Subject: [MMUSIC] SDP media stream with non symmetric codec

From: "SOLLAUD Aurelien RD-TECH-LAN" <[email-masked]>
Date: Wed, 11 Jul 2007 15:38:29 +0200

Hi

When reading RFC 3388 (grouping of media lines in SDP), in 7.4.1 there
is an example of use of the FID grouping :
 <quote>
   The next example shows how the "group" attribute used with FID
   semantics can indicate the use of two different codecs in the two
   directions of a bidirectional media stream.
       [...]
       a=group:FID 1 2
       m=audio 30000 RTP/AVP 0
       a=mid:1
       m=audio 30002 RTP/AVP 8
       a=recvonly
       a=mid:2
 </quote>

What I would like to express in SDP is a non symmetric stream, on a
single RTP port, with one codec (or set of codecs) in one direction, and
another codec (or set of codecs) in the other direction. It is close to
the previous example, but Section 7.5.3 explicitly forbids to have the
same transport address for media lines under a FID group.

So Say I define a new XYZ grouping that allows that.

I would have 
       [...]
       a=group:XYZ 1 2
       m=audio 30000 RTP/AVP 0
       a=sendonly
       a=mid:1
       m=audio 30000 RTP/AVP 8
       a=recvonly
       a=mid:2

The problem I see is if the receiver of this SDP offer does not
understand grouping, it will see 2 media lines with the same transport
address, so the behavior is undefined (RFC 4566). In addition, if the
receiver is designed to support only one media stream, it will take the
first one and the result will be a mono-directional stream, which is not
what was desired.

A solution with a better fallback would be to define a new SDP attribute
'xyz' to express the non-symmetry. Something like:
       [...]
       m=audio 30000 RTP/AVP 0 8
       a=xyz send=0;recv=8

If the receiver of this offer does not understand the a=xyz, it will
ignore it and we have a simple case with one bidirectional stream, which
is not so bad.

Any comments on the idea ?
new grouping vs new attribute ?

Thanks
Aurelien

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