preferred mitigation strategy is to block scanning 

activity at the network perimeter. Other methods include generating addresses using a cryptographic (i.e., 

non-predictable) algorithm or assigning addresses with DHCPv6.

6.2.4 Address Management

Address management for IPv6 has security implications. Various mechanisms are available for address 

management: 

 Managed addresses 

 Autoconfiguration 

 Manual.

DHCP is widely used in IPv4 networks to allocate managed addresses. IPv4 administrators understand its 

configuration and use. IPv6 provides a similar mechanism called DHCPv6. DHCPv6 is not the same 

protocol as DHCP, but it does provide similar functionality. DHCPv6 RFC 3315124 is not a widely 

implemented protocol for address management. Many operating systems do not provide client 

configuration or DHCPv6 services. Organizations may need to rely on alternate DHCPv6 servers like 

routers or open source software to support address management using DHCPv6. DHCPv6 is a different 

protocol than DHCPv4. DHCPv6 is still susceptible to the same security vulnerabilities affecting 

DHCPv4. Using DHCPv6 has the following security advantages:
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 DHCPv6 can limit smaller ranges of valid IPv6 addresses. A smaller range allows administrators to 

implement better access control rules.

 DHCPv6 allows administrators to identify managed clients more easily than autoconfiged or 

manually configured clients, and to follow their network usage through logging.

With autoconfiguration, the host machine automatically generates an IPv6 address on its own. 

Susceptibility to host initialization attacks poses one security risk. With autoconfiguration, generated 

addresses are in a wider range of valid addresses. Administrators have to create broader rules for access 

control lists if they are using range rules. This allows more addresses for rogue devices to connect to the 

network, and scanning the network is harder. Benefits of using autoconfiguration include:

 Addresses are deterministic. The same machine will always generate the same IP address. 

Administrators can pre-populate DNS and other logging systems with valid hosts.

 If administrators use host enumeration rules, tighter access controls are possible than with range rules.

With manual address management, administrators configure each node manually. This method is more 

resource intensive than the other two methods described in this section. Organizations often use manual 

address management with well-known services to make them easier to discover or manage. 

Address management in IPv6 will probably look similar to methods of address management used for 

IPv4. After deploying IPv6, organizations should use a combination of methods for address management. 

Administrators should use manually configured addresses for servers. Client machines will use either 

DHCPv6 or autoconfiguration. The decision of which method to use (DHCPv6 or autoconfiguration) will 

depend on how administrators want to manage addressing. If administrators want to manage their 

addresses as pools of resources, they would opt for DHCPv6 address management. On the other hand, if 

administrators want to allow equipment to assign itself an IP address without additional network 

resources or administration, they would opt for autoconfiguration. The security implications for either 

method are about the same. 

6.2.5 Privacy Extensions 

People accessing the Internet are concerned about their privacy and having their IP addresses tracked 

when they are online. In response, the IETF released an RFC specifying the mechanism to create 

temporary addresses, thereby affording the end user a degree of anonymity when online. RFC 4941, 

Privacy Extensions for Stateless Address Autoconfiguration in IPv6, specifies the mechanism for 

autoconfiguration to generate a temporary address that inhibits device or user tracking of the end users. 

Organizations that want to implement privacy extensions should refer to RFC 4941 for more 

information.140 The temporary interface ID is a randomly generated 64-bit value. Local policies control 

the regeneration schedule of temporary interface IDs.

Organizations should use privacy extensions for external communications but not for internal 

communications. Privacy extensions thwart correlation of a host with the use of a service. With internal 

IT systems, privacy extensions affect logging and prevent administrators from properly tracking which 

systems are accessing which services. Many internal resources require the ability to track the end user’s 

use of services for correct operations. Organizations should document where and how it intends to use 

privacy extensions in the numbering plan. This will allow an organization to evaluate how it will use 

privacy extensions and assess the security impact. Privacy extensions can be configured to reallocate 

addressing on a scheduled interval; this rotation policy should also be documented in the numbering plan.
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6.3 Transition Mechanisms

IPv6 is not backwards compatible with IPv4, and IPv4 systems cannot use IPv6 services or communicate 

with IPv6 hosts. The transition from IPv4 to IPv6 is expected to take a significant amount of time. As 

long as systems require interoperability between IPv4 and IPv6, transition mechanisms are needed. In the 

transition environment, three different types of hosts exist: IPv4 only, IPv6 only, and dual stack 

IPv4/IPv6. 

Transition mechanisms support interoperability between IPv4 and IPv6 hosts. Multiple transition 

mechanisms may be deployed with any IPv6 transition. Client capabilities, the particular transition 

strategy chosen, time frame, and the transition stage all impact which transition mechanisms make the 

most sense. Transition mechanisms fall into three categories:

 Dual stack

 Tunneling

 Translation.

A typical transition will transform an organization’s network from an all IPv4 environment to one where 

there are isolated IPv6 hosts or small islands of IPv6 hosts. The term island often appears in RFCs and 

refers to a small collection of like protocol hosts (either IPv4 or IPv6). 

This example demonstrates how an organization could use the different transition mechanisms during this 

process:

 Early in a transition, IPv4 tunnels will connect IPv6 islands. Translation and dual stack mechanisms 

allow IPv6 hosts to use IPv4 resources. 

 As the transition progresses from IPv4 dominant to IPv6 dominant, the organization will configure 

the network core to either IPv6 or a dual stack. This will allow the organization to dispense with 

some of the tunnel mechanisms installed in the earlier stage of IPv6 transition. Translation 

mechanisms will be required to allow IPv4 hosts to use new IPv6 services. 

 In the last phase of the migration, most equipment and services will support IPv6. Now isolated 

islands of IPv4 legacy services remain. IPv4 traffic will tunnel over IPv6 and translation services will 

allow IPv6 clients to access legacy services. 

6.4 Dual Stack IPv4/IPv6 Environments

In the dual stack method of IPv4 to IPv6 transition, each host is both IPv4 and IPv6 aware. Dual stack 

hosts run both IPv4 and IPv6 protocols and allocate addresses for both protocols. RFC 4213, Basic IPv6 

Transition Mechanisms, further explains the dual stack method.170 In most cases, IPv4/IPv6 dual stack 

deployments rely on tunneling and translation mechanisms for interoperability for parts of the network 

that are not dual stack. 

The purpose of a dual stack method is to minimize the number of tunnels used in a transition. 

Organizations use dual stack when the majority of an organization’s equipment is dual stack capable and 

they want a rapid deployment. In the following sections, we cover deployment, addressing, and security 

issues for a dual stack environment. 

 

170 IETF RFC 4213, Basic Transition Mechanisms for IPv6 Hosts and Routers, is available at 

http://www.ietf.org/rfc/rfc4213.txt.
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6.4.1 Deployment of a Dual Stack Environment

When considering a deployment of a dual stack environment, one must consider the following issues:

 Shared infrastructure

 Need for more resources

 Application protocol preference.

Logically, the IPv4 and IPv6 infrastructures are different. Dual stack devices require routing and 

switching infrastructures that are protocol aware. RFC 4554, Use of VLANs for IPv4-IPv6 Coexistence in 

Enterprise Networks, describes how 802.1q tags can be used to divide a network logically into IPv4 

routing and IPv6 routing domains.171

Dual stack environments use more resources than a single protocol environment. The following are 

examples of how a dual stack environment uses more resources:

 Each protocol stack must share the available network bandwidth

 Routers need to:

- Maintain forwarding tables for both IPv4 and IPv6

- Run routing protocols for both protocols

- Implement packet filtering for both protocols

- Provide for congestion control for both protocols

- Handle special cases (IPv4 Router Alerts and IPv6 Hop-by-Hop Options) for both

- Forward packets for both protocols. 

 Hosts must devote resources to both protocol stacks (for example, processing, memory, and network 

infrastructure traffic). 

 Administrative and security staff must maintain concurrent environments as well.

Within a dual stack environment, some applications are IPv4 only, some are IPv6 only, and some 

applications may be IPv4/IPv6. The host must use the correct protocol to access each. The administrator 

using DNS record order or translation mechanisms can influence protocol selection. 

Applications are written to query only A, only AAAA, or both A and AAAA records for name resolution. 

The administrator can influence the service called by ordering the records returned by DNS, giving 

precedence to the preferred service. Except for IP addressing, DNS is the same protocol for both IPv4 

and IPv6. For example, to allow an IPv4 host to locate a service, create a DNS A record, and to allow an 

IPv6 host to locate a service create a DNS AAAA record. Order the DNS records so that dual stack hosts 

are resolved to the preferred service. When configuring DNS in a transition environment, administrators 

should set the preference for IPv6. 

 

171 IETF RFC 4554, Use of VLANs for IPv4-IPv6 Coexistence in Enterprise Networks, is available at 

http://www.ietf.org/rfc/rfc4554.txt.
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6.4.2 Addressing in a Dual Stack Environment

Each protocol stack is responsible for configuring its own addresses. The administrator can configure 

static addresses or configure the host to receive dynamic addressing. If DHCP is used, then each protocol 

stack must access a DHCP server for address allocation. DHCP has different protocols for IPv4 and IPv6. 

Each protocol must access its own DHCP server. 

6.4.3 Security Implications of a Dual Stack Environment

A dual stack strategy is useful in making a transition between protocols, but the approach exposes every 

dual stack node to the vulnerabilities of both protocols, plus any new vulnerabilities resulting from 

unintended interactions between them. 

RFC 4852, IPv6 Enterprise Network Analysis - IP Layer 3 Focus, contains sound general advice on 

securing dual stack systems172. RFC 4942, IPv6 Transition/Coexistence Security Considerations,173

includes many specific details:

 Organizations need to implement a consistent security policy for both IPv4 and IPv6 (including 

firewalls and packet filters).

 Organizations should account for new IPv6 functionality. This functionality may include mobility, 

stateless autoconfiguration, neighbor discovery, privacy addresses, and end-to-end encryption with 

IPsec.

 Because both protocols are running, unexpected tunneling between the hosts may occur. The result 

may violate security policies. 

 Organizations must upgrade intrusion detection or intrusion prevention systems, monitoring, logging, 

and auditing to provide IPv6 protection equivalent to what was available for IPv4. 

 The performance of security systems may degrade when handling IPv6 (when using the same 

resources compared to IPv4).

Good security practice dictates disabling unneeded services. Network administrators deploying IPv6 dual 

stack should configure nodes (hosts, servers, routers, etc.) to treat the IPv6 protocol as preferred and 

phase out remaining instances of the IPv4 protocol in a timely manner. When a given IPv4/IPv6 node no 

longer needs IPv4 services, administrators should disable the IPv4 protocol. Employing both protocols is 

useful during the early phases of IPv6 deployment, but the practice becomes a security risk because of 

increased complexity.

Administrators also need to watch for an unintended dual stack transition due to IPv6 being enabled 

prematurely. Security organizations should monitor for IPv6 traffic. Organizations should also audit 

router and neighbor solicitations to detect the insertion of rogue routers and devices on the network. 

Security and network administrators should have an incident response plan174 in place for responding to 

violations of the configuration and security policies.

 

172 RFC 4852, IPv6 Enterprise Network Analysis - IP Layer 3 Focus, is available at http://www.ietf.org/rfc/rfc4852.txt. 

173 RFC 4942, IPv6 Transition/Coexistence Security Considerations, is available at http://www.ietf.org/rfc/rfc4942.txt.

174

 See also NIST SP 800-61, Computer Security Incident Handling Guide, http://csrc.nist.gov/publications/nistpubs/800-61-

rev1/SP800-61rev1.pdf.
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6.5 Tunneling

Tunneling is the encapsulation of one protocol inside of another. The tunneling protocol carries the 

tunneled protocol. The tunneled protocol is unaware of the tunnel and will not incur hop counts while in 

transit. This section provides an overview of tunneling either IPv6-over-IPv4 or IPv4-over-IPv6 packets. 

Tunneling is a flexible transition mechanism and supports several scenarios including:

 As part of a dual stack IPv4/IPv6 transition strategy 

 As a stand-alone transition method 

 Used together with protocol translation. 

At a high level, tunnels are either configured tunnels or automatic tunnels. Configured tunnels require 

system administrators to configure the endpoints of the tunnel. With automatic tunnels, the nodes 

configure the endpoints themselves. Typically, an organization uses configured tunnels for infrastructure 

tunneling. Hosts use automatic tunnels to tunnel back to IPv4 or IPv6 islands. Configured tunnels are 

static in nature, and automatic tunnels are dynamic in nature. 

Figure 6-1 shows a generic case in which two hosts on IPv6 networks can only reach each other through 

an IPv4 network. Each host can reach a dual stack IPv4/IPv6 router. This dual stack router in turn knows 

how to reach another IPv4/IPv6 dual stack router over an IPv4 network. The second router can forward 

the IPv6 packet successfully. 

These routers are also set up to serve as tunnel endpoints. As tunnel endpoints, they encapsulate the IPv6 

packet inside an IPv4 packet and forward it to the other tunnel endpoint. Then the tunnel endpoint 

decapsulates the IPv6 packet for further processing. The paths do not need not to be symmetric. The 

end-point of the tunnel found in the left-to-right direction may be different from the starting point of the 

tunnel found in the right-to-left direction. From the IPv6 perspective, the IPv4 tunnel looks like a single 

hop.

Figure 6-1. Example of Tunneling IPv6 over IPv4 Networks

Figure 6-1is a simplified view of tunneling. It illustrates the general concept. There are, in fact, many 
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6-13

different mechanisms for tunneling. 

