Re: [MMUSIC] Wisdom sought: Prioritization of codecs

["Horvath, Ernst" <[e-mail]>]

Harald,

RFC 3264 states that codecs are listed in order of preference, most preferred first, in both offer and answer. When selecting the acceptable codecs from the offered list the answerer should keep the order unless there is good reason not to do so. The offerer must be prepared to receive an answer with codecs arranged in a different order. Although both sides can then use any of the negotiated codecs it is recommended they use the first in the list.

Regards,
Ernst

________________________________
From: [e-mail] [mailto:[e-mail]] On Behalf Of Harald Alvestrand
Sent: Donnerstag, 15. November 2012 09:47
To: mmusic
Subject: [MMUSIC] Wisdom sought: Prioritization of codecs

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?

2) in offer/answer, does this sequence in an answer mean that the answerer prefers that the offerer use codec 111?

3) is there any preference expressed between codec 112 and codec 113?

4) Should the sequence of a=rtpmap: lines be completely ignored, or does it matter?

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

               Harald