Soft Switch and SIP that is mainstream application protocol in NGN are studied. The structure of SIP message and process of building connection are introduced, a SIP stack model is set to program.
其次，对SIP这个NGN软交换的核心协议进行深入的研究，分析SIP消息包的结构，理解SIP协议中规定的通信连接流程和状态机维护过程，建立了一个遵循协议规定的SIP协议栈模型。
The article introduce SIP protocol, design stack of sip and realize sip user proxy;
本文所作主要工作为引入了SIP协议，设计了SIP协议栈，并有SIP用户代理的粗步实现；
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