When an IPv4 header directly encapsulates IPv6, it uses protocol number 41. When an IPv6 header 

directly encapsulates IPv4, it uses Next Header 4. It is possible to use other encapsulation methods such 

as GRE175 or IPsec ESP152. 

IPv6 packets can be up to four gigabytes in size. The maximum size for an IPv4 packet is 64k. 

Fragmentation may be required when an IPv6 packet is encapsulated in IPv4. The IPv6 layer should 

discover the IPv4 path maximum transmission unit (MTU). Using this information, it should create the 

most efficient packets for transmission. The encapsulated protocol (IPv4 or IPv6) sees the tunnel as a 

single hop. The hop limit or TTL for the encapsulating protocol is independent of the TTL for the 

encapsulated protocol. 

6.5.1 General Security Considerations for Tunneling

The following are security items to consider for tunneled environments:

 Tunnel endpoints

 Inspection

 Access control

 Termination.

Tunnel endpoints are always a focal point for security; attackers frequently target them in attacks. 

Potentially, traffic from anywhere can arrive at tunnel endpoints. Tunnels should be treated as an external 

link. The tunneled traffic should not be trusted without inspection. This requires examining IPv6 traffic 

within IPv4 packets and subjecting it to security controls at the same place, logically, that IPv4 is 

controlled. Packet filters, network ingress filtering, virus protection, application proxies, and intrusion 

detection systems need to apply the same security policy to what is inside tunneling protocols as they do 

to IPv4. Enforcing security policies at both the ingress and egress of the tunnel will protect the tunnel 

provider from attacks and protect hosts from traffic coming through the tunnel. Even if the traffic going 

through the tunnel is protected with end-to-end IPsec, additional security controls such as authorization 

should be applied at the tunnel endpoints.

Routers, firewalls, and security devices at the end of an enterprise network may not be technically capable 

of inspecting the IPv6 payload contained within IPv4 packets entering and exiting the network. In 

particular, security devices are unable to inspect encrypted tunnel traffic. By default, tunneling protocols 

do not encrypt traffic. Organizations should deploy security devices that can understand tunneled traffic 

or can inspect traffic once decapsulated.

The access control lists (ACL) on IPv4 routers may be incapable of blocking access to the network from 

specific IPv6 addresses or from entire blocks of IPv6 addresses. They may not be able to recognize IPv6 

next header numbers, ICMPv6 message types, or port numbers for TCP or UDP over IPv6. The reason is 

that the IPv6 header and addresses are not visible to the router; they are part of the packet payload, which 

is the functional intent of any tunneling process. 

Network administrators must understand that IPv6 tunneling may be taking place without their 

knowledge. IPv4 can tunnel IPv6 traffic through security controls and violate normal access control 

 

175 RFC 2784, Generic Routing Encapsulation, is available at http://www.ietf.org/rfc/rfc2784.txt.
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policies. In effect, the IPv6 over IPv4 tunnel becomes a backdoor into the network. Figure 6-2 illustrates 

the relationship between IPv6 tunnels and the existing IPv4 network.

Figure 6-2. IPv6 over IPv4 Tunnels Transparent to the IPv4 Infrastructure

IPv6 over IPv4 tunneling protocols themselves are visible to edge devices such as routers and firewalls, 

even if those devices cannot apply security to encapsulated IPv6 traffic. For this reason, IPv4 access 

control lists and firewalls should block well-known tunneling protocols and ports or ensure that these 

protocols are destined to an IPv6-capable firewall. Of course, this raises the question of whether an attack 

could get through the IPv6 firewall with, for example, a type 0 routing header. Many firewalls can block 

IPv4 protocol 41 (used in the 6over4, ISATAP, and 6to4 tunnel protocols). Network and security 

administrators should confirm that their infrastructure appropriately filters protocol 41 by verifying 

security device configurations and evaluating existing traffic.

Blocking protocol 41 with access control lists or firewalls does not prevent tunneling IPv6 over IPv4 

UDP tunnels such as Teredo. It is hard to identify and filter all Teredo traffic. Teredo uses a well-known 

outbound UDP port 3544 to locate Teredo servers; simply blocking port UDP 3544 does not guarantee 

that Teredo traffic is blocked, because it is easy to change the port assignment. A more effective 

technique is to use an IDPS that can identify the protocols to block regardless of the port assignment. 

Tunnels encapsulating IPv6 in SSL-TLS or IPsec are more problematic. Because the payload may be 

encrypted, no mechanism exists that can inspect the traffic. Organizations must possess the decryption 

keys on the inspection device or terminate tunnels in front of the security devices and evaluate plain text. 

Better security for tunnels is available if both endpoints are under the same administrative control or have 

a trust relationship sufficient to set up cryptographic security. RFC 4891, Using IPsec to Secure IPv6-inIPv4 Tunnels,

176 explains the easiest way to set up IPsec ESP to secure IPv6 over IPv4 encapsulation.

RFC 4213, Basic Transition Mechanisms for IPv6 Hosts and Routers,170 recommends sending the same 

response as for ―destination unreachable‖ when dropping a packet for security reasons. Otherwise, an 

attacker could probe for the existence of the tunnel endpoint in preparation for other attacks.

 

176 IETF RFC 4891, Using IPsec to Secure IPv6-in-IPv4 Tunnels, is available at http://www.ietf.org/rfc/rfc4891.txt.

IPv4 

Internet or 

IPv4 

Backbone 

Network

IPv4 Firewall IPv4 Firewall

Dual Stack IPv4/IPv6 LAN

Dual Stack

Edge Router Dual Stack 

Edge Router

Dual Stack IPv4/IPv6 LAN

= IPv6 over IPv4 Tunnel Transparent to Firewalls
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The following sections address specific deployment and security issues for various tunneling 

mechanisms. Section 6.5.2 covers configured tunneling of IPv6 over IPv4. Automatic tunneling 

mechanisms in general are discussed in Section 6.5.3, following by detailed discussions of various 

automatic tunneling mechanisms in Sections 6.5.4 through 6.5.10. 

6.5.2 Configured Tunneling

Configured tunnels establish connections before traffic using the tunnel arrives. Configured tunnels are 

set up and maintained by network administrators. Establishing remote endpoints outside the organization 

may require peering agreements and may not use optimal paths between peering sites. The general model 

for configured tunnels is to set them up and use them router-to-router for regular traffic between sites.

6.5.2.1 Using Configured Tunnels

Configured tunnels provide a straightforward way to use the IPv4 routing and forwarding infrastructure to 

carry IPv6 traffic to IPv6 islands. RFC 4213 further describes configured tunnels. This document 

contains details about maintaining state information at the tunnel endpoints, constructing the IPv4 

encapsulation, handling MTUs and fragmentation, running neighbor discovery through a tunnel, and 

dealing with ICMPv4 error messages. 

For instance, in tunneling with IPv6 over IPv4 using protocol 41, the endpoint routers at both ends of a 

configured tunnel are provided with the four parameters defining the tunnel (IPv4 and IPv6 source and 

destination addresses of both endpoints). Commonly, IPv6 traffic on a link reaches a default router that 

serves as the endpoint of a configured tunnel and knows how to use IPv4 as the next hop to forward the 

IPv6 traffic. The tunnel will not work with NAT filtering of protocol 41. Configured tunnels can also 

tunnel IPv4 over IPv6. Tunnels may use GRE encapsulation or MPLS instead of protocol 41. 

Using configured tunnels does not scale well, because of the need to set up and maintain each one 

manually. However, configured tunnels provide a much greater degree of control than using automatic 

tunneling.

6.5.2.2 Security Considerations for Configured Tunnels

General security considerations for tunneling are covered in Section 6.5.1 and in RFC 4942. The 

following points apply to security for configured tunnels:

 After decapsulation, packets with a multicast, loopback, IPv4 compatible or IPv4 mapped source 

address must be discarded (but the unspecified address :: should be allowed). Apply normal ingress 

filtering, which may require manually maintaining IPv6 prefix lists. 

 The tunnel must allow the encapsulated packet to get through any instances of security policy 

enforcement: packet filters, application layer gateways, intrusion detection or prevention systems, and 

so forth.

 Additional restrictions may be applied to using the tunnel. For example, the tunnel endpoints may 

accept encapsulated packets only from a list of approved senders.

 Tunnel endpoints handling multiple configured tunnels must treat each one as a separate interface. 

Treating each tunnel endpoint as a separate interface prevents violating scoping rules and isolates 

neighbor discovery traffic.
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 If the encapsulated packet uses IPsec for integrity or confidentiality, little is gained by applying IPsec 

again for the same services through the tunnel, but if IPsec along the tunnel is desired, then the 

methods in RFC 4891 may be used.

6.5.3 Automatic Tunneling

Due to the administrative overhead of maintaining configured tunnels, and because configured tunnels do 

not cover all transition scenarios, the IETF defined additional automatic tunneling mechanisms. As 

transition problems were encountered, the IETF created additional transition mechanisms to solve them.

The general automatic tunneling model consists of isolated hosts with limited communications needing 

dynamic tunneling capabilities. With automatic tunnels, one tunnel endpoint can find the other end 

without pre-configuration. One way for IPv6 over IPv4 tunnels to accomplish this is by embedding IPv4 

addresses in IPv6 addresses. This may be a good method for jump-starting a transition to IPv6, but it is 

not recommended as a long-term solution. Besides the overhead involved with establishing the tunnels, 

this approach limits the use of new IPv6 capabilities, does not solve the IPv4 address space problem, and 

imports the IPv4 routing table into IPv6. Automatic tunneling is more likely needed in the earlier stages 

of IPv6 transition. 

Deployment and security considerations for the following automatic tunneling mechanisms are discussed 

in subsequent sections:

 6over4 - host to router or router to host

 6to4 and 6rd - router to router

 ISATAP - intra-site tunnels

 Teredo - UDP encapsulation intended for tunneling through IPv4 NATs

 Tunnel brokers - using a server for automatic tunneling

 DSTM (Dual Stack Transition Mechanism)

 Carrier-Grade NAT (CGN) and Dual-Stack Lite

6.5.4 6over4 Protocol

The 6over4 protocol (RFC 2529177) is a relatively old and simple transition mechanism that relies on IPv4 

multicast as a virtual link layer. Also known as IPv4 multicast tunneling or virtual Ethernet, it is a hostto-host, host-to-router, and router-to-host automatic tunneling technology supplying unicast and multicast 

IPv6 connectivity between IPv6 nodes within an IPv4 site. It is not intended for connecting an isolated 

IPv6 user to the rest of the IPv6 Internet. Because of its dependence on IPv4 multicast, it has not been 

widely implemented or deployed.

6.5.4.1 Using the 6over4 Protocol

The 6over4 protocol assigns two IPv6 addresses per interface, a unicast address and a link-local address. 

Hosts use a valid 64-bit IPv6 prefix for unicast addresses and set their interface ID simply to their 32-bit 

IPv4 address (e.g., A.B.C.D). They also configure the link-local address FE80::A.B.C.D on each 6over4 

interface. Figure 6-3 shows the two 6over4 IPv6 address sets for an interface.

 

177 IETF RFC 2529, Generic packet Tunneling in IPv6 Specification, http://www.ietf.org/rfc/rfc2529.txt
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 +-------+-------+-------+-------+-------+-------+------+------+

 | FE 80 00 00 00 00 00 00 |

 +-------+-------+-------+-------+-------+-------+------+------+

 | 00 00 | 00 | 00 | IPv4 Address |

 +-------+-------+-------+-------+-------+-------+------+------+

Figure 6-3. 6over4 Interface Addresses

The 6over4 protocol regards the IPv4 network as a link layer with multicast capability (like Ethernet). 

This means that neighbor discovery processes (such as address resolution and router discovery) work as 

they do over a physical link with multicast capabilities. The entire IPv4 infrastructure used for the 

tunneling must be multicast enabled. 

6.5.4.2 Security Considerations for the 6over4 Protocol

Security considerations for the 6over4 protocol include:

 In addition to attacks against IPv6, attacks against IPv4 are also possible, so standard IPv4 security 

controls apply.

 Boundary routers need to apply normal ingress and egress filtering rules to the IPv4 addresses and 

filter incoming unicast IPv4 packets with protocol type 41 from unknown sources. Configure 

boundary routers to accept IPv6-in-IPv4 tunnels only from trusted sources. Boundary routers also 

need to reject multicast IPv4 packets with a more restricted scope than appropriate for the interface on 

which they arrive.

 Do not treat decapsulated IPv6 packets with a hop count of 255 as locally generated packets.

 If IPsec is desired, it is better to run it in the IPv6 domain, end to end or nearly end to end. Little is 

gained (except perhaps some protection against traffic analysis) by adding IPsec in the IPv4 domain.

6.5.5 6to4 and 6rd Protocols

The 6to4 mechanism is designed to provide IPv6 site-to-site and site-to-existing-IPv6-network 

connectivity across an IPv4 network by embedding IPv4 addresses in IPv6 prefixes. The 6to4 protocol is 

intended to be a start-up technique for providing IPv6 connectivity. One example is when an ISP does 

not provide an IPv6 prefix. The 6to4 protocol is appropriate with an IPv6-dominant transition strategy 

without a global IPv6 prefix. It is not a permanent solution. More information on the 6to4 protocol is 

available in RFC 3056, Connection of IPv6 Domains via IPv4 Clouds.

