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Re: [MMUSIC] BUNDLE: Session bandwidth (Re: comments on draft-ietf-mmusic-sdp-bundle-negotiation-01)

"Charles Eckel (eckelcu)" <eckelcu@cisco.com> Mon, 05 November 2012 20:04 UTC

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From: "Charles Eckel (eckelcu)" <eckelcu@cisco.com>
To: Harald Alvestrand <harald@alvestrand.no>, "mmusic@ietf.org" <mmusic@ietf.org>
Thread-Topic: [MMUSIC] BUNDLE: Session bandwidth (Re: comments on draft-ietf-mmusic-sdp-bundle-negotiation-01)
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Date: Mon, 05 Nov 2012 20:04:24 +0000
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Subject: Re: [MMUSIC] BUNDLE: Session bandwidth (Re: comments on draft-ietf-mmusic-sdp-bundle-negotiation-01)
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> -----Original Message-----
> From: mmusic-bounces@ietf.org [mailto:mmusic-bounces@ietf.org] On
> Behalf Of Harald Alvestrand
> Sent: Monday, November 05, 2012 10:45 AM
> To: mmusic@ietf.org
> Subject: [MMUSIC] BUNDLE: Session bandwidth (Re: comments on draft-
> ietf-mmusic-sdp-bundle-negotiation-01)
> 
> On 11/04/2012 07:23 AM, Charles Eckel (eckelcu) wrote:
> > 2) Section 6.4.2, reads:
> >
> >     The total proposed bandwidth is the sum of the proposed bandwidth for
> >     each "m=" line associated with a negotiated BUNDLE group.
> >
> > I'm afraid this is an overly simplistic approach. For example, a typical video
> call with content sharing includes one audio m-line and 2 video m-lines. The
> bandwidth is declared at the session level as well as at the media level, but
> the sum of the media levels is typically greater than that of the session
> level. When content sharing occurs, it steals some bandwidth from the main
> video. In the case of BUNDLED m-lines, the proposed bandwidth should
> therefore be the lesser of the sum of the BUNDLE m-lines and the session
> bandwidth.
> >
> I'm not sure this really matters; what we're trying to compute is the b=
> parameter for the "implied RTP session" in the BUNDLE group. What
> matters is that it's computed consistently, which argues for the
> simplest possible computation.
> 
> Last time I looked b=CT was scoped to a whole conference ("supplied for
> the session"), while b=AS was scoped to a single RTP session. But I may
> be misinterpreting RFC 4566 section 5.8.

I recommend using b=TIAS, as defined in RFC 3890. This parameter may be used at either the session or the media level, and it scope accordingly. 
What needs to be avoided is summing up the b=TIAS/AS values from the bundled media lines and using that as that bandwidth for the bundle even if it exceeds the b=TIAS/AS/CT value from the session level.

> But yes, that's one of the advantages of the MMT approach - the sender
> gets to specify instead of the receiver having to compute.

The MMT approach has different challenges when it comes to bandwidth. That is being tracked in another thread (http://www.ietf.org/mail-archive/web/mmusic/current/msg09817.html).

Cheers,
Charles

> 
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