Section 9.1 bis-09 gives the following reason why the CANCEL should not be
sent by the UAC to the UAS for an INVITE before a provisional response is
received.
If it was allowed to send the CANCEL before receiving a
response for the previous request, the server could receive
the CANCEL before the original request.
I believe another important reason is to handle the race condition where the
UAS has already sent a response which has not yet been received by the UAC.
In that case, the UAC would not know the tag value to use in the To field
and the UAS would reject the CANCEL (not being able to match it (including
tags) to the INVITE).
Is this true? Or would the UAS only match against the original request
(pre-response)?
Thanks
Ken
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