IETF提出的会话初始协议(SIP)，是在IP网上进行多媒体通信的应用层控制协议，可以用来发起、建立以及释放会话。SIP协议灵活简单的特性以及其灵活强大的呼叫控制的功能吸引了越来越多的厂商和运营商。SIP协议还可以与SDP协议配合使用，用来协商会话的媒体属性，因此更易于实现第三方呼叫控制。 -IETF' s Session Initiation Protocol (SIP), IP network multimedia communication application layer control protocol that can be used to initiate the establishment and release session. Flexible SIP protocol characteristics as well as its flexible and powerful call control functions to attract a growing number of vendors and operators. SIP protocol can also be used in conjunction with the SDP protocol, used to negotiate the session' s media properties, it is easier to achieve third-party call control.