178

The 6rd mechanism is designed to allow IPv4 service providers to offer IPv6 to their customers with 

minimum delay and expense. The name ―6rd‖ stands for ―IPv6 rapid deployment.‖ Its main idea is to use 

the same protocol mechanisms as 6to4 but with the service provider’s IPv6 prefix. Two Internet Drafts on 

6rd exist: (1) IPv6 Rapid Deployment on IPv4 infrastructures (6rd)179 describes the motivation for and 

user experience with 6rd; and (2) IPv6 via IPv4 Service Provider Networks180 defines the protocol. The 

 

178 RFC 3056, Connection of IPv6 Domains via IPv4 Clouds, is available at http://www.ietf.org/rfc/rfc3056.txt.

179 IETF work in progress IPv6 Rapid Deployment on IPv4 infrastructures (6rd), is available from 

http://tools.ietf.org/html/draft-despres-6rd. 

180 IETF work in progress IPv6 via IPv4 Service Provider Networks, is available from http://tools.ietf.org/html/draft-ietfsoftwire-ipv6-6rd.
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advantages of 6rd are that it automatically delegates IPv6 prefixes to customers’ sites, operates statelessly, 

allows customers to use native IPv6, and can be provisioned with a few simple steps. The problem 6rd 

solves that 6to4 does not is route discovery. A packet originating from any native IPv6 address needs to

traverse, somewhere, a relay router to obtain IPv6 over IPv4 encapsuation. Using the service provider’s 

IPv6 prefix provides such a route. In all other respects, except as noted below, 6to4 and 6rd work 

similarly. 

6.5.5.1 Using 6to4 and 6rd

Each 6to4 border router must have a globally unique IPv4 address (e.g., w.x.y.z.). The IPv6 network 

connected to that router uses the IPv6 prefix 2002:w.x.y.z/48181. An entire /48 IPv6 prefix can be routed 

through one IPv4 address. 6to4 IPv6 sites can automatically connect to each other over the IPv4 Internet. 

This prefix can be advertised to the IPv6 network or sub-allocated as desired. IPv6 hosts can use 

autoconfiguration to generate interface IDs for 6to4 prefixes. The 6to4 border routers tunnel IPv6 over 

IPv4 using protocol 41. 

The IPv6 destination does not need to have a 6to4 prefix. It may be any globally unique IPv6 address. 

Similarly, IPv6 packets can be sent from any IPv6 address to a 6to4 address. Relays, which are IPv6 

routers with at least one 6to4 address and one native IPv6 address, are used to connect 6to4 domains with 

the native IPv6 Internet. Each 6to4 domain must have at least one relay router, which is normally reached 

with an (IPv4) anycast address (RFC 3068, An Anycast Prefix for 6to4 Relay Routers182). 

The 6to4 mechanism has been widely implemented and is in use in both production and trial networks. 

Sites that have globally routable IPv6 prefixes should avoid using the 6to4 protocol. Note that 6to4 sites 

require a globally unique IPv4 address. The 6to4 and 6rd protocols presume that IPv4 is still being used 

and do not solve the IPv4 space exhaustion problem. Relay routers that serve a large number of 6to4 

routers have to import that part of the IPv4 forwarding table into IPv6.

The 6to4 and 6rd mechanisms can work on networks using IPv4 NAT as long as these two conditions are 

met:

 The NAT boxes are fully functional IPv6 routers 

 The globally unique IPv4 address of the outermost NAT box is used to construct and advertise the 

IPv6 6to4 prefix. (For 6rd, this condition is not necessary.) 

6.5.5.2 Security Considerations for 6to4 and 6rd

In addition to the general security advice for tunneling mechanisms, RFC 3056 contains an overview of 

6to4 security. RFC 3964, Security Considerations for 6to4, examines many 6to4 security concerns more 

thoroughly.183 In particular, it considers the following cases:

 6to4 routers not being able to identify whether relays are legitimate

 Wrong or partially implemented 6to4 protocol on the router or relay security checks

 

181 For 6rd, the IPv4 address may be globally unique or a non-routable RFC 1918 address used only within the service 

provider’s network. The exact layout and length of the 6rd IPv6 prefix varies according to the length of the service 

provider’s IPv6 prefix and other tradeoffs.

182 RFC 3068, An Anycast Prefix for 6to4 Relay Routers, is available at http://www.ietf.org/rfc/rfc3068.txt.

183 RFC 3964, Security Considerations for 6to4, is available at http://www.ietf.org/rfc/rfc3964.txt.
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 The 6to4 architecture being used to participate in DoS or reflected DoS attacks or made to participate 

in ―packet laundering,‖ i.e., making another attack harder to trace

 The 6to4 relays being subject to administrative abuse, e.g., theft of service, or being seen as a source 

of abuse.

Complete solutions to all of these abuses do not exist, but basic guidelines for using 6to4 effectively 

follow:

 If IPsec is desired, it is better to run it in the IPv6 domain, either end to end or nearly end to end. 

Little is gained (except perhaps some protection against traffic analysis) by adding IPsec in the IPv4 

domain.

 If IPv6 source address spoofing is a problem, check that the IPv4 and encapsulated IPv6 prefixes 

agree with the necessary exceptions for relay routers.

 Many address spoofing and denial of service attacks stem from the requirements that 6to4 routers 

must accept and decapsulate IPv4 packets from any other 6to4 router or relay, and 6to4 relays must 

accept traffic from any IPv6 node. Administrators should apply normal ingress and egress filtering 

rules to IPv4 addresses.

 Any 6to4 prefix must correspond to a globally routable unicast IPv4 address. The IPv4 address must 

not be private, loopback, unspecified, multicast, broadcast, DHCP link local, or a reserved IPv4 

address. 

 The IPv6 address must not be IPv4 compatible, IPv4 mapped, loopback, unspecified, link local, site 

local, or multicast. 

Many of the threats against 6to4 do not apply to 6rd, because the IPv6 over IPv4 tunnels exist only within 

a single service provider. First, the customer’s end of a 6rd tunnel only needs to accept packets from a 

single or small set of known 6rd relay routers, Second, the tunnel endpoint within the service provider’s 

network can ensure that tunneled traffic arriving at its IPv4 address comes from a customer and not an 

outsider. Third, IPv6 packets can be protected against source address forgery to the same extent that IPv4 

packets are within a service provider’s network.

6.5.6 Automatic Intra-Site Tunnel Addressing Protocol (ISATAP)

ISATAP allows isolated IPv6 hosts within a site running IPv4 to construct an automatic IPv6-in-IPv4 

tunnel. ISATAP uses IPv6 without depending on IPv4 multicast, as required with 6over4, but ISATAP 

requires more setup work than 6over4. All hosts using ISATAP must be dual stack IPv4/IPv6. The 

network may only route IPv4. Most dual stack IPv4/IPv6 operating systems implement ISATAP. More 

information about ISATAP is available in RFC 5214, Intra-Site Automatic Tunnel Addressing Protocol 

(ISATAP).184

ISATAP is a useful mechanism promoting early deployment of IPv6 within a site with an IPv6 prefix. 

Consider ISATAP a bootstrap mechanism for connecting IPv6 hosts as the other parts of the network 

become IPv6 enabled. Remove ISATAP when there is no longer a need. ISATAP is not appropriate for 

IPv6 dominant networks. Once IPv6 is available in a dual stack environment, disable all tunneling of 

IPv6 over IPv4. 

 

184 RFC 5214, Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) is available at http://www.ietf.org/rfc/rfc5214.txt.
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6.5.6.1Using ISATAP

ISATAP hosts communicate by tunneling IPv6 packets over IPv4 using protocol 41. The IPv4 addresses 

are encoded in the low-order bits of the IPv6 addresses, allowing automatic tunneling. Globally routable 

IPv4 addresses are not needed. If using a private IPv4 address to create the ISATAP address, then that 

IPv6 address cannot be used out of its IPv4 scope. The local IPv4 network is represented in IPv6 as a 

single subnet.

ISATAP assumes that some dual stack IPv4/IPv6 router exists and that it advertises an IPv6 prefix. A 

host with an IPv4 address w.x.y.z then performs autoconfiguration with interface ID = ::0:5EFE:w.x.y.z.

The example in Figure 6-4 shows:

 IPv4 address: 192.168.1.10

 Global IPv6 prefix: 2001:1C3:1:200F::/64

 Link-local address: FE80::5EFE:65:C0A8:010A = FE80::5EFE:65:192.168.1.10

 Global IPv6 address: 2001:1C3:1:200F::5EFE:C0A8:010A = 

2001:1C3:1:200F::5EFE: 192.168.1.10.

To make this work, one needs to ensure that:

 All IPv6 hosts run dual stack IPv4/IPv6 with support for ISATAP.

 Each ISATAP host must know at least one dual stack IPv4/IPv6 router.

 All traffic is constrained to a single administrative domain.

 There is no need for IPv4 NAT traversal.
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Figure 6-4. Example - Tunneling IPv6 over IPv4 Networks with ISATAP

6.5.6.2 Security Considerations for ISATAP 

Security considerations for ISATAP include:

 ISATAP does not assume that multicast works. ISATAP foregoes IPv6 autoconfiguration and 

neighbor discovery protocols. It automatically knows the correspondence between the IPv6 and IPv4 

(link layer) addresses. Administrators statically configure the address of the default router. Hence, 

many of the security considerations with autoconfiguration and neighbor discovery do not exist with 

ISATAP.

 The appropriate use of IPsec is the same as with 6to4. Implement IPsec in the IPv6 domain.

 Certain IPv6 addressing capabilities such as privacy addresses or CGA cannot be used. 

 All of the usual protocol 41 concerns apply. Various spoofing attacks may try to take advantage of 

the protocol 41 tunneling mechanism. Firewalls need to perform ingress and egress filtering on either 

the encapsulating and encapsulated addresses or block protocol 41, as appropriate. 

 ISATAP clients are vulnerable to an internal attack in which an imposter pretends to be an ISATAP 

router. The recommended defense is for administrators to ensure that the ISATAP potential routers 

list (PRL) is accurate and protected from tampering. 
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6.5.7 Teredo Protocol

The 6over4 mechanism requires IPv4 multicast. The 6to4 and 6rd protocols require public IPv4 

addresses. Although ISATAP solves both of these problems, ISATAP does not work across NATs. 

Microsoft developed the Teredo protocol as a solution, submitting it to IETF. The IETF approved it as 

RFC 4380, Teredo: Tunneling IPv6 over UDP through Network Address Translations (NATs).23

Teredo’s functionality and usage scenarios are similar to 6to4, but 6to4 requires support from a router, 

which serves as the edge device connected to the Internet. IPv4 NAT devices do not usually support 6to4 

router functionality. Even if the NAT device supported 6to4, the 6to4 protocol would still not work for 

configurations with multiple NATs between a site and the IPv4 Internet or for NATs that do not handle 

protocol 41 packets, since some NATs only handle TCP and UDP. 

Teredo is a useful mechanism for promoting early deployment of IPv6 on hosts behind a NAT, prior to 

having an IPv6 network, in the case where an IPv6 prefix has been provided to the site. Production and 

pre-production networks have been using Teredo.

Because Teredo has more overhead than other methods, Teredo is considered a technology of last resort 

for IPv6 connectivity. The design of Teredo favors robustness over efficiency. If native IPv6, ISATAP, 

or 6to4 connectivity can be used, Teredo is not needed. The use of Teredo will decrease as more IPv4 

NATs support 6to4 and IPv6 connectivity becomes more common.

Teredo provides address assignment and automatic tunneling for host-to-host unicast IPv6 traffic across 

an IPv4 Internet even if the dual stack IPv4/IPv6 hosts are located behind one or more IPv4 NATs. 

Teredo works for the various types of cone NATs, but it does not work, in general, for symmetric NATs. 

See RFC 5389, Session Traversal Utilities for NAT (STUN), for definitions of these types of NAT 

devices.185 To get through the IPv4 NATs, IPv6 packets are tunneled over IPv4 and UDP.

6.5.7.1 Using Teredo

The general approach to NAT traversal starts with detecting the presence of NAT. Then the protocol is 

started from inside the NAT and encapsulated in UDP. There is a need for some mechanism to keep track 

of the NAT mappings.

Figure 6-5 shows the components of Teredo and the protocol messages used to set up communications 

between a Teredo client and a remote IPv6 interface. A Teredo client is a dual stack IPv4/IPv6 node with 

an IPv4 address behind a NAT device. A Teredo server is also dual stack IPv4/IPv6, but it has a globally 

routable IPv4 address. The Teredo server listens to UDP port 3544 for client requests and uses a Teredo 

service prefix to construct an IPv6 address for the Teredo client. A Teredo relay is a dual stack IPv4/IPv6 

router that provides the remote tunnel endpoint for the Teredo client. Together, the Teredo server and 

Teredo relay provide IPv6 connectivity for the client with the IPv6 Internet and with mechanisms like 

6to4.

 

185 RFC 5389, Session Traversal Utilities for NAT (STUN), is available at http://www.ietf.org/rfc/rfc5389.txt.
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Figure 6-5. Example - Tunneling IPv6 over IPv4 Networks with Teredo

In Figure 6-5, the messages work as follows:

1. The Teredo Client sends an IPv6 ping to the remote interface through its Teredo server (encapsulated 

in UDP and IPv4).

2. The Teredo server decapsulates and forwards the ping.

3. The ping reply goes to a Teredo relay.

4. The ping reply goes to the Teredo server.

5. The Teredo server tells the Teredo client what Teredo relay to use.

6. The Teredo client sends normal (encapsulated) traffic to the Teredo relay.

7. The Teredo client receives normal (encapsulated) traffic from the Teredo relay.

Separating the Teredo server and Teredo relay reduces the load on servers and provides a more direct path 

for tunneled traffic. Servers do not forward ordinary traffic for clients. The Teredo client and Teredo 

server only communicate when:

 Getting an IPv6 address for the client

 Helping the client get through the NAT with Teredo Bubbles, empty (protocol 59) IPv6 packets 

designed to create mappings through the NAT, or
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 Sending an IPv6 ping to other IPv6 nodes to discover the best Teredo relay for that address.

