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An alternate perspective... (was RE: What is at the end of the n amespace?)

From: <Patrick.Stickler@nokia.com>
Date: Mon, 19 Nov 2001 10:52:42 +0200
Message-ID: <2BF0AD29BC31FE46B78877321144043114C0B0@trebe003.NOE.Nokia.com>
To: uri-request@w3.org, uri@w3.org, www-talk@w3.org

Thank you, David, for saying so well what I've only been
able to seemingly mumble poorly. 

Spot on! An HTTP URL is intimately tied to the HTTP protocol.
An HTTP URL should not have interpretation that is not 
meaningful within the scope of the purpose and functionality
of the HTTP protocol. It is not logical to define an HTTP
URL for something which an HTTP deamon cannot provide
access to within the bounds of the HTTP protocol.

And it is the (IMO unreasonable) presumption that HTTP URLs
can identify *anything* whatsoever that is at the root of all
of the confusion about dereferencing of namespaces, the
relationship between ontological terms and explicit schemas,
the (mis)use of URLs as URNs, etc.

--

Perhaps the following will offer a useful alternate perspective
on the matter:

I've been thinking about URIs alot lately in terms of agents
(human, organization, or software) which are involved in the
definition of URI schemes, the minting of URIs, and the
interpretation/application of URIs, and have see that a
key difference between HTTP (and other) URLs and URNs is 
that (in addition to the name vs. location issue) URLs denote 
both the agency which mints the URI (implicitly, the authority 
owning the web domain) and the agency which must interpret the 
URI (the server which is located at the specified domain name).
URNs on the other hand only denote (implicitly) the agent
minting the URI (which is the owner of the URN scheme or 
namespace).

   For the HTTP URL "http://xyz.foo.com/bar", the defining
   agent is the "owner" of the 'http:' URI scheme, the minting
   agent is the "owner" of the subdomain 'xyz.foo.com' and
   the interpreting agent is the machine residing at the 
   subdomain 'xyz.foo.com'.

   For the URN "urn:foo:bar", both the defining agent and the
   minting agent is the owner of the namespace 'urn:foo'.

I've identified a third class of URIs, which I call URPs
(Uniform Fesource Primitives) c.f.
http://www-nrc.nokia.com/sw/X_Values_URI.pdf which differ from
both URLs and URNs in that they denote neither the agency
which mints the URI nor the agency which interprets the URI,
but only the agency which defines the URP scheme.

   For the URP 'xsd:integer:10', the defining agent is the
   owner of the URP scheme 'xsd:'. No minting or interpreting
   agents are denoted in the URI itself.

This gives us the following matrix, showing where agents
are denoted (either implicitly or explicitly) in the URI itself:


                      URL   URN   URP
                     ------------------
    Definition         X     X     X
    Minting            X     X
    Interpretation     X


Thus, from the perspective of the indentity of the agencies
involved, we can see how these three classes of URI differ
in a manner that is quite clear, and also which is significant 
to web architecture in general, yet not based at all on the
notions of name, location, or nature of the resource itself.

Thus, rather than saying that URNs differ from URLs because
they define only a name, rather than a location. One can say
that URNs differ from URLs because the agency which is intended
to interpret the URI is not identified in terms of the URI itself,
whereas for a URL, the agency of interpretation is identified in
the URI itself.

Thus, the "fuzzy" notions of name, location, resource, etc.
can be left out of the "argument", per se, and the focus put
on more real nuts-n-bolts issues relating to how different classes 
of URIs are defined, created, and used and to what extent the
agencies participating in the lifespan of the URI are identified
by the URI itself. 

Eh?

Cheers,

Patrick

--
               
Patrick Stickler              Phone: +358 50 483 9453
Senior Research Scientist     Fax:   +358 7180 35409
Nokia Research Center         Email: patrick.stickler@nokia.com



