Neelakantan Balasubramanian wrote:
> See below.
>
> Thanks,
> Neel.
> ________________________________________
> From: [EMAIL PROTECTED] [EMAIL PROTECTED] On Behalf Of Iñaki Baz Castillo
> [EMAIL PROTECTED]
> Sent: Friday, November 28, 2008 9:31 AM
> To: sip-implementors@lists.cs.columbia.edu
> Subject: [Sip-implementors] Different SDP in same early-dialog (same To_tag)
>
> Hi, I'm 100% sure that an UAC receiving two different SDP's during the same
> early-dialog (SDP_1 in 183 and SDP_2 in 200) should ignore/discard the second
> SDP_2.
>
>>>> This is not allowed from the same UAS.
This is called out in 3261 section 13.2.1:
o If the initial offer is in an INVITE, the answer MUST be in a
reliable non-failure message from UAS back to UAC which is
correlated to that INVITE. For this specification, that is
only the final 2xx response to that INVITE. That same exact
answer MAY also be placed in any provisional responses sent
prior to the answer. The UAC MUST treat the first session
description it receives as the answer, and MUST ignore any
session descriptions in subsequent responses to the initial
INVITE.
> The offer/answer is atomic. If the endpoint want to change the >>> SDP, I
> have seen few endpoints change the To tag and send a different SDP_2 in 200
> OK. So, to the >>> UAC it is a forked response.
>
> But could I know in which section of RFC 3261 (or other) is it specified? I
> don't remember it now...
This isn't *explicitly* called out in 3261.
You can figure it out by studying forking, and how that results in early
dialogs.
A UAS that changes the to-tag is simply pretending to be multiple UASs
that have received forked copies of the request, each of which then
establishes an early dialog.
Thanks,
Paul
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