Fortinsky Michael wrote:
I have a question about SDP offer-answer, particularly in the special
case of hold and unhold.
Consider the following sequence of messages:
Message #1: INVITE from A to B with an SDP offer that (among other
things) puts B on hold
Message #2: 200 OK from B to A with an answer to the offer
So far so good.
Message #3: ACK from A to B with a second offer that removes the
hold condition (but does not change anything else)
(1) Is this legal (i.e., the second offer in the ACK that removes the
hold)?
No, this is not legal. If the ACK had a 2nd offer, where would the
answer be?
Nor would this make much of any sense. There can't be a lot of time
between the 200 and the ACK. What would change to clear the hold in that
brief time?
(2) Can anyone provide a reference to the section(s) in the RFC's that
discuss this (sections that either allow it or not)?
3261 talks about two cases:
- offer in invite, answer in first reliable response
(which is only 2xx in 3261, extended in other rfcs.)
- no offer in invite, offer in 200, answer in ACK.
This also permits the answer to be in a provisional response, but then
repeated in the 200.
3262 talks about reliable provisional responses. It allows:
- offer in invite, answer in reliable provisional response
- no offer in invite, offer in first reliable provisional response,
answer in PRACK to the provisional with offer
3311 defines UPDATE. It allows:
- an offer in an UPDATE with answer in the 200 of the UPDATE.
- this can occur in an early dialog (before ACK of the INVITE)
or in a final dialog (after ACK).
In all cases the offer/answer protocol must be followed as defined in 3264.
Paul
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