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

From: Paul Kyzivat <[email-masked]>
Date: Tue, 05 Aug 2014 11:48:26 -0400

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 ietf.org 
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

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From: Colin Perkins <[email-masked]>
Date: Wed, 6 Aug 2014 10:48:05 +0100

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 http://tools.ietf.org/id/draft-ietf-mmusic-sdp-01.txt (draft-ietf-mmusic-sdp-00 is missing from the archive). Mark Handley’s slides from the MMUSIC meeting at IETF 33 have some details ftp://ftp.ietf.org/ietf-online-proceedings/95jul/area.and.wg.reports/tsv/mmusic/mmusic.handley.slides.ps

Colin

On 5 Aug 2014, at 16:48, Paul Kyzivat <[email-masked]> wrote:
> I volunteered to draft ABNF syntax for the attributes defined in RFC4566 (SDP).
> 
> 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 ietf.org 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.
> 
> Is there anybody still here who remembers when these were defined, and what the expectations were about the syntax?
> 
> 	Thanks,
> 	Paul
> 
> _______________________________________________
> mmusic mailing list
> [email-masked]
> https://www.ietf.org/mailman/listinfo/mmusic

-- 
Colin Perkins
http://csperkins.org/

---

From: Paul Kyzivat <[email-masked]>
Date: Wed, 06 Aug 2014 10:18:00 -0400

> The a=tool: attribute was introduced in draft-ietf-mmusic-sdp-03; the others are both in http://tools.ietf.org/id/draft-ietf-mmusic-sdp-01.txt (draft-ietf-mmusic-sdp-00 is missing from the archive)…

Thanks for the pointers. From the slides:

   a=cat:<category>
   * 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.

	Thanks,
	Paul

---

From: Paul Kyzivat <[email-masked]>
Date: Wed, 06 Aug 2014 10:20:28 -0400

On 8/6/14 7:01 AM, Rémi Denis-Courmont wrote:

> VLC uses a=tool and a=cat. The dot separator is handled. Of course, tool
> can be replaced with User-Agent and Server headers in the case of SIP
> and RTSP, but that just does not fly in the case of SAP. So please keep
> them.
>
> Ideally, a=tool would use the User-Agent syntax, but it's too late. Just
> leave it as free form. And anyway, the User-Agent syntax is violated as
> often as it's respected.

OK. I am inclined to define the value for tools as <byte-string>, which 
is as flexible as it can be.

	Thanks,
	Paul