# URNs are unambiguous rather than unique (was Re: Current URN syntax)

Owen Rees (rtor@ansa.co.uk)
Mon, 31 Oct 1994 13:50:15 GMT

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    Message-Id: <9410311350.AA10849@plato.ansa.co.uk>

    To: uri@bunyip.com

    Subject: URNs are unambiguous rather than unique (was Re: Current URN syntax)

    In-Reply-To: Message from fielding@avron.ICS.UCI.EDU of Thu, 27 Oct 1994

                 <9410270942.aa27557@paris.ics.uci.edu>

    Date: Mon, 31 Oct 1994 13:50:15 GMT

    From: Owen Rees <rtor@ansa.co.uk>

"Roy T. Fielding" <fielding@avron.ICS.UCI.EDU> writes:
> The important requirement for URNs is that there be a one-to-one
> correspondence per URN *scheme*. This needs to be considered when
> defining the structure of URCs. It should also be noted that only the
> client knows which URN scheme is preferable for any given situation.
>
> We could, however, obtain a one-to-one correspondence between
> each URN and each named object, but only if there is a strict partition
> of all objects among the naming authorities. Unfortunately, that would
> be quite impossible with grandfathering, and unlikely in any case.

Making URNs unambiguous is relatively easy, but making a URN the *only* URN
for a