会话发起协议 Session Initiation Protocol
（重定向自SIP proxy）
会话发起协议（Session Initiation Protocol，缩写SIP）是一个由IETF MMUSIC工作组开发的协议，作为标准被提议用于创建，修改和终止包括视频，语音，即时通信，在线游戏和虚拟现实等多种多媒体元素在内的交互式用户会话。2000年11月，SIP被正式批准成为3GPP信号协议之一，并成为IMS体系结构的一个永久单元。SIP与H.323一样，是用于VoIP最主要的信令协议之一。
![Example: User1’s UAC uses an Invite Client Transaction to send the initial INVITE (1) message. If no response is received after a timer controlled wait period the UAC may chose to terminate the transaction or retransmit the INVITE. Once a response is received, User1 is confident the INVITE was delivered reliably. User1’s UAC must then acknowledge the response. On delivery of the ACK (2) both sides of the transaction are complete. In this case, a dialog may have been established.[18]](/uploads/202502/11/SIP_signaling4122.png)
