This alerts devices that this is not a brand new session but a modification
to an existing session. The new request will identify the participants and the
media to be added, and any other modifications to be made.
The SDP in the message body will contain a description of the entire session, as well
as any changes being made to the session. This is to keep all devices in synch and prevent
any confusion between devices.
The SDP will also contain a version field to indicate a change is being made to the
original session. For example, the original session would be version=1, while a change
to that session would be version=2. The entire session description is checked for any
changes from the original INVITE.
Typically the INVITE method is used to modify a session. However, this is not always
the case. The general rule of thumb is to use the INVITE method if there is to be some
form of user interaction (such as acceptance from the user prior to starting a video session).
Otherwise, the UPDATE method can be used.
All the procedures discussed earlier regarding routing and session establishment apply
to session modification as well. Devices must still accept the request by responding
with a successful response; otherwise, the session modification is rejected. The original
session is not usually interrupted if the modification is
136 Chapter 5
Terminating SIP Sessions in the IMS
Any participant in a session can terminate the session.
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