Re: [MMUSIC] QUERY: info about a=cat, a=keywds, a=tool

Paul Kyzivat <> Wed, 06 August 2014 14:18 UTC

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Date: Wed, 06 Aug 2014 10:18:00 -0400
From: Paul Kyzivat <>
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Subject: Re: [MMUSIC] QUERY: info about a=cat, a=keywds, a=tool
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On 8/6/14 5:48 AM, Colin Perkins wrote:
> Paul,
> I think these come from the Mbone tools (Van Jacobson’s sd and Mark Handley’s sdr), which used the combination of SDP and the Session Announcement Protocol (SAP) for multicast session directories, TV guide-style.
> The a=tool: attribute was introduced in draft-ietf-mmusic-sdp-03; the others are both in (draft-ietf-mmusic-sdp-00 is missing from the archive). Mark Handley’s slides from the MMUSIC meeting at IETF 33 have some details

Thanks for the pointers. From the slides:

   * dot separated hierarchical category
     (similar to usenet news groups)
   * Experimental - hence not a field in its own right.
   * Need to sort out first level of hierarchy!
   * Is it any use if not compulsory?
   * Can have several cat attributes

   a=keywds:<list of keywords>
   * a space separated list of keywords
   * Experimental - hence not a field in its own right.
   * Can only have one keywds attribute

Nothing about a=tools in those slides.

The above gives some insight, but I don't know if it is enough to nail 
down a syntax more specific that byte-string without running the risk of 
breaking something.

If I were to hazard a guess, it would be:

cat-value = token
keywds-value = single-keywd *(SP single-keywd)
single-keywd = non-ws-string

Since <token> allows dot, it can represent a dot separated hierarchical 
category for cat, while being fairly lenient. (But only ascii graphics.)

Using <non-ws-string> is hopefully lenient enough to cover the intent.


> Colin
> On 5 Aug 2014, at 16:48, Paul Kyzivat <> wrote:
>> I volunteered to draft ABNF syntax for the attributes defined in RFC4566 (SDP).
>> There are some that I have very little information about:
>>    a=cat:<category>
>>    a=keywds:<keywords>
>>    a=tool:<name and version of tool>
>> They have all been in SDP since RFC2327 in 1998. And I don't think I have ever seen any of them used. I did a google search over looking for usages without luck. And they are all very vague about their syntax.
>> The safe answer is to just define the value syntax for each of then as <byte-string>. But at least for a=cat the value (category) is specified as "dot-separated hierarchical category", so it seems reasonable that the syntax ought to involve dots in some way.
>> Perhaps these should all be deprecated.
>> Is there anybody still here who remembers when these were defined, and what the expectations were about the syntax?
>> 	Thanks,
>> 	Paul
>> _______________________________________________
>> mmusic mailing list