[MMUSIC] syntax for attributes in 4566bis
Paul Kyzivat <pkyzivat@alum.mit.edu> Mon, 06 May 2013 18:42 UTC
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Date: Mon, 06 May 2013 14:42:20 -0400
From: Paul Kyzivat <pkyzivat@alum.mit.edu>
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Subject: [MMUSIC] syntax for attributes in 4566bis
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I have been working on an SDP Directorate review of a draft that defines some new SDP attributes. In the process of doing that I came across some things that I think ought to be improved in 4566bis: 1) The formal syntax for SDP in 4566 defines a generic syntax for attributes: attribute-fields = *(%x61 "=" attribute CRLF) attribute = (att-field ":" att-value) / att-field att-field = token att-value = byte-string This is fine. Elsewhere it defines a number of specific attributes: a=cat:<category> ... a=keywds:<keywords> .... These are defined with informal patterns such as above, along with text. There is no formal specification of the syntax of the values of these attributes. Then in the IANA considerations section these attributes are added to the IANA registry of SDP attributes. The main problem with the above is the lack of syntax for the values. With newer attributes in other drafts we have typically required ABNF. I think 4566bis ought to provide ABNF syntax for all of the attributes it defines. 2) If syntax is to be provided, then there is a question of *how* it should be done. There are a number of possibilities. Here are two: - link it right into the full SDP syntax. E.g., cat-attr = "cat:" category category = token attribute =/ cat-attr - define the syntax of the attribute, but *don't* link it into the the full SDP syntax. In my opinion it is better to *not* link into the general syntax. It doesn't matter much for this draft, but it sets a bad precedent for other drafts that define attributes. Each draft that does so (via use of 'attribute =/ ...' is an *update* to the rfc that defines 'attribute', and should be marked as such. The existence of the IANA registry for attributes means that it is not necessary to formally extend the syntax of 'attribute' and so the draft need not be an update. Whatever we do here will become a defacto template for how future drafts define attributes. So we should make it a good example. 3) The IANA registration of attributes has become a mess. Section 8.2.4 both defines the IANA registry for attributes and populates it with the attributes defined in the draft. But it doesn't precisely specify the name or format of the table. It *does* specify what information must be provided. What is actually present in IANA are *four* different tables for attributes: SDP Parameters att-field (both session and media level) SDP Parameters att-field (media level only) SDP Parameters att-field (session level) SDP Parameters att-field (source level) SDP Parameters att-field (unknown level) This is hard to read, and its hard for a draft to specify precisely what table things should go in. I suggest it would be good to reorganize the IANA registry, so that it only has one table, with a separate column (or columns) to indicate where the attribute may be used: session, media, source. Then ensure that the registrations for the attributes defined in the bis are good examples of how to use this, including explicitly referencing the table to be updated. 4) RFC5576 introduced the notion of source-level attributes. In principle the same attributes may be used at source level and the other levels, so they should be incorporated into the same table structure. Some thought ought to be given to how to ensure 4566bis integrates well with 5576 regarding definition and registration of attributes. Perhaps it would be better to make 4566bis replace 5576 as well. Or maybe that isn't necessary. Should be discussed. Thanks, Paul
- [MMUSIC] syntax for attributes in 4566bis Paul Kyzivat