Rather than send an INVITE, which was developed to establish a session,
a new method was created for the simple task of delivering a text message.
The MESSAGE method is also used for sending instant messages through the IMS,
but only when the IM is being sent in ???page??? mode. In other words, the IM is being sent in
connectionless mode without establishing a session with the destination. If connectionoriented
service is required, then an INVITE is used to establish a dialog between the
sender and receiver.
The content of the IM (the text itself) is carried in the message body of the MESSAGE
method. Again, this is only when the IM is being sent using a connectionless service.
When a session is established using the INVITE method, the text itself is placed in a
Message Session Relay Protocol (MSRP) after the session is established.
Notify NOTIFY is used by the S-CSCF to notify application servers (or any other entity)
that have ???subscribed??? to event notification (registration updates) that a subscriber
has changed his or her registration. The NOTIFY method will contain the changes that
were made through the new registration.
One example of how this method is used is message waiting. A message waiting
server would ???SUBSCRIBE??? to event notification. When a message was left on the
subscriber??™s voicemail, the message waiting server would then send a NOTIFY method
to the subscriber device, with the appropriate header identifying the type of event that
Session Initiation Protocol (SIP) 63
has occurred.
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