Paul Kyzivat <pkyzivat@xxxxxxxxx> 写于 2010-04-14 22:12:48:
>
>
> Somogyi, Gabor (NSN - HU/Budapest) wrote:
> > Hi,
> >
> > RFC3261: "...MUST ignore any session descriptions in subsequent
> > responses..."
> > I think that the common industry understanding of RFC3261 is that 1
> > offer has 1 answer, even though that 1 answer may be transmitted several
> > times.
>
> Yes. Well, actually one answer per dialog. (With forking, an offer in
> the initial invite will get a separate answer per-forked-dialog.)
>
> > And the 1st transmission is used (treated as THE answer). While
> > you are speaking about several answers with 1 matching offer. That is a
> > fundamental difference.
>
> This of course only makes sense if the sdp in all unreliable responses
> is the same as the sdp in the first reliable response. That is so
> because any/all of the unreliable responses may be lost. You cannot
> count on the UAC using the SDP from the first transmission.
>
> And because of that, a valid implementation could drop all the SDP
> received unreliably and only process the one received reliably.
Supporting of this.
My point here is that making UAC's using of the SDP in first reliable response normatively, no matter how many SDP it received before the *real* answer. And how UAC handle SDP(from UAS) before the real answer is another issue, it can be BCP issue.
>
> > In your chart SDP4 is a reliable answer. Therefore SDP5 might be
> > interpreted as a new offer, hence UAC could send an answer in PRACK.
> > Quite similarly to 3PCC cases, where 200 contains the offer and ACK the
> > answer.
>
> That has been investigated. Its not allowed. (Unfortunately I cannot
> recall the chain of reasoning that derived its illegality - it wasn't
> obvious but it was sound. It was worked out a *long* time ago.)
>
> Thanks,
> Paul
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