# URN Requirements

Jared_Rhine@hmc.edu
Sat, 14 May 1994 21:57:32 -0700

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    Date: Sat, 14 May 1994 21:57:32 -0700

    Message-Id: <199405150457.VAA08446@osiris.ac.hmc.edu>

    From: Jared_Rhine@hmc.edu

    To: avatar@notebook.aus.xanadu.com (Andrew Pam)

    Subject: URN Requirements

Andrew> Other software will almost certainly have to use a local Xanadu
Andrew> server to access Xanadu resources, so the gateway could just produce
Andrew> locally valid URNs. Of course, I raised the issue because I think
Andrew> "locally valid URNs" would kind of spoil the point of the whole
Andrew> scheme...

Jared> Actually, upon a rereading of your response, I'm almost sure we have
Jared> different ideas. How could you possibly produce a "locally valid URN"?

Andrew> Well, the other alternative (which may have been what you were
Andrew> getting at in the first place) is that Xanadu URNs are globally
Andrew> valid, but are then not unique, since each document may be referred
Andrew> to by an infinite number of URNs - one for each server.

If I read the Sollins and Masinter draft correctly, the URI architecture
does not require require a one-to-one relationship between resources and
URNs. The "uniqueness" meant by the URN model is explained in section two
of the draft:

o Global uniqueness: The same URN will never be assigned to two
different resources.

The fact that URNs do not require a one-to-one mapping is further supported
on page 4:

One of the ways in which naming authorities, the assigners of names, may
choose to make themselves distinctive is by the algorithms by which they
distinguish or do not distinguish resources from each other. For example,
a publisher may choose to distinguish among multiple printings of a book,
in which minor spelling and typographical mistakes have been made, but a
library may prefer not to make that distinction.

As you can see, two different naming authorities are naming the same object.
Since naming authorities cannot interfere with each other's name spaces,
they will end up assigning different names to the same object. Thus the
same resource can have an "infinite number of URNs". Does this knowledge
help alleviate the Xanadu<->URI mapping?

I may be mistaken in this evaluation. Perhaps it would be best to wait
until Monday to get some other viewpoints on this issue.

    --
    Jared_Rhine@hmc.edu | Harvey Mudd College | http://www.hmc.edu/~jared/

    "Better to be of a rare breed than a long line." -- TDK

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