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   RE: Just require URLs

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  • From: "Carl Hage" <carl@chage.com>
  • To: xml-dev@ic.ac.uk
  • Date: Tue, 1 Jun 1999 10:28:29 -0800

From:           	"Jonathan Borden" <jborden@mediaone.net>
> By definition XML namespace URI's are URNs *not* URLs regardless of which
> URI scheme/namespace they use.

A URL can be a URN if the creator declares it to remain unique.

From:           	"Mark D. Anderson" <mda@discerning.com>
> But it would be great if there were a standard that also
> allowed for greater precision, so that one could distinguish
> the notions of:
> - a URN
> - a dtd or xml schema
> - an english spec
> - a document describing rights for some TBD rights protocol

It's hard to tell if a URL has the intent to be permanent (is a URN), 
though if it is used in a context requiring a URN, one should 
assume it is. A META or LINK tag on an HTML document could 
declare a URL to be permanent. (In case of a redirect or access via 
mirror, the original URI must be stored in the retrieved document.)

Using HTTP, a language specification and MIME type can be 
selected in a request (with the Accept). If the URI lead to an HTML 
document with a human-readable title page, the <LINK> tag could 
identify the DTD, Schema, Copyright, alternative languages, etc. A 
<META keywords="" scheme=""> could identify that page as a 
URI for use in an xml DTD.

The HTTP server could return an HTML document for a namespace 
URI unless an Accept was received indicating XML, where an XML 
document could contain all the metadata encoded in XML. (Maybe 
in RDF).

Such a scheme could provide all and more capabilities than a non-
retrivable URN, plus is backwards compatible with browsers and 
search tools.

From:           	"Didier PH Martin" <martind@netfolder.com>
> Why should URN usage for name space identification shouldn't
> be used? Do you have a good argument against it? If so, let's
> share it. I have an argument against URLs: location dependency.

URLs are location independent via DNS and/or redirection. An 
unretrievable URN is location nonexistant.

If you use a DTD in spam.org, your software deserves to fail. 
Instead of using some top-level IANA "urn: my:spam/carl.dtd", an 
http URI-space can be used with an http server behind it to receive 
queries. For example, "http://urn.xml.org/my/spam/carl.dtd" could 
act as a URN/URL registrant and redirect the /my paths to 
somewhere else, or return a filed document. I suspect 
www.cpan.org is more permanent than arbitrary IANA name 

URLs are permanent, persistent, non-reusable, and location-
independent as long as the creator maintains it to be so. Anyone 
can create a URN registry based on http (e.g. purl.org). That 
doesn't mean people won't violate the URN requirements. We'd 
need an XML Crimes Tribunal in the Hague to fix that.

From:           	Paul Prescod <paul@prescod.net>
> urn:urn-<assigned number>:<addr-spec>:<date> [ : <qualifier> ]

 With the IANA URN scheme, it is not possible to retrieve anything 
given the URI unless you customize your software for each URN 
registrant. However, IANA could create an http URI space which 
could at least return the name of the registrant, and usually 
something better.

The urn above can be converted into
http://urn.iana.org/<assigned number>:<addr-spec>:<date>: 

in order to make it useful.
Carl Hage                                              C. Hage Associates
<mailto:carl@chage.com> Voice/Fax: 1-408-244-8410      1180 Reed Ave #51
<http://www.chage.com/chage/>                          Sunnyvale, CA 94086

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