> -----Original Message-----
> From: ext uri-request@w3.org [mailto:uri-request@w3.org]
> Sent: 17 November, 2001 14:05
> To: uri@w3.org
> Subject: Re: What is at the end of the namespace?
> 
> 
> Hi Roy!
> 
> This has my views as to why URNs were not a waste of time. 
> I've included 
> stuff that you surely know better than I do, but explained more 
> sympathetically. I'm trying to talk to the larger group, not 
> down to you.
> 
> At 12:40 PM -0800 11/16/01, Roy T. Fielding wrote:
> >  > Are you saying that HTTP URLs are also URNs?
> >
> >No, URNs are only those URI that start with a "urn" scheme.  
> What I said is
> >that HTTP URLs are identifiers, and hence names, and 
> therefore capable of
> >being a symbolic replacement for any other identifier, 
> including URNs.
> 
> I was involved in the URN stuff for a long time, though 
> peripherally. I 
> stopped eventually because the other folks were more than 
> competent to move 
> things along without me, and because, like you, I got tired 
> of the endless 
> discussions like this. Discussions in which I've seen almost 
> no-one change 
> their minds, even about the problem's definition, because of 
> the very wide 
> divergences in perspective the people bring to the problem.
> 
> However, it's been some years, so here's my take on the 
> issues and positions.
> 
> The nature of the resource identified is a red herring. The 
> question is 
> what method, if any, is suitable for obtaining the 
> representation of a 
> resource.
> 
> This is the place that URNs, http: URLs, and other URL 
> formats _may_ be 
> seen to differ.
> 
> >  > Does that mean that
> >  > all of the work being done by the URN WG is for nothing? 
>  Are they
> >>  just wasting their time, since we already have HTTP URLs and can
> >>  just use those?
> >
> >I have been saying that for the past eight years.  That 
> doesn't mean it is
> >a waste of their time, only that the solution to persistent 
> naming isn't
> >obtained merely by changing the scheme name.
> 
> It is indeed possible to use any string as a name, and any 
> anme must be 
> supported by a social/technical infrastructure that defines 
> its properties 
> and utilities.
> 
> The http: scheme is different from the ftp: scheme, although 
> both can serve 
> as a name infrastructure (given social/technical support). 
> The difference 
> between them is that each has a formal, standard definition of how to 
> request a digital representation (message body/file contents) 
> for a given 
> resource. The protocol for FTP is very limited, supporting 
> binary transfer 
> of data, and character conversion. The protocol for http: is 
> very rich, 
> supporting independent data format, and character encoding 
> conversions as 
> well as caching, etc. These schemes differ in their technical 
> infrastructure, but they both provide a mapping from 
> identifiers to data, 
> based on a standard protocol.
> 
> A user-agent is free in principle to resolve http: URLs in 
> any convenient 
> way. However, if that user agent resolves a URL in a way 
> which returns 
> different results than would be obtained by using HTTP, then 
> that agent can 
> be plausibly said to be broken. A great deal of HTTP 1.1 is 
> devoted to 
> enabling "correct" caching of data by arbitrary programs, 
> within parameters 
> of correctness as set by the server and conveyed by HTTP headers.
> 
> In other words one is free to resolve http: URLs by any means 
> one wants to 
> use, but use of any other method than HTTP is not 
> standardized, and thus is 
> not interoperable between applications. At some future date, 
> there may some 
> "redirection registry" that will resolve old URLs in a canonical way 
> (perhaps by date?).
> 
> URNs were created to satisfy a different set of needs, and, 
> in consequence, 
> make a radically different tradeoff between social and technical 
> infrastructure. URNs are specifically intended for names that 
> are intended 
> to be _persistent_ and _location independent_.
> 
> "Persistent" means that there is no upper bound on the 
> lifetime of a URN -- 
> librarians like to think in terms of decades to hundreds of years. To 
> guarantee that a name won't be re-used over that kind of 
> timespan is not 
> basically a technical issue, but a social one, because our 
> software seems 
> almost certain to undergo radical change over that timescale.
> 
> "Location independent" means that when you assign a URN, you 
> are naming 
> something, but _not_ picking a preferred protocol for 
> fetching it. You can 
> commit to persistence with http:, and the w3c advocates this, 
> but currently 
> this is rarely done. There are social obstacles, as well, as 
> there's no 
> guarantee that you can keep a domain name forever; nor is 
> there a standard 
> way to indicate to software that HTTP is _not_ a suitable 
> protocol for 
> fetching the resource at the end of an http: URL. And of course, it's 
> possible that maintaining a web server will no longer be the 
> preferred way 
> to provide a resource, because of software changes.
> 
> Of course, resolving names is nice, and there is a network 
> protocol (NAPTR) 
> that can be used to turn a URN into a URL -- using any scheme. That's 
> great, because it provides a technical infrastructure for making the 
> retrieval of URNs easy, _if the owners choose to use it_. The 
> point of a 
> URN is to have a scheme that exploicitly warrants the use of 
> arbitrary 
> retrieval mechanisms.
> 
> There was another comment that I wanted to respond to:
> 
> At 1:05 PM -0800 11/16/01, Roy T. Fielding wrote:
> >The only chaos I have seen is in the writings of more recent 
> specifications
> >that ignore the research and experience of the Web 
> developers in favor of
> >their own personal view of an ideal world.  When they 
> implement something
> >that works and has the same expressive power as the Web 
> itself, then I will
> >take their writings seriously.
> 
> A lot of the issues raised in the URN debate were raised by 
> people from a 
> library background, and librarians have been devising 
> reference systems for 
> a long time.
> 
> I could counter your ad-hominem argument by saying "I'll 
> listen to the web 
> folks when they've created a URL that was usable after 50 
> years." I'm not 
> saying that, and the IETF groups didn't either, because it's not 
> productive. Both perspectives have good ideas and techniques to offer.
> 
> URNs are just names with an agreement in advance that any 
> resolution method 
> that "works" is acceptable, where "works," like the notion of 
> "resource" is 
> a fuzzy one, ultimately defined by human beings.
> 
>    -- David
> --
> -------------------------------
> David Durand
> Chief Scientist, Scholarly Technology Group
> Adjunct Associate Professor, Computer Science.
> Brown University
> Cell: 401-935-5317
> email: David_Durand@brown.edu
> 
> commercial .sig:
> VP, Software Architecture
> ingenta plc
> 
> 
Received on Monday, 19 November 2001 03:52:35 GMT

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