[MEDIACTRL] Issue with mixer package <codec> element

Jonathan Lennox <jonathan@vidyo.com> Thu, 25 March 2010 20:57 UTC

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From: Jonathan Lennox <jonathan@vidyo.com>
To: "mediactrl@ietf.org" <mediactrl@ietf.org>
Date: Thu, 25 Mar 2010 16:58:16 -0400
Thread-Topic: Issue with mixer package <codec> element
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Subject: [MEDIACTRL] Issue with mixer package <codec> element
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I'm sorry this is being raised so late, but I was looking over the mixer package's definition of <codec> (to recommend that MRB copy its definition for its <rtp-codec>), and realized there was a problem.

The problem is that mixer package's definition of <codec> includes only the media subtype (e.g. pcmu or h264), not its top-level media type (audio or video).  In most cases subtypes are unique, but there are a few "container" subtypes (e.g. rtx, fec, or red) that are registered as multiple top-level types.  I believe this is only by convention and good practice, however; RFC 4288 requires only that the type/subtype pair be unique. 
My recommendation would be to add a <type> element under <codec>, whose definition is the top-level content type, leaving <subtype> as the subtype.  (An alternative would be to change to only using <type> as type/subtype, e.g. audio/pcmu, which would probably be equally good, but this might make life unnecessarily harder for implementations that want to try to maintain backward compatibility with the draft as-is.)

It is probably also worth pointing out that both type and subtype names are not case-sensitive.

The other (minor) problem is that I'm not sure the defining the values of <subtype> by reference to a column IANA's RTP Payload Format media types table is the best choice, among other reasons being that that table is very incomplete, missing, for instance, video/h264.  (This has been raised with IANA.)  I would instead define the value simply as "the name of the subtype of the codec's media format".  The reference to RFC 4855 can remain.

Jonathan Lennox
Vidyo, Inc