Re: [Sipping] PRACK: Does non-200 response cease re-transmissionofreliable 18x?

["Dale Worley" <dworley at nortel.com>]

On Fri, 2009-04-03 at 13:39 -0500, Adam Roach wrote:
> I agree that a UAC can't *count* on a non-200 response stopping the 
> provisional retransmission, since the error may have come from an 
> intermediary. Of course, if the error has an obvious recovery path (e.g. 
> 407), then the UAC should attempt recovery (largely because it doesn't 
> know whether the error came from the UAS or from an intermediary).
> 
> However, if the PRACK makes it all the way to the UAS, reliability has 
> been achieved. Continued retransmission of the provisional response by 
> the UAS at that point provides no benefit. It does, however, consume 
> gratuitous bandwidth. (The UAS does need to deal with the possibility 
> that the UAC will re-attempt the PRACK, however).

Adam's response seems to be the obvious solution to this problem.  The
only thing I would clarify is that if the UAC sends a PRACK which
receives a 4xx response, and the UAS *didn't* send the response, the UAC
will discover this directly by receiving another copy of the reliable
provisional response, and then the UAC will then know that it has to
send a new PRACK acknowledging that reliable provisional resopnse.

I admit that this may have complicated interactions with the
offer/answer model, but otherwise, it seems straightforward, and indeed,
it seems to be the only solution.

But perhaps I am overlooking something.

Dale