 of the media. In telephony, this is described as the separation of control channel and bearer channel. The media session is ended when Heisenberg sends a BYE message: BYE sip: schroed5244@pc33.aol.com SIP/2.0 Via: SIP/2.0/UDP 200.201.202.203:5060;branch=z9hG4bK4332 Max-Forwards: 70 To: E. Schroedinger <sip:schroed5244@aol.com>;tag From: Heisenberg k<sip:werner.heisenberg@munich.de>, tag=314159 Call-ID: 10@100.101.102.103 CSeq: 2000 BYE Content-Length: 0 Introduction to SIP Note that Heisenberg's CSeq was initialized to 2000. Each SIP device maintains its own independent CSeq number space. This is explained in some detail in Section 6.1.5. The Request-URI is set to Schroedinger's Contact URI. Schroedinger confirms with a 200 OK response: SIP/2.0 200 OK Via: SIP/2.0/UDP 1200.201.202.203:5060;branch=z9hG4bK4332 To: E. Schroedinger k<sip:schroed5244@aol.com>;tag=42 From: Heisenberg <sip:werner.heisenberg@munich.de> tag=314159 Call-ID: 10@100.101.102.103 CSeq: 2000 BYE Content-Length: 0 Not discussed in the previous example is the question of how the database accessed by the proxy contained Heisenberg's current IP address. There are many ways this could be done using SIP or other protocols. The mechanism for accomplishing this using SIP is called registration and is discussed in the next section. 2.3 SIP Registration Example In this example, shown in Figure 2.3, Heisenberg sends a SIP REGISTER request to a type of SIP server known as a registrar server. The SIP registrar server receives the message uses the information in the request to update the database used by proxies to route SIP requests. Contained in the REGISTER message To header is the SIP URI address of Heisenberg. This is Heisenberg's "well-known" address, perhaps printed on his business card or published on a Web page or in a directory. Also contained in the REGISTER is a Contact URI, which represents the current device (and its IP address) that the user Heisenberg Registrar server REGISTER Contact: sip:werner.heisenberg@200.201.202.203 200 OK Figure 2.3 