The Design And Realization Of SIP's Travesing NAT

With the rapid development of IP networks, more and more Service Providers and IP network users give their attention in VoIP (Voice over IP). VoIP allows users to make telephone calls by using a computer network. VoIP converts voice and multimedia signals to digital signals that can travel over the internet, and then converts them back at the other end, so users can speak to anyone with a regular phone number. VoIP may also allow users to make a call directly by using a microphone with a computer.The initiators of VoIP mainly consider that what important is low inputs and operation costs, the telephone system structures based on the SIP have the most obvious superiority in this aspect. SIP is called the Session Initiation Protocol, which is proposed by The Internet Engineering Task Force (IETF), which using signal command. SIP is used in creating, modifying and terminating the conversations participated by one or more users.However, in present network environment, the NAT equipments existence widely, because the IPv4 address resources are limited or because the security of network is considerrd. Well, how the correspondence software based on SIP pass through NAT is one important question in the SIP applied research. On the one hand, NAT only carries on the transformation of the address and the port in the IP level and the TCP/UDP level, On the other hand, SIP is one control protocol on applying level, the address and the port included in the SIP message can't be transformed when passing through NAT, which will result in failing in call, by reason of SIP message making the addressing unsuccessful, or establishing the media channel unsuccessful.Presently, the many resolutions were already applied to traverse the NAT, for example: ALGs (Application Layer Gateways), Middlebox Control Protocol, STUN (Simple Traversal of UDP through NAT), TURN (Traversal Using Relay NAT) and so on. However, when these technologies apply to the different network, all of them have the remarkable advantages and disadvantages, so, user can just apply the...