Christer,
Your comments indicate how including SDP in extra places would not
constitute offers or answers and so how it ought not cause problems.
But regardless of that, SDP in those places does not seem to be allowed
by existing standards. There is already enough problem with redundant
SDP that was grandfathered in by the wording of the specs, and it has
proven to cause trouble. So I would not want to explicitly allow this in
other places.
If you want to put SDP in these places, in a non-conformant way, with
the expectation that others will not be bothered by it, they you are
free to try that. But don't complain if somebody handles it in a way
different from what you expect.
Paul
Christer Holmberg (JO/LMF) wrote:
> Hi,
>
>>> All,
>>>
>>> This was talked before but it still looks not clear:
>>>
>>> UAC UAS
>>>
>>> ----------->F1 INVITE w/o SDP
>>>
>>> <-----------F2 18X w/ SDP1
>>>
>>> ----------->F3 PRACK w/ SDP2
>>>
>>> <-----------F4 200 for PRACK
>>>
>>> <-----------F5 200 for INVITE w/ SDP1
>>>
>>> ----------->F6 ACK (w/ SDP2 ???)
>>>
>>>
>>>
>>> My questions:
>>>
>>> 1. Could 200 for INVITE in F5 carry an SDP?
>
>> No. There has already been one o/a in the transaction initiated F1.
>> There cannot be another offer in the same transaction.
> As far as I understand, the 200 for INVITE does not carry a new offer. It
> carries the same answer that was carried in 18x (F2).
>
>
>> In the case where the invite carries an offer sdp, and there is SDP in a
>> provisional response that same sdp can be in the 200. That wording in
>> 3261 that specifies this is intended for unreliable provisionals, but
>> has been interpreted to also be possible when there was a reliable
>> response with an answer. But I can find no rationale for allowing an
>> offer that was sent in a reliable provisional to be sent again in the
>> 200 response to the invite.
>
> Since SIP only allows at most one o/a exchange per SIP transaction, I think
> the o/a state machine should know that the SDP in 200 is of no meaning - no
> matter if it's a copy of a previously sent (in a reliable 18x) offer or anwer.
>
> It's the same thing if you send an offer, or answer, in a reliable 18x, and
> then copy it in subsequent reliable 18x responses for the same transaction -
> from an o/a perspective they have no meaning.
>
>
>> Also I can find no rationale for allowing a *new* offer to be included
>> in the 200 response to an invite if the initial o/a has been completed.
>
> Correct. Again, at most one o/a exchange per SIP transaction. Keeping that in
> mind will take you pretty far, I think :)
>
>
>>> 2. If 200 in F5 carries the same SDP with that in 18X, should UAC send
>>> an ACK with SDP?
>>>
>> Definitely not.
>
> Agree.
>
> Regards,
>
> Christer
>
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