A Teredo client can learn about other Teredo clients on its own IPv4 network by using the IPv4 multicast 

address 224.0.0.253. 

IPv6 addresses for Teredo clients are comprised of the following five parts:

 Prefix: the 32-bit Teredo service prefix 2001:0000::/32

 Server IPv4: the 32-bit IPv4 address of a Teredo server

 Flags: 16 bits set to 8000 for cone NATs and 0000 otherwise

 The Teredo client’s 16-bit UDP port number, inverted bit by bit

 The Teredo client’s 32-bit IPv4 address (behind the NAT), inverted bit by bit.

The parts are illustrated in Figure 6-6.

 +-------------+-------------+-------+------+-------------+

 | Prefix | Server IPv4 | Flags | Port | Client IPv4 |

 +-------------+-------------+-------+------+-------------+

Figure 6-6. Teredo Address

6.5.7.2 Security Considerations for Teredo

The Teredo specification, RFC 4380, Teredo: Tunneling IPv6 over UDP through Network Address 

Translations (NATs) and a subsequent Internet Draft, Security Concerns with Tunneling186, contain 

lengthy descriptions of Teredo security considerations, including:

 Teredo allows the use of end-to-end IPsec for the encapsulated IPv6 traffic, thereby preventing the 

inspection of Teredo encapsulated traffic. 

 No effective method exists to disable Teredo and filter all Teredo traffic. The initial communication 

with the Teredo server (UDP Port 3544) is easily recognizable, but the port assignment can easily be 

changed.

 Subsequent traffic has no immediately distinguishing feature. Heuristic checks have been devised, 

but these are not likely to be practical.

 Teredo requires that external UDP ports are open and are not filtered at a NAT device or firewall. 

This could allow port scans to identify openings they would not see otherwise. Teredo bubbles 

increase this exposure.

 

186 The Internet Draft, Security Concerns with Tunneling, is available at http://tools.ietf.org/html/draft-ietf-v6ops-tunnel-securityconcerns.
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 Teredo’s IPv6 addresses are more predicable than native IPv6 addresses. Techniques like privacy 

addressing and CGA are not usable with Teredo.

 A third party may impersonate a Teredo server and carry out a DoS attack by sending wrong 

information to the Teredo client. To counter impersonation attacks, use two-way authentication 

between Teredo client and server.

 Teredo relays must protect themselves against being used as anonymous senders in distributed DoS 

attacks.

 The Teredo service must not bypass or disable ingress filtering, whatever firewall service was 

available in the NAT box, or other firewalls, virus checkers, or intrusion detection systems. This is a 

common security concern for all tunneling protocols. Ingress filtering may show up particularly with 

Teredo, because the system was assumed to be behind a NAT. Such security checks should also look 

for unexpected effects due to IPv6 type 0 routing headers. 

 Services that listen to the Teredo IPv6 address become potential targets of attacks from the entire 

IPv6 Internet. This vulnerability can be reduced in three ways. First, it is possible to restrict some 

services to accept traffic only from local neighbors (for example, by using link-local addresses). 

Teredo does not support link-local addresses, so link-local services cannot be accessed through 

Teredo and are restricted to whatever other IPv6 connectivity may be available, (for example, direct 

traffic with neighbors on the local link, behind the NAT). Second, a local firewall can perform the 

same kind of filtering otherwise performed in a perimeter firewall. Third, use IPsec with Teredo to 

protect clients from intermediate nodes.

 An attacker may intercept a router solicitation from a Teredo client, respond with a spoofed router 

advertisement, and provide a Teredo client with an incorrect default router address. This may simply 

deny service to the Teredo client, or the attacker may try to insert itself as a relay for all client 

communications, effectively enabling a variety of man-in-the-middle attacks.

 A simple nonce-based verification procedure described in RFC 4380 provides a first level of 

protection against attacks in which a third party tries to spoof the Teredo server. In practice, the 

nonce procedure can be defeated only if the attacker observes both the client and server 

communications.

 If client and server share a secret and agree on an authentication algorithm, the Teredo’s secure 

qualification procedure provides further protection. The most likely way to obtain the shared secret is 

to listen to the traffic and run an offline dictionary attack. To protect against this attack, the secret 

shared between client and server should contain sufficient unpredictability. 

 A more sophisticated attack can be carried out, but it is somewhat difficult and it does not gain too 

much. Actually, it works because the communications with the NAT are unprotected. It goes as 

follows:

1. The client (victim) prepares router solicitation, including authentication.

2. The attacker intercepts the solicitation and somehow manages to prevent it from reaching the 

server. One way to intercept the solicitation is by running a short-duration DoS attack against the 

server.

3. The attacker replaces the source IPv4 address and source UDP port of the request by one of its 

own addresses and port and sends the modified request to the server.
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4. The server sees the IPv4 address from which the packet is received, verifies that the 

authentication is correct, prepares a router advertisement, signs it, and returns it to the attacker.

5. The attacker receives the advertisement, remembers the mapping, replaces the IPv4 address and 

UDP port by the original values in the intercepted message, and sends the response to the client.

6. The client receives the advertisement and uses the proposed prefix and the mapped addresses in 

the origin indication encapsulation. The attacker can now run a DoS or man-in-the-middle attack.

 Other attacks may try to spoof a Teredo relay. An attacker may try to spoof another IPv6 host or to 

place itself as a man-in-the-middle between a Teredo host and a native IPv6 host. The attacks work 

by convincing the Teredo client that packets bound to the native IPv6 host should be relayed to the 

IPv4 address of the attacker. These attacks are possible because there is no obvious relationship 

between the IPv4 address of a relay and the native IPv6 addresses it serves. A Teredo client cannot 

decide by looking at the encapsulating IPv4 and UDP headers whether or not a relay is legitimate. 

The proper relay is identified by a direct IPv6 connectivity test. Traffic sent to this relay will be out 

of reach of attackers that are not on the direct path between the Teredo client and its IPv6 peer. To 

stop on-path attacks, end-to-end security such as IPsec is needed. IPsec is the best defense against 

many of these attacks.

 A Teredo client keeps a cache of recently used peers. It is possible to provoke it to respond to packets 

that appear to come from a large number of Teredo peers, thus overloading this list. 

 A DoS attack against the local peer discovery procedure exists if attackers can send spoofed local 

discovery bubbles to a Teredo client. Mitigate DOS attacks by packet filtering, restricting local peer 

discovery to the local link, or turning off support for discovery bubbles.

 An attacker may try to overwhelm and take down a Teredo server. Having a failover server can help, 

but addresses will have to be renumbered. A similar attack against a Teredo relay may cause the 

Teredo relay to temporarily stop announcing the reachability of the Teredo service prefix to the IPv6 

network. The traffic will be picked up by the next relay. An attacker may also try to overwhelm the 

state table in a Teredo relay. Relays may have to be selective about whom they serve.

 Three classes of DoS attacks attempt to inject traffic at locations where it is not expected. The first 

uses a Teredo server as a reflector in a denial of service attack. The second uses a Teredo server to 

carry out a denial of service attack against IPv6 nodes. The third uses a Teredo relay to carry out a 

denial of service attack against IPv4 nodes. Adding appropriate filters are effective at mitigating 

these attacks.

 An attacker can use a Teredo server as a reflector in a DoS attack on an IPv4 target in two ways. The 

first is to construct a router solicitation message and post it to a Teredo server, using as the IPv4 

source address the spoofed address of the target. The Teredo server will then send a router 

advertisement message to the target. The second is to construct a Teredo IPv6 address using the 

Teredo prefix, the address of a selected server, the IPv4 address of the target, and an arbitrary UDP 

port, and to send packets bound to that address to the Teredo server. Again, adding appropriate filters 

can defend against these.

 An attacker may use a Teredo server to launch a DoS attack against an arbitrary IPv6 destination. 

The attacker builds an IPv6 packet bound for the target. The attacker sends that packet to the IPv4 

address and UDP port of a Teredo server to be relayed from there to the target over IPv6. Servers 

should check that the IPv4 and IPv6 source addresses are consistent, and routers or security devices 

should perform ingress filtering for the IPv6 addresses.
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 An attacker with IPv6 connectivity may use a Teredo relay for a DoS attack against an arbitrary IPv4 

destination. The attacker may compose a Teredo IPv6 address using the Teredo prefix, a ―cone‖ flag 

set to 1, the IPv4 address of the target, and an arbitrary UDP port. First, relays should not allow the 

attacker to use multicast, broadcast, or non-routable IPv4 addresses. Second, IPv6 ingress filtering 

and trace back could be used (if enabled).

6.5.8 Tunnel Brokers

IPv6 tunnel brokers provide dual stack IPv4/IPv6 nodes on IPv4 networks with a way to obtain IPv6 

connectivity without the administrative support of a large site running, for example, 6to4. It is intended 

for small sites or individual hosts. The IPv6 tunnel broker method requires the deployment of a tunnel 

broker server. The tunnel broker server can be viewed as a ―virtual IPv6 ISP.‖ More information on IPv6 

tunnel brokers is available in RFC 3053, IPv6 Tunnel Broker.

187

6.5.8.1 Using IPv6 Tunnel Brokers

The client connects to a tunnel broker, which sets up and manages tunnels between clients and tunnel 

servers. Tunnel servers are dual stack IPv4/IPv6 routers connected to the global Internet. These proxy 

servers reside at well-known IPv4 addresses. A list of public tunnel broker servers is available at 

http://www.ipv6.org. A tunnel broker’s clients can be hosts or routers. The tunnel broker performs 

authentication and access control. The tunnel broker assigns the IPv6 tunnel endpoints and creates 

records for those endpoints in the DNS. The tunnel broker allocates relatively long-lived IPv6 addresses 

from its own global IPv6 address space. It does not work for clients behind NATs.

The tunnel broker description leaves many feature choices to individual implementations. Typically, 

clients communicate with tunnel brokers using HTTP. A tunnel broker may help the client with the actual 

tunnel setup by sending automated scripts, but this raises security concerns. The client may run dynamic 

DNS and may communicate with tunnel servers via SNMP. These are just examples, and other 

implementation details are possible. 

6.5.8.2 Security Considerations for IPv6 Tunnel Brokers

Security must be a consideration for all the interactions between the tunnel broker and other entities: 

clients, tunnel servers, and the DNS. The appropriate security depends on how each of these interfaces is 

implemented:

 For HTTP clients, using SSL/TLS to protect the username and password seems appropriate. HTTP is 

a clear text protocol. When sending confidential information, an encrypted channel should be used. 

Organizations should choose implementations that are inherently more secure. For example, 

delivering lists of tunnel parameters is safer than downloading executable scripts that run on the 

client.

 SNMPv1 and SNMPv2 are clear text protocols. As such, they can disclose confidential information. 

SNMPv3 provides encryption but lags behind SNMPv1 and SNMPv2 in support and deployment. 

SNMP interfaces to tunnel servers should use IPsec or SNMP over SSH if SNMPv3 is not an option. 

Using this option will support confidentiality and integrity of the information passed on the channel. 

 

187 RFC 3053, IPv6 Tunnel Broker, is available at http://www.ietf.org/rfc/rfc3053.txt.
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 For DNS updates, either Secure Dynamic Update (RFC 3007, Secure Domain Name System (DNS) 

Dynamic Update 188) or command scripts protected by SSH or IPsec could be used.

 If a host disconnects from the Internet and its IPv4 address is reallocated, the tunnel server may not 

find out about this and may keep forwarding IPv6 traffic to that address. 

 Tunnel broker servers maintain state for each client. As such, they are susceptible to resource 

exhaustion attacks and other types of DoS attacks.

6.5.9 Automatic Tunneling of IPv4 over IPv6 (Dual Stack Transition Mechanism 

(DSTM))

As IPv6 local networking and internetworking become more prevalent, tunneling IPv4 over IPv6 will 

increase in importance. One approach, called DSTM (for Dual Stack Transition Mechanism), defines an 

IPv6-dominant approach: convert the infrastructure to IPv6 and force IPv4 to run in tunnels over IPv6. 

All of the tunneling mechanisms discussed above (6over4, 6to4, 6rd, ISATAP, Teredo, and IPv6 tunnel 

broker) tunnel IPv6 packets within an IPv4 network. DSTM does the opposite, tunneling IPv4 packets 

within IPv6.

Consider DSTM where there is an IPv6 network without any IPv4 infrastructure and a relatively small

number of nodes needing IPv4 support. Such networks, called IPv6 dominant, can be deployed to 

encourage a rapid migration to IPv6. DSTM is functionally similar to the IPv6 tunnel broker with the 

roles of IPv4 and IPv6 reversed. One major difference is that DSTM presumes clients, servers, and 

border routers belong to the same enterprise’s intranet. 

RFC 4852, IPv6 Enterprise Network Analysis—IP Layer 3 Focus172, discusses the IPv6-dominant 

transition strategy and a research report189 contains the details. DSTM has received wide implementation 

and support on multiple platforms (see http://www.dstm.info).

Running DSTM has several advantages:

 It makes IPv4 available end to end without NAT.

 It conserves IPv4 addresses by reusing them.

 It is transparent to IPv6 applications.

 It promotes a rapid transition to IPv6.

6.5.9.1 Using DSTM

The idea is to run IPv6 as the local networking protocol and still be able to access IPv4 services as needed 

with IPv4 over IPv6 tunneling. When an IPv6 address corresponding to an IPv4 address is needed, the 

IPv4 address is embedded in the IPv6 interface ID as the low order 32-bits. The high-order 16 bits are set 

to all ones, so the IPv4 address w.x.y.z yields the IPv6 interface ID FFFF:w.x.y.z. These are called IPv4-

mapped IPv6 addresses. The IPv6 packet encapsulating the IPv4 packet has Next Header = 4.

