Re: [Sipping] About offeranswer draft:
My previous mail to Christer is about
the same topic.
To be honest, your agree almost all
of your reply. And I once have the same understanding of RFC3261.
But when I reviewed again during the
interworking-testing(in new interworking-testing, we always face some new
equipment and hear some new understanding of RFC3261 from them), I feel
it seams need correction.
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|Paul Kyzivat <pkyzivat@xxxxxxxxx>
|Christer Holmberg <christer.holmberg@xxxxxxxxxxxx>,
"Somogyi, Gabor (NSN - HU/Budapest)" <gabor.somogyi@xxxxxxx>,
OKUMURA Shinji <shinji.okumura@xxxxxxxxxxxx>, "sipping@xxxxxxxx"
|Re: [Sipping] About offeranswer draft:|
> sipping-bounces@xxxxxxxx 写于 2010-04-15 04:38:21:
> > Hi,
> > >> 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
> > >recall the chain of reasoning that derived its illegality
- it wasn't
> > >obvious but it was sound. It was worked out a *long*
> > The rule we have agreed to is very simple: at most one
> > exchange per SIP transaction.
> Yes, it is. And what I talked here is also under this basic rule.
> > Whether it's clearly specified somewhere needs to be checked,
> > I know there are implementations that "update"
the SDP answer from
> > one reliable response to another (within the same transaction),
> > the same transaction, but that is certainly nothing we
> I think it should be mentioned here, what is the *lawful* answer?
> It is the one in the first reliable response.
> As it is the *lawful* answer, I think the UAC should using it when
> get the answer. And this seems *should* be made normative.
> While how UAC handle SDP(from UAS) before the real answer, it can
I think you are arguing that it is "lawful" for the UAS to send
differing values for the SDP successive unreliable responses and in the
subsequent reliable response. And that it is then the responsibility of
the UAC to make this work "right" and "deterministically"
the first and ignoring the subsequent ones. Is that right?
But that makes no sense. The UAS cannot know if the UAC will receive the
first, or any of, the unreliable responses. So if it were to do this odd
behavior it must be satisfied that *any* of then are the one that the
UAC uses. Or else it must be assuming that the UAC *might* use one or
more of the unreliable ones, and eventually *switch* to the one in the
But 3261 is clear that the UAC should use the first one it receives, and
ignore the remainder, *including* the reliable one. There is no
provision for *switching*.
Hence, its a corollary that the only behavior that the UAC can follow
that is valid and consistent in the face of loss of unreliable responses
is for them all to contain the same SDP. So that is the *lawful*
behavior - a UAS that violates this is unreasonable.
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