VOCAL是vovida.org开发的SIP系统，应该是目前功能最完善，使用者最多的开源SIP协议栈，这是一些最新相关的文档说明，相关协议实现可以参考网站：http://www.huihoo.com/telecom/vovida_sip/-development of the SIP system, it should be is the most complete, most users of the open-source SIP protocol stack, which is a document related to the latest statement, related agreements can achieve reference website : http://www.huihoo.com/telecom/ vovida_sip /