Re: [MMUSIC] Wisdom sought: Prioritization of codecs

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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