Subject: [SIP] 183 in call flows.

From: "Fairlie-Cuninghame, Robert" <[email-masked]>
Date: Fri, 13 Apr 2001 04:45:11 -0700

Hi,

I recently noticed in the call flow documents that it states that early
media sessions can be established by simply returning a 183 without SDP
(giving the argument that the early media is unidirectional and thus only
needs the caller's SDP).

I don't remember this change being discussed on the list and I have not seen
this behavior stated in any of the bis or old-183 drafts. Where did it come
from  and why?

Although strictly speaking, the SDP is not required I still think it should
still be required in the 183 response because:
a)	it  now forms a response-code specific behavior that is in contrast
to the general early media handling for other provisional responses, eg SDP
in 180. 
b)	Does not allow symmetric RTP or any other type of NAT friendly
processing to occur (because the source address is unknown).
c)	Is an unnecessary "saving". Why not send the SDP? Sending the
reverse SDP doesn't hurt and is in line with processing for other early
media sessions (eg 180 with SDP). Just because the both SDP are specified,
does not mean that the UAS need accept any media in the forward direction.

What do other people think? I know we all hate early media but at least
let's make the handling of what we have consistent across all response
codes.

Cheers,

Robert.

---

From: Alan Johnston <[email-masked]>
Date: Tue, 17 Apr 2001 13:49:55 -0500

Robert,

See my comments below.

Thanks,
Alan Johnston
sip:[email-masked]

"Fairlie-Cuninghame, Robert" wrote:

>
>
> Hi,
>
> I recently noticed in the call flow documents that it states that
> early media sessions can be established by simply returning a 183
> without SDP (giving the argument that the early media is
> unidirectional and thus only needs the caller's SDP).
>
> I don't remember this change being discussed on the list and I have
> not seen this behavior stated in any of the bis or old-183 drafts.
> Where did it come from  and why?
>

It was discussed at the interim meeting in Plano and also at
Minneapolis.  I did not agree with the conclusion, but I have updated
the call flows draft to reflect the consensus.  The consensus was that
early media is problematic, and as a result the 183 Session Progress
approach described in the expired 183 draft
(http://www.softarmor.com/sipwg/drafts/draft-ietf-sip-183-00.txt) will
be abandoned in favor of a yet to be designed early media approach that
is friendly to forking, hold, etc.  As a result, 2543bis will list the
183 code, but without any requirements.  Also, putting SDP in a 183 also
becomes undefined by any SIP WG document.

>
> Although strictly speaking, the SDP is not required I still think it
> should still be required in the 183 response because:
>
>   a. it  now forms a response-code specific behavior that is in
>      contrast to the general early media handling for other
>      provisional responses, eg SDP in 180.
>   b. Does not allow symmetric RTP or any other type of NAT friendly
>   c. Is an unnecessary "saving". Why not send the SDP? Sending the
>      reverse SDP doesn't hurt and is in line with processing for other
>      early media sessions (eg 180 with SDP). Just because the both SDP
>      are specified, does not mean that the UAS need accept any media
>      in the forward direction.
>
>
I agree that the 183 with SDP is useful, however, others were insistent
that it not be standardized.

>
>
> What do other people think? I know we all hate early media but at
> least let's make the handling of what we have consistent across all
> response codes.
>
> Cheers,
>
> Robert.