For example, if a cell phone is attempting to
set up a call and sends an INVITE, but the user then hangs up their phone immediately
after dialing, the call is released and the session is ended. However, the session was
never really established, because a response was never received by the originator.
When these scenarios happen, the device will send a CANCEL method to the destination,
which then cancels the original request and discards the request message.
What happens if the CANCEL is received by the destination after the response was
sent (they cross each other)? When the response is received by the originating device,
it is ignored.
Info The INFO method was added as an extension to the SIP protocol for the purpose
of communicating mid-call information between endpoints. Information could include
dialed digits, possibly a picture file being exchanged between the two callers, or even
ISUP signaling parameters that one of the endpoints needs to process to support the
call in progress.
62 Chapter 3
The RFC does not define exact uses for the method, other than to outline the possible
uses for this method and how it should be processed by the various entities within the
network. Since the contents of the INFO method may contain information that needs to
be processed by entities supporting the call, it must be routed using the same path as the
call setup.
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