tion server, making it available to any SIP server trying to find a user. Laura Figure 5-6 Laura sends a BYE (1) INVITE when she hangs up. (2) 180 Ringing (3) 200 OK (4) ACK Conversation (5) BYE (6) 200 OK Chapter 5 Registrar at Figure 5-7 Bob registers his company.com current location to (1) REGISTER the registrar at company.com. (2) 200 OK REGISTER messages also contain the times when the registration per- tains. For instance, Bob can register his present location until four o'clock in the afternoon because he knows that's when he will leave the office. A user can also be registered at several locations at the same time, indicating to the server that it should search for the user at all registered locations until he or she is reached. OPTIONS OPTIONS requests query a server about its capabilities (Fig- ure 5-8), including which methods and which session description protocols it supports. One SIP server might answer to an OPTIONS request that it supports SDP as session description protocol and five methods: INVITE, ACK, CANCEL, BYE, and OPTIONS. Because the server does not support the REGISTER method, I can deduce that it is not a registrar. The OPTIONS method might not look useful now, but as new extensions add new methods to SIP, the OPTIONS method is a great way to discover which methods a certain server supports. An OPTIONS method also returns data that specifies which encodings for message bodies the server understands. If a certain server understands, for instance, a certain compression scheme, the client will be able to send the session descriptions compressed and take the opportunity to save some bandwidth. Types of Proxy Servers Proxy servers can be classified according to the amount of state information that they store during a session. SIP defines three types of proxy servers: call stateful, stateful, and stateless. SIP: Protocol Operation SIP server Figure 5-8 Bob queries a server (1) OPTIONS about its capabilities. (2) 200 OK Call Stateful Proxy Call stateful proxies need to be informed of all the S