Patent Publication Number: US-7917948-B2

Title: Method and apparatus for dynamically securing voice and other delay-sensitive network traffic

Description:
BENEFIT CLAIM 
     This application claims the benefit as a Divisional of prior application Ser. No. 10/305,762, filed Nov. 27, 2002, now U.S. Pat. No. 7,366,894, which is a continuation-in-part of application Ser. No. 10/247,695, filed Sep. 18, 2002, now U.S. Pat. No. 7,447,901 which claimed the benefit of provisional application 60/391,745, filed Jun. 25, 2002, the entire contents of which are hereby incorporated by reference in their entirety for all purposes as if fully set forth herein. The applicant(s) hereby rescind any disclaimer of claim scope in the parent application(s) or the prosecution history thereof and advise the USPTO that the claims in this application may be broader than any claim in the parent application(s). 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to communication networks. The invention relates more specifically to a method and apparatus for dynamically securing voice and other delay-sensitive network traffic. 
     BACKGROUND OF THE INVENTION 
     The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. 
     Virtual Private Network 
     A virtual private network (VPN) is a private data network that makes use of the public packet-switched telecommunication infrastructure, maintaining privacy through the use of a tunneling protocol, such as GRE, and encryption or security protocols, such as IPsec. A virtual private network can be contrasted with a system of privately owned or leased lines that can only be used by one organization or entity. VPNs give an organization the same capabilities at much lower cost by using the shared public infrastructure rather than a private one. 
     Tunneling, and the use of a VPN, is not intended as a substitute for encryption/decryption. In cases where a high level of security is necessary, other encryption should be used within the VPN itself. 
     IPsec Protocol 
     The IPsec protocol and related protocols such as IKE and ISAKMP (collectively referred to as IPsec) provides a standards-based method of providing privacy, integrity, and authenticity to information transferred point-to-point among peers across IP networks, such as the public Internet and private local networks. IPsec provides IP network-layer encryption. That is, it provides security at the packet-processing layer of network communication. 
     IPsec defines formats of packet headers to be added to IP packets, including the authentication header (AH) to provide data integrity and the encapsulating security payload (ESP) to provide confidentiality and data integrity. Furthermore, key management and security associations are negotiated with the Internet Key Exchange (IKE). A security association (SA) is a set of IPsec parameters between two devices. Because the encrypted packets appear to be ordinary packets, they can easily be routed through any IP network without changes to the intermediate network equipment. 
     Several papers on various aspects of IPsec are available at the time of writing, and can be located via the document “ipsec.html” in directory “ids.by.wg” of domain “ietf.org”. In addition, numerous RFCs (Request For Comment) are available from the Network Working Group of the IETF (Internet Engineering Task Force), and can be located via the document “rfc.html” of domain “ietf.org”. 
     IPsec provides two modes of operation: transport mode and tunnel mode. In transport mode, only the IP payload is encrypted, with the original IP headers left intact. This mode adds minimal bytes to each packet. In tunnel mode, the entire original IP packet is encrypted and it becomes the payload in a new IP packet. This allows a network device, such as a router or gateway, to act as an IPsec proxy and perform encryption on behalf of the hosts. The source router or gateway encrypts packets and forwards them along the IPsec tunnel, and the destination router or gateway decrypts the original packet and forwards it to the destination host. 
     Hub and Spoke Network Architecture 
     Currently IPsec VPN networks are established using point-to-point links among routers or switches that participate in the VPNs. This is a natural way to set up encrypted networks since encryption involves establishing a shared secret between the two endpoints so that each end can decrypt what the other end has encrypted. The most efficient way to manage larger and larger collections of these point-to-point links is to arrange them into hub-and-spoke networks. 
     In hub-and-spoke networks, all traffic from behind one spoke to behind another spoke traverses first to the hub and then back out to the other spoke. Thus, packet latency is increased because all network traffic between end points is routed through the hub. Furthermore, secure traffic is encrypted and decrypted twice: first, between the source spoke and the hub; and second, between the hub and the destination spoke. This is because encryption/decryption keys must be exchanged between only two points. Hence, this architecture causes increased load on the hub router, which is required to perform many encryption operations. 
     Multicasting is communication between a single source and selected multiple destinations on a network. Teleconferencing and videoconferencing, for example, are technologies that may utilize multicasting protocols. Broadcasting is communication that is simultaneously transmitted from a source to all destinations on a network. IPsec does not readily support IP multicast or broadcast packets, due to challenges with managing the encryption keys associated with IPsec secure associations with respect to such packets. 
     Since IPsec does not readily support broadcasting of IP packets, it also does not support any interior dynamic routing protocol (e.g., RIP, OSPF, EIGRP), since these protocols rely on broadcasting/multicasting for their operation. Thus, currently all routing of packets over an IPsec VPN utilizes static routing. Consequently, any time there is a change, addition or removal of equipment in the network, routing information must be updated manually, which is not manageable in a large VPN network. 
     One technique to overcome the above multicast/broadcast restriction is to use another tunneling protocol such as GRE to first tunnel the IP data packets, including multicast/broadcast packets, and then use IPsec to encrypt (transport mode) the GRE encapsulated packets. This technique, therefore, allows the support of dynamic routing protocols and IP multicast over the VPN network. However, this technique requires the hub router to know the IP address of all the spoke routers, since the GRE tunnel endpoints are configured manually. Often, the spoke routers are connected to network via DSL or cable modem links. It is typical for such routers to be assigned an IP address dynamically, that is, each time they reboot or reload. Implementing a network in which the hub router knows the IP address of all the spoke routers increases costs significantly since the spoke routers need to have static IP addresses. Furthermore, the hub router needs to be larger with respect to, for example, configuration information and computational capability, since it will be one endpoint of all the point-to-point links and is in the path for all spoke-to-spoke traffic. 
     Full Mesh Network Architecture 
     A typical approach to solving the foregoing shortcomings of having a single hub router utilizes a static full-mesh VPN network architecture. In full-mesh architecture, each router or switch has a link to every other router or switch in the VPN. However, a static full-mesh network requires all nodes in the network to be configured with information about all other nodes in the network. The resulting configuration files are large and diffulcult to manage. Also, all nodes must set up VPN point-to-point links with all other nodes in the network by negotiating encryption keys, which are maintained at all times whether they are needed or not. 
     Currently, the maximum size of IPsec full mesh networks is limited by the number of simultaneous IPsec tunnels that must be supported on each node in the mesh. In practice, the limiting factor is the number of tunnels that can be supported by the smallest hardware platform used in the mesh. An additional problem is the size of the routter configuration files for mesh networks, and the size of the hub router in hub-and-spoke networks. In both cases, each configuration must include numerous lines per tunnel for defining crypto-maps, access control lists (ACLs), and definitions of tunnel interfaces for GRE tunnels. As the number of peers gets large, the configuration becomes huge. 
     Hence, instead of having &lt;n&gt; IPsec VPN links to connect &lt;n&gt; remote sites, there are &lt;(n 2 −n)/2&gt; IPsec VPN links to connect &lt;n&gt; remote sites. To support this architecture, all routers in the VPN network must be as large, in terms of processing power and storage capacity, as the hub router in the hub-and-spoke network, since all nodes must be the end point for &lt;n&gt; links. This significantly increases the cost to deploy the IPsec VPN network. Furthermore, the complexity of the IPsec VPN network increases dramatically, which decreases the manageability of the VPN network significantly. Also, when adding a new node to the full-mesh VPN network, all other nodes in the network must also be modified, that is, they need to be reconfigured to add information regarding connecting to the new node. 
     Network Configuration and Performance-General 
     Generally, performing encryption of any form is a CPU-intensive process and introduces latency into transmission of the traffic. Network latency represents the amount of time it takes a bit of information to travel a network link. Jitter represents the change in network latency over time and is typically measured over short periods of time. 
     Latency, and likewise, jitter, affect the performance of real time network applications. Most data traffic is typically tolerant to latency and jitter introduced into a packet-switched network, such as the Internet. For some applications, such as voice and video, packets need to arrive at their destination within a certain timeframe or they become useless. As a result, many voice and streaming audio and video applications can be greatly affected by significant jitter. 
     No prior approach to securing the transport of non-data traffic such as voice traffic is known to exist. Hence, based on the foregoing, there is a clear need for a technique for securing the transport of delay-sensitive IP traffic, such as voice traffic, which reduces the latency and CPU overhead associated with existing IP security approaches. 
     A Fully Meshed network configuration requires provisioning and maintaining a separate virtual circuit for each of the possible connections between all network end nodes. The number of virtual circuits required in a fully meshed network of N nodes is [(N*(N−1))/2]. Therefore, provisioning a public network such as the Internet in a fully meshed configuration for secure voice traffic is certainly not a practical endeavor, and may not even be feasible, due to the myriad of end nodes that would require such provisioning. 
     A Hub and Spoke configuration is more scalable than a Fully Meshed configuration, requiring only (N−1) virtual circuits. However, this configuration is not suitable for delay-sensitive traffic between spokes because of the additional hop to the hub that is required for all traffic between endpoints. 
     Based on the foregoing, there is a clear need for a technique for securing the transport of delay-sensitive IP traffic, such as voice traffic, which minimizes provisioning and maintenance overhead and is highly scalable. 
     One method of securing delay-sensitive network traffic, such as voice and video, includes efforts to change the current standard signaling and transport protocols used for voice and multimedia traffic, to include integrated security mechanisms for these types of network traffic. For example, the standards boards could be solicited to implement encryption fields in the relevant protocols. This is not considered an optimum method, especially in view of the re-provisioning that the existing network infrastructure would be expected to undergo. 
     Signaling Protocols 
     In order to provide IP telephony and multimedia capabilities in a network, a signaling protocol and signaling information are needed to perform various functions for the endpoint devices, such as session establishment, negotiation, and termination. Several protocols exist which provide the foregoing functions, for example, ITU H.323, IETF SIP (Session Initiation Protocol), IETF MGCP (Media Gateway Control Protocol), and their respective associated protocols. For example, each SIP request consists of a set of header fields that describe the call as a whole, followed by a message body that describes the individual media sessions (e.g., audio video) that make up the call, and utilizes a request/response process to establish, negotiate, and tear down a session. The actual real-time media is typically exchanged between participants using a suitable protocol, such as RTP (Real-Time Transport Protocol) for real-time media or RTSP (Real-Time Streaming Protocol) for stored media. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
         FIG. 1  is a block diagram illustrating an example of an operational environment in which an embodiment may be implemented; 
         FIG. 2  is a block diagram of an example network in which a dynamic multipoint encrypted virtual private network may be established; 
         FIG. 3  is a block diagram of a hub router illustrating elements involved in communicating data on multipoint tunnels using IPsec encryption; 
         FIG. 4  is a flow diagram illustrating a high-level view of a process for establishing and using dynamic multipoint encrypted virtual private networks; 
         FIG. 5  is a flowchart illustrating a process for dynamically securing delay-sensitive network traffic; 
         FIG. 6  is a flowchart illustrating a process for tearing down a secure virtual circuit; and 
         FIG. 7  is a block diagram that illustrates a computer system upon which an embodiment may be implemented. 
     
