ion and invites users to participate in the session. Suddenly, in the middle of the audio-video session, Laura decides she is having a bad-hair day and wants to kill the video component. She modifies the session for audio only. When Bob then decides the conversation is over (I can't imagine why), the session is terminated. Just as telephone systems inform a caller about the status of his or her call setup by playing different tones (busy tone or ringing tone), SIP pro- vides the session initiator with information about the progress of his or her session setup (Figure 4-5). User Mobility SIP can't deliver a session description to a potential participant until he or she has been located. Frequently, a single user might be reached at several locations. For instance, a student using a computer room in the university typically works on a different workstation every day. Thus, he or she is reachable at different Internet Protocol (IP) addresses depending on which computer is available and wants to receive incoming session invitations only at his or her current location. Another person might want, for instance, to receive session invitations on his or her workstation in the morning when the user arrives at the office, on his or her desktop at home in the evening, and on his or her mobile terminal when the user is traveling. The Session Initiation Protocol: SIP Figure 4-5 SIP informs how I will call Laura to see how session establishment she is doing. progresses. Laura Would you like to join a voice session? Laura is being alerted. Her phone is ringing. Laura is willing to join the session you proposed. She has picked up the phone. Conversation SIP URLs We've already mentioned that SIP provides some user mobil- ity. Users in a SIP environment are identified by SIP Uniform Resource Locators (URLs). The format of a SIP URL is similar to an e-mail address, generally consisting of a username and a domain name, which looks some- thing like this: SIP:Bob.Johnson@company.com. In the previous example, if we consult the SIP serve