Study On The Traversing Of SIP Through NAT Based On Soft Switch Network

NGN (Next Generation Network) is the network based on grouping technology, which can provide many services, thus make it applicable in grouping switch net to provide real time voice and multimedia service as a soft switch network. Because the shortage of IP address, from the commission of soft switch network till the IPv6, plentiful terminals have to use the private IP address. As a result, the soft switch stream will pass the NAT (Network Address Translation) equipment when the consumers who owned the private network connect the soft switch network.The protocols, such as SIP (Session Initiation Protocol), H323 and H248, are all carried on the UDP/IP in technology, as the NAT equipment only replace the header of packet data in network layer and transmission layer, the private address in the command information, which is sent out by the consumer, can not be replaced, which make it impossible to orient the address of users exactly and the session can not be build successfully. Some proper methods should be used to realize the communication among operations so that it will support the pass of private network in the soft switch network. As the NGN is used more and more in business, it is meaningful to study the pass of NAT in the SIP protocol, which is one of the most useful protocols.Fistly, the paper introduced the SEP protocol and NAT technology systemically, presented the in-depth study of the operation during which the SIP protocol passed the NAT, analyzed the mechanism of NAT, compared and discussed several methods which can realize the traversing of SIP through NATThe paper mainly studied the Full Proxy methods which can realize the traversing of SIP through NAT, presented the practical settlement model of software in Full Proxy gateway, and introduced the practical function of the gateway. The pass of private network in SIP was realized an last, which was carried out in the following procedure: the fully Proxy gateway was made to be a Sip Server which have the function of B2BUA (Back to Back User Agent), the description information of SDP (Session Description Protocol) in the SEP was modified when the SEP informationarrived the Full Proxy gateway, the transfer of information and relay of media was realized in the full representative gateway,.The advantage of this method is that the pass of private network in SIP can be realized and the NGN service can be carried out without any change of the network in existence and the NAT equipment, and it is not necessary for the private network terminals to support any other protocols. The operator's investment cost is much lower than with other methods. Because of the Full Proxy gateway can analyze and deal with the QoS (Quality of Service) information such as every call bandwidth, the security of the NGN can be ensured in Edge-Access layer. Besides, it can realize the dynamic fire wall of the media stream, ensure the network's security and prevent the bandwidth's embezzlement through controlling the call state.In the end, the conclusion is presented and the further work is brought forward.