    
    
     DETAILED DESCRIPTION 
     A method and apparatus for dynamically securing delay-sensitive network traffic are described. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present invention. 
     Overview 
     According to one aspect of the invention, a request for secure network traffic is received from a device having a private network address at a source node. The private network address of a requested destination device at a destination node is obtained from a route server, based on signaling information associated with the request. The public network address of the destination node associated with the private network address is obtained. 
     In one embodiment, the public network address of the destination node is obtained from a next-hop server. In another embodiment, the public network address for the destination node is obtained from a cache at the source node. In still another embodiment, the public network address for the destination node is obtained from a call setup signal, such as a multimedia call setup signal conforming to the H.225 signaling protocol. 
     Furthermore, in response to the request, a virtual circuit between the source node and the destination node is created based on a mapping of the public network address of the destination node and an associated virtual tunnel address. Network traffic is encrypted for transporting at least from the source node to the destination node through the virtual circuit. In one embodiment, the encryption process utilizes the IPsec Protocol. 
     The process is dynamic in that a virtual circuit is created in response to a request. Hence, the process operates as if a fully meshed network exists, but requires less provisioning and maintenance than a fully meshed network architecture. Furthermore, the process is readily scalable, as if a hub and spoke network exists, but is more suitable for delay-sensitive traffic than a hub and spoke network architecture. 
     In one embodiment, the step of creating the virtual circuit comprises the steps of encapsulating a payload packet of the network traffic with a first protocol header, e.g., GRE, which is in turn encapsulated with a second protocol header, e.g., IPsec ESP. For example, a payload packet may be encrypted and encapsulated using the IPsec protocol, and transported from the source node to the destination node through a GRE (Generic Routing Encapsulation) tunnel by encapsulating the IPsec packet using a GRE delivery protocol packet. 
     Operational Context 
       FIG. 1  is a block diagram illustrating an example of an operational environment in which an embodiment may be implemented. The operational environment of  FIG. 1  is described herein to provide an example context, however, it is not intended to limit the scope or use of the invention. 
       FIG. 1  illustrates a computer network environment  100 , which comprises routers  102 A,  102 B,  102 C communicatively coupled to respective local networks  104 A,  104 B,  104 C and to a public network  108 , such as the Internet. Each of the routers  102 A,  102 B,  102 C can be, for example without limitation, a head-end router of a cable network or other similarly functioning broadband device, a gateway, a combination of router and gateway, a gatekeeper, or a similarly functioning device. In one embodiment, routers  102 A,  102 B,  102 C provide a tunnel interface between a public network  108  and one or more private networks  104 A,  104 B,  104 C. A tunnel interface is, generally, a logical interface that facilitates creation, maintenance, and tear-down of a virtual point-to-point circuit between two nodes in a network. The private networks  104 A,  104 B,  104 C can be a LAN, implementing conventional technology such as Ethernet or token ring. 
     A plurality of end devices, collectively referred to as  106 A,  106 B,  106 C, are connected to private networks  104 A,  104 B,  104 C, respectively. Each end device  106 A,  106 B,  106 C can be any device that is capable of communicating through a network. More specifically, in order to fully benefit from the processes described herein, the end devices should be capable of communicating with delay-sensitive traffic, such as voice, fax, or other IP telephony, video, multimedia, and the like. Non-limiting examples of end devices include IP phones, computers, workstations, personal digital assistants (PDA) or other handheld computing device, videophones, etc. 
     The network environment  100  further includes a next-hop server (NHS)  110  or a similarly functioning device, a route server  112  or similarly functioning device, and a virtual circuit  114 . A virtual circuit  114  is also termed a tunnel or a virtual point-to-point circuit. 
     A next-hop server such as NHS  110  typically utilizes NHRP (Next-Hop Resolution Protocol) to maintain precise information about network configurations, such as how to route packets to a particular IP address, by transmitting NHRP queries and replies between IP subnets. Thus, the functionality of an NHS  110  includes awareness of public next-hop devices for any given device. 
     A route server such as route server  112  is, generally, a device that runs one or more network layer routing protocols, maintains routing tables, and uses a route query protocol in order to provide network layer routing forwarding descriptions to clients. Thus, the functionality of a route server  112  in an IP telephony network includes resolving telephone numbers to private network addresses. In this context, a private network address is an address on a private network behind a firewall, gatekeeper, or other security/authorization device. 
     Generally, a virtual circuit  114  is a path between points in a network that appears to be a discrete, physical path but is actually a managed pool of circuit resources from which specific paths, channels or circuits are allocated as needed to meet traffic requirements. A switched virtual circuit is a virtual circuit in which a connection session is set up for a user only for the duration of a connection. 
     Detailed descriptions of the functionality of NHS  110 , route server  112 , and virtual circuit  114 , with respect to embodiments of the approaches herein, are provided below. 
     Network environment  100  may also include an analog end device  106 D connected to a PBX (Private Branch Exchange)  107 , which is a private phone system switch that connects to the public telephone network and offers in-house connectivity. The PBX  107  is connected to the public network  108  via a T1 line  105  to one or more other devices, such as TDM (Time Division Multiplexing) device  109 . TDM device  109  is connected to the public network  108 . In such a scenario, the TDM device  109  would be publicly routable, that is, it is known by the network routing devices. Furthermore, end device  106 D would not have a private IP address, hence, IP network communications would end at or near TDM device  109 , typically with conversion of digital signals to analog signals. 
       FIG. 1  is not a comprehensive illustration of a network environment, but illustrates example network components that are useful in describing embodiments herein. For example, numerous other devices may be utilized to direct network traffic associated with communication between end devices  106 A and  106 B, such as registration servers, proxy servers, universal access servers, redirect servers, and the like. 
     Operating Environment for Establishing a Dynamic Multipoint Encrypted Virtual Private Network 
       FIG. 2  is a block diagram of an example network in which a dynamic multipoint encrypted virtual private network may be implemented. In  FIG. 2 , a router  202  is located in the position of hub for a hub-and-spoke arrangement of other routers S 1 , S 2 , S 3 , S 4 . Because of its logical role as a communications hub in such a system, in this description, router  202  is sometimes termed a hub; however, this terminology is distinct from a hardware hub as known in the networking field for interconnecting end stations, as in an Ethernet hub. 
