ts reserved CHAPTER 3.6 Real-time communication over networks IP phone Gate- Internet Traditional phones Figure 3.6-1 A typical configuration for a VolP system with both IP-based devices and traditional PSTN phones. widely used signaling protocol for real-time conversa- There are two types of SIP messages: request and tional applications over IP networks. As a signaling pro- response. The request message is initiated by a user agent tocol, SIP provides the following functions: client (UAC) for registering, call setup, tear down, ac- Call setup and tear down; knowledgment, etc., while the response message is gen- Advanced features, such as call hold, call waiting, call erated by a user agent server (UAS) or an SIP server in forwarding, and call transfer; response to the request. Capability exchange; The request message in SIP, as with the other IETF protocols (e.g., RTSP), is called a "method." There are Interoperability between different types of networks six fundamental SIP methods considered as basic sig- (e.g., PSTN) and different signaling protocols (e.g., naling for call setup and tear down, which are defined in H.323); IETF RFC 3261 [1]: INVITE, ACK, BYE, REGISTER, Multicasting. CANCEL, and OPTIONS. Specifically, the INVITE Moreover, SIP has been designed to be scalable enough to method is used to initiate a call. The ACK method is used support simultaneous calls for a substantial number of by the call originator to acknowledge the final response to users and to be extensible enough to include more fea- the INVITE request. The BYE method is used to ter- tures and functions in the future. minate a call. The REGISTER method is used by a user SIP may be transported by either TCP or UDP. TCP agent (UA) to register itself to an SIP server with the provides a reliable, connection-oriented transport, while addressing information (contact URI). The CANCEL UDP provides a best-effort, connectionless transport method is used to cancel the request sent earlier. The across the Internet. Port numbers 5060 and 5061 are th