I have the following questions on CANCEL issued by a UAC (not a proxy
client)
1. If the UAC has received a provisional response with a TO
tag, should it use the tag in the TO of the CANCEL?
This is what bis-04 has to say on this, in section 5.2 -
The Request-URI, topmost Via, Call-ID, To, the numeric part
of CSeq
and From header fields in the CANCEL request are identical to those
in the original request being cancelled, including tags. This allows
a CANCEL request to be matched with the request it cancels.
The plural tags implies "FROM and TO tags", but the original request
may not have had a TO tag, if it is the request initiating the session.
2. If the UAC has received provisional responses from multiple UASes
due to proxy forking, and then received a final response from one of them,
can it send a CANCEL followed by BYE to each of the other call legs?
The SIP spec specifies that a proxy SHOULD (not MUST) CANCEL other branches
if a 2xx or 6xx response is received. So, it is up to the UA to CANCEL other
branches and reclaim its own resources associated with other call legs since
the other branches may have the phones ringing for a long while.
3. If the UAC sends a CANCEL to the proxy with a TO tag, what is the
mandated behavior of the proxy? Is it to CANCEL just the branch which is
associated with that call leg?
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