Tunneling is set up automatically. A DSTM server receives a request from a client for a temporary, 

dynamically allocated IPv4 address, and it sets up the IPv4 over IPv6 tunnel to a dual stack IPv4/IPv6 

 

188 RFC 3007, Secure Domain Name System (DNS) Dynamic Update, is available at http://www.ietf.org/rfc/rfc3007.txt.

189 See Xia, H., J. Bound, and Y. Pouffray, The Evaluation of DSTM: An IPv6 Transition Mechanism., is available at 

http://www.moonv6.org/lists/att-0314/pdfdstmpaper.pdf
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DSTM border router. 

The requirements for running DSTM are:

 An IPv6-only network infrastructure

 A dual stack IPv4/IPv6 client needing IPv4 connectivity and running DSTM with either DHCPv6 (the 

recommended choice) or Tunnel Setup Protocol (TSP) 190. 

 A DSTM server

 A DHCPv6 relay or server, or a TSP server

 An available pool of IPv4 addresses

 A dual stack IPv4/IPv6 border router capable of handling DSTM.

The dual stack IPv4/IPv6 client on the IPv6 network gets an IPv4-mapped IPv6 address and tunnel 

endpoint (TEP) for a DSTM router from the DSTM server. The client-server protocol is either DHCPv6 

or TSP.

6.5.9.2 Security Considerations for DSTM

DSTM does not use NAT, so IPsec, SSH, and other IPv4 security services can be used end to end. 

DSTM can use any other security functionality that supports parts of its operation: DHCPv6 

authentication, DNSSEC, or TSP over TLS. 

Tunnel endpoint security is an issue, as always. DSTM routers should be configured together with IPv4 

firewalls and intrusion detection systems to secure IPv4 tunnel endpoints from IPv4-based attacks. They 

should also check the consistency of the IPv4 and IPv6 addresses used in tunneled traffic and apply 

ingress and egress filtering. Finally, they should only accept DSTM traffic on their internal interfaces to 

avoid being used as an open relay. 

Perhaps the most serious resource exhaustion DoS attack is against the supply of IPv4 addresses. 

Authenticating the client-server protocol allows such attacks to be tracked down easily. 

6.5.10 Carrier-Grade NAT and Dual-Stack Lite

For several years, dual stack hosts and routers have been promoted as the most practical transition plan. 

Howver, this approach faces a new problem today. Dual stack means having IPv4 and IPv6 addresses, but 

the world is running out of IPv4 addresses. It is expected that IPv4 will be needed to support IPv4-only 

devices in the home and IPv4-only content on the Internet long past the exhaustion of the pool of globally 

routable IPv4 addresses. 

The emphasis in this transition strategy is on helping service providers plan for a situation in which not 

enough globally-routable IPv4 addresses are available for the WAN-facing side of customers’ NAT 

boxes. 

Several proposals have been made to deploy native IPv6 along with some method of sharing globally 

reachable IPv4 addresses among broadband customers needing both IPv4 and IPv6 access. All of these 

 

190 An Internet Draft, IPv6 Tunnel Broker with the Tunnel Setup Protocol (TSP), is available at http://tools.ietf.org/html/draftblanchet-v6ops-tunnelbroker-tsp
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involve some kind of centralized network address translation (NAT), which is frequently called ―carriergrade NAT.‖ In this context, ―carrier grade‖ essentially means centralized, and it may also imply high 

capacity, shared, or multiplexed.

At least four choices for how a customer’s site achieves IPv4 connectivity with a carrier-grade NAT191

have been described. One, called ―IPv4->IPv4->IPv4,‖ provides the customer with a non-routable 

(typically network 10.0.0.0/8) IPv4 address between the customer’s gateway and the carrier-grade NAT. 

This can cause new problems traversing an additional layer of NAT, handling the possibility of address 

conflicts, and even dealing with a shortage of 10.0.0.0/8 addresses for large service providers. A second 

proposal, called ―IPv4->IPv6->IPv4,‖ is to run IPv6 between a customer’s gateway and carrier-grade 

NAT. IPv4 traffic then needs to be translated twice. A third choice is to run IPv4-over-IPv6 tunnels 

between a customer’s gateway and carrier-grade NAT. This combination of carrier-grade NAT and IPv4-

over-IPv6 tunneling, called ―dual stack lite,‖ has perhaps gathered the most support, although no 

approved specifications exist yet. Finally, a fourth proposal tries to improve on dual-stack lite by treating 

portions of the port numbers effectively as extensions of the IPv4 address.192

Carrier-grade NAT does, however, pose restrictions and present some problems for users. Applications 

relying on port mapping or port opening (e.g., the current UPnP specification) may not work with 

multiple NATs. IPv4 multicast may not be supported. Finally, the total number of TCP ports for the 

shared IPv4 address is limited. 

6.5.10.1 Using Carrier-Grade NAT and Dual-Stack Lite

Dual-stack lite193 stretches the supply of routable IPv4 addresses by letting a service provider share them 

among customers. The customer is provided an IPv6 prefix and uses IPv4 over IPv6 tunnels as described 

in RFC 2473194 with Next Protocol 4 in the outer header to reach the IPv4 Internet. The service provider’s 

carrier-grade NAT performs encapsulation and decapsulation of tunneled traffic and network address 

translation. It shares its routable IPv4 addresses among multiple customers, and it uses the IPv6 source 

address, inner IPv4 address, and port numbers of tunneled traffic to maintain a NAT mapping table and to 

keep track of which customer is using which TCP and UDP ports. 

From the service provider’s viewpoint, sharing these IPv4 addresses is an advantage. From the customer’s 

viewpoint, only one NAT operation is performed. Transition is facilitated, because standard, native IPv6 

is provided to the customer. On the other hand, IPv4 applications that require specific port assignments or 

specific port mapping from the NAT operation likely will not work. As user experience grows and the 

specifications for dual-stack lite evolve, several challenges remain: 

 Keeping peer-to-peer applications, which often use only one or only a few port numbers, working.

 Providing application layer gateways for applications needing special handling.

 Defining a scheme whereby most ports are maintained by the carrier-grade NAT, but some are 

dedicated to and controlled by individual customers.

 Ensuring sufficient non-routable IPv4 address space is available for large service providers. The 

proposal asks IANA to assign new address space for this purpose. 

 

191 See IETF work in progress draft-durand-v6ops-natv4v6v4 for a discussion of the first three of these.

192 See Olaf Maennel, Randy Bush, Luca Cittadini, and Steven M. Bellovin, A Better Approach than Carrier-Grade-NAT, 

Columbia University Technical Report CUCS-041-08, 2008.

193 See IETF work in progress draft-ietf-softwire-dual-stack-lite.

194 IETF RFC 2473, Generic packet Tunneling in IPv6 Specification, http://www.ietf.org/rfc/rfc2473.txt
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 Handling MTU efficiently.

 Providing IPv4 multicast service.

6.5.10.2 Security Considerations for Carrier-Grade NAT and Dual-Stack Lite

Using NAT for IPv4 is likely not new, so general NAT security issues are not repeated here. (See, for 

example, RFC 2663195 and RFC 2993196.)

Some of the security issues with carrier-grade NAT result directly from the sharing of the routable IPv4 

address. Addresses and timestamps are often used to identifiy a particular user, but with shared addresses, 

more information (i.e., protocol and port numbers) is needed. This impacts software used for logging and 

tracing spam, denial of service attacks, and other abuses. 

Devices on the customer’s side may try to carry out general attacks against systems on the global Internet 

or against other customers by using inappropriate IPv4 source addresses inside tunneled traffic. The 

carrier-grade NAT needs to protect against such abuse. 

One customer may try to carry out a denial of service attack against other customers by monopolizing the 

available port numbers. The carrier-grade NAT needs to ensure equitable access. At a more sophisticated 

level, a customer may try to attack specific ports used by other customers. This may be more difficult to 

detect and to mitigate without a complete system for authentication by port number, which would 

represent a huge security requirement. For a description of an analogous attack, see the Java-based attack 

described by Martin et al.197

Some attack mitigation strategies block an IPv4 address (temporarily or permanently) after detecting an 

improper use. Thus, the carrier-grade NAT needs to protect other customers using the same address by 

making sure that a single user of a shared address cannot trigger such a response.

Finally, a carrier-grade NAT needs to enforce ingress filtering of IPv6 traffic to prevent IPv6 spoofing.

6.6 Translation

Translation consists of transforming IPv4 or IPv6 packets into the other protocol so they can be routed or 

transmitted across a network. Network Address Translation—Protocol Translation (NAT-PT) allows 

IPv6 and IPv4 devices to communicate via an intermediate translation device. Transport Relay Translator 

(TRT) is another mechanism to allow IPv6 hosts to communicate with IPv4 hosts through an 

intermediary. Translation mechanisms (IPv4 to IPv6 and IPv6 to IPv4) introduce new methods to 

construct networks and systems and thus enlarge the set of possible attacks against those networks and 

systems. 

Protocol translation is not recommended as a strategic approach to transitioning from IPv4 to IPv6 for a 

variety of reasons. Translating IPv6 into IPv4 effectively negates most of the compelling reasons for 

transitioning to IPv6 in the first place. Hierarchical routing, expanded address space, streamlined packet 

headers, and IPv6 mobility are all lost when translating IPv6 into IPv4. Translation does not solve the 

IPv4 address space exhaustion problem. Nevertheless, an IPv6-only system cannot communicate with an 

 

195 IETF RFC 2663, IP Network Address Translator (NAT) Terminology and Considerations, is available at 

http://www.ietf.org/rfc/rfc2663.txt.

196 IETF RFC 2993, Architectural Implications of NAT, is available at http://www.ietf.org/rfc/rfc2993.txt.

197 Martin, D., S. Rajagopalan, and Aviel D. Rubin, ―Blocking Java Applets at the Firewall,‖ Proceedings of the Internet Society 

Symposium on Network and Distributed System Security, pp. 16–26, 1997.
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IPv4-only system unless translation occurs somewhere, so translation is necessary to keep isolated IPv4

legacy applications running in an otherwise IPv6 world.

Because protocol translation is not exact, but merely the best approximation of one protocol with another, 

the translation of header fields, addresses, extensions, options, fragmentation, and error reporting can be 

exploited to try to circumvent security policies. 

Translation mechanisms can work at different layers: 

 Network layer translators

- Stateless IP/ICMP Translation Algorithm (SIIT) (RFC 2765)

- NAT-PT (RFC 2766 and RFC 4966) 

- Bump in the Stack (BIS) (RFC 2767)

 Transport layer translators

- Transport Relay Translator (TRT) (RFC 3142)

 Application layer translators

- Bump in the API (BIA) (RFC 3338)

- SOCKS64 (RFC 3089)

- Application Level Gateways.

6.6.1 SIIT

SIIT stands for Stateless IP/ICMP Translation. It replaces headers, IPv4 for IPv6 and vice versa. It also 

translates ICMP messages and adds the ICMP pseudo-header checksum. If necessary, it fragments and 

reassembles IPv4 messages.

6.6.1.1 Using SIIT

SIIT uses IPv4-translated addresses to refer to IPv6-enabled nodes. When it needs to create an IPv6 

address from an IPv4 address w.x.y.z, it uses 0:0:FFFF:0:0:0:w.x.y.z. SIIT uses IPv4 mapped addresses 

to refer to IPv4-only nodes: 0:0:0:0:0:FFFF.w.x.y.z. SIIT thus requires IPv6 interfaces to obtain an IPv4 

address and is not a convenient transition mechanism for an IPv6 dominant network.

SIIT also adjusts the payload length appropriately and equates the following fields:

 IPv4 traffic class and IPv6 TOS

 IPv4 protocol number and IPv6 next header number

 IPv4 TTL and IPv6 hop limit. 

6.6.2 NAT-PT

NAT-PT stands for Network Address Translation—Protocol Translation. It uses SIIT and performs NAT 

traversal for IPv4. NAT-PT allows IPv6 systems and services to communicate with IPv4 systems and 
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vice versa. NAT-PT is the successor to IPv4 NAT, which is well understood and widely deployed.

6.6.2.1 Using NAT-PT

NAT-PT is controversial for may reasons, which are explained in detail in RFC 4966. The IETF 

recommends that it no longer be used, and discussions of how it may be revised are ongoing. Its biggest 

benefit is that IPv6 connectivity becomes possible for legacy applications that may never have IPv6 

support. The main objections to NAT-PT are that it reintroduces the scaling and operational issues of 

NAT into IPv6 and may make full migration to IPv6 more difficult. For example:

 It breaks end-to-end security: DNSSEC has to terminate on the NAT if it performs translation and 

IPsec cannot transit NAT-PT.

 It prevents full deployment of new applications.

 It may be a single point of failure and performance bottleneck.

 Protocols with embedded IP addresses do not work. 

 Protocols like RSVP that do not use port numbers cannot be multiplexed.

In addition, translation is incompatible with multicast, mobility, and multihoming. Where possible, a 

tunneling method such as Teredo or IPv6 tunnel broker may be preferable for coping with IPv4 NAT.

NAT-PT has three variations:

 Basic NAT-PT provides translation of IPv6 addresses to a pool of IPv4 addresses. It tracks and 

possibly alters IPv6 port numbers so that multiple IPv6 interfaces can share a single IPv4 address.

 NAPT-PT (Network Address Port Translator—Protocol Translator) is similar to NAT-PT, but it 

translates port numbers as well to avoid port number collisions that may break applications or even be 

exploited in an attack.

