Design And Implementation Of A Scheme For SIP Traversing NAT Based On Application-Layer Routing

The purpose of this paper is to seamlessly connect SIP services in the network containing NAT devices. Although SIP is a newly proposed network protocol, it is able to combine all the communication capabilities into a larger scale seamlessly, like VoIP, multi-media conference, push-to-talk, locating service, online messaging and instant messaging. Nevertheless, the NAT services having been deployed in the network make transferring SIP messages difficult, thus prohibit the popularity of SIP applications. In order to solve this problem, this paper will introduce SIP in detail firstly, and find the key of the issue is the inability of NAT devices to translate the special information containing SIP application-layer routing. We investigate present researches on NAT, and discover that present researches do not solve the problem correctly. On the contrary, they make assumptions on the network topologies. As a result, the cost for applying SIP technology in an organization is increased greatly, which will discourage enterprise from deploying SIP applications. Basing on our analysis, we implement an effective scheme for SIP message traversing NAT, which focuses on network address translation for application-layer routing information in SIP messages and designs a core algorithm for it. The advantage of our scheme includes following aspects: Our implementation is not restricted to any NAT architecture, it could be used a SIP processing component in any present application-layer NAT scheme; It is applicable to any SIP topologies, that is to say no additional servers are required; There is no need to recognize the entire SIP message, but only those that must be translating, therefore it could not only make NAT process SIP packets more quickly, but also lower the occupancy of NAT storage. Moreover, this paper makes use of "stack" to analyze the application-layer routing of SIP, and reduces the topology space of SIP NAT network, which will be used for validation of our scheme.Generally, this paper not only solve the problem of SIP traversing in NAT network, but also increase our knowledge of SIP and NAT, thus makes rich references for designing and developing for both fields.