[Serusers] SER Nokia CANCEL Problem

[Jiri Kuthan]

At 16:56 19/02/2007, Bogdan Pintea wrote:
>As a side observation regarding to protocol's fault tolerance / race prone, it would seem there's no way for a stateful proxy to let a callee (1) whose provisional reply got lost (or not sent, in the first place, as provisionals is a MUST only for stateful proxies), but (2) whose final answer was received, about the fact that the call was canceled in the meantime, even though the proxy has the clear picture - and the only one that can have it - at the moment it sees the final reply from callee;

Hi Bogdan,

just thinking loudly how to best narrow down the problem. I think there are two:
one (2) is race-condition. This occurs even if no provisional answer is lost --
it is just that CANCEL meets 200 on the net and client has then to sort it out.

The other problem (1) is a kind of amplifier in that it enlarges the race condition
window through lacking reliability of provisional answers.

So I think that the solution is the same ... put the burden on UAC. It is then
UAC's responsibility to terminate a too-late-cancelled call (even if "too late"
is actually caused by reliability issues).

   RFC3261
   "If the INVITE results in 2xx final response(s)
   to the INVITE, this means that a UAS accepted the invitation while
   the CANCEL was in progress.  The UAC MAY continue with the sessions
   established by any 2xx responses, or MAY terminate them with BYE."

So I guess that the harm is sustainable even though it indeed doesnt look
too nice.
> (callee which will, however, time out, eventually, waiting for the ACK).

How come? UAS keeps retransmitting 200s till ACK comes. The ACK should come
rather early. It is then UAC's choice to BYE the too-late-cancelled 
call or not.

-jiri