     Hub router  202  is communicatively coupled to a packet-switched network  204  that may contain any number of network infrastructure elements including routers, switches, gateways, etc. Such elements are omitted from  FIG. 2  for clarity, because they are not pertinent to the embodiments described herein. In one embodiment, network  204  is a TCP/IP network in which infrastructure elements execute a conventional routing protocol, such as RIP, EIGRP, OSPF, BGP, etc., for routing packets among the infrastructure elements. Hub router  202  also is communicatively coupled to a local area network  206  from and to which the hub router receives and routes data packets, respectively. LAN  206  comprises one or more hosts  208   a ,  208   b ,  208   n . A first host  208   a  is also designated in this description as host H 0 . 
     The other routers S 1 , S 2 , S 3 , S 4  also are communicatively coupled to network  204 . Each of the other routers S 1 , S 2 , S 3 , S 4  also may route data packets to a local area network, or to other network infrastructure elements. As an example, router S 1  receives and routes from and to LAN  210  having hosts  212   a ,  212   n . Host  212   a  is also referred to herein as host H 1 . 
     Further, each of the other routers S 1 , S 2 , S 3 , S 4  is identified by a routable network address R 1 , R 2 , R 3 , R 4 , respectively. The designation “R” in R 1 , R 2 , R 3 , R 4  is used to signify that such addresses are routable and “real,” as opposed to virtual. Addresses R 1 , R 2 , R 3 , R 4  are IP addresses, and may be dynamically assigned. For example, routers S 1 , S 2 , S 3 , S 4  may communicate with address servers that conform to Dynamic Host Control Protocol (DHCP) and that assign a dynamic network address R 1 , R 2 , R 3 , R 4  to the routers when they power-up or initialize. Although embodiments are described herein with reference to IP addresses and the IP protocol, implementations are not limited to use of IP. Rather, other packet-based protocols, even protocols that are not yet developed, are specifically contemplated. 
     Hub router  202  further comprises a GRE module  220 , NHRP module  222 , and IPsec module  224 A. Each such module comprises one or more computer programs or other software elements for implementing the functions described further herein. Modules  220 ,  222 ,  224 A may form components of an operating system for hub router  202 . Each of the spoke routers S 1 , S 2 , S 3 , S 4  are similarly configured with a GRE module  220 , NHRP module  222 , and IPsec module  224 A. 
     For purposes of illustrating a clear example, limited numbers of routers, LANs, and hosts are shown in  FIG. 2 . However, in a practical embodiment, there may be any number of such elements, and the use of hundreds or thousands of routers is specifically contemplated. 
     The hub router  202  participates in a point-to-multipoint (i.e., “multipoint”) Generic Routing Encapsulation (GRE) tunnel with routers S 1 , S 2 , S 3 , S 4 . A protocol for establishing GRE tunnels is described in IETF Request for Comments (RFC)  1701 . Thus, in an embodiment, GRE module  220 , which implements the functions and protocols of RFC 1701, is used to set up a multipoint GRE tunnel having one endpoint at a logical GRE interface in hub router  202 , and multiple other endpoints at logical GRE interfaces of routers S 1 , S 2 , S 3 , S 4 . In this arrangement, the GRE tunnel interface at router  202  has a static virtual tunnel IP address of TH, and the GRE tunnel interfaces of routers S 1 , S 2 , S 3 , S 4  have static virtual tunnel IP addresses of T 1 , T 2 , T 3 , T 4 , respectively, which are not conventionally routable over a public network. Use of a point-to-multipoint tunnel allows for a single tunnel interface on each router  202 , S 1 , S 2 , S 3 , S 4 , rather than an interface for each point-to-point link in a point-to-point tunnel network. Hence, configuration information associated with and residing on each router is minimized. Furthermore, each tunnel interface can have any number of destinations configured or dynamically learned thereon. 
     Typically, tunnel addresses T 1 , T 2 , T 3 , T 4 , which are associated with routers S 1 , S 2 , S 3 , S 4  of the virtual private network, are selected in an address range that places the addresses within the same subnet. Techniques are well-known in the art for assigning addresses to network devices such that they appear on the same subnet. Thus, the address TH of hub router  202  appears to be one hop away from address T 1 , even though multiple real infrastructure elements of network  204  may be interposed among the hub router  202  and endpoint router S 1 . The GRE tunnel may be established by providing appropriate GRE tunnel configuration commands to routers  202 , S 1 , S 2 , S 3 , S 4 , which commands are interpreted by a configuration interpreter and executed by respective GRE modules  220 . 
     NHRP module  222  of hub router  202  enables hub router  202  to resolve “non-routable” virtual tunnel addresses into real routable addresses so that infrastructure elements in network  204  can route packets to a tunnel endpoint. However, the real address of a tunnel endpoint may be assigned dynamically when an endpoint device initializes, with the exception of the hub router  202 , which typically is configured with a static real address. Therefore, to facilitate such address resolution, upon power-up or initialization, routers S 1 , S 2 , S 3 , S 4  register with hub router  202 , which serves as a next hop server (NHS), and provide their real addresses and information about networks to which they can route packets. Such network information is typically provided by running a dynamic routing protocol over the VPN network. Hub router  202  stores the real addresses in a mapping of virtual tunnel addresses to real addresses, and stores the network information in a similar mapping, such as a routing table. 
     For example, assume that router S 1  initializes and determines from its configuration information that NHRP is enabled thereon. In response, NHRP module  222  of router S 1  sends an NHRP registration packet to hub router  202  that contains the real address R 1  and tunnel virtual address T 1  of S 1 . NHRP module  222  of hub router  202  stores R 1  in a mapping that associates real address R 1  to virtual tunnel address T 1 . Use of this arrangement enables hub router  202  to forward packets from one host to another host across a multipoint GRE (“mGRE”) tunnel. 
     For example, assume that one host, such as H 0 , generates IP packets that are directed to host H 1 , and therefore have a source IP address value of H 0  and a destination IP address value of H 1 . The packets arrive from LAN  206  at hub router  202 . Hub router  202  looks up host H 1  in a routing table and determines that host H 1  is associated with a tunnel endpoint having virtual tunnel address T 1 . 
     Address T 1  is a virtual address that is not routable by infrastructure elements in network  204 , and therefore hub router  202  requests NHRP module  222  to resolve the virtual tunnel address. As a result, real routable address R 1  is identified in association with virtual address T 1 . Hub router  202  encapsulates the packets from host H 0  in a GRE header, and adds a new IP header having a source address of RH and a destination address of R 1 . Hub router  202  forwards the modified packet to network  204 . The modified packet is structured as follows, with real source and destination IP addresses (RH and R 1 ), a GRE header, and encapsulated IP host addresses (H 0  and H 1 ). 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 IP S:RH D:R1 
                 GRE 
                 IP S:H0 D:H1 
               
