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[MEDIACTRL] Control Framework SDP Review - Issue 3

Issue 3:

One other concern I have is how CFW interacts with SIP forking.  As far
as I can tell, if an initiator sends an initial SDP offer with setup
mode "passive" or "actpass" (which, as the draft mentions, may be
necessary for topology reasons), and the INVITE is forked, the initiator
could receive multiple answers and thus multiple incoming connections.

[Chris] That is possible.

As far as I can tell, all such answers will have identical cfw-id
values, and thus the initial offerer has no way to correlate them.  (One
solution to this problem would be for answerers to have their own unique
cfw-id values, separate from those of offerers; the CFW Dialog-ID header
field would then need to be sent in the 200 response to SYNC as well as
in SYNC request.)

[Chris] Agreed.  I would propose that we use Jonathan's suggestion of making the cfw-id unique per endpoint.  The endpoint initiating the connection then inserts its own cfw-id into the SYNC message.  This will then allow an endpoint receiving connections to cope with a forked SIP call.  We would also need guidelines that inform SIP UAS acting in the 'active' role that they must wait for the incoming ACK before sending the SYNC message otherwise we create a race between the SIP response and the SYNC.  Using multiple entities to generate the cfw-id also means that we need to tighten up on exactly how its generated - I will propose some text in the next couple of days.  This is a worthwhile change and provides a more complete solution.  Any comments?  Suggestions or problems?

This raises another interesting point that the document is not very strong on procedures for terminating.  I also think we need to add some text to the end of both sections 4.1 and 4.2 on how to act when terminating a session.  While we infer its linked to the SIP session etc I think we need to be more explicit.

--
Chris Boulton
CTO & Co-founder
NS-Technologies <http://www.ns-technologies.com>
m: +44.7876.476681