nection reuse will reduce the number of times it has to be run. Connection reuse is mainly defined between proxy servers. The usage of connection reuse combined with registration is defined for UAs as SIP outbound in the next section.

10.11.3 SIP Outbound

SIP outbound is a mechanism that combines connection reuse, registration through multiple proxies, and keep alives. It solves many of the existing problems with SIP NAT traversal, although it requires both UAs and registrars support the extension. SIP outbound is currently being finalized by the IETF as [14]. The basic configuration is shown in Figure 10.9. UAs have an instanceid, which uniquely identifies them. Multiple registrations by the UA through different proxy servers will have different reg-id. STUN keep alive messages are used between the UA and the proxy to monitor the flow and keep the NAT mappings active. If the STUN checks reveal the flow has failed (perhaps due to a failure in the UA, the NAT, or the registrar), the UA registers again using the same instance-id but a new flow-id. This is shown in Figure 10.10.

10.12 Media NAT Traversal Solutions

There are a number of methods used for media traversal of NAT, which will be discussed in the following sections. These solutions involve getting RTP and RTCP negotiated using SIP to work.

10.12.1 Symmetric RTP

Symmetric RTP involves sending RTP from the same port that it is listening for RTP. This results in a UDP connection that the NAT will understand and allow

Figure 10.9 SIP outbound.

Figure 10.10 SIP outbound call flow.