Subject: [Sipping] draft-ietf-sipping-offeranswer-02 scenario

From: "Alexeitsev, D" <[email-masked]>
Date: Fri, 27 Jul 2007 14:55:31 +0200

I could not find the answer to the following scenario in the
draft-ietf-sipping-offeranswer-02

The UAS receives INVITE with an initial offer and answers it with a
valid answer, but also changes the version number in SDP o-line of the
answer. 

How this should be understood by the UAC

A) Ignore the change of the version number as it is a valid answer to
the valid initial offer. Offer-answer exchange is successfully finished.

B) Due to the change of the version number consider this answer to be an
offer, forget about the initial offer that was already sent and send the
answer to this considered offer.

C) something else???

Greetings,
Denis Alexeitsev
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From: [email-masked]
Date: Fri, 27 Jul 2007 23:31:13 -0400

From: "Alexeitsev, D" <[email-masked]>

In essentially all cases, the "origin" listed in the o= line of the
answer will be different from the origin listed in the offer, and so
the version numbers going in the two directions on the dialog will be
unrelated.

See section 6 of RFC 3264:

   The answer to an offered session description is based on the offered
   session description.  If the answer is different from the offer in
   any way (different IP addresses, ports, etc.), the origin line MUST
   be different in the answer, since the answer is generated by a
   different entity.  In that case, the version number in the "o=" line
   of the answer is unrelated to the version number in the o line of the
   offer.

Dale

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From: Paul Kyzivat <[email-masked]>
Date: Sun, 29 Jul 2007 21:56:10 -0400

I agree with Dale. Upon reexamining 3264 I agree that it isn't explicit 
about how the o-line may/must change in subsequent *answers*. It is 
however plausible to assume that the intent was that the o-line in a 
subsequent have the same relationship to the prior o-line in that 
direction as the o-line in a subsequent *offer* has to the previous 
o-line in that direction.

NOTE: I refer to the prior sdp in "that direction" because the prior sdp 
may have been in either an offer or an answer. E.g.

Example 1:

A --(offer1,  sdp1)-> B
A <-(answer1, sdp2)-- B
A --(offer2,  sdp3)-> B
A <-(answer2, sdp4)-- B

Example 2:

A --(offer1,  sdp1)-> B
A <-(answer1, sdp2)-- B
A <-(offer2,  sdp3)-- B
A --(answer2, sdp4)-> B

So in each case sdp1 is the prior sdp to sdp3,and sdp2 is the prior sdp 
to sdp4. According to 3264, the o-line of sdp3 must be the same as in 
sdp1, except an increment to the version number if the sdp is not 
otherwise the same.

While it doesn't say so, it is reasonable to assume that the o-line of 
sdp4 must be the same as that of sdp2 except an increment to the version 
number if the sdp is not otherwise the same.

	Paul