Subject: RE: AW: [MMUSIC] I-D ACTION:draft-ietf-mmusic-sdp-new-11.txt

From: "Kevin Boyle" <[email-masked]>
Date: Thu, 7 Nov 2002 16:22:16 -0500

So instead of redefining ptime, create a new "mptime" parm that has a
construct similar to the m line:

m=audio 49170 RTP/AVP 0 4 18
A=mptime:10 30 30

(or whatever... This is a really contrived example, since the ptime for
G.723.1 is always 30).

The feedback I've seen on previous proposals to this end have met with some
resistance, but I don't see why.  Earlier I made the mistake of saying that
QoS functions could use this to great benefit (which they can), although
there are other non-QoS uses as well, so I think it is too narrow a scope to
consider only the QoS aspect.  For example, if a Megaco or MGCP controller
wants to explicitly limit bandwidth usage due to network load, then it could
use this to restrict packetization times for certain codecs to make the most
efficient use of the available bandwidth.

This issue has already come to light in PacketCable's work, and no doubt
will be run into in both Megaco and SIP eventually, if they aren't already
trying to "fix" this shortcoming.  It would be nice to get a common solution
in the base SDP specification (or a sidekick draft), rather than having each
SDP client application define its own way of doing things, and thereby
diverging from each other.

Kevin

-----Original Message-----
From: Colin Perkins [mailto:[email-masked]] 
Sent: Thursday, November 07, 2002 10:53 AM
To: Kreuter Ruediger
Cc: [email-masked]
Subject: Re: AW: [MMUSIC] I-D ACTION:draft-ietf-mmusic-sdp-new-11.txt 

--> Kreuter Ruediger writes:
>If I understand this correct, then it is allowed to signal several 
>codecs with a session description, e.g. m=audio 49170 RTP/AVP 0 4 18
>indicating that PCMU G.723.1 or G.729 may be used within the session;
>
>But there is only one media description for packetization time, e.g. 
>a=ptime:10 which is then the recommendation for encoding the packet.
>
>So, if it is allowed to use any codec in the list we get in trouble by 
>using 10ms packets with G.723.1 On the other side, if we use 
>a=ptime:30, since this is the smallest common value, then we add extra 
>delay to G.711 packets without need.
>
>That is, it would be useful to have a ptime per codec.

Changing the definition of ptime would break backwards compatibility. If
this functionality is needed, you should make sure that it's in SDPng.

Colin
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From: Colin Perkins <[email-masked]>
Date: Thu, 07 Nov 2002 16:33:27 -0500

We'd be happy to consider any draft you submit on a=mptime. I have no plans
to add extensions to the base SDP spec.

Colin