[MMUSIC] Comments on draft-holmberg-mmusic-sdp-mmt-negotiation-00
Paul Kyzivat <pkyzivat@alum.mit.edu> Thu, 01 November 2012 21:16 UTC
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Date: Thu, 01 Nov 2012 17:16:39 -0400
From: Paul Kyzivat <pkyzivat@alum.mit.edu>
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Subject: [MMUSIC] Comments on draft-holmberg-mmusic-sdp-mmt-negotiation-00
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Here are a number of comments on draft-holmberg-mmusic-sdp-mmt-negotiation-00, written as I prepare for the meeting. Section 5.2.2.1: I understand why an offerer would typically *want* to group one or more media descriptions for specific media types with an "anymedia" media description. But why MUST that be done? Why can't an offerer just offer the anymedia description if he isn't interested in backward compatibility? Section 6.1 It is unclear to me whether the <media> value "anymedia" that is defined he is legitimate according to 4566. Section 8.2.1 of 4566 says: The set of media types is intended to be small and SHOULD NOT be extended except under rare circumstances. The same rules should apply for media names as for top-level media content types, and where possible the same name should be registered for SDP as for MIME. For media other than existing top-level media content types, a Standards Track RFC MUST be produced for a new top-level content type to be registered, and the registration MUST provide good justification why no existing media name is appropriate (the "Standards Action" policy of RFC 2434 [8]. The above suggests that the name needs to be a top level media content type, except for the "where possible". (And it's unclear what sort of exceptions "where possible" was intended to cover. The above uses the iana registry for "Media Types", which is shared with MIME usage. And from my quick scan it is not evident how to register a new top level type. So, I think it might actually require a revision to 4566 to make this change - breaking this case out and exempting it from the registration process. If so then I think it would be helpful to choose something more distinctive than "anymedia" to denote this quite different case. I suggest "*". (This is syntactically valid in the <media> field according to the existing 4566 syntax, so it *ought* to be backward compatible.) Section 7 - mmtype attribute: Note that the use of payload types in m-lines is only for those with one of the RTP proto values. For UDP the media subtype is placed in the <fmt> field of the m-line rather than in rtpmap. But with some rewording and explanation what you propose should work in most practical cases using the <fmt> value directly. E.g. a=mmtype: PCMU/8000 audio a=mmtype: H261/90000 video The only time it wouldn't work is if you wanted to use a common subtype for two top level (e.g. both audio/foo and video/foo). That isn't likely. (I'm having a separate discussion about how the new SCTP... <proto> values might describe the assignment of SCTP streams for particular purposes. At some point we *might* end up having a way to specify use of RTP over an SCTP stream. That then might start to interact with this if we wanted to mix audio/video in one such stream. But maybe it is premature to deal with that.) Section 7.3.2: I think you need to say *something* about O/A. I guess what makes sense is that 7.3.1 applies independently to the offer and the answer. Section 10: *Something* needs to be done about "anymedia" or whatever replaces it. As currently written you would need to register a new media type value. With my suggestions you might avoid that but have to identify this draft as an extension to 4566. Unfortunately it is a bad time to revise 4566, since there is a 4566bis pending. Conceivably this whole thing could be added in to 4566bis, if the schedule works for that. Thanks, Paul
- [MMUSIC] Comments on draft-holmberg-mmusic-sdp-mm… Paul Kyzivat
- [MMUSIC] Comments on draft-holmberg-mmusic-sdp-mm… Dale R. Worley