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 t