               
                   
                   
               
            
           
         
       
     
     The packet is routed through network  204  to arrive at real address R 1  of router S 1 , which detects the GRE header in the packet. Router S 1  drops the new IP header (i.e., IP S:RH D:R 1 ) and consults the encapsulated original IP header to identify the destination address (i.e., IP S:H 0  D:H 1 ) of H 1 . Router S 1  then routes the packet to host H 1  via LAN  210 . Note that both hosts and host addresses are referred to similarly, as in host H 1  has an associated address H 1 . 
     According to an embodiment, hub router  202  and the other routers S 1 , S 2 , S 3 , S 4  can communicate encrypted data traffic on the multipoint GRE tunnel using the IPsec protocol by communicating certain messages and information among NHRP module  222  and IPsec module  124 A. 
     Router Functional Components 
       FIG. 3  is a block diagram of a hub router illustrating elements involved in communicating data on multipoint tunnels using IPsec encryption. Other routers configured with the hub router in a hub-and-spoke network architecture, for example, spoke routers S 1 , S 2 , S 3 , S 4  ( FIG. 2 ), are configured similarly to the hub router, as depicted in  FIG. 3 . However, in certain embodiments, the functionality of the hub and spoke routers is different. For example, in one embodiment, the hub router is designated as the next hop server for the spoke routers, thereby facilitating the resolution and mapping of associations between public and private network addresses. In that context, the hub router functions as a server and the spoke routers function as associated clients. 
     In the example of  FIG. 3 , hub router  202  executes an operating system  301  that includes NHRP module  222  and IPsec module  224 A. Note that implementations are not limited to a configuration as depicted in  FIG. 3 . For example, NHRP module  222  and IPsec module  224 A are not limited to being an operating system  301  function, but may be installed and configured separately from the operating system. NHRP module  222  and IPsec module  224 A have a direct programmatic or messaging connection, as indicated by line  223 . Using connection  223 , as described below, NHRP module  222  can inform IPsec module  224 A when mappings of virtual tunnel addresses to real addresses are created or deleted. Alternatively, in an embodiment, tunnel interface  302  can inform IPsec module  224 A when address mappings are created or deleted. Operating system  301  also instantiates and manages, as a data structure or other logical construct, a tunnel interface  302  that represents an endpoint of a GRE tunnel having virtual address TH. In the course of operations and as depicted in  FIG. 3 , tunnel interface  302  communicates a message to IPsec module  224 A that includes VPN tunnel information, real routable address information, and encryption policy information (e.g., GRE; RH:R 1 ; PH, as depicted). 
     Further, IPsec module  224 A is coupled to an Internet Key Exchange (IKE) module  224 B. In the course of operations, as described further below, IPsec module  224 A may create and manage one or more security associations with other end points, such as a security association  224 C, for tunnel interface  302  associated with tunnel address TH and a tunnel interface at router S 1  associated with tunnel address T 1 . In one embodiment, IPsec module  224 A and IKE module  224 B implement the functions and protocols described in IETF RFC 2401 to RFC 2411, inclusive. 
     Processes for Dynamically Establishing a Secure VPN 
       FIG. 4  is a flow diagram illustrating a high-level view of a process for establishing and using dynamic multipoint encrypted virtual private networks, according to an embodiment. For purposes of illustrating a clear example, the method of  FIG. 4  is described with reference to  FIG. 2  and  FIG. 3 . However, embodiments are not limited to the context of  FIG. 2  and  FIG. 3 . 
     At block  402  a network security policy, such as an IPsec policy, is associated with a virtual private network tunnel interface at a first network device, such as hub router  202  or spoke router S 1  of  FIG. 2 . In this context, “IPsec policy” refers to associated information, which may be encapsulated in an IPsec policy data structure, that specifies one or more encryption parameters and related metadata. For example, an IPsec policy may comprise values indicating that the encryption methodology is DES, IPsec transport mode is used, specific key lifetime values, etc. These values are typically established in a static configuration step as part of a router configuration. Each router may have a different IPsec policy for each of its interfaces. As an example,  FIG. 3  illustrates IPsec policy PH  303 , which is associated with tunnel interface  302 . Thus, for example, block  402  involves associating information with tunnel interface  302  that instructs tunnel interface  302  to perform IPsec encryption using policy PH when GRE traffic is sent from the tunnel interface  302 . 
     At block  404 , input specifying a new association of a VPN endpoint address to a corresponding real routable address of a second network device, such as spoke router S 1  or S 2 , is received. In one embodiment, such input is received at tunnel interface  302  ( FIG. 3 ) when NHRP module  222  ( FIG. 2 ) generates a new mapping of a GRE tunnel address for a destination spoke router, such as T 2 , to a corresponding real routable address R 2  for the destination spoke router. Such input may be received either at a spoke router or a hub router. Further, the input may be transmitted and received in response to a spoke router sending a resolution request message to the hub router acting as next-hop server (NHS), in the form of a resolution reply message. This activity may occur, for example, when one spoke router, such as router S 1 , wants to communicate with another spoke router, such as router S 2 . Alternatively, such input may occur when a hub router, such as hub router  202  ( FIG. 2 ), receives a registration request from a spoke router, such as router S 1 , and consequently performs an address resolution and generates an address mapping. For example, this activity may occur upon initialization activity as part of a power up sequence for the spoke router. 
     An mGRE tunnel between a given spoke router and the hub router can be established upon power-up of the spoke router, so that subsequent NHRP resolution traffic is IPsec encrypted. For example, router S 1  is aware of its real address R 1  and the static hub real address RH. Thus, upon power-up and establishment of the VPN tunnel interface on S 1 , a IPsec module  224 A listener socket at S 1  is created, S 1  registers with the hub router  202  ( FIG. 2 ) as NHS, thus triggering a T 1 :R 1  mapping and reception thereof at S 1 . Consequently, an IPsec state between S 1  and hub router  202  is established, as described further below. Hence, subsequent network traffic transmitted between S 1  and the hub router  202 , including NHRP registration and/or resolution messages, are protected through IPsec encryption. 
     The real IP address of spoke router S 1  is sent to hub router  202  in NHRP registration packets, which is used to create the T 1 :R 1  mapping for S 1 . Consequently, spoke routers&#39; real addresses can dynamically change (e.g., due to a reboot or reconnect to the network), and a new address mapping and IPsec state will automatically be generated. Further, once hub router  202  ( FIG. 2 ) receives a NHRP registration from a given spoke router, the hub router  202  enters unicast and multicast NHRP mappings for the given spoke router. The unicast mapping is used when sending IP unicast packets over the VPN tunnel and the multicast mapping is used when sending multicast packets over the tunnel, most notably for the dynamic routing protocol packets (e.g., RIP, EIGRP or OSPF). 
     This process is repeated as each spoke router powers up. Thus, the hub-and-spoke part of the VPN network, although built dynamically, will stay up all the time since the network paths are used for propagation of dynamic routing information from spoke routers S 1 , S 2 , S 3 , S 4  to the hub router  202  ( FIG. 2 ) and back out to spoke routers. 
     Typically, for hub routers, there is a block of configuration code that defines the crypto map characteristics for each spoke router. The characteristics code includes “set peer . . . ” commands for each peer router. In an embodiment in which IPsec is running in transport mode, IPsec peer addresses must match the IP destination address on each packet to be encrypted, which is the GRE tunnel address. Thus, for example, for purposes of negotiating a security association, the IPsec module  224 A ( FIG. 2 ) can obtain the appropriate peer address from the GRE tunnel interface or NHRP rather than requiring specification in the configuration code. Consequently, the number of lines of configuration code on a hub router is significantly reduced, in relation to prior approaches. This approach also reduces the configuration on a spoke router, but to a lesser degree. 
     The above dynamic hub-and-spoke network facilitates the dynamic creation of direct dynamic spoke-to-spoke tunnels. This allows for the forwarding of spoke-to-spoke data packets directly between spokes without having to manually setup a full-mesh VPN network. For an example, assume that the embodiment of  FIG. 4  is used when a first spoke router S 1  (“spoke 1 ”) is dynamically establishing a direct encrypted mGRE tunnel to a second spoke router S 2  (“spoke 2 ”). Assume also that there is traffic destined from host H 1  behind S 1  to host H 2  behind S 2 . S 1  knows, possibly from a dynamic routing protocol, that to route to S 2  it is supposed to send packets to VPN tunnel address T 2 , but S 1  does not have an NHRP mapping for T 2 . That is, S 1  does not know the real routable address R 2  of S 2 , only the VPN tunnel address T 2 . Therefore, it sends an NHRP resolution request to its NHS. As described above, hub router  202  may function as the NHS. Regardless, upon S 1  receiving the NHRP resolution reply from its associated NHS, S 1  will create an internal NHRP mapping for
         &lt;T 2 &gt;==&gt;&lt;R 2 &gt;;
 
