IPv4 Address Sharing Technology in the IPv6 Era

We examine the characteristics of “stateful” and “stateless” address sharing technologies, and discuss thecurrent state of IPv4 address sharing technology based on our experience with experimental implementation of
4rd using SEIL*1 series routers that we presented in IIR Vol.13*2.

Network Technology

3. Network Technology

3.1 A Post IPv4 Address Exhaustion WorldAs has already been announced, the pool of IPv4 addresses at the IANA (Internet Assigned Numbers Authority)*3 wasexhausted on February 3, 2011. Following this, addresses at the Asian RIR (Regional Internet Registry) APNIC were exhausted
on April 15, 2011. The measures suggested by JPNIC*4 in response to this consist of the following three pillars:
1. Promote the efficient use of allocated IPv4 addresses
2. Use NAT technology to accommodate new hosts without using global addresses
3. Implement IPv6 to accommodate new hosts
The third measure, migration to IPv6, is considered the ultimate solution. However, most business is currently conducted
over IPv4, so the first and second measures are urgently required. Regarding the first measure, address space is being
actively consolidated at organizations with a large number of addresses, and address blocks of a certain size can now betransferred between organizations. In cases where a global address is really required, it is likely that this framework will beused. This will require more time and money than in the past, but once an address is acquired there is little difference. Forclient use it may be possible to connect to the Internet while keeping consumption of global addresses to a minimum byapplying the second measure.

3.2 Stateful vs. Stateless
CGN/LSN*5 is the oldest form of IPv4 large-scale address sharing technology. With CGN/LSN an ISP can minimize IPv4
global address consumption by allocating private addresses after NAT conversion, instead of directly allocating IPv4 globaladdresses. The IPv4 access network-based “NAT444*6” and the IPv6 access network-based “DS-Lite*7” are becoming
accepted as standard CGN/LSN usage models.
A number of “stateless” approaches to this CGN/LSN group of technologies have been proposed, and are discussed ona regular basis. I am sure many of you have heard of the broad categorization of methods as either stateful or stateless.
Currently a large number of stateless methods have been proposed, and although the details of standardization have still notbeen hammered out, some form of standard will no doubt be agreed upon.
In this report we examine the characteristics of stateful methods including CGN/LSN and stateless methods such as 4rdbased on experience gained through experimental implementation of 4rd using SEIL series routers that we presented in IIR
Vol.13. We also discuss the current state of IPv4 address sharing technology from the perspective of a developer of routers.

*1

Portal site for the “SEIL” high-performance routers developed by IIJ using its expertise as an ISP (http://www.seil.jp/) (in Japanese).

*2

IIR Vol.13 “Internet Topics: 4rd Proof-of-Concept Tests for IIJ’s Proprietary “SEIL” Routers” (http://www.iij.ad.jp/en/company/development/iir/pdf/iir_vol13_topic_EN.pdf).

*3

A faculty for managing and regulating the Internet resources operated by ICANN.

*4“Regarding the exhaustion of IPv4 addresses” (http://www.nic.ad.jp/ja/ip/ipv4pool/) (in Japanese).

*5

CGN = Carrier Grade NAT, LSN = Large Scale NAT. A system for conducting the NAT conversion of IPv4 addresses at an ISP. CGN and LSN are two separate*6“NAT444 addressing models draft-shirasaki-nat444-isp-shared-addr-07” (http://tools.ietf.org/html/draft-shirasaki-nat444-isp-shared-addr-07).

*7“Dual-Stack Lite Broadband Deployments Following IPv4 Exhaustion” (http://tools.ietf.org/html/rfc6333).

terms that refer to the same technology.

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Network Technology

3.3 An Attempt to Compare Methods
When comparing IPv4 address sharing technologies, methods are often categorized as either stateful or stateless. However,these categories are not always sufficient when comparing the details of each technology. For this reason it is common toalso take into account the following factors when comparing technologies in detail.
• Where will the NAT device be placed?
• How will communicating entities be identified?
• What communication method will be used between customers?
• What packet format will be used?

