le, the UAS could leave an unanswered INVITE request displayed on a screen for the duration of specified in the Expires header. Once a session is established, the Expires header has no mean- ing-the expiration of the time does not terminate the media session. Instead, a Session-Expires header (Section 6.2.26) can be used to place a time limit on an established session. An example INVITE request with a SDP message body is shown below: INVITE sip: 411@salzburg.at;user=phone SIP/2.0 Via: SIP/2.0/UDP salzburg.edu.at:5060; branch=z9hG4bk1d32hr4 Max-Forwards: 70 To: <sip: 411@salzburg.at;user=phone> From: Christian Doppler <sip:c.doppler@salzburg.edu.atx :tag=817234 Call-ID: 12-45-A5-46-F5@salzburg.edu.at CSeq: 1 INVITE Subject: Train Timetables Contact: sip:c.doppler@salzburg.edu.at Content-Type: application/sdp Content-Length: 151 o=doppler 2890842326 2890844532 IN IP4 salzburg.edu.at s=Phone Call C=IN IP4 50.61.72.83 SIP: Understanding the Session Initiation Protocol m=audio 49172 RTP/AVP 0 a=rtpmap 0 PCMU/8000 In addition to the required headers, this request contains the optional Subject header. Note that this Request-URI contains a phone number. Phone number support in SIP URIs is described in Section 4.2. The mandatory and header field in an INVITE request are shown in Table 4.1. 4.1.2 REGISTER The REGISTER method is used by a user agent to notify a SIP network of its current Contact URI (IP address) and the URI that should have requests routed to this Contact. As mentioned in Section 2.3, SIP registration bears some similarity to cell phone registration on initialization. Registration is not required to enable a user agent to use a proxy server for outgoing calls. It is nec- essary, however, for a user agent to register to receive incoming calls from prox- ies that serve that domain unless some non-SIP mechanism is used by the location service to populate the SIP URIs and Contacts of end-points. A REGISTER request may contain a message body, although its use is not defined in the standard. Depending on the use 