that is, a T 2 :R 2  mapping.
       

     The creation of the NHRP mapping for T 2  will trigger IPsec module  224 A of S 1  to set up state with S 2 . Specifically, in response to the input received at block  404 , at block  406  the tunnel interface  302  ( FIG. 3 ) of source router S 1  sends a message to its associated IPsec module  224 A ( FIG. 2 ), requesting it to generate new encryption state information with destination router S 2 , as depicted at block  408 . In the message, the tunnel interface  302  also provides at least the real IP addresses of the tunnel endpoints, such as R 1  and R 2 , and the IPsec policy, such as policy PI similar to policy PH on hub router  202 . 
     At block  408 , new encryption state information is generated for use in encrypting traffic directed from the first or source network device, such as spoke router S 1 , to the second or destination network device, such as spoke router S 2 . The encryption state is represented as a data structure or other logical construct, which specifies parameters used to encrypt and transmit packets between the tunnel endpoints. The encryption state information includes, for example, routable network address information, VPN encapsulation protocol (e.g., GRE) information, and security policy information. In one embodiment, block  408  is triggered when a listener socket connection of IPsec module  224 A ( FIG. 2 ) for a given router receives the foregoing message of block  406  from its associated tunnel interface  302  ( FIG. 3 ). Further, the IPsec module  224 A listener socket for a given router may be automatically created upon initialization of the tunnel interface  302  for that router. 
     Block  410  is described for an embodiment in which IPsec is used to secure a GRE or other VPN tunnel. A similar step, with respect to secure association establishment and key exchange, could be performed utilizing appropriate protocols in implementations that do not use the IPsec protocol. 
     At block  410 , IPsec module  224 A ( FIG. 2 ) of the source router S 1  initiates a communication with the destination spoke router S 2 , utilizing an appropriate protocol such as ISAKMP (Internet Security Association and Key Management Protocol) to perform a key exchange such as IKE. As a result, the source spoke router authenticates itself to the destination spoke router, exchanges encryption key information and negotiates encryption parameters. As a result of block  410 , pairwise keys are generated for use in encrypting traffic among the pertinent spoke routers S 1 , S 2 . 
     At this point in the process, the source router S 1  can transmit encrypted IP packets encapsulated in a GRE tunnel directly to the destination router S 2 . Upon receiving the first packet from S 1  at S 2 , S 2  initiates a similar process with respect to address resolution for S 1 , so that it knows to where a return packet should be transmitted. Further, an encryption state associated with the two spoke routers has already been established, therefore return data packets to the source spoke router can be encapsulated and encrypted from the destination spoke router to the source spoke router. Due to the ability to dynamically build spoke-to-spoke links, load on an associated hub router, as well as network latency, is reduced. 
     If the networks change on either side of the encrypted VPN tunnel, the other side will dynamically learn of the change through NHRP registration and mapping propagation and through propagation of dynamic routing information. Thus, encrypted connectivity will be established without any router configuration changes. 
     The procedure described with respect to blocks  402 - 410  may also be used when the originating or source node is a hub router and the destination node is a spoke router. 
     Assume, for purposes of illustrating an example with reference to  FIG. 4 , that an IPsec policy PH is created for use when GRE traffic is routed from hub router  202  ( FIG. 2 ) at real address RH to router S 1  at real address R 1 . Whether initiated by a spoke-to-spoke communication or a hub-to-spoke communication, at block  412  an IPsec security association (“SA”) is stored. In one embodiment, the security association is passed from IPsec module  224 A to tunnel interface  302 . The SA associates policy PH with traffic-identifying information. Thus, for example, an SA  224 C may indicate that policy PH is used when traffic has a source address of RH, a destination address of R 1 , and a protocol of GRE. The SA also typically includes the keys that were generated at block  410  and a security parameter index (SPI) value, which is used as an identifier. 
     At block  414 , encrypted traffic is passed on the VPN tunnel from the first device to the second device, based on the encryption state generated at block  408 . For example, when tunnel interface  302  ( FIG. 3 ) of hub router  202  receives data packets for forwarding on the tunnel to spoke router S 1  (e.g., a packet from H 0  destined to H 1 ), the tunnel interface  302  invokes the associated IPsec module  224 A ( FIG. 2 ), which determines that security association  224 C ( FIG. 3 ) specifies how to encrypt the traffic. In turn, IPsec module  224 A encrypts the traffic according to the SA  224 C, and passes the packets to the real interface RH of the hub router  202  ( FIG. 2 ) out through the tunnel to the real address R 1  of destination router S 1 , whereby it is routed to the final destination H 1 . 
     The process described dynamically establishes a secure VPN by generating an encryption state for network traffic over a VPN link in response to notification of a virtual address-to-real address mapping. It is further dynamic with respect to spoke-to-spoke VPN links, in that network traffic between two spokes can trigger generation of an encryption state and a security association among the two spokes, via NHRP resolution requests and replies between spoke routers and their associated NHS. Therefore, significantly, a statically configured full mesh network is unnecessary. Note that hub-to-spoke links are normally more lasting than spoke-to-spoke links due to the repetitive dynamic routing protocol traffic and NHRP registration and resolution traffic between a hub router and its related spoke routers. 
     An encrypted packet, according to the techniques described herein, is structured as follows, with real source and destination IP addresses (RH and R 1 ), a conventional transport mode IPsec ESP (Encapsulating Security Payload) header, a GRE header, and encapsulated IP host addresses (H 0  and H 1 ). 
     
       
         
           
               
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 IP S:RH D:R1 
                 ESP 
                 GRE 
                 IP S:H0 D:H1 
               
               
                   
                   
               
            
           
         
       
     
     IPsec does not readily support IP multicast traffic. Further, dynamic routing protocols typically use IP multicast traffic to communicate among network devices for dynamic routing purposes. Significantly, utilizing the techniques described herein, an IP multicast packet can be encapsulated into an IP unicast GRE packet, which can be encrypted using IPsec. Thus, the capability is provided for using IPsec with multicast traffic and, therefore, for using dynamic routing protocols. Consequently, dynamic discovery of network destinations over a VPN is facilitated. 
     Furthermore, in an implementation that utilizes a dynamic routing protocol, when the hub router  202  ( FIG. 2 ) reflects routes advertised from one spoke router out to the other spoke routers, the hub router  202  may set the next-hop for such routes to another spoke router. That is, the hub uses the originating spoke router rather than itself as the next-hop for the route. Thus, spoke routers can route traffic directly to another spoke router rather than through the hub router. 
     In an alternative embodiment, an IPsec interface configured in IPsec tunnel mode is used in addition to encapsulation of a GRE tunnel. Thus, IPsec is used to implement both encryption and encapsulation functionality. This is useful, for example, when there are network modules in packet network  204  ( FIG. 2 ) that will do Network Address Translation (NAT) of the IP header of the IPsec packets, which could break the IPsec transport mode tunnel and thus break the VPN network. 
     In an alternative approach, IPsec-related operations (steps  408 - 414  of  FIG. 4 ) may be triggered by initialization of the mGRE interface of a spoke router, such as router S 1 , rather than upon reception of an address mapping. For example, assume that a spoke router is powered up. On the spoke router, in its VPN tunnel configuration, are the CLI commands:
         ip nhrp map &lt;TH&gt;&lt;RH&gt;   ip nhrp nhs &lt;TH&gt;
 
The spoke router configuration is parsed, and the first command above causes creation of a static NHRP mapping entry of the hub tunnel address to the hub real routable address, at the spoke router. When the NHRP mapping entry is created, the IPsec module  224 A ( FIG. 2 ) of the spoke router S 1  is triggered to create an IPsec state with the hub router  202 .
       

