[MMUSIC] M-lines: Wikifying it

Harald Alvestrand <harald@alvestrand.no> Wed, 28 November 2012 14:50 UTC

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Date: Wed, 28 Nov 2012 15:50:52 +0100
From: Harald Alvestrand <harald@alvestrand.no>
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Subject: [MMUSIC] M-lines: Wikifying it
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Perhaps we can do some real work rather than just disagreeing on principles.

Perhaps the most contentious issue now is "what should an M-line be".
As far as I can tell, M-lines and their parameters describe 3 things:

- Properties of the transport flow / RTP session (Transport, or T for short)
- Properties that make sense to apply to an individual media stream, and 
may need to be different for different media streams in the same 
combined RTP session (Flow, or F for short)
- Properties that make sense to apply to all streams of a given type 
(Media, or M for short)

The properties that can be represented in M-lines are found in two IANA 
registries, both on

- att-field (both session and media level)
- att-field (media level only)

I make it 179 entries in these categories, combined.

I see the same kind of work being necessary no matter which way we go:

- If we go with M-lines describing a whole RTP session (the MMT 
proposal), each property of the "F" class that we stil need needs to be 
copied onto a property in the "att-field (source level)" registry.

- If we go with M-lines describing groups of media streams, degenerating 
to a single media stream in the case of using "a=inactive" for stream 
muting (the BUNDLE and TOGETHER proposals), each property of the "T" 
class needs to be described as "MUST have consistent values for all the 
combined M-lines", "MUST be taken only from the first M-line in the 
bundle argument", or "must get special treatment".

- In both cases, the "M"-class lines don't need any special treatment. 
They just need to be present in all the M-lines where they're relevant.

I've created a spreadsheet here:


with 2 extra columns: "Class" (M, F or T) and "Importance" (1 for 
"necessary" and 0 for "will never matter").

Anyone should be able to write it.
If a few of us do a quick job of markup on it, I think we should be able 
to have a more realistic assessment in a few days.

I've started, but much of this is unknown to me. Can others help?