Subject: Re: RFC3108 - Possible issue with media line syntax

From: Rajesh Kumar <[email-masked]>
Date: Tue, 06 Nov 2001 19:53:46 -0800

David,
This was discussed. In Hh248, one need not view the SDP returned by a media 
gateway as a verbatim response to the SDP in the add command. When the 
media gateway indicates
         m=audio $ $ $
it means, among other things, that it has chosen to not specify the 
transport and format list. In response, the media gateway may return one or 
more transport/format list pairs.

Similarly, even if the the media gateway controller indicates m=audio $ $ $ 
$ $,  the media gateway would be right in returning a single 
transport/format list pair.

Since the local descriptor provided by MGC is not necessarily a template 
for the MG to fill in, there is no need for the media gateway to send 
m=audio $ $ $ $ $ or m=audio $ $ $ $ $ $ $ etc., even though they are 
syntactically valid. We recommend that it use '$ $'  to indicate that is it 
nor specifying one or more transport/format list pairs.

Does this sound reasonable?

Rajesh

At 02:06 PM 11/6/2001 +0000, you wrote:

>Hello,
>
>I am sending this email to you because you are the authors of 
>RFC-3108.  There may be a problem with the syntax of the media ("m=") 
>line, and I'd appreciate your opinion.
>
>In RFC-3108, the SDP media line has the form:
>
>m=<media> <virtualConnId> <transport#1> <formatlist#1> <transport#2> 
><formatlist#2> ... <transport#M> <formatlist#M>
>
>Now for AAL2, <transport#X> may be: "AAL2/ATMF", "AAL2/ITU", 
>"AAL2/custom", "AAL2/<corporateName>, or "AAL2/IEEE:<oui>".  I.e. there is 
>potentially an infinite number of <transport#X> types.
>
>In H.248, the MGC will often provide a SDP (Local descriptor) in an Add 
>command.  Many of the fields are specified as "$" to allow the MG to 
>select the value.  This may include the transport and formatlist parameters.
>
>If the MGC wants to leave the MG to select the AAL2 profiles in the Add 
>command, what would it specify?
>m=audio $ $ $
>OR
>m=audio $ $ $ $ $
>OR
>m=audio $ $ $ $ $ $ $
>OR ... etc, etc.
>
>Since, in theory, a MG may return multiple <transport#X> types, and the 
>MGC does not know how many, then how many "$ $" pairs should the MGC put 
>in the request?
>
>What is the correct syntax for this case?
>
>Thanks,
>David Barr
>Nortel Networks

--- Rajesh
---------------
Rajesh Kumar
Cisco Systems