     The second command above instructs the NHRP module  222  ( FIG. 2 ) of the spoke router that the hub router  202  ( FIG. 2 ) is its next-hop-server. This instruction causes NHRP module  222  of the spoke router to send NHRP registration packets to the hub router  202 , as the NHS. This will use the NHRP mapping entry above to actually send the tunnel packet to the router (hub) associated with RH. 
     Hence, in this approach, initiation of IPsec operations may occur when a configuration interpreter executes a configuration command at a spoke router, which sets an NHRP mapping for the hub router and sets the hub as the NHS for the spoke. Consequently, NHRP is not used as a signaling protocol to aid in establishment of a VPN tunnel. Contrastly, NHRP is used for network address resolution, which predominately occurs within a VPN tunnel (e.g., a GRE tunnel) that is already established, and subsequently, as a trigger mechanism for IPsec state generation for traffic through the tunnel. Hence, NHRP resolution traffic can be exchanged through the tunnel rather than in the clear. 
     In yet another alternative approach, steps  404  to  414  of  FIG. 4  may be performed in an embodiment that operates with a point-to-point tunnel in which NHRP is not used. In this alternative, a peer router is manually configured on the tunnel interface of a given router. For example, router S 2  is manually configured with VPN tunnel address information associated with router S 1 . In response, the tunnel interface  302  ( FIG. 3 ) at router S 2  creates a message based on information known to it from its router configuration, such as its real IP address R 2 , the peer router real IP address R 1 , IPsec policy, etc. The tunnel interface  302  at router S 2  sends the message to its associated IPsec module  224 A ( FIG. 2 ). The IPsec module  224 A receives this message and creates the encryption state information with respect to the peer router, in a manner as described above in reference to block  408 . 
     Although certain embodiments have been illustrated in the context of IPsec encryption, the invention is not limited to that context. Further, mechanisms other than NHRP alone may be used to resolve addresses of remote routers. For example, Tunnel Endpoint Detection (“TED”) protocol may be used in combination with NHRP module  222  ( FIG. 2 ) and multipoint GRE tunnels as described herein in order to obtain one or more remote router addresses, and to communicate such addresses to IPsec module  224 A ( FIG. 2 ). 
     Embodiments herein provide for enhanced scalability in full mesh or partial mesh IPsec VPNs. Embodiments are especially useful when spoke-to-spoke traffic is sporadic (i.e., every spoke is not constantly sending data to every other spoke), such as in a VoIP (Voice Over Internet Protocol) context. Any spoke may send data directly to any other spoke, as long there is direct IP connectivity between the spokes. 
     In prior approaches to full mesh networks, all point-to-point IPsec (or IPsec+GRE) tunnels must be configured on all the routers in the mesh network, even if some or most of these tunnels are not running or needed at all times. Utilizing an embodiment described herein, one router is designated the “hub”, and all the other routers (“spokes”) are configured with tunnels to the hub. The spoke-to-hub tunnels are up continuously. However, the spoke routers do not have nor need configuration for tunnels to any of the other spoke routers. Instead, when a spoke router wants to transmit a packet to the subnet behind another spoke router, it uses NHRP to dynamically determine the required destination address of the target router. The hub router acts as the NHRP server and handles this request for the source router. The two spokes then dynamically create an IPsec tunnel between them (via the single mGRE interface) and data can be directly transferred. 
     An idle or other timeout function will automatically tear down the encrypted VPN tunnel after a period of inactivity. In an embodiment, the timeout function is triggered by an NHRP mapping timeout, wherein the tunnel interface becomes aware of the NHRP timeout and notifies the IPsec module, which in turn deletes its state information/data structure relative to the particular tunnel. 
     Furthermore, multiple hub routers can be implemented in the network, each supporting a large number of spokes. The hubs in this “partial temporal mesh” could be interconnected using a mesh of permanent GRE+IPsec tunnels, local LAN interfaces (if the hubs are co-located), or these hubs could serve as spokes for another tier of hub routers to create a multiple tier hub-and-spoke VPN network. 
     Embodiments herein support IPsec nodes with dynamically assigned addresses (e.g. Cable, ISDN, DSL). This applies to hub-and-spoke as well as mesh networks. Consequently, the cost of provisioning spoke routers to an underlying network is reduced due to the lower costs associated with dynamic addresses than with static addresses. 
     Embodiments herein simplify the addition of VPN nodes. When adding a new spoke router, only the spoke router is configured and plugged into the network, and possibly ISAKMP authorization information for the new spoke is added at the hub router. The hub router will dynamically learn about the new spoke router and the dynamic routing protocol will propagate routing paths to the hub and all other spokes. 
     Embodiments herein significantly reduce the size of the configuration needed on all the routers in the VPN. This is also the case for GRE+IPsec hub-and-spoke only VPN networks. 
     Embodiments herein support IP multicast and dynamic routing traffic across the VPN through utilization of GRE, which encapsulates the IP multicast packets into IP unicast tunnel packets. Hence, a dynamic routing protocol can be used, and redundant “hubs” can be supported by the protocol. Multicast applications are also supported. 
     Embodiments herein support split tunneling at the spokes. Furthermore, Embodiments herein support CEF (Cisco&#39;s Express Forwarding) and other fast switching techniques. The mGRE/NHRP solution can CEF switch the mGRE traffic, resulting in much better performance than with typical process switching in mGRE interfaces. 
     Process for Dynamically Securing Delay-Sensitive Network Traffic 
       FIG. 5  is a flowchart illustrating a process for dynamically securing delay-sensitive network traffic. For purposes of illustrating a clear example, some of the components of  FIG. 1  are referred to in describing the processes of  FIG. 5 . However, the invention is not limited to the implementations in these examples. 
     At block  502 , a request is received for secure network traffic. The request is received from a source device having a private network address at a source node and is directed to a destination device having a private network address at a destination node. For example, a request from an end device  106 A, such as an IP phone on a private LAN  104 A, is received at router  102 A, requesting secure voice communication with an end device  106 B, such as a voice-enabled PDA on a private LAN  104 B, through a public network  108  such as the Internet. Typically, a user at end device  106 A would perform conventional dialing of an IP phone, thus transmitting data that represents the telephone number associated with the end device  106 B to the router  102 A. Additionally, a user at end device  106 D could dial a conventional analog phone, transmitting DTMF (Dual Tone Multi-Frequency) signals that represent a telephone number to the TDM device  109 , through PBX  107  and T1 line  105 . 
     At block  504 , the private network address of the destination device is obtained from a route server or gatekeeper, based on signaling information associated with the request of block  502 . For example, router  102 A communicates with route server  112  according to a particular signaling protocol, thereby exchanging protocol-specific signaling information that includes a representation of the end device  106 B telephone number. A request/response transaction may be performed to obtain the signaling information. Any suitable signaling and/or transport protocol associated with particular media types may be used with the invention, such as H.323, SIP, RTP, Q.931, and their respective associated protocols. Examples of media types include voice and video. The responsibility for understanding the specific transport protocol lies with the route server  112 , or with some other network device that can convert the source protocol to a protocol understood by the route server  112  in cases in which the router  102 A and the route server  112  do not support the same protocols. 
     In the public Internet, a packet cannot be routed to a private network address entirely through the public network. That is, conventional routers only route to public IP addresses. Therefore, a route server  112  is used to resolve the requested telephone number of the destination end device  106 B, received via router  102 A, to a private network address for the destination end device  106 B associated with the telephone number. The route server  112  identifies and returns the private network address back to a requesting client, such as router  102 A. 
     Once the private network address of the destination end device  106 B is known, the router  102 A needs to know how to reach that private network address, or at least how to get as close to the private network address as possible via the public network  108  (i.e., to the destination node). Therefore, at block  506  a public network address of the destination node that is associated with the private network address obtained in block  504  is obtained. In this context, “destination node” refers to the node of the public network  108  that interfaces with the private network  104 A,  104 B,  104 C, respectively. For example, router  102 B is the interfacing device between the public network  108  and the private network  104 B. 
