Resolve The SIP-NAT Problem With STUN Protocol

SIP is an application-layer control (signaling) protocol, but it includes IP address and the Port number in its messages for addressing purpose. Because the NAT devices translate only the IP address in the IP layer and the Port number in the TCP/UDP layer, when the SIP message passes NAT device, the IP address and port number in IP layer and TCP/UDP layer are translated but the IP address and port number embedded in SIP messages are not, resulting inconsistency of the addressing information which leads SIP addressing to failure and the medium channel can not be setup. We name this as the SIP-NAT problem in the paper. The SIP-NAT problem prevents the SIP-based VoIP from popularizing in Intranet.In order to identify and resolve this problem, after brief introduction of VoIP technology, the paper analyzes SIP protocol and the NAT operation thoroughly, and three catagories of solutions for SIP-NAT problem are investigated:1. the solutions at the user agent2. the solutions at the routers3. the solutions at the serversBased on the investigation the author picked up the sulotion based on STUN protocol and disgned a implementation scheme for a VoIP terminal. The detailed disgn is presented in the paper.