The Research And Implementation Of SIP Traversal NAT On Linux

VoIP is a technology that mostly utilized in IP phone, and relevant value-added services. The most advantage of VoIP is that it can make use of the global IP internetworking environment, to provide the more and more,the better and better services than tradition PSTN network. It is also the hot research field of Internet. At the same time, as SIP has more advantages than H.323, VoIP trends to adopt SIP protocol instead of H.323. This thesis does some research on the VoIP system based on SIP.The wide application of VoIP greatly increases the demand of IP addresses. VoIP mainly considers using NAT traversal to solve this problem. Compared to general data traversal such as HTTP, the VoIP application based on H.323 and SIP uses the IP address and port in the message body of singnaling to route. So besides transforming the TCP/UDP layer port and IP layer address, VoIP NAT traversal needs to transform parts of the payload of IP packages. NAT traversal is now the biggest drawback in VoIP application and should be solved in short time.This thesis first analyses the related protocols in VoIP system based on SIP, introduces the firewall and NAT including the NAT and firewall implementation in Linux, and discusses some kinds of method to solve this problem. The thesis mainly implements a prototype of an Application Level Gateway (ALG) for SIP traversal. The implementation summaries the modifications to SIP messages and puts forward an ALG state machine. This state machine not only passes SIP signaling through NAT gateway but also records the SIP communication states and controls the RTP channel. Following the request of software engineering, this thesis takes a function testing and a performance testing on ALG; the testing result shows that the ALG has fulfilled the requirement of SIP traversal NAT. It will be a good help to wide usage of VoIP applications.