RE: [xri-editors] XRIs and canonical form

["Wachob, Gabe" <gwachob@visa.com>]

I think canonical form is sort an arbitrary, but well understood "state" of an identifier. 

When an identifier is in canonical form, it should be possible to compare it with another identifier in canonical form and the process of comparing the two character-by-character (or in the case of canonicalized URIs, byte for byte) is exactly the process of applying the built-in equivalence rules in the XRI spec. 

Does this make sense? I mentioned the leading-. issue, the $! and ! cross references. One other thing that would be useful to describe for canonicalization is the uppercasing of %HH (hex digit)..

If we define resolution to operate only on canonicalized forms of identifiers, it potentially makes the deployment of XRI local access servers MUCH simpler as they don't have to apply any of the "built-in" equivalence rules themselves. They just have to make sure that they resolve the one canonical form...

	-Gabe

> -----Original Message-----
> From: Dave McAlpin [mailto:dave.mcalpin@epokinc.com]
> Sent: Tuesday, November 11, 2003 10:50 AM
> To: xri-editors@lists.oasis-open.org
> Subject: [xri-editors] XRIs and canonical form
> 
> 
> I've been asked to draft text specifying a "canonical form" 
> for XRIs. I
> wanted to start by understanding what canonical form meant for URIs in
> general, and in searching the web I came across the following 
> exchange. The
> initial question is from Terence Spielman of Visa, followed 
> by Gabe's and my
> responses. Just interesting that we've considered this 
> question before.
> 
> Dave
> 
> >>>In addition, aside from unresolvable references, is it possible
> >>> to canonicalize XRIs?  This is a highly desireable feature
> >>> (for equivalence, at a minimum).
> 
> >>We talked quite a bit about this. The decision was made to 
> be silent on
> >>canonicalization because equivalence is actually 
> unambigious given the
> >>rules stated. Now, that doesn't mean that its at all obvious.
> >>
> >>I do think giving names to the escaped vs. unescpaed forms 
> of XRI, at
> >>least, would be useful.  Canonicalization would then just 
> be transforming
> >>an identifier into one of those forms. We didn't want to 
> mandate a single
> >>canonical form because different environments would need 
> XRIs in different
> >>levels of escaping and it would be unfortunate to require a specific
> >>canonicalization form that would require otherwise-unneeded
> transformation.
> >>
> >>Again, Dave McAlpin probably has better input on this. 
> 
> >A canonical representation might be useful for comparison, 
> but it would
> >involve a formal definition of things like "minimally 
> escaped", which would
> >be fairly difficult to nail down. It would also depend on 
> the existence of
> >a canonical form for URIs used as cross-references. In other 
> words, an XRI
> >wouldn't have a canonical form if it contained 
> cross-references that didn't
> >define a canonical form.
> >
> >Note that equivalence rules are generally problematic. The 
> IRI proposal,
> >for example, completely dodges the question of equivalence 
> when it says,
> >"There is no general rule or procedure to decide whether two 
> arbitrary IRIs
> >are equivalent or not... Each specification or application 
> that uses IRIs
> >has to decide on the appropriate criterion for IRI 
> equivalence." 2396bis
> >notes that even terms like "different" and "equivalent" are 
> fuzzy in the
> >general spec and ultimately application dependent.
> 
> 
> 
> 
> 
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