一个例子 自己动手写sip。 包括设计文档和部分源码。由于是毕设文档，重在设计，代码仅起示范作用，希望能起到启迪作用。-An example to write their own sip. Including design documentation and some source code. Because it is located to complete the document, focusing on design, code only the role model, hoping to play a role in inspiration.