 DNS Application Level Gateway (DNS-ALG) is also specified, but gaps exist, a number of problems 

with it were indentified, the fixes are rather complicated, and it has not been widely used. The 

specifications are unclear about using NAT-PT without DNS-ALG.

6.6.2.2 Security Issues for NAT-PT

IPv4 NAT is often deployed as a security control, a function it was explicitly not intended to perform. 

This opinion is widely stated in IETF documents, and it continues to be a point of controversy. Whether 

deployed as a security device or to facilitate IPv4/IPv6 interoperability, NAT-PT has security 

implications. An Internet Draft198 documents security issues related to NAT-PT. This Internet Draft, 

although expired, contains lengthy descriptions of security considerations and some mitigation strategies 

that will influence follow-on work. These issues include:

 IPsec, in all modes except ESP tunnel mode, cannot be translated.

 If the translator attempts to reassemble fragments, this allows a DoS attack.

 

198 Expired Internet Draft, NAT-PT Security Considerations Concerns, is available at http://tools.ietf.org/html/draft-okazakiv6ops-natpt-security.
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 If the translator can be tricked into using a multicast address, this allows an amplified DoS attack.

 NAT-PT is vulnerable to DoS attacks that deplete its address pools.

 NAT-PT is incompatible with DNSSEC.

6.6.3 Replacing NAT-PT

The IETF is defining two complementary network layer translation mechanisms to replace NAT-PT and 

NAPT-PT. Five Internet Drafts199 defining these exist:

 Framework for IPv4/IPv6 Translation, draft-ietf-behave-v6v4-framework

 IP/ICMP Translation Algorithm, draft-ietf-behave-v6v4-xlate

 NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers, draft-ietfbehave-v6v4-xlate-stateful

 IPv6 Addressing of IPv4/IPv6 Translators, draft-ietf-behave-address-format

 DNS64: DNS extensions for Network Address Translation from IPv6 Clients to IPv4 Servers, draftietf-behave-dns64

The Framework recommends a dual stack coexistence and transition strategy and supports the transition 

plan in RFC 5211200, but it describes translation scenarios and guidelines for cases in which the dual stack 

approach may not be a practical medium-term coexistence plan. Two translation mechanisms are being 

defined because stateless and stateful translation have different characteristics, and each may be 

appropriate in certain settings. 

The scenarios differ according to whether or not the IPv4 and IPv6 sides of the translator are on the global 

Internet or on an enterprise or ISP’s network.

The mapping between IPv4 and IPv6 addresses is either maintained at the translator or conveyed by using 

IPv6 addresses with embedded IPv4 addresses. In the stateless translation mechanism, all address 

mapping is computed algorithmically. The stateful translator also uses algorithmic translation of IPv4 to 

IPv6 addresses, but it maintains a translation table from IPv6 addresses to, for example, a service 

provider’s pool of IPv4 addresses. Other header fields are treated as in SIIT.

Once the address mappings are defined, perhaps the trickiest aspect of translation is handling DNS 

correctly. Consider, for example, an IPv6-only client accessing an IPv4-only server. The client expects a 

AAAA record, but the server’s DNS knows nothing of this. A new DNS ALG is being defined to handle 

different possible configurations securely. Two plausible choices, depending on the particular scenario, 

are:

 For internal use, say a legacy printer pool, simply add the necessary static AAAA records.

 In the general case, the application asks for A and AAAA records, an A record is retrieved, and, 

assuming none exists, the ALG generates and adds the appropriate AAAA record. Resolvers have to 

be careful about caching such artificially generated records.

 

199 All five of these Internet Drafts are work in progress being developed in the IETF’s behave Working Group. To access the 

working group charter and email discussions, see http://www.ietf.org/dyn/wg/charter/behave-charter.html. To obtain the 

official status and revision history of these documents, see http://tools.ietf.org/wg/behave. To view presentation materials 

and minutes of discussions at recent meetings, follow the links under http://www.ietf.org/meeting/proceedings.html.

200 IETF RFC 5211, An Internet Transition Plan, is available at http://www.ietf.org/rfc/rfc5211.txt.
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Similar techniques with a few more wrinkles can be applied to make dynamic DNS work as well.

Some applications need special handling during translation. One good example is FTP passive mode, 

which works differently in IPv4 and IPv6. It is anticipated that a separate Internet Draft will be written to 

specify how to handle each such case as it arises. 

Two aspects of translation for future study are (1) handling multicast; and (2) sharing IPv4 addresses and 

port numbers for cases where these are in short supply. 

Security has been an important concern in this new design, and security considerations have influenced 

the design of aspects such as the address mappings. No other new security issues have arisen. 

6.6.4 TRT 

Transport Relay Translator (TRT) is defined by RFC 3142. Rather than simply rewriting headers, TRT 

keeps track of the state of TCP and UDP flows and relies on DNS translation between AAAA and A 

records. DNS translation is defined by RFC 2694201, DNS-ALG.

6.6.4.1 Using TRT

TRT works much like a proxy firewall, except that it can use IPv4 on one side and IPv6 on the other. The 

TRT establishes connections to the ―inside‖ and ―outside‖ nodes, and no IPv4 or IPv6 packets go through 

the TRT. The TRT does not translate DNS records; it handles DNS itself. It only works with TCP and 

UDP.

6.6.4.2 Security Issues for TRT

Security Considerations for TRT include:

 IPsec cannot traverse TRT.

 Because of the way TRT handles DNS, DNSSEC is incompatible with TRT.

 Protocols that base authentication on IP address (e.g., rsh, rlogin) do not work across TRT.

 The translator must retain state, so it is vulnerable to various DoS attacks.

 Attackers may use the translator to subvert address filtering or hide the true source of traffic.

6.6.5 Application Layer Translation

Application gateways provide application-specific translation, which is necessary when the application 

contains IP addresses. They are similar to the application gateways used in some firewalls. 

6.6.5.1 Using Application Layer Translation

Three generic approaches to building application gateways are:

 

201

 IETF RFC 2694, DNS extensions to Network Address Translators (DNS_ALG), http://www.ietf.org/rfc/rfc2694.txt
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 RFC 2767 covers Bump in the Stack (BIS). The translation takes place in an endpoint, so IPv4 

applications can run on an IPv6 host. The translation uses SIIT and maintains address mappings from 

an IPv4 address pool. BIS converts DNS AAAA records to A records.

 BUMP in the API is designed to provide the same capability as BIS, but the translation occurs 

between IPv4 and IPv6 APIs, that is between the sockets layer calls and TCP/IP implementation. It 

uses the same translation conventions as SIIT, but no actual IPv4 header exists. IPv4 DNS requests 

are translated into IPv6 requests. An artificial pool of internal IPv4 addresses is mapped to IPv6 

addresses.

 RFC 1928202 defines the Socksv5 protocol. Firewall support is part of this protocol's design. The 

SOCKS library is installed on the client between the application layer and the socket layer. It 

maintains an artificial pool of IPv4 addresses and keeps a mapping of these to domain names. 

SOCKS Gateways relay connections at the application layer. 

6.6.5.2 Security Considerations for Application Layer Translation

Application layer translation can offer good security, even if it is of limited value in transitioning to IPv6. 

Host-resident translation allows use of IPv6 security mechanisms, whereas gateway-resident translation 

breaks end-to-end connectivity. Gateways may be subjected to DoS, address spoofing, and open relay 

attacks if they do not use authentication. 

6.7 Other Transition Mechanisms

Dual stack operation, tunneling, and translation help to provide interoperability between IPv4 and IPv6 

nodes while the transition to IPv6 is taking place. Other possible methods exist:

 Host-based translation is possible by modification of application layer software or network layer 

protocols. This approach is not common, perhaps because if the changes needed for translation could 

be made, a dual stack system could be implemented just as simply instead. 

 VLANs or MPLS can carry IPv4 and IPv6 traffic. These approaches are viable ways to use Layer 2 

technology instead of Layer 3 encapsulation, potentially reducing overhead. A downside is that 

Layer 2 technology cannot be routed effectively. All of the security concerns remain, so it is just as 

important in this case to ensure that security policy applies equally for IPv4 and IPv6.

 SSL-TLS encapsulation of IPv6 has been implemented and is available as a secure tunneling 

technology. The advantage of this approach is good security between tunnel endpoints. The 

disadvantage is that many of the properties of IP are lost by forcing it to run over TCP, and typical 

SSL-TLS mechanisms are oriented towards client-server systems rather than peer-to-peer networking.

6.8 The IPv6 Deployment Planning Process for Security

The Office of Management and Budget (OMB) developed the Enterprise Architecture Assessment 

Framework203 to help organizations transitioning to IPv6. The deployment of IPv6-enabled equipment is 

a small part of the OMB framework. In the framework, most of the tasks deal with documenting the 

environment, communicating the transition both internally and externally, and making the strategic 

 

202

 IETF RFC 1928, SOCKS Protocol Version 5, http://www.ietf.org/rfc/rfc1928.txt.

203 OMB, Enterprise Architecture Assessment Framework, is available at 

http://www.whitehouse.gov/obm/memoranda/fy2005/m05-22.pdf.
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decisions. The framework outlines high-level tasks that should be accomplished during the transition 

effort:

 Conduct a requirements analysis to identify the current scope of IPv6 within an organization, current 

challenges using IPv4, and target requirements. 

 Develop a sequencing plan for IPv6 implementation, integrated with the organization’s Enterprise 

Architecture. 

 Develop IPv6-related policies and enforcement mechanisms. 

 Develop training material for stakeholders. 

 Develop and implement a test plan for IPv6 compatibility/interoperability. 

 Deploy IPv6 using a phased approach. 

 Maintain and monitor networks. 

 Update IPv6 requirements and target architecture on an ongoing basis.

The Architecture Assessment Framework can also be used to measure an organization’s progression from 

an IPv4 operational environment to an IPv6 operational environment. Although the framework is not a 

detailed transition plan, a transition plan should address each of the framework’s objectives.

6.9 IPv6 Deployment

This document discusses IPv6 deployment, rather than IPv6 transition. The basic assumption is that, for 

the foreseeable future, organizations will either operate dual stack networks or accommodate both IPv4 

and IPv6 networking through other means. However, the eventual goal is to transition to an IPv6 only 

network, or at least an IPv6-centric one. This section is written with that ultimate goal in mind.

This section covers the deployment of IPv6. The IPv6 deployment should follow the NIST guidelines for 

a secure information system life cycle204 and should include the following stages:

 Initiation Phase

 Acquisition/Development Phase

 Implementation Phase

 Operations/Maintenance Phase

 Disposition Phase.

The framework calls for a phased approach or a gradual transition from IPv4 to IPv6. The use of a 

phased implementation will enable an organization to implement IPv6 with as little disruption to the 

current environment as possible. Existing users should be unaware of the new protocol until they require 

its use. The phased approach will minimize the affect on day-to-day operations. There are two main 

approaches to transition deployment:

 Pervasive IPv6 deployment

 

204 See NIST SP 800-64, Security Considerations in the Information System Development Life, which is available at 

http://csrc.nist.gov/publications/nistpubs/800-64-Rev2/SP800-64-Revision2.pdf.
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 Sparse IPv6 deployment.

In a pervasive approach, the organization will enable dual (IPv4/IPv6) stack equipment rapidly 

throughout the entire enterprise. This scenario is appropriate when an organization has mostly new 

equipment that supports both IPv4 and IPv6. After the organization validates core services and 

translation mechanisms are functioning properly, IPv4 is disabled on all equipment, leaving an IPv6 

dominant network.

Edge to core describes a sparse IPv6 deployment. In this approach, organizations enable groups or 

islands of IPv6 equipment in an IPv4 dominant network. After most of the edge devices transition to 

IPv6, the network core transitions to either dual stack or IPv6 only. A sparse IPv6 deployment requires 

supporting both IPv4 and IPv6 traffic throughout the duration of the deployment life cycle. This 

approach will make extensive use of IPv4/IPv6 and IPv6/IPv4 tunneling. This scenario is appropriate 

when an organization has a large installed base of older equipment or services that cannot transition to 

IPv6.

Software or applications may be more important than equipment when selecting a transition approach. 

Upgrades for hardware and embedded operating systems can be quicker than custom or off the shelf 

applications. The hardware vendors have been working towards IPv6 support for longer than application 

software vendors have. Many vendors may not be able or willing to upgrade software to support IPv6 

and many organizations do not have the expertise in house to upgrade the code base. The more legacy 

applications and custom code an organization supports (either developed in house or highly customized 

off the shelf software) the greater the risk that the software will not support IPv6. Transition planners 

must address software in the approach to IPv6 transition.

The two main differences between an IPv6 pervasive deployment and an IPv6 sparse deployment are:

 The IPv6 pervasive deployment has a shorter lifecycle than an IPv6 sparse deployment. 

 An IPv6 sparse deployment will take longer and make use of tunneling mechanisms. 

Both deployment scenarios (IPv6 pervasive deployment and IPv6 sparse deployment) will be covered by 

the same general deployment plan. All phases of the lifecycle are the same regardless of approach.

6.9.1 Initiation Phase 

The initiation phase is concerned with requirements gathering. It is important for an organization to 

understand its current environment before deploying IPv6. By understanding the current environment, 

the correct transition approach can be selected, and an organization can ensure that it maintains security 

parity between its IPv4 and IPv6 environment. 

An organization must also begin to plan for new IPv6 features. The transition to IPv6 gives an 

organization an opportunity to fix problems in its current environment. IPv6 addressing will allow more 

opportunities to aggregate addressing and simplify routing and access control rules if requirements are 

collected early.

