> I hope this holds good, even if there is change
> of offer/answer in between (18X reliable & 200 ok).
> My doubt is that, the UAS can still send the same
> session description in 200 ok (of Invite) as in 18x
> (reliable). This session description could be
> different from what was exchanged previously using
> PRACK/UPDATE. Or there should not be any session
> description at all in 200 ok (of Invite) in this case?
Including the SDP is optional within subsequent 18x/2xx and discouraged by RFC
6337.
> Or if present, the UAC should ignore it?
Yes; it MUST be ignored per RFC 3261 (although RFC 6337 indicates should be
ignored). However to accommodate interoperability with some vendors that don't
support 100rel, forking, and UPDATE, some vendors have configuration options to
not ignore the SDP change.
RFC 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.
RFC 6337 section 3.1.1:
3. If the second and subsequent SDP (including a real answer) is
different from the first SDP, the UAC should consider that the
SDP is equal to the first SDP. Therefore, the UAC should not
switch to the new SDP.
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