SIP registration example. SIP: Understanding the Session Initiation Protocol Heisenberg is currently using. The registrar binds the SIP URI of Heisenberg and the IP address of the device in a database that can be used, for example, by the proxy server in Figure 2.2 to locate Heisenberg. When a proxy server with access to the database receives an INVITE request addressed to Heisenberg's URI (i.e., an incoming call), the request will be proxied to the Contact URI of the currently registered device. This registration has no real counterpart in the telephone network, but it is very similar to the registration a wireless phone performs when it is turned on. A cell phone sends its identity to the base station (BS), which then forwards the location and phone number of the cell phone to a home location register (HLR). When the mobile switching center (MSC) receives an incoming call, it consults the HLR to get the current location of the cell phone. Further aspects of SIP mobility are discussed in Chapter 9. The REGISTER message sent by Heisenberg to the SIP registrar server has the form: REGISTER sip:registrar.munich.de SIP/2.0 Via: SIP/2.0/UDP )200.201.202.203:5060;branch=z9hG4bKus19 Max-Forwards: 70 To: Werner Heisenberg <sip:werner.heisenberg@munich.de> From: Werner Heisenberg <sip:werner.heisenberg@munich.dex tag=3431 Call-ID: 23@200.201.202.203 CSeq: 1 REGISTER Contact: sip:werner.heisenberg@200.201.202.203 Content-Length: 0 The Request-URI in the start line of the message contains the address of the registrar server. In a REGISTER request, the To header field contains the URI that is being registered, in this case sip:werner.heisen- berg@munich. de. This results in the To and From header fields usually being the same, although an example of third-party registration is given in Section 4.1.2. The SIP URI in the Contact address is stored by the registrar. The registrar server acknowledges the successful registration by sending a 200 OK response to Heisenberg. The response echoes the Contact informa- tion that has jus