# Re: Grainularity of URN

Putz.PARC@xerox.com
Thu, 20 May 1993 14:23:25 PDT

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    Date: 	Thu, 20 May 1993 14:23:25 PDT

    From: Putz.PARC@xerox.com

    Subject: Re: Grainularity of URN

    In-Reply-To: <9305201915.AA07832@joe.uwex.edu>

    To: Dirk Herr-Hoyman <hoymand@joe.uwex.edu>

    Message-Id: <93May20.142345pdt.2445@spoggles.parc.xerox.com>

Dirk Herr-Hoyman wrote:
> But, it's not clear to me how an anchor fits into URNs?

Well, a URN is supposed to be a name that can be turned into a URL when
necessary (via some kind of name lookup service), and if you believe (I
don't) that named anchors in URLs are a sufficient way to refer to portions of
a document, then I suppose the trick is just getting another URN assigned to
the portion of the document you are interested in.

But it is not really that simple. Even with URLs to HTML documents, you
cannot refer to a portion of document unless the author has put in a named
anchor where you want it. And there is no way to refer to a segment or
range of a document, rather than a single point. Using character positions
is not robust across end-of-line conversions for ASCII files, much less
across versions of a document or conversions into different formats.

Which brings up the question of whether a given URN should be allowed
to map to multiple URLs for different representations of a document (e.g.
FrameMaker, PostScript, TIFF, ASCII).

That is, are URNs just location independent, or are they also format
independent? If a URN for a document is to be useful over a long
time span, then it should survive format changes (or multiple
formats) as well as location changes (or multiple locations).

Format negotiation is mentioned ocasionally, but should it happen at
URN -> URL lookup time or at URL -> file retrieval time?

Either way, I suspect that the combination of a URN plus a format name
(e.g. MIME content type) may be yet another important kind of resource
identifier. (Oh no, do we need another acronym?)

Steve Putz
Xerox Palo Alto Research Center

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