Re: [MMUSIC] M-lines: Wikifying it - a=label

Paul Kyzivat <> Mon, 03 December 2012 21:37 UTC

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Date: Mon, 03 Dec 2012 16:37:52 -0500
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Subject: Re: [MMUSIC] M-lines: Wikifying it - a=label
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On 12/3/12 6:06 AM, Harald Alvestrand wrote:

>>>> Regarding a=label: This could be any of T/F/M, depending on
>>>> application use. I think we must assume T, and consider an alternative
>>>> for F. (And for M if we think it is needed.)
>> I may have misunderstood your intended meaning for M. I assume you
>> meant all RTP streams of the same media type (audio or video) within a
>> single 5-tuple.
> I was thinking "same payload type", since the payload type is the
> distinguisher used for groups of flows that should have the same
> parameters in many cases.
>>> The claim that a=label can be used to mark flows makes it F.
>> a=label is a media-level attribute, so it is bound to an m-line.
>> In the absence of bundle, regardless of whether the corresponding RTP
>> session has one or several flows. The m-line has a particular media
>> type, so I guess it could be viewed as either T or M.
> The reason why we're having the discussion is that we have two competing
> definitons of what an M-line should be, and need to choose one.
> So arguing from whether or not something associates with an M-line is
> not useful.
>> Some applications of a=label may assume or require that the labeled
>> m-line contain only one RTP stream. If so, then perhaps the label
>> could be considered to be F. But we can't really know that.
> What applications of a=label do you know about?

I know of two: BFCP and SIPREC.

In BFCP the label is used to associate a bfcp floor with an m-line. I 
don't think it matters whether that m-line carries one or several RTP 

We can speculate how we would want this to work in the case that several 
media m-lines were bundled together. I don't know - too many hypotheticals.

In SIPREC, the label is used to associate an m-line with certain 
metadata that describes the source of the media being sent via that 
m-line. The siprec group has intentionally kept the question of whether 
the content in that m-line is a single RTP stream or if it can be 
several multiplexed together. There is nothing to forbid multiplexing, 
but no specification of what it means either.

SIPREC *might* have specified use of bundle, or explicitly specified 
multiplexing of RTP streams over a single m-line, if those things were 
better defined. But how it might eventually use bundle or mmt is again a 
hypothetical right now.

A problem for siprec is that it is recording media from existing sip 
sessions, without any knowledge of how they use SDP and RTP. We could 
find it recording a call initiated with RTCWEB that uses bundle or mmt. 
Most likely siprec will need to be extended to support that, though I 
wish that wasn't so.