Re: [MMUSIC] Wisdom sought: Prioritization of codecs

[Paul Kyzivat <[e-mail]>]

Harald,

While the specs aren't entirely clear, after some searching and study 
they become *reasonably* clear as you have found. But this is only true 
in the context of a particular usage profile - that the RTP session will 
contain only one RTP stream, and that stream will use only a single codec.

If you move outside of that usage profile then the existing text is 
really deficient. And we are often outside that profile. One example is 
telephone-events. That is a separate "codec" that is never used alone, 
it is always used in conjunction with another codec. You may prefer G711 
for most audio, but still prefer telephone-events for DTMF tones.

The whole notion of codec preference breaks down even more for 
multiplexed streams. You may very explicitly prefer/require different 
streams to use different codecs.

So I don't think we can rely too much on parsing the existing text for 
guidance on how to use SDP to represent multiplexed sessions. (Unless of 
course we use bundle, with a separate m-line for each stream.) AFAIK we 
are breaking new ground here.

	Thanks,
	Paul

On 11/15/12 3:57 AM, Harald Alvestrand wrote:
> Found some answers in RFC 3264.
>
> On 11/15/2012 09:47 AM, Harald Alvestrand wrote:
>> I've been searching RFC 4566 (SDP) for this info, but came up blank....
>>
>> in an M-line, we typically have
>>
>> m=video blabla 111 112 113
>> a=rtpmap:111 ....
>> a=rtpmap:112 ...
>> a=rtpmap:113 ....
>>
>> Section 5.14 says this:
>>
>>        If the <proto> sub-field is "RTP/AVP" or "RTP/SAVP" the <fmt>
>>        sub-fields contain RTP payload type numbers.  When a list of
>>        payload type numbers is given, this implies that all of these
>>        payload formats MAY be used in the session, but the first of these
>>        formats SHOULD be used as the default format for the session.
>>
>> Now, the questions...
>>
>> 1) In offer/answer, does this sequence in an offer mean that the
>> offerer prefers that the answerer use codec 111, if supported?
>
> 3264 5.1:
>
>     In all cases, the formats in the "m=" line MUST be listed in order of
>     preference, with the first format listed being preferred.  In this
>     case, preferred means that the recipient of the offer SHOULD use the
>     format with the highest preference that is acceptable to it.
>
>
>>
>> 2) in offer/answer, does this sequence in an answer mean that the
>> answerer prefers that the offerer use codec 111?
> 3264 6.1:
>
>    The
>     media formats in the "m=" line MUST be listed in order of preference,
>     with the first format listed being preferred.  In this case,
>     preferred means that the offerer SHOULD use the format with the
>     highest preference from the answer.
>
>
>
>>
>> 3) is there any preference expressed between codec 112 and codec 113?
> Clear from 3264. If we ever update 4566, might want to harmonize those.
>>
>> 4) Should the sequence of a=rtpmap: lines be completely ignored, or
>> does it matter?
>
> I hope the answer is "completely ignored", have found no evidence to the
> contrary.
>
>>
>> I'm trying to fix a bug, and want to make sure I fix it according to spec.
>>
>>                Harald
>>
>>
>>
>>
>>
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