Resources, XPointer, and Fragment Identifiers

[I'm raising this issue while it's fresh in my mind and to put it on the
issues list of the WG once it is formed.]

I was talking to Philipp Hoschka (W3C Architecture Domain Lead) about some
of the coordination issues with XML and we spent a small amount of time
talking about filters, selectors, and XPointer when I said signed-XML was
probably going to punt on the issue of using "filters" to say which elements
and sub-blocks are signed. Or, only go as far as requiring that if an
element block wants to be signed, it should have an ID attribute specified
that can be referenced in a locator in the manifest. Philipp responded by
stating that if you permit URIs, then you permit XPointer. [1] Were we going
so far as to say only URIs, or only certain types of URIs. (He also
mentioned that XSL selectors and XPtr should converge.)

This seems odd to me in the following way. signed-XML will be a XML 1.0
application, but need it be an XPtr application, do they mean the same
thing? Imagine one implementor who supports XPtr and uses them in his
manifest, and another who doesn't. The semantics are not shared and in a way
that is dangerous to interoperability.  I'm comfortable with the idea of a
signed-XML application failing on the fact that it doesn't recognize a HTTP
scheme (e.g., locator="foobar:ASF*&@#$~~"). But less comfortable on failing
because of fragment identifiers.

Presently, Richard Brown's signed-XML draft states that:

      Locator: Locator value that contains either a URI [RFC 2396], a
           fragment identifier, or both. Notice that making use of a
           fragment identifier for a document content other than XML is
           out of the scope of this draft proposal and may lead to
           inconsistent results.

This certainly seems true. But I think we should be more specific. Also,
this might even be true in XML if XPtr is used, because the semantics of the
content after the "#" (i.e., "http://*#stuff") is defined by the MIME type.
Does the registered MIME type specify XPtr?

Joseph Reagle Jr.
Policy Analyst

Received on Thursday, 6 May 1999 16:20:57 UTC