 Protocol IM Client 1 Proxy Proxy IM Client 2 MESSAGE MESSAGE 200 OK 200 OK MESSAGE MESSAGE 200 OK 200 OK Figure 4.9 SIP instant message example. IM Client 1 Proxy IM Client 2 INVITE INVITE 200 OK 200 OK Instant message session Figure 4.10 Using SIP to establish an instant messaging session. An example MESSAGE request is shown below: MESSAGE sip :editor@rcs.org SIP/2.0 Via SIP/2.0/UDP lab.mendeleev.org : 5060; branch=z9hG4bk3 Max-Forwards : 70 SIP Request Messages To: <editor@rcs.org> From: "D. I. Mendeleev" <dmitry@mendeleev.org>;tag=1865 Call-ID: 93847197172049343 CSeq: 5634 MESSAGE Subject: First Row Contact: <sip:dmitry@lab.mendeleev.org> Content-Type: text/plain Content-Length: 5 H, He Table 4.11 lists the mandatory header fields in a MESSAGE request. 4.1.11 INFO The INFO [16] method is used by a user agent to send call signaling informa- tion to another user agent with which it has an established media session. This is different from a re-INVITE since it does not change the media characteristics of the call. The request is end-to-end, and is never initiated by proxies. A proxy will always forward an INFO request-it is up to the UAS to check to see if the dialog is valid. INFO requests for unknown dialogs receive a 481 Transac- tion/Dialog Does Not Exist response. An INFO method typically contains a message body. The contents may be signaling information, a midcall event, or some sort of stimulus. INFO has been proposed to carry certain PSTN midcall signaling information such as ISUP USR messages. The INFO method always increments the CSeq. An example INFO method is: INFO sip poynting@mason.edu.uk SIP/2.0 Via: SIP/2.0/UDP cavendish.kings.cambridge.edu.uk branch=z9hG4bK24555 Max-Forwards: 70 Table 4.11 Mandatory Header Fields for a MESSAGE Call-I Max-Forwards SIP: Understanding the Session Initiation Protocol To: John Poynting <sip:nting@mason.edu.uk> ;tag=3432 From: J.C. Maxwell <sip:james.maxwell@kings.cambridge.edu.uk> ; tag=432485820183 Call-ID: 18437@cavendish.kings.cambridge.edu.uk CSeq: 6 INFO Content-