 network games let players send short messages to their opponents while play is in progress. Instant messages can also be used in combination with voice. Say Bob is on the phone with Laura trying to explain the correct spelling of a word to her. A short message is much faster than speaking all the letters of the word one by one. SIP with some extensions can provide simple instant- messaging services. How It's Done A new method called MESSAGE was defined [draft- rosenberg-impp-im] to carry in its body the message that the sender wrote. The big advantage of using SIP for delivering instant messages is that prox- Chapter 6 ies route a MESSAGE request as a BYE request. Therefore, the same infra- structure deployed for establishing multimedia sessions can be reused for providing instant-messaging services without any modifications. Proxies do not need to be aware of this new service in order to route MESSAGE requests. Figure 6-8 shows Bob and Laura in the midst of a conversation. They establish a SIP session using an INVITE and begin talking. While they're discussing vacation plans, Bob needs to explain to Laura how to spell a word. He sends her an instant message and then they resume conversing by voice. Automatic Configuration of UAs One of the main features of SIP is that it provides user mobility through registrations. UAs register their current location to SIP servers SO that these servers can route INVITE requests correctly. It is essential for a SIP UA to know which server is handling a particular domain in order to regis- ter with that SIP server. The more mobile the user, the more difficult is for him or her to remem- ber all of the data needed to configure his or her SIP UAs to work in each different domain where the user can be reached. Figure 6-8 How do you spell the Bob sends a short name of the place we are message to Laura in going on vacation? the middle of a voice Laura session. (1) INVITE (2) 200 OK (3) ACK (4) MESSAGE "Costa del Sol" (5) 200 OK Extending SIP: The SIP Toolkit Bob, for example, has a 