3.3.1 Considering NAT Device Placement
NAT device placement (Figure 1) is the closest measure for the classification of stateless and stateful methods. NAT devices
can be placed on the ISP side, on the customer's network, or both. When NAT devices are placed on the ISP side, the ISP must
manage a vast amount of shared resources, but they are able to exert fine control. Placing the NAT device on the customer's
network simplifies resource management for the ISP, but at the same time reduces flexibility.

3.3.2 Considering Methods for Identifying Communicating Entities
When unauthorized access is detected, it is important for ISPs and server administrators to be able to identify the entity
causing the issue from logs. When address sharing is used the communicating entity cannot be identified simply from IP
address records, so a system that makes it possible to track who was using a specific port number is required.
When using a stateful method NAT session information is required for this process, while for a stateless method static
allocation rules are all that is needed. This is one of the most significant differences between the two methods.

3.3.3 Considering Methods for Communications Between Customers
The methods available for communications between customers are of particular importance for P2P communications such asIP telephony. P2P communications technology is inherently incompatible with NAT, but its use has now been made practicaldue to various NAT bridging technology. If this existing NAT bridging technology can continue to be used, we can expect
communications to become more efficient. Network usage efficiency and user comfort will change significantly based onwhether a remote conference application operates in P2P communications mode or via an intermediate server.

3.3.4 Considering Packet Format
Broadly speaking, packet format options include transferring IPv4 packets as-is over an IPv4 network, translating them toIPv6 for transfer over an IPv6 network, or encapsulating them into IPv6 packets for transfer over an IPv6 network. When IPv6
is used, even if IPv4 addresses overlap, packets can be delivered to the right network based on IPv6 address information. This
allows for more flexible placement of NAT devices.

NAT
Information

ISP Equipment(Stateless Mapping Device)
Static Resource
Allocation Rules(Mapping Rules)

NAT(SharedResource)
Existing
NAT

ISP Equipment(NAT Device)
Customer
Equipment

NAT(AllocatedResource)

Existing
NAT

Allocated
Allocated
Resource Customer
Customer Resource
Equipment
Equipment
Shared Resource:
Address + Port

Customer
Equipment
Shared Resource:Address + Port

TerminalTerminal

Stateful Method

Figure 1: NAT Device Placement

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Stateless MethodWe have selected NAT444 and DS-Lite for comparison as examples of the address sharing methods categorized as stateful(Table 1).

3.4.1 Stateful Method Example: DS-Lite
DS-Lite is a technology based on CGN/LSM that uses an IPv6 access network. An ISP's CGN/LSN device and the CPE areNetwork Technology

3.4 An Overview of Stateful Methods

connected using IPv4 over IPv6 tunneling. NAT conversion including the IPv6 global address is carried out as shown inTable 2.
Packets transferred from the CPE to the access network are encapsulated using IPv6, so there is no need for NAT at the CPE.
Packets returned from the Internet are encapsulated inside IPv6 packets using the IPv6 global address recorded in the NAT
session. The CPE can be identified by the IPv6 address, so the private address for the CPE can overlap within the ISP.

3.5 An Overview of the Stateless Method
Currently a large number of stateless methods have been proposed in parallel, and it is not known which will ultimately
emerge as a standard. Here we examine the methods touched upon at IETF 82 in November 2011 (Table 3).

Table 1: Address Sharing Methods (NAT444, DS-Lite)
MethodAccess Network

NAT444

IPv4DS-Lite

IPv6

NAT

Identification of Communicating Entity Communication Between Customers

ISP, customer
ISP

Packet Format

Referencing NAT session information

Via ISP device*8

IPv4 native

Referencing NAT session information

Via ISP device*9

IPv4 over IPv6 tunnel*10

Table 2: NAT Conversion
Private Side (CPE)

Global Side (IPv4 Internet)

IPv6 global address

IPv4 global address

IPv4 private address

Port number

Port number

Table 3: Stateless Methods
Access Network

NAT4rd-E*11MethodIPv6Customer

Static rules • Prefix × 2• Integer × 1

Connection possible

IPv4 over IPv6 tunnel

4rd-U*12

IPv6Customer

Static rules • Prefix × 2• Integer × 1

Connection possible

IPV4 - IPv6 double translation w/
IPv6 option header

SA46T-AS*13

IPv6Customer

Static rules • Integer × 2

Connection possible

IPv4 over IPv6 tunneldIVI*14

IPv6Customer

Static rules • Integer × 2

Connection possible

IPv4-IPv6 double translation

IPv4 IPv6

Customer (ISP)

