The Design And Implementation Of NAT Traversal In SIP Based MoIP Gateway

With the rapid development of network and communication technologies, Internet is applied in more and more realms. The multimedia communication over Internet Protocol (MoIP) is a typical example among the applications. MoIP means transmitting voice, video, data and other multimedia information over Internet protocol (IP) network. At present, the primary standards of MoIP include H.323, Session Initiation Protocol (SIP), H.248 and Media Gateway Control Protocol (MGCP). SIP has become more and more popular in MoIP communication because of its simplicity, flexibility and extendability.The IPv4 address resource is facing exhaustion with the rapid increase of internet users. To overcome the lack of IPv4 addresses, the Network Address Translation (NAT) technology was proposed. But the addresses of the IP based voice and video devices behind NATs are private, and therefore they aren't routable in public internet. Furthermore, SIP and H.323 used for multimedia communication over IP network can't traversal NAT because of protocol designing. So the research on SIP traversing NAT is an important issue of MoIP application, and is of critical practical significance.In this thesis, the author discusses the related research on NAT traversal technology, analyzes the key technology referring to NAT traversal according to the requirement and characteristic of NAT traversal in MoIP. With the expatiation and comparison of present primary NAT traversal schemes, the author proposes a solution based on Simple Traversal of UDP through NAT (STUN) to solve this problem. The author also designs and implements the STUN client and STUN server. The solution proposed in this thesis can traverse most kinds of NATs, requireing no modification and extention of related protocols or NAT devices. It can work through series of NAT, thus users can successfully initiate a call to connect the users behind NAT. This solution has been adopted successfully in the MoIP Gateway based on embedded Linux platform, and it can also be applied to other SIP protocol based terminals, which cleans out the obstacle for applying MoIP.