Can you provide the complete INVITE and CANCEL messages?
Normally, when a CANCEL is sent there is as yet no dialog.
The rules for forming the CANCEL message mean that in such cases there
will be no to-tag in the CANCEL. The exception to that would be if you
sent a re-INVITE within an established dialog, and then a CANCEL for it.
So, if this was a CANCEL to the initial INVITE, and it was correctly
formed, then the UAS is broken.
If this was a CANCEL to a re-INVITE, and was well formed, and it
occurred before the INVITE transaction timed out, then the UAS is still
wrong. But if the dialog was gone at the time the re-INVITE was
received, then I can at least understand how a UAS might send a 481 to both.
But you asked what you should do. From a practical perspective I would
just consider that the CANCEL failed, and wait for the response to the
INVITE. This is pretty much the only thing that it makes sense to do for
*any* response to CANCEL.
Thanks,
Paul
On 4/27/13 3:39 AM, Aman wrote:
> Hi All,
>
> Stuck on a scenario, want to understand what should be the UAC behavior if
> it receive "481 Call/Transaction Does Not Exist" for the CANCEL it sent to
> terminate the transaction.
>
> RFC3261 is not much clear on this part, here is the call flow,
>
> UAC UAS
> ------------------------> INVITE
> <------------------------ 100 Trying
> <------------------------ 180 Ringing
>
> ------------------------> CANCEL
> <------------------------ 481 CANCEL
>
>
> As per my understanding, the primary purpose of the CANCEL request is to
> terminate a pending transaction. The CANCEL request is never sent to
> terminate a SIP dialog or, a session. The UAC upon receiving the error
> response to a CANCEL request should destroy the transaction ( e.g. INVITE )
> for which it issued the CANCEL request.
>
> INVITE transaction should be destroyed on the UAC side?
> UAC must send the ACK for 481 to clear the transaction/ dialog at UAS side?
>
>
> Cheers,
> Aman
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