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RE: URN as namespace URI for RDF Schema

From: Hammond, Tony <T.Hammond@nature.com>
Date: Tue, 5 Oct 2004 10:20:50 +0100
Message-ID: <125F7834E11A5741A7D79412EE3504F90CE55992@UK1APPS2.nature.com>
To: "'mdirector@iptc.org'" <mdirector@iptc.org>, www-rdf-interest@w3.org
Cc: Graham Klyne <GK@ninebynine.org>

Well, Michael, it certinly is a tricky one. I take Graham's point about '#'
not being part of any URI scheme *as far as RFC 2396 is concerned*. RFC 2396
supported this view in distinguishing between 'URI-references' and 'URIs':

	URI-reference = [ absoluteURI | relativeURI ] [ "#" fragment ]

In fact, you might be surprised to learn that there was/is no single
production for a generic URI! Instead RFC 2396 deferred to URI schemes to
specify which components would be available to them. Although it did not
provide a URI production the closest it got to defining a generic URI syntax
was the following:

   This "generic URI" syntax consists of a sequence of four main components:

      <scheme>://<authority><path>?<query>

   each of which, except <scheme>, may be absent from a particular URI.

So clearly in RFC 2396 terms fragment does not belong to URI but to
URI-reference.

Now, however, the revision of this RFC (2396bis, currently verion -07 and
close to final call), see

	
ftp://ftp.ietf.org/internet-drafts/draft-fielding-uri-rfc2396bis-07.txt

normalizes these various constructs and does provide for a single, generic
URI production

	URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ]

As you can see a fragment is an optional but integral component of all URI
schemes. The URN scheme as currently stands does not support either '/' or
'#' - although there's no reason not to now seek for a revision on that.
Note that we have been pursuing registration of a new URI scheme - INFO (see
http://info-uri.info/) - which is a kind of URN lite and that does allow for
both '/' and '#' characters - one of the shortcomings IMO of URNs. Of
course, one could just go ahead and use the URN for an RDF namespace without
the '/' or '#' chars.

Cheers,

Tony
 


> -----Original Message-----
> From: Michael Steidl/MDir IPTC [mailto:mdirector@iptc.org] 
> Sent: 05 October 2004 05:48
> To: www-rdf-interest@w3.org
> Cc: Graham Klyne; Hammond, Tony
> Subject: RE: URN as namespace URI for RDF Schema
> 
> 
> Graham, Tony,
> 
> good to see that this fragment identifier issue is ambiguous 
> not only for us. 
> Would strongly support to have this clarified at the level of 
> specifications - 
> not only as "best practice".
> 
> Michael
> 
> On 4 Oct 2004 at 20:54  Graham Klyne wrote:
> 
> > Tony,
> > 
> > Thanks for pointing that out.
> > 
> > My immediate thought is that RFC2141 is "exceeding its 
> authority" as a 
> > URI
> > scheme definition, but, IIRC, that RFC was written before 
> the general 
> > consensus (as I perceive it) emerged that URNs are just 
> another kind of URI.
> > 
> > I guess this is something that should probably be clarified in the 
> > wake of
> > the revised URI specification [1] that's about to be (or is 
> being) last-called.
> > 
> > Another way of looking at this is that an (unescaped) '#' 
> cannot be a 
> > part
> > of *any* URI scheme, so in that respect there's nothing 
> special about 
> > URNs.  I think I prefer that view.
> > 
> > #g
> > --
> > 
> > [1] http://gbiv.com/protocols/uri/rev-2002/rfc2396bis.html:
> > 
> > (Note the final sentence below...)
> > 
> > [[
> > 3.5 Fragment
> > 
> > The fragment identifier component of a URI allows indirect 
> > identification
> > of a secondary resource by reference to a primary resource 
> and additional 
> > identifying information. The identified secondary resource 
> may be some 
> > portion or subset of the primary resource, some view on 
> representations of 
> > the primary resource, or some other resource defined or 
> described by those 
> > representations. A fragment identifier component is 
> indicated by the 
> > presence of a number sign ("#") character and terminated by 
> the end of the URI.
> > 
> >     fragment    = *( pchar / "/" / "?" )
> > 
> > The semantics of a fragment identifier are defined by the set of
> > representations that might result from a retrieval action 
> on the primary 
> > resource. The fragment's format and resolution is therefore 
> dependent on 
> > the media type [RFC2046] of a potentially retrieved 
> representation, even 
> > though such a retrieval is only performed if the URI is 
> dereferenced. If no 
> > such representation exists, then the semantics of the fragment are 
> > considered unknown and, effectively, unconstrained. 
> Fragment identifier 
> > semantics are independent of the URI scheme and thus cannot 
> be redefined by 
> > scheme specifications.
> > ]]
> > 
> > 
> > At 17:30 04/10/04 +0100, Hammond, Tony wrote:
> > 
> > >Hi Graham:
> > >
> > > > It's true that URN's don't (strictly) allow '/' signs, 
> but they do 
> > > > not prohibit '#' signs, as the fragment is not part of the main 
> > > > URI.  See http://www.w3.org/DesignIssues/Model.html for some
> > > > discussion.  You could
> > > > include an escaped (using %hh) '/' in a URN.
> > > >
> > >
> > >This from RFC 2141 (both "/" and "#" are reserved chars):
> > >
> > >http://www.ietf.org/rfc/rfc2141.txt
> > >
> > >2.3.2 The other reserved characters
> > >
> > >    RFC 1630 [2] reserves the characters "/", "?", and "#" 
> for particular
> > >    purposes. The URN-WG has not yet debated the applicability and
> > >    precise semantics of those purposes as applied to 
> URNs. Therefore,
> > >    these characters are RESERVED for future developments. 
>  Namespace
> > >    developers SHOULD NOT use these characters in 
> unencoded form, but
> > >    rather use the appropriate %-encoding for each character.
> > >
> > >Cheers,
> > >
> > >Tony
> > >
> > >
> > >
> > 
> >*************************************************************
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> > 
> > ------------
> > Graham Klyne
> > For email:
> > http://www.ninebynine.org/#Contact
> > 
> > 
> 
> 



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Received on Tuesday, 5 October 2004 09:21:26 GMT

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