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Subject: RE: [xri-editors] XRIs and canonical form
It's interesting that neither 2396 nor 2616 defined a canonical form. 2396bis defines some "good practices" for making URIs "reasonably canonical", but they don't attempt anything normative. The following post by Larry Masinter is instructive. > In general, URLs do _not_ have a canonical form. However, HTTP > defines some equivalences for URLs (e.g., that http://host is > equivalent to http://host/, and by using the generic > syntax for host names, the host part is case insensitive). > > Some particular HTTP servers MAY define other equivalences, > e.g., that http://host/dir is equivalent to http://host/dir/ > and to http://host/dir/index.html. > Given that URIs don't have a normative canonical form, it's hard to see how we can define a canonical form for XRIs that contain cross-references. Dave > -----Original Message----- > From: Wachob, Gabe [mailto:gwachob@visa.com] > Sent: Tuesday, November 11, 2003 10:58 AM > To: 'Dave McAlpin'; xri-editors@lists.oasis-open.org > Subject: RE: [xri-editors] XRIs and canonical form > > 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. > > > > > > > > > > > > To unsubscribe from this mailing list (and be removed from > > the roster of the OASIS TC), go to > > http://www.oasis-open.org/apps/org/workgroup/xri-editors/membe > rs/leave_workgroup.php. > > To unsubscribe from this mailing list (and be removed from the roster of > the OASIS TC), go to http://www.oasis-open.org/apps/org/workgroup/xri- > editors/members/leave_workgroup.php.
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