Subject: [SIP] SDP: Sub-set or not in the 200 OK?

From: Gonzalo Camarillo <[email-masked]>
Date: Sun, 01 Apr 2001 06:17:50 +0300

Hello,

There have been already several mails showing certain confusion about
SDP usage in SIP.
In the last IETF the conclusion was:
For sendrecv flows (uni-directional flows are easy), the caller puts in
the INVITe the codecs that he can both send AND receive.

The open issue is what the callee returns:
1) a sub-set of the codecs listed in the INVITE that he can send AND
receive.
2) all the codecs that he can send AND receive.

Note that caller and callee have reversed roles if the SDP from the
client to the server is sent in the ACK instead of in the INVITE.

I would like to ask folks not to mix this open issue with another
similar issue: I have a DSP that cannot change codecs on-the-fly, and I
want to pick up just one codec. Let us discuss this issue at other time.

My proposal is to go for solution number 2. This gives a symmetrical
negotiation. Besides, it covers more cases than solution number 1). It
can happen that for a particular codec, the callee can send AND receive
it but the caller can just send it. Therefore, the caller does not
include it in the INVITE. However, if the callee lists it in the 200 OK,
the caller will be able to send it.

Opinions?

Gonzalo
-- 
Gonzalo Camarillo                    Phone :   +1 212 939 71 71
Columbia University                  Mobile:  +358 40 702 35 35
472 Computer Science Building        Fax   :  +358  9 299 30 52
1214 Amsterdam Ave., Mail Code 0401  http://www.hut.fi/~gonzalo
New York, NY 10027                   
USA                              [email-masked]

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