Hi Paul,
inlines.
On Thu, Apr 15, 2010 at 10:42 PM, Paul Kyzivat <pkyzivat@xxxxxxxxx> wrote:
Eric - inline
Eric wang wrote:
HI,
inlines.
BR.
2010/4/15 <gao.yang2@xxxxxxxxxx <mailto:gao.yang2@xxxxxxxxxx>>
Paul Kyzivat <pkyzivat@xxxxxxxxx <mailto: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.
Eric: I think, the UAC should listen the SDP sending packets,and choose another to listen according to rfc3264.
Reuse Shinji 's chart, if SDP2 sends packets while SDP3/SDP4 don't,
the UAC should listen SDP2 and SDP5.
Eric,
I think you may be talking about cases where the call has been forked to different destinations, and the distinct answers are coming from them. Is that right?
No. I meant there may be several non-reliable responses in a *single* dialog.
There maybe several servers that want to send non-reliable response with SDP(neither offer nor answer) to the caller.I accept it as non-reliable responses with SDP can also establish early dialog for tone/announcement and do nothing with offer/answer negotiation between the caller and the called.
I know it violate rfc3261, but it's useful and simple.
I know it violate rfc3261, but it's useful and simple.
Because in that case the responses should have different to-tags, thus becoming distinct (early) dialogs. The discussion we are having is about what happens in a *single* dialog. And in a single dialog the behavior you describe is *wrong*.
But I cannot accept that several reliable responses with SDP appear in a single dialog, and it seems be allowed in Shinji's chart.
BR
Eric
When there are multiple early dialogs, it is indeed a challenge for the UAC to figure out what to do. And one of the things it might choose is to listen to one input stream and ignore the others. Unfortunately, there is universal and certain way to associate the incoming media streams you are receiving with the answers you have received in the signaling. You can do so in certain cases that may apply to you, such as when symmetric RTP is used (address/port of sender is same as address/port that is listened on.)
Thanks,
Paul
UAC UAS<mailto:sip-implementors@xxxxxxxxxxxxxxx> for questions on current sip
| F1 INVITE (SDP1) | <-- offer
|-------------------->|
| F2 1xx (SDP2) |
|<--------------------|
| F3 1xx (SDP3) |
|<--------------------|
| F4 1xx (SDP4) | |<--------------------|
| F5 1xx-rel (SDP5) |<-- answer
|<--------------------|
>
> > 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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