The Design And Implementation For NAT/Firewall Traversal System Based On SIP

With the next network technology developing and maturing further, VOIP technologies have become a research hotspot. At present, there are two kinds of technical specifications used in VOIP:ITU's H.323 protocol and the IETF's SIP protocol. H.323 protocol uses the traditional telephone signal, which can easily connect to the traditional telephone network, Therefore it was used widely. However, the SIP protocol draws on the ideas of the other Internet standards and protocols. It is famous for its simple, flexible, compatible and scalability. It has been the future direction of development in VOIP.Internet's rapid expansion causes a crisis over exhausted IPv4 address. NAT is an important technology which can solve the problem. According to its mapping features, NAT can be divided into two major kinds:Asymmetric NAT and Symmetric NAT. The SIP message header and message body contains the local IP address. If one client is an internal host in the network, its local IP address is private. Then the SIP session can't be established. These addresses in SIP signal have become an barrier of traversal NAT.Therefore, according to the features of SIP protocol and various traversal protocols, this paper proposes a comprehensive and efficient traversal method. It is based on an extension protocol of SIP, STUN protocol, and TURN protocol. When SIP message carrying SDP in asymmetric NAT network, it uses STUN protocol to get the local media address as exit NAT address. It uses TURN protocol to get the transit address in symmetric network. Practice has proved that the SIP soft-phone integrated with this method can traverse all kinds of network.The dissertation is comprised of six parts. The first part introduces the present development of the project and the work in this dissertation. The related technologies are introduced in the second part. The third part is the design of the NAT traversal system. The fourth part is the implementation of all modules in the traversal system. The fifth part is the test of the total system. The last part concludes the dissertation, and gives some suggestions for the future work.