[Sip] Re: draft-ietf-sip-replaces-04.txt

Rohan Mahy wrote:
> Paul,
> 
> Thanks for bringing this up.  I think the draft needs to distinguish 
> between these cases:
> 
> 1) The UA receives an offer with no compatible media and does not wish 
> to maintain a session with no associated media.
> 
> 2) The UA receives an offer but does not wish to send/receive media at 
> the moment (possibly because it is negotiating a session using 3pcc on 
> behalf of another UA).
> 
> I think the same distinction applies to UAs that receive "ordinary" 
> INVITEs.  They can accept an offer but reject all the m-lines, or they 
> can send a 4xx response. So...

I agree that this is a general issue for invites - or perhaps more 
generally for offer/answer.

> - Do you think 3261 needs any clarification to make this distinction 
> obvious?
> - Can you propose some text for Replaces that makes clear my original 
> intention (what you do in case 1), while still allowing for case 2?

I have been thinking about this problem for some time. I have an idea, 
but I have been reluctant to offer it because it seems to be overly 
complex. Nevertheless...

How about preconditions?

- In the absence of preconditions to the contrary, the assumption would 
be that the offerer is satisfied with whatever it can get, including 
nothing at all. It is up to the answerer to decide what is sufficient 
and what is not.

- some kind of new media precondition would provide the opportunity for 
the two sides to do an extended negotiation for mutually satisfactory 
media before alerting anyone. If it becomes apparent that the conditions 
cannot be met, then the invitation containing the offers and answers fails.

I haven't thought enough about this to go further and define exactly how 
the precondition would be expressed. But how does the general concept sound?

An advantage to this is that it doesn't change 3261, and is upward 
compatible with current practice.

	Paul

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