Chapter 22 - NAT Traversal

Abstract

This chapter presents an overview of the various problems caused by network address translation (NAT), which are specifically related to session initiation protocol (SIP) signaling and media transfer, and presents several possible solutions. NAT connects an isolated internal realm with private IP addresses to an external realm with globally unique registered addresses and it is typically implemented in a private domain's edge router that connects to the public domain. Traditional NAT (or simply NAT) is split into two types, the basic NAT and the network address and port translation (NAPT). The two main issues posed by NATs to SIP traversal are routing of SIP responses and routing of incoming requests. The first problem can be solved with a new SIP extension called “Symmetric response routing,” whereas the second problem can be solved by slightly modifying the core SIP registration process. Session border controllers (SBC) represent the industry's response to solve, among other things, the NAT traversal problem is coined by the industry to refer to signaling and media intermediaries that implement generic border control functions. The two scenarios for SBC deployment are the interconnect scenario and access scenario. In the interconnect scenario the SBC sits between two service provider networks in a peering environment, whereas in the access scenario, SBC is placed at the border between the access network and the service provider's network. SBCs also typically handle both signaling and media.