Subject: Media negotiation failure in ACK

From: "Adam B. Roach" <[email-masked]>
Date: Tue, 25 May 1999 14:11:18 -0500 (CDT)

I'm trying to figure out how to handle media negotiation failures
for SDP transmitted in a 200 response.  There seem to be two options,
but neither allows for a Warning: header.

C1            C2
|---INVITE--->| No SDP
|<----200-----| Contains SDP which C1 does not support
|-----ACK---->| Contains SDP with no media sections

C1            C2
|---INVITE--->| No SDP
|<----200-----| Contains SDP which C1 does not support
|---CANCEL--->|
|<----200-----|

Is there a better solution to this?

(As Jonathan Rosenberg pointed out earlier, this type of situation 
occurs when interworking with systems which don't indicate media 
preference until call acceptance, such as H.323v1).

Also, as a straw poll for those of you who have already implemented 
clients: does your client support this style of media negotiation
(i.e. INVITE with no SDP)?

-- 
Adam Roach, Ericsson Inc. |  Ph: +1 972 583 7594 | 1010 E. Arapaho, MS L-04
[email-masked]   | Fax: +1 972 669 0154 | Richardson, TX 75081 USA

---

From: Jonathan Rosenberg <[email-masked]>
Date: Tue, 25 May 1999 22:27:32 -0400

> |<----200-----| Contains SDP which C1 does not support

Its worth pointing out that this problem arises only when there are no
media in common at all; since its only in this case that the call is
rejected. I think the aim is to try and emulate what happens in the case
when there are media sections in the SDP in the INVITE (normal case).
When there is, and there is no media in common, the UAS returns an error
and the call is over. So, to emulate this, I think the right solution is
a third, which is to send a BYE after receiving the 200. Note that
sending a CANCEL accomplishes nothing, since the 200 has already been
sent, and it therefore has no effect.

-Jonathan R.
-- 
Jonathan D. Rosenberg                       Lucent Technologies
Member of Technical Staff                   101 Crawfords Corner Rd.
High Speed Networks Research                Holmdel, NJ 07733
FAX: (732) 834-5379                         Rm. 4C-526
EMAIL: [email-masked]
URL: http://www.cs.columbia.edu/~jdrosen