Static rules • Address × 2• Integer × 2

Via ISP deviceIPv4 nativeIPv4 over IPv6 tunnel

IPv6Customer

Static rules • dIVI compliant

Via ISP device*17

IPv4 over IPv6 tunnel

SD-NAT*15
Stateless 4over6*16

Identification of Communicating Entity Communication Between Customers*8However, it is difficult to apply existing NAT bridging technology because multiple NAT layers are used.

*9

Extended specifications are also being standardized.

*10

A specific packet format is not assumed, but in typical cases a simple IPv4 over IPv6 tunnel is used.

*11

http://tools.ietf.org/html/draft-murakami-softwire-4rd-01*12

http://tools.ietf.org/html/draft-despres-softwire-4rd-u-02*13

http://tools.ietf.org/html/draft-matsuhira-sa46t-as-02*14

http://tools.ietf.org/html/draft-xli-behave-divi-04*15

http://tools.ietf.org/html/draft-penno-softwire-sdnat-01*16

http://tools.ietf.org/html/draft-sun-softwire-stateless-4over6-00*17

A route optimization device can also be added.

Packet Format

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Network Technology

Stateless methods identify communicating entities by referencing rules statically determined by the ISP. This reduces
operational costs because a record of NAT sessions is not needed. Currently the various methods are differentiated by theirapproach to defining rules, but there have also been moves to create a standard that is separate from individual methods.

3.5.1 Stateless Method Example: 4rd (4rd-E)
Here we take a quick look at the specific behavior of system operation using 4rd (4rd-E), stateless method, which is currently
undergoing experimental implementation via SEIL.
When using 4rd, the CPE (CE in 4rd terms) is connected to an IPv6 access network, and an IPv6 global address is assigned. Atthe same time, the CPE is also assigned an IPv4 global address and a range of port numbers available for IPv4 communications.
A customer identifier (EA-bits) is embedded in part of the IPv6 address that is assigned. Similarly, an identifier that covers
the IPv4 global address and port numbers is also embedded. The static relationship between the identifier (EA-bits), the IPv6
global address, the IPv4 global address, and the port numbers serves as the mapping rules for 4rd. The IPv4 global address
may be shared between different CPE, but when combined with the port numbers each unique CPE can be identified.
It is generally not possible to select a route by referencing port numbers on an IPv4 network. For this reason, IPv6 is used forpacket delivery over 4rd. As mentioned above there is a static relationship between the CPE's combination of IPv4 address
and port numbers and the IPv6 address, so by encapsulating packets using the post-conversion IPv6 address it is possible toperform route selection based on IPv4 packet port numbers.
n NAT with 4rd
An IPv4 global address and port numbers are allocated to the CPE. NAT (NAPT) is used when sharing these resources
between CPE terminals. Unlike standard IPv4 NAT, there are restrictions on the port numbers that can be allocated. Other
than this point, there are no differences with existing NAT.
n Server Log Analysis
To identify the communicating entity from the IP address and port numbers recorded on the server, the mapping rules
and IPv6 allocation information are required. The former is a set value unique to the access network, and does not change
dynamically. The latter can be determined by referring to the connection log for the access network. Connection logs can be
managed using the same system as current IPv4 connection logs.

3.6 Conclusion
Here I have given a brief explanation of the distinguishing characteristics of IPv4 address sharing technology. There are likely
to be many changes due to forthcoming standardization, but I hope that this report provides some food for thought regarding
future IPv4 environments.

Author:Hiroki Suenaga
Technical Manager, Product Technology Section, Product Development Department, IIJ SEIL Business Unit. Mr. Suenaga joined IIJ in 2004, and sincethen has been engaged in the development of the SEIL series and SMFv2. He is also involved in experimental implementation and proof-of-concept
tests for 4rd, as part of research and development for future NGN.

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