One of the key tasks in the initiation phase is to conduct an extensive inventory of the IP equipment and 

services. RFC 4057, IPv6 Enterprise Network Scenarios,

205 covers the types of questions that an 

organization needs to answer to plan a successful IPv6 transition. These questions are broken out by 

different scenarios, because each type of organization will have unique transition requirements.

 

205 RFC 4057, IPv6 Enterprise Network Scenarios, is available at http://www.ietf.org/rfc/rfc4057.txt.
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What all organizations will have in common are IPv4 assets that will require transition to IPv6. In an 

effort to understand the transition requirements, an organization must perform an asset discovery. While 

it is feasible that asset discovery could populate a configuration management database (CMDB), the 

primary purpose of asset discovery in an IPv6 transition plan is to gather transition requirements. 

Transition requirements determine which assets will transition to IPv6, the order assets will transition, the 

transition methods selected, and the security controls implemented. The type of information that can help 

determine if an asset or service will transition to IPv6, require some other coexistence mechanism, or be 

replaced include system security categorization, operating system and applications, lifecycle replacement, 

and dependencies.

Each information system should designate a security categorization. FIPS PUB 199, Standards for 

Security Categorization of Federal Information and Information Systems,

206 is the document that guides 

an organization in the determination of a system’s category. The category of the system will determine 

which security controls must be implanted to protect the system appropriately. The controls implemented 

for IPv6 will be similar to the controls implemented for IPv4. 

Most organizations have a large installed base of legacy equipment and applications. Many legacy 

systems cannot support IPv6. IPv6 is widely supported by network equipment vendors, and desktop 

systems and productivity applications generally support IPv6. Firewall and IDS implementations also 

have good support for IPv6. Where organizations have difficulties supporting IPv6 is with management 

applications, embedded systems, and legacy applications. Legacy applications that have implemented 

network protocols or process network addresses were not written to support IP agnostic addressing 

(IPv4/IPv6). There were no requirements for IPv6 address handling. These applications’ code assumes 

32-bit addressing. The application code logic and storage allocations work with IPv4 addressing. It will 

take a long time to update legacy applications to support IPv6. Applications that cannot support IPv6 will 

require the evaluation and selection of co-existence mechanisms. Organizations should plan that a 

significant number of legacy applications will not natively support IPv6 and include requirements to 

support accessing IPv4 only applications and service by IPv4 and IPv6 clients.

The information captured in this inventory should populate the organization’s IPv6 configuration 

management database and be maintained by configuration management. Configuration management data 

will not be perfect and will become out of date with the production environment. An organization must 

decide the acceptable level of accuracy needed to make good decisions and build processes to maintain 

the inventory data within the established parameters. 

The following decisions need to be made for each piece of equipment:

 Will the equipment be replaced to support IPv6?

 Can a service be upgraded to support IPv6?

 Will a translation mechanism be necessary?

The asset inventory is the main input into the organization’s decision process concerning the deployment 

scenario. If the inventory demonstrates mostly new equipment, an IPv6 pervasive scenario is available as 

a deployment option. On the other hand, if the inventory demonstrates mostly older equipment that 

cannot readily support IPv6, then an IPv6 sparse deployment scenario is the more likely choice. 

 

206 See NIST Federal Information Processing Standards Publication (FIPS) 199, Standards for Security Categorization of 

Federal Information and Information Systems, which is available at http://csrc.nist.gov/publications/fips/fips199/FIPS-PUB199-final.pdf.
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Organizations should considering the life cycle replacement of equipment in conjunction with the IPv6 

rollout schedule. When possible, the IPv6 rollout schedule should align with the lifecycle replacement to 

reduce costs of the overall transition. Organizations that do not align schedules should plan for the 

additional costs.

Other requirements subject areas are:

 Security

 Routing

 Network Management

 Host Configuration

 Address Planning

 Application Support and Development

 Performance and Bandwidth.

6.9.2 Acquisition / Development Phase

The acquisition and development phase is concerned with taking the requirements gathered during the 

initiation phase and developing the IPv6 enterprise architecture. When developing the IPv6 environment, 

the current enterprise architecture should be considered. The acquisition/development phase will work 

with three different architectures: the ―as is‖ IPv4 based enterprise architecture, the ―to be‖ IPv6, and the 

transitional architecture that bridges the ―as is‖ and ―to be‖.

During the development phase, an organization should plan for an IPv6 evaluation pilot. The goals of an 

IPv6 pilot are to test IPv6 configuration and design assumptions against existing equipment, test and 

evaluate new IPv6 equipment and begin training staff. The pilot is the time to test the IPv6 numbering 

plan and other design assumptions. It will be easier and less expensive to correct design deficiencies 

earlier in the transition lifecycle. As the requirements are evaluated against the existing equipment, 

inventory gaps may be discovered. It should be expected that some equipment will not make the 

transition; either it does not support IPv6 or will not meet performance expectations. The pilot should be 

used to evaluate new equipment and its ability to coexist in the ―to be‖ and transition architectures. The 

pilot is a great opportunity to allow technical staff to develop their IPv6 skills. A pilot affords the 

opportunity to combine classroom training with a hands-on environment.

IPv4 and IPv6 are different protocols; they behave differently on similarly configured equipment. The 

IPv6 pilot should include testing to validate the performance of IPv6 in the environment and to develop 

strategies to mitigate any problems. IPv6 generally performs worse on equipment that was designed for 

IPv4. This is mainly the result of IPv6 header manipulation in software instead of hardware. Other 

factors besides header size can degrade performance, including extension headers, large packet sizes, and 

differences in fragmentation characteristics. This performance difference can vary. 

The pilot should establish the baseline performance of IPv4 only, IPv6 only, and dual stack equipment 

and services. The baseline performance will allow the enterprise architects to understand the different 

performance characteristics and design capacity into the ―to be‖ and transition architectures. When 

possible the testing should use production configuration to reduce the variation between the production 

and the test environment.
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When establishing a baseline for security equipment, performance and coverage measures must be 

established. Security equipment evaluates traffic, which will introduce latency into the network and 

reduce network throughput. IPv6 security equipment tends to have fewer signatures available than 

equivalent IPv4 equipment which will reduce the level of coverage and potentially increase risk. Testing 

should be performed after establishing equivalent levels of coverage between IPv4 an IPv6. All security 

controls should be tested to include access control list, firewall rules, and IDPS signatures. The goal 

should be to design an IPv6 environment that is as secure as or more secure than the IPv4 environment.

Another area the pilot should address is the validation of tunnel performance. A mixed IPv4/IPv6 

environment will use tunnels to connect the islands of similar protocol equipment. The performance of 

these tunnels will affect the user experience, availability, and scalability of the network. Each transition 

mechanism will have different performance characteristics. The pilot will have the enterprise architecture 

team select the appropriate mechanism based on both functionality and performance. The testing of 

tunnels requires a realistic environment. 

The final area the pilot should address is baseline transition mechanisms. Translation is performed in 

software and will incur a performance penalty. 

In addition to developing the ―to be‖ and transition enterprise architectures and running the IPv6 pilot, the 

other activities of the acquisition and development phase include:

 Develop a risk assessment.

 Develop a sequencing plan for IPv6 implementation.

 Develop IPv6 related policies and enforcement mechanisms. 

 Develop training material for stakeholders. 

 Develop and implement a test plan for IPv6 compatibility/interoperability. 

Enterprise Architecture IPv6 will introduce new risks into the current environment. These risks can be 

the result of many different factors, including reduced security coverage, new IPv6 features, lack of IPv6 

experience, degraded performance, and dual operations. A formal risk assessment must be performed 

with a risk mitigation strategy to address the identified risks. The risk assessment will drive several other 

deliverables, including the security plan and certification and accreditation.

The transition to IPv6 offers an opportunity to implement restrictive ingress and egress firewall rules. 

Organizations have few IPv6 source or destination addresses that traverse the perimeter. Organizations 

should be able to implement a deny all permit by exception IPv6 firewall policy. In the future, when an 

external connection is required, either in bound or out bound, this requirement can be documented and 

allowed by exception.

As the transition enterprise architecture is developed, a transition plan must also be developed. The 

transition plan will detail which equipment and servers are to be transitioned and in which order. The 

transition plan will drive the project plan. Part of the transition plan will be the coexistence plan. The 

coexistence plan will document the type of transition mechanism used to connect islands of like protocol 

hosts and will also document the translation mechanisms for enterprise services. 

Existing policies must be reviewed to determine whether they provide adequate coverage for IPv6. New 

policies should be developed to address IPv6 specific areas or areas where IPv6 has a significant impact. 
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Senior leadership should support the new policies and sign off on them. Once policies are established,

standards, procedures, and guidelines can be developed to provide guidance for equipment configuration 

and operation. The procurement policies should be amended early in the transition to require that all new 

equipment introduced to the environment support IPv6. This requirement should include both equipment 

that is lifecycle replacement and equipment introducing new capabilities. In many cases, existing policies 

can simply be amended.

IPv6 will affect a broad range of support and operations personnel. During the acquisition and 

development phase, individual jobs should be evaluated, and training material should be developed or 

identified. IPv6 skills are not ready available in the job candidate pool, and organizations should expect 

to increase training budgets for IPv6 training for existing staff and new hires. 

Key to the success of the IPv6 transition will be proven connectivity and interoperability of the enterprise 

architecture. To ensure a successful transition, develop measures and test procedures for key services, 

applications, and capabilities. These measures and procedures collectively are the test plan. After a 

service, application, or capability is migrated, the test plan will validate that the IPv6 equivalent provides 

service equal to or better than the IPv4 service. 

At the conclusion of the acquisition and development phase, an organization should have produced the 

following artifacts:

 Enterprise Architecture

 Address Allocation Plan

 Address Management Plan

 Routing Plan

 Training Plan

 Security Plan

 Coexistence Plan.

The first design decision is to determine the organization’s IPv6 address allocation. The size of the 

address space required to support the deployment depends on the number of devices and how many 

networks those devices require For example, in an office scenario, a desktop would only need a single IP 

address, whereas an ISP supporting a cable modem would need four to eight IP addresses to support 

different services. 

At this point, the organization that is doing an IPv6 deployment should request the IP allocation from 

their regional internet registrar (RIR). 

The organization will also need to create an address management plan. The address management plan 

documents the following:

 How devices allocate an address (for example, clients could use autoconfiguration or DHCPv6)

 Which nodes will have fixed IP addresses 

 How to update DNS with address ranges.

The training plan documents the training requirements across the organization as related to the IPv6 
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deployment. Typically, core engineers, help desk support, application developers, the security team, and 

system administrators will need training. The training plan will include both the types of training and 

length of training required. Training should allow IT operations to support IPv6 in production along with 

their current job specifications. Poor security practices, misconfiguration, and operator error are some of 

the leading causes of system compromise and loss of availability. Increasing staff knowledge about 

security and the computing environment will increase security and availability.

The goal of the security plan is to ensure that the IPv6 environment has the same level of security or 

better than the existing IPv4 environment. It should document how an organization plans to maintain 

security parity during the IPv6 deployment. The security plan should address the following areas:

 Equipment configuration

 Perimeter defense (firewall, ACL, IDPS)

 Content filtering

 Mail filtering

 Patch management

 Vulnerability management (scanning)

 Certification and accreditation of the new systems

 AAA (authentication, auditing, and accounting)

 Rogue detection

 Infrastructure protocol security. 

The coexistence plan documents which mechanisms support IPv6/v4 internetworking. The coexistence 

plan should detail how IPv6 clients will access legacy IPv4 services (i.e., deployment of which translation 

mechanisms) and how existing IPv4 clients will access IPv6 services. By planning the coexistence 

mechanism in advance, an organization is able to leverage economy of scale and select technology that 

can be a repeatable solution. This will reduce the amount of required training and increase operator 

familiarity with the mechanisms.

6.9.3 Implementation Phase 

The implementation phase involves the secure installation and configuration of IPv6 equipment, tunnels, 

and translation mechanisms. The deployment stage differs depending on which deployment scenario is 

used (IPv6 pervasive deployment or IPv6 sparse deployment). In both scenarios, the actual IPv6 roll out 

will involve a phased deployment. The first three steps are the same regardless of deployment scenario. 

The steps for the IPv6 pervasive deployment are as follows:

1. Enable perimeter firewall IPv6 policies and IPv6 access control lists. Configure devices in 

accordance with security plan, standards, and procedures.

2. Deploy external IPv6 connectivity with exterior IPv6 routing.

3. Deploy basic IPv6 services (DNS, DHCPv6, and NTPv6).
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4. Deploy IPv6 interior routing.

5. Enable management monitoring (SMP, service monitoring, IDPS, authentication, statistical 

monitoring, and netflow).

6. Enable IPv6 hosts.

7. Deploy IPv6 to IPv4 translation mechanisms.

The steps for the IPv6 sparse deployment are as follows:

1. Enable perimeter firewall IPv6 policies and IPv6 access control lists. Configure devices in 

accordance with security plan, standards, and procedures.

2. Deploy external IPv6 connectivity with exterior IPv6 routing.

3. Deploy basic IPv6 services (DNS, DHCPv6, and NTPv6).

4. Enable dual protocols on core routers. (Optionally at this point, enable the dual core after 

completing Step 5).

5. For each IPv6 island:

a. Enable IPv6 on hosts. 

b. Enable transition mechanism (tunnels) to other IPv6 islands.

6. Enable management monitoring (SMP, service monitoring, IDPS, authentication, statistical 

monitoring, and netflow).

7. Deploy IPv4 to IPv6 translation mechanisms.

With Step 5 above, the process is iterative until all the hosts are either dual stack or IPv6 enabled. 

