There is nothing that forbids changing any of the Allow-* / Accept-* /
etc. headers within a dialog. If you examine the definition of a dialog
you will not find any of them listed as part of dialog state.
From a practical perspective they sometimes need to change, because
circumstances change at or beyond the UA. This shows up often in 3pcc
call flows, such as when a B2BUA is carrying out a transfer on one side
while retaining a single session on the other side.
The scope of these things is very unclear. About all you can reliably
assume is that they are true at the time they are conveyed. Note that
these are carried in OPTIONS too, and it is typically sent out-of-dialog.
Thanks,
Paul
Jagan Mohan wrote:
> Hi All,
>
> I would like to know whether the content (SIP Methods supported) of Allow
> header can change within a dialog.
>
> For example, can 100 Trying message have "Allow: INVITE ACK BYE CANCEL
> OPTIONS UPDATE" and
> 180 Ringing message have "Allow: INVITE ACK BYE
> CANCEL OPTIONS" in the same dialog?
>
> I don't see any specific mention of this in RFC 3261. Logically, I feel
> the behavior of UA sending the above messages is not correct.
>
> Thanks,
> Jagan
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