     In one embodiment, the public network address of the destination node is obtained from a next-hop server, such as NHS  110 . Many network devices register with a next-hop server upon initialization, and next-hop servers are capable of exchanging information with other next-hop servers. Thus, NHS  110  has information about every site in the public network, and is a source of the information that router  102 A uses to facilitate provisioning of secure delay-sensitive network traffic (e.g., voice) over public network  108 . As described above, a router configured as a hub router in a hub-and-spoke network, such as hub router  202  ( FIG. 2 ) can be configured as NHS  110 , and can, therefore, provide a virtual address to real address mapping. Further, this mapping triggers creation of an encryption state for network traffic between the source and destination nodes, as depicted at block  408  of  FIG. 4 . 
     Alternatively, public address information may be obtained from a device other than NHS  110 , using protocols other than NHRP, for example, the TED protocol. Furthermore, other devices may contribute to determining a route between router  102 A and router  102 B on which a tunnel through the public network  108  is established. As a non-limiting example, a GLP (Gateway Location Protocol)-enabled gateway and/or a directory server may be used. 
     In one implementation, a public address corresponding to the private address of a destination node is obtained from the NonstandardData fields in multimedia call setup signaling. For example, the H.225 Recommendation (“Call Signaling Protocols and Media Stream Packetization for Packet-Based Multimedia Communication Systems”; dated Nov. 2000; section 7.11.2) from the ITU-T refers to the capability of carrying public IP addresses in a nonStandardData field of a “confirm” signal (ACF), where the destCallSignalAddress field carries the original private IP address of the endpoint. H.225 messages are exchanged between endpoints if there is no gatekeeper; however, if there is a gatekeeper, the H.225 messages are exchanged either directly between endpoints or between the endpoints after being routed through a gatekeeper or other call control server, such as a with gatekeeper-routed call signaling, a SIP proxy, or a MGCP (Media Gateway Control Protocol) CallAgent. 
     Implementation goals include minimizing the overhead from call/tunnel set-up processing. Therefore, in another embodiment, the public network address of the destination node is obtained from a cache at the source node that stores mappings of private network addresses to public network addresses. For example, the router  102 A may cache IP address information to limit its communications with the NHS  110 . Thus, the relevant public address information may already be available locally to the router  102 A, without requiring communication with the NHS  110  in response to every request (e.g., for a commonly called number, or a commonly requested end device). For another example, the cache may be populated through the public-to-private address mapping acquired from the multimedia signaling, as described above in reference to the H.225 call signaling protocol. Accordingly, in this approach, block  506  involves searching a cache for a public network address. 
     At block  508 , in response to the request for secure network traffic of block  502 , a virtual circuit is created between the source node and the destination node, based on a mapping of the public network address of the destination node that is obtained at block  506  and an associated virtual tunnel address of the destination node. For example, a bi-directional tunnel is created through public network  108  between source router  102 A and destination router  102 B, as depicted as tunnel  114  of  FIG. 1  and as described in reference to  FIGS. 2-4 . Since the virtual circuit, or tunnel  114 , is created in response to the request, the virtual circuit is dynamically created. 
     In one embodiment, the tunnel  114  is a dynamic GRE tunnel. GRE protocol is used for encapsulation of an arbitrary network layer protocol over another arbitrary network layer protocol, and is described in IETF RFC 2784. In the most general case, a system that has a packet that needs to be encapsulated and delivered to some destination (a payload packet) is first encapsulated as a GRE packet. The resulting GRE packet can then be encapsulated as some other protocol (delivery protocol) packet and then forwarded to its destination via a GRE “tunnel” or virtual circuit. 
     In one implementation, a point-to-multipoint GRE communication link is created and maintained as part of the process for securing delay-sensitive network traffic. For example, a mGRE tunnel is originated at router  102 A to communicate with the NHS  110 . In this scenario, the same mechanism is used at block  508  to create a virtual circuit or tunnel  114  between router  102 A and router  102 B. Thus, use of a point-to-multipoint mechanism minimizes the amount of provisioning needed to create the tunnel  114  and other similar tunnels originating at the same source node. Furthermore, redundant provisioning is eliminated. 
     At block  510 , network traffic is encrypted for transport at least from the source node to the destination node over the virtual circuit. In one embodiment, the packets representing a voice call are encrypted using IPsec protocol, using either transport or tunnel modes, depicted as blocks  406 - 412  of  FIG. 4  and described in reference thereto. Another example of a secured transport protocol that can be implemented is SRTP (Secure Real-Time Transport Protocol). However, the invention is not limited to use of the IPsec or SRTP protocols. 
     For full bi-directional encrypted communication, both the source and destination nodes are provided with both encryption and decryption capabilities. For example, both source and destination routers support IPsec. 
     In one embodiment, an optional step to the process illustrated in  FIG. 5  is to ensure the integrity of the network traffic transported from the source node to the destination node over the virtual circuit. In another embodiment, an optional step is to ensure the authenticity of the network traffic transported from the source node to the destination node over the virtual circuit. The preceding optional steps can be implemented into the process either alone or in combination. Using IPsec for encryption of the network traffic is one, but not the only, technique for implementing the preceding optional steps. 
     In tunnel mode of IPsec, the entire original IP packet is encrypted and it becomes the payload, i.e., is encapsulated, in a new IP packet. Hence, in one embodiment, encrypting network traffic for transporting over the virtual circuit at block  510  comprises: (1) encapsulating an encrypted payload packet of the network traffic in an encapsulating protocol packet; and (2) encapsulating the encapsulating protocol packet in a delivery protocol packet. For example, router  102 A receives a request for secure voice communication between end device  106 A and end device  106 B. After performing steps  502 - 506  of  FIG. 5 , steps  508  and  510  can be performed generally as indicated by steps (1) and (2) above (and as described in more detail in reference to blocks  406 - 412  of  FIG. 4 ). That is, using IPsec, the original payload RTP packet is encrypted, keys established and exchanged, and headers added (ESP header and possibly an AH header), thereby generating an encrypted IPsec-encapsulated RTP voice packet. Then, using GRE, a GRE header is added to the encrypted IPsec-encapsulated RTP packet and the resulting packet is encapsulated in a delivery protocol packet (e.g., IPv4), thereby generating an encrypted, twice-encapsulated RTP voice packet. 
     Another process is provided for dynamically securing delay-sensitive network traffic that is directed to an end device  106 D behind a PBX  107  or similar device, in which the end device does not have an associated IP address. According to this embodiment, a request is received for secure network traffic between a source device at a source node and a destination device at a destination node. A public network address for the destination node is obtained from a next-hop server, and, in response to the request, a virtual circuit is created between the source and destination nodes based on a mapping of the public destination address to an associated virtual tunnel address. Network traffic is then encrypted for transporting at least from the source node to the destination node, and the PBX  107  at the destination node is responsible for forwarding (i.e., switching) the traffic to the end device  106 D. This process is applicable if one or both of the participating end devices do not have a private IP or other packet-switched network address. 
     Process for Tearing Down Secure Virtual Circuit 
       FIG. 6  is a flowchart illustrating a process for tearing down a secure dynamically created virtual circuit. At block  602 , an indication is received that a virtual circuit between a source node and a destination node is no longer needed for secure network traffic between a source device at the source node and a destination device at the destination node. For example, an IP phone in the position of end device  106 A is hung up, and router  102 A receives this indication that telephone communication with end device  106 B over virtual circuit  114  is finished. The virtual circuit is a dynamically created virtual circuit that was previously created using the process illustrated in  FIG. 5 . 
     At decision block  604 , it is determined whether any other devices are currently using the virtual circuit. For example, router  102 A determines whether any network traffic other than that between particular end devices  106 A,  106 B is currently using virtual circuit  114  between the two network nodes. If any other devices are still using the virtual circuit, then the process returns to wait for the other network traffic to end, and to receive such indications, such as at block  602 . 