Organizations should have a change control board that enforces the change control process. The security 

manager should be a member of the change control board. An IPv6 transition should follow established 

change control processes. Once a system is transitioned, it should be accredited and certified using 

inspection and acceptance testing. The measures and test procedures developed during the acquisition 

and development phase are used to certify that the transition equipment performs in the environment as 

intended.

Validate IPv6 migrated equipment by inspecting the configuration and running test procedures before 

transitioning devices to production. Device configurations must comply with established security and 

operations standards. Device configuration settings can be validated using either manual inspection or 

automated inspection tools. When possible, the use of automated compliance management solutions will 

increase the accuracy and consistency of configuration compliance.

Transitioned equipment must integrate and interoperate with other production and migrated equipment 

and systems. Integration will validate that equipment is able to communicate with other systems and 

equipment and correctly exchange data. Checklists should be consulted that list the services with which a 

transitioned device must interoperate. Some examples include routing neighbors, DNS, DHCPv6, 

NTPv6, SNMP trap servers, syslog servers, mail servers, and application gateways. Integration and 

interoperations testing is difficult and usually performed manually.
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Transitioned equipment must perform at the levels established in the test plan. Performance testing is 

validated using traffic or load generators. Failure to comply with test plans could result in a loss of 

availability when the equipment is placed into production. 

IPv6 migration efforts will require the certification and accreditation of systems. The migration to IPv6 

(either IPv6 or dual stack) has the potential to significantly change the existing security posture. While 

the same security controls required for IPv4 are also required for IPv6, existing controls will require 

rework and reconfiguration to support IPv6, and new security controls are required to mitigate new IPv6 

vulnerabilities. Organizations should plan on certifying and accrediting systems that have been migrated 

to IPv6 and systems that interact with IPv6 systems. Organization should evaluate their certification 

support tools, techniques and procedures to ensure that these support IPv6. Organizations should ensure 

auditors performing the certification function are knowledgeable about IPv6 security and have the tools to 

look for IPv6 traffic and vulnerabilities. If two separate environments are supported (IPv4 and IPv6), 

then a system accreditor may require two separate certifications and accreditations; with a dual stack 

environment only a single certification and accreditation may be required. The goal should be to achieve 

security as good as or better than the IPv4 network.

6.9.4 Operations / Maintenance Phase

The operations phase often begins concurrently with the implementation phase. During operations, the 

focus is to operate securely a dual stack or mixed IPv6/IPv4 environment. One of the most difficult 

challenges facing the operations staff in a mixed IPv6/IPv4 environment will be keeping the two 

environments synchronized. 

When operations makes changes to security controls such as firewall rule sets, access control lists, and 

IDS signatures, they must ensure the change occurs on both the IPv4 and IPv6 networks. Translate rules 

and signatures between IPv4 and IPv6 syntax and deploy as a single coordinated process. If strict change 

control is not followed, the two environments will have non-overlapping protections. 

In a dual stack environment, the physical topologies of the equipment will be the same, but the logical 

topologies can be very different. Configuration changes can have unpredictable or unforeseen 

consequences. Structure configuration management controls to prevent configuration changes on IPv4 or 

IPv6 networks that affect the other network. Organizations should manage and monitor their IPv4, IPv6, 

and dual stack environment as a single environment. 

6.9.5 Disposition Phase

A migration from IPv4 to IPv6 will result in displacement or retirement of equipment. Some equipment 

will not support IPv6 and will be retired, while other equipment will be transferred to IPv4 islands or 

other organizations. Organizations must plan for the secure disposition of this influx of obsolete 

equipment, ensuring that no confidential data is released. Organizations place themselves at great risk for 

exposing confidential information when disposing of obsolete equipment.

A key decision concerning sanitization is whether the equipment is planned for reuse or retirement. 

Often, equipment is reused to conserve an organization’s resources. Equipment should be sanitized 

before it is reused or retired. Most organizations have policies, regulations or standards that require the 

proper disposition of obsolete equipment. It is important that these processes be followed.

Failure to follow proper disposition procedures could result in the disclosure of confidential information. 

Organizations store and process information such as personal records of employees, personal records of 

clients, credit card numbers, social security numbers, medical information, trade secrets, classified data, 
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system passwords, system configuration, network documentation, and many other examples of data that 

must be kept confidential. Network equipment can expose administrative accounts, passwords, 

authentication credentials, certificates, pre-shared keys, and community strings.

Failure to maintain confidentiality could result in embarrassment to the organization, system intrusions, 

software and hardware license violations, legal fees and fines, and other avoidable consequences. Some 

data requires protection under State law or Federal law or both (e.g., the Health Insurance and Portability 

and Accountability Act (HIPAA) and the Family Education Rights Privacy Act (FERPA)) or industry 

governance boards like Payment Card Industry Security Standards Council. 

There are several methods available for sanitizing storage media. Evaluate equipment carefully for the 

type of media employed for persistent information. Some equipment like personal computers employs a 

single persistent storage mechanism (hard drive); other equipment such as a router could employ multiple 

methods including hard drives, flash memory, EEPROM, and other non-volatile memory. It is important 

to address the sanitation of solid-state memory. The method of media destruction should insure the 

destruction of the information or media so that the information is unrecoverable. NIST SP 800-88, 

Guidelines for Media Sanitization,

207 provides information about the proper sanitation or destruction of 

equipment. SP 800-88 includes pointers to other resources such as the U.S. Department of Defense 

5220.22-M Clearing and Sanitization Matrix.208 

One overlooked aspect of disposition is the management of software and hardware license and service 

agreements. Licensing agreements often dictate the terms under which software or hardware are 

transferable. Once equipment is at end-of-life, it may be possible to remove that equipment from support 

agreements, potentially saving money.

Organizations should adopt a proactive approach to media sanitation. Sanitize equipment removed from 

the network before the installation of replacement equipment. This will reduce the window of 

vulnerability and will greatly decrease the risk of information disclosure. Installers should provide proof 

of sanitation, including a certificate of destruction with serial numbers and asset tags. 

6.10 Summary

Security risks are inherent with IPv6 but mitigation strategies exist and many of the residual risks are not 

different from existing IPv4 networks. 

IPsec is a major component of IPv6 security and is something organizations should deploy to secure IPv6 

networks. Transition mechanisms allow existing IPv4 networks to coexist and interoperate with IPv6 

networks, systems, and services. These transition mechanisms cover a wide range of technologies and 

transition scenarios. Organizations should plan their deployment and account for the full lifecycle of 

equipment from inception to disposal.

 

207 See NIST SP 800-88, Guidelines for Media Sanitization, which is available at http://csrc.nist.gov/publications/nistpubs/800-

88/NISTSP800-88_rev1.pdf.

208 See DOD 5220.22-M, Clearing and Sanitization Matrix, which is available at 

http://www.dtic.mil/whs/directives/corres/pdf/522022ms1front.pdf.
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Appendix A—Acronyms and Abbreviations

Selected acronyms and abbreviations used in the guide are defined below.

AAA Authentication, Authorization, and Accounting

AAAA Quad-A DNS Resource Record

ACL Access Control List

AES Advanced Encryption Standard

AH Authentication Header

AfriNIC Africa Network Information Centre

ALG Application Layer Gateway

API Application Program Interfaces

APNIC Asia Pacific Network Information Centre

ARIN American Registry of Internet Numbers

ARP Address Resolution Process

AS Automomous System

ATM Asynchronous Transfer Mode

BGP Border Gateway Protocol

BIA Bump in the API

BIS Bump in the Stack

BTNS Better Than Nothing Security

BU Binding Update

BUA Binding Acknowledgement

CGA Cryptographically Generated Addresses

CIDR Classless Inter-Domain Routing

CN Correspondent Node

CoA Care of Address

CVE Common Vulnerabilities and Exposures

DAD Duplicate IP Address Detection

DDoS Distributed Denial of Service

DES Data Encryption Standard

DHAAD Dynamic Home Agent Address Discovery

DHCP Dynamic Host Configuration Protocol

DHCPv6 Dynamic Host Configuration Protocol version 6

DNS Domain Name System

DNSKEY Domain Name System Key

DNSSEC DNS Security Extensions

DOI Domain of Interpretation

DoS Denial of Service

DS Delegation Signers

DS Field Differentiated Services Field

DSTM Dual Stack Transition Mechanism

DUID DHCP Unique Identifier

EAP Extensible Authentication Protocol

EGP Exterior Gateway Protocols

EH Extension Header
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EIGRP Enhanced Interior Gateway Routing Protocol

ESN Extended Sequence Number

ESP Encryption Security Payload

EUI-64 Extended Unique Identifier 64 bit

FQDN Fully Qualified Domain Name

FTP File Transfer Protocol

GAO Government Accountability Office

GSEC Group Security Research Group

GRE Generic Routing Encapsulation

GTSM Generalized TTL Security Mechanism

HA Home Agent

HIP Host Identity Protocol

HMAC Hashed Message Authentication Code

HoA Home Address

HTML HyperText Markup Language

HTTP HyperText Transfer Protocol

IANA Internet Assigned Number Authority

ICANN Internet Corporation for Assigned Names and Numbers

ICMP Internet Control Message Protocol

ICMPv6 Internet Control Message Protocol version 6

ICSA International Computer Security Association

ID Identification

IDPS Intrusion Detection and Prevention System

IDS Intrusion Detection System

IETF Internet Engineering Task Force

IGP Interior Gateway Protocol

IID Interface Identifier

IKE Internet Key Exchange

IKEv1 Internet Key Exchange version 1

IKEv2 Internet Key Exchange version 2

IP Internet Protocol

IPComp IP Payload Compression Protocol

IPng Internet Protocol Next Generation

IPv4 Internet Protocol version 4

IPv6 Internet Protocol version 6

IPS Intrusion Prevention System

IPsec Internet Protocol Security

IPX Internetwork Packet Exchange

IRTF Internet Research Task Force

ISAKMP Internet Security Association and Key Management Protocol

ISATAP Intra-Site Automatic Tunnel Addressing Protocol

ISP Internet Service Providers

IS-IS Intermediate System to Intermediate System

KCN Key CN

KBN Key BN

KINK Kerberized Internet Negotiation of Keys
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LACNIC Latin America and the Caribbean Network Information Centre

LIR Local Internet Registries

LAN Local Area Network

MAC Media Access Control

MD5 Message-Digest algorithm 5

MH Mobility Header

MIPv4 Mobile Internet Protocol version 4

MIPv6 Mobile Internet Protocol version 6

MLD Multicast Listener Discovery

MLDv2 Multicast Listener Discovery version 2

MN Mobile Node

MOBIKE IKEv2 Mobility and Multihoming Protocol

MPA Mobile Prefix Advertisement

MPLS Multiprotocol Label Switching

MPS Mobile Prefix Solicitation

MSDP Multi-source Discovery Protocol

MSEC Multicast Security

MTU Maximum Transmission Unit

MX Mail Exchange

NA Neighbor Advertisement

NAT Network Address Translation

NAT-PT Network Address Translation—Protocol Translation

ND Neighbor Discovery

NH Next Header

NIS Network Information Service

NIST National Institute of Standards and Technology

NS Name Server

NS Neighbor Solicitation

NSA National Security Agency

NSEC Next Secure

NTP Network Time Protocol

NUD Neighbor Unreachability Detection

OMB Office of Management and Budget

OS Operating System

OSI Open Systems Interconnection

OSPF Open Shortest Path First

OSPFv2 Open Shortest Path First version 2

OSPFv3 Open Shortest Path First version 3

PA Provider Assigned

PAD Peer Authorization Database

PDA Personal Digital Assistant

PIM Protocol Independent Multicast

PIM-SM Protocol Independent Multicast—Sparse Mode

PKCS Public Key Cryptography Standard

PKI Public Key Infrastructure

PMTU Path Maximum Transmission Unit
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PPP Point-to-Point Protocol

POA&M Plan of Action and Milestones

QoS Quality of Service

RA Router Advertisement

RADIUS Remote Authentication Dial-In User Service

RFC Request for Comment

RIP Routing Information Protocol

RIPng Routing Information Protocol next generation

RIPv2 Routing Information Protocol version 2

RIPE NCC Réseaux IP Européens Network Coordination Centre

RIR Regional Internet Registries

RPF Reverse Path Forwarding

RR Resource Record

RRSIG Resource Record Signature

RS Router Solicitation

RSVP Resource Reservation Protocol

SA Security Association

SAD Security Association Database

SHA Secure Hash Algorithm

SEND Secure Neighbor Discovery

SCTP Stream Control Transmission Protocol

SHA-1 Secure Hash Standard 1

SHIM6 Site Multihoming by IPv6 Intermediation

SIIT Stateless IP/ICMP Translation Algorithm

SMTP Simple Mail Transfer Protocol

SNTP Simple Network Time Protocol

SOCKS Sockets

SOCKS64 Sockets 64-bit

SP Special Publication

SPD Security Policy Database

SSH Secure Shell

SSL Secure Sockets Layer

TCP Transmission Control Protocol

TCP/IP Transmission Control Protocol/Internet Protocol

TKEY Transaction Key

TLD Top Level Domain servers

TLS Transport Layer Security

ToS Type of Service

TRT Transport Relay Translator

TSIG Transaction Signature

TTL Time to Live

US-CERT United States Computer Emergency Readiness Team

UDP User Datagram Protocol

ULA Unique Local Address

ULP Upper Layer Protocols
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VLAN Virtual LAN

VoIP Voice over Internet Protocol

VPN Virtual Private Network

WiFi Wireless Fidelity

3G Third Generation Wireless Technologies

3GPP2 Third Generation Partnership Project 2
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Appendix B—Resources

The lists below provide examples of resources that may be helpful. 

TBD