Re: [MMUSIC] Wisdom sought: Prioritization of codecs

[Flemming Andreasen <[e-mail]>]

On 11/15/12 3:57 AM, Harald Alvestrand wrote:
> Found some answers in RFC 3264.
>
That's the right place to look assuming this is all for use in an 
Offer/Answer context.


> 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.
4566bis is work-in-progress - do you see a disconnect between 4566 and 
3264 (or are you just looking to repeat the information in 4566) ?

>>
>> 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.
>
It should not matter, but I don't think we have any text explicitly 
stating that anywhere.

-- Flemming


>>
>> I'm trying to fix a bug, and want to make sure I fix it according to 
>> spec.
>>
>>                Harald
>>
>>
>>
>>
>>
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