     If the determination at block  604  is negative, then at decision block  606  it is determined whether the virtual circuit has an associated teardown policy. For example, router  102 A may access stored configuration information at the router to determine whether to tear down the virtual circuit. If the virtual circuit has no teardown policy, then the virtual circuit is torn down at block  608 . In one embodiment which utilizes cache at router  102 A to store, among other things, the public network address information used to create the virtual circuit  114  between  102 A and  102 B, tearing down the virtual circuit  114  is performed by clearing the cached information from the cache of the participating routers  102 A,  102 B. 
     If the determination at block  606  is positive, then at block  610  it is determined, from the applicable teardown policy, whether the virtual circuit should be torn down. A teardown policy may indicate, for example, that all virtual circuits are kept up for 24 hours minimum. A teardown policy may consider, for example without limitation, the amount of network traffic using the given virtual circuit. Such a policy consideration may consider the frequency of calls or other network traffic over the virtual circuit, as well as the duration of such traffic. In general, creating a virtual circuit results in some processing overhead, but maintaining or tearing down the virtual circuit is relatively simple and computationally inexpensive. Hence, in some instances, it is a more efficient use of resources to maintain a virtual circuit with no current traffic over it rather than to automatically tear it down when traffic ends, only to recreate another virtual circuit upon a subsequent request for secure network traffic. Thus, the processing and network traffic overhead associated with virtual circuit creation and teardown is minimized. 
     Implementation Mechanisms—Hardware Overview 
       FIG. 7  is a block diagram that illustrates a computer system  700  upon which an embodiment of the invention may be implemented. Embodiments are implemented using one or more computer programs running on a network element such as a router device. Thus, in this embodiment, the computer system  700  is a router. 
     Computer system  700  includes a bus  702  or other communication mechanism for communicating information, and a processor  704  coupled with bus  702  for processing information. Computer system  700  also includes a main memory  706 , such as a random access memory (RAM), flash memory, or other dynamic storage device, coupled to bus  702  for storing information and instructions to be executed by processor  704 . Main memory  706  also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor  704 . Computer system  700  further includes a read only memory (ROM)  708  or other static storage device coupled to bus  702  for storing static information and instructions for processor  704 . A storage device  710 , such as a magnetic disk, flash memory or optical disk, is provided and coupled to bus  702  for storing information and instructions. 
     A communication interface  718  may be coupled to bus  702  for communicating information and command selections to processor  704 . Interface  718  is a conventional serial interface such as an RS-232 or RS-722 interface. An external terminal  712  or other computer system connects to the computer system  700  and provides commands to it using the interface  714 . Firmware or software running in the computer system  700  provides a terminal interface or character-based command interface so that external commands can be given to the computer system. 
     A switching system  716  is coupled to bus  702  and has an input interface  714  and an output interface  719  to one or more external network elements. The external network elements may include a local network  722  coupled to one or more hosts  724 , or a global network such as Internet  728  having one or more servers  730 . The switching system  716  switches information traffic arriving on input interface  714  to output interface  719  according to pre-determined protocols and conventions that are well known. For example, switching system  716 , in cooperation with processor  704 , can determine a destination of a packet of data arriving on input interface  714  and send it to the correct destination using output interface  719 . The destinations may include host  724 , server  730 , other end stations, or other routing and switching devices in local network  722  or Internet  728 . 
     The invention is related to the use of computer system  700  for dynamically securing delay-sensitive network traffic. According to one embodiment of the invention, a multipoint IPsec VPN is established by computer system  700  in response to processor  704  executing one or more sequences of one or more instructions contained in main memory  706 . Such instructions may be read into main memory  706  from another computer-readable medium, such as storage device  710 . Execution of the sequences of instructions contained in main memory  706  causes processor  704  to perform the process steps described herein. One or more processors in a multi-processing arrangement may also be employed to execute the sequences of instructions contained in main memory  706 . In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement the invention. Thus, embodiments of the invention are not limited to any specific combination of hardware circuitry and software. 
     The term “computer-readable medium” as used herein refers to any medium that participates in providing instructions to processor  704  for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, optical or magnetic disks, such as storage device  710 . Volatile media includes dynamic memory, such as main memory  706 . Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise bus  702 . Transmission media can also take the form of acoustic or light waves, such as those generated during radio wave and infrared data communications. 
     Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, or any other magnetic medium, a CD-ROM, any other optical medium, punch cards, paper tape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read. 
     Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to processor  704  for execution. For example, the instructions may initially be carried on a magnetic disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system  700  can receive the data on the telephone line and use an infrared transmitter to convert the data to an infrared signal. An infrared detector coupled to bus  702  can receive the data carried in the infrared signal and place the data on bus  702 . Bus  702  carries the data to main memory  706 , from which processor  704  retrieves and executes the instructions. The instructions received by main memory  706  may optionally be stored on storage device  710  either before or after execution by processor  704 . 
     Communication interface  718  also provides a two-way data communication coupling to a network link  720  that is connected to a local network  722 . For example, communication interface  718  may be an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface  718  may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface  718  sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information. 
     Network link  720  typically provides data communication through one or more networks to other data devices. For example, network link  720  may provide a connection through local network  722  to a host computer  724  or to data equipment operated by an Internet Service Provider (ISP)  726 . ISP  726  in turn provides data communication services through the world wide packet data communication network now commonly referred to as the “Internet”  728 . Local network  722  and Internet  728  both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link  720  and through communication interface  718 , which carry the digital data to and from computer system  700 , are exemplary forms of carrier waves transporting the information. 
     Computer system  700  can send messages and receive data, including program code, through the network(s), network link  720  and communication interface  718 . In the Internet example, a server  730  might transmit a requested code for an application program through Internet  728 , ISP  726 , local network  722  and communication interface  718 . In accordance with the invention, one such downloaded application provides for dynamically securing delay-sensitive network traffic as described herein. 
     The received code may be executed by processor  704  as it is received, and/or stored in storage device  710 , or other non-volatile storage for later execution. In this manner, computer system  700  may obtain application code in the form of a carrier wave. 
     Extensions and Alternatives 
     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. 
     In addition, in this description certain process steps are set forth in a particular order, and alphabetic and alphanumeric labels may be used to identify certain steps. Unless specifically stated in the description, embodiments of the invention are not necessarily limited to any particular order of carrying out such steps. In particular, the labels are used merely for convenient identification of steps, and are not intended to specify or require a particular order of carrying out such steps. 
     With respect to dynamically securing video traffic over a network using the techniques described herein, an extension to the invention includes ensuring that the end devices (e.g., end devices  102 A, B) are capable of applying QoS (Quality of Service) processes to the secure network traffic over the virtual circuit  114 . One example of QoS support includes the capability of fragmenting video traffic (in general, due to the size of some video frames), possibly in conjunction with prioritizing voice traffic over video traffic. The foregoing is but one example of possible QoS policies that may be implemented with the techniques described herein.