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Re: When are two URIs equivalent?

From: Tim Berners-Lee <timbl@w3.org>
Date: Tue, 23 May 2000 09:43:22 -0400
Message-ID: <006d01bfc4bc$ddefcfc0$2a7b0a0a@ridge.w3.org>
To: "Josef Dietl" <josef@mozquito.com>, "Eric van der Vlist" <vdv@dyomedea.com>
Cc: <xml-uri@w3.org>

-----Original Message-----
From: Josef Dietl <josef@mozquito.com>
To: Eric van der Vlist <vdv@dyomedea.com>
Cc: Tim Berners-Lee <timbl@w3.org>; xml-uri@w3.org <xml-uri@w3.org>
Date: Tuesday, May 23, 2000 6:03 AM
Subject: When are two URIs equivalent?


>Eric,
>
>ok, you caught me - I meant to say "scheme".
>
>Still: would somebody mind telling me when two URIs are the same?


A URI is a (syntax constrained) string used to identify something.

A resource is that which, being in general abstract, is identified by the
URI.

Two URIs are the same when they compare character for character.

When two URIs are the same they identify the same resource.

(NB. There are many cases in which the resources identified by two
different URIs are the same. Software is not required to know
all (or for xml well-formedness checking, any)  these cases. They include
knowledge that the hex %nn encoding for non-reserved charecter
is an arbitrary choice; the knowledge that if the scheme is HTTP or
FTP then the domian name part is not case sensistive. They include
information obtained from a name server returning a "Found" response
to an HTTP request. They include metata gained from a third party.
There is no defiitive list of these. Some of them are a function of the
URI scheme.)

                         u1 = u2  =>  R1 = R2

but not the reverse implication does not hold.  On the left hand side,
"=" means string equality; on the right hand side, "=" stands for
equivalence for any operation at all.

For example, u1 may be the absolutized URI from the namespace name
of a namespace in an xml document, in which case R1 is the namespace.
u2 may be the absolutized URi from an XSLT style sheet, and R2
the namespace which h stylesheet is giving a particular style to.

The name for a string returned in the body of a successful HTTP GET request
is an HTTP "entity body".

(Yes, the XML, HTTP and URI communities have to learn a certian amount
of each others' jargon).

Tim BL
Received on Tuesday, 23 May 2000 09:41:38 UTC

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