Hi Paul,
While considering one problem in our production's interoperability testing, I re-read some parts of offeranswer draft and find something might be deserving discussion.
//begin of text(part):
For example, in Figure 1, only the SDP in F6 is the answer. The SDP
in the non-reliable response (F2) is the preview of the answer and
Kyzivat & Sawada Expires September 9, 2010 [Page 8]
Internet-Draft SIP Usage of the Offer/Answer Model March 2010
must be the same as the answer in F6. Receiving F2, the UAC should
act as if it receives the answer.
//end of text(part)
[Gao] In fact, UAS sending SDP in non-reliable response is for potential early media usage. Considering some UAS may have different address for early media channel and the final session, some UAS may send different SDP(compare with the answer) in non-reliable response. And I really found such equipment inside and outside of ZTE. And considering UAC, I think we should allow the UAC ignore the SDP in non-reliable response, while some UAC really do not handle any SDP which is not offer or answer.
But the permissibility of the degree of the difference might be delicate. If the non-answer SDP just has different ip address or port, it seams OK. If the non-answer SDP has different media streams, it would be hard to handle for UAC.
And I re-read correlative part of RFC3261. I don't know that whether allowing different SDP(compare with the answer) in non-reliable response is violation/correction of current text or not.
//correlative part of RFC3261
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.
Thanks,
Gao
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