| With the development of communication and IP technology, the IP-based distributed communication mode is more and more popular. As a signal-command protocol on application layer, SIP (Session Initiation Protocol) is discussed in amount of science literatures. The SIP stack here referred, is one of the sub-projects of video surveillance system, and is the control stack carried out for corresponding the modules and devices of the whole system. The video surveillance system under the control of the SIP, is able to achieve all the functions expected, reached the requirement set by the R&D plan. Therefore, because of the standardization of SIP, the video surveillance system has a well expansibility. As an extended demonstration analysis of SIP, this paper studies the functions and characters of SIP, with comparision to the VoIP based on SIP and H.323. Further more, this paper discusses the reliability and security of this protocol as well, and suggests the design solution, framework and carry-out method of SIP stack, particularly on the structure,functions and working-flow of UA (User Agent) module. We divided the SIP stack into several levels according to the position of SIP in network and clearly defined the functions to be realized and the borderline of the stack..We divided the code into an amount of modules for realization on the layer which UA is in, and worked them out respectively. Under the framework of SIP stack, we emphasized and compared the achievement methods.With the object-oriented method being selected. We realized the administration, management and functions of UA, and finally completed the resolution and coding task of UA in SIP. The test results taking on the UA module indicate that, UA is able to accomplish the communication with multithreading, multi-user and asynchronous mode. Based on these studies, we completed the whole SIP stack system. Since this system is so large, there are still existing many repetitive and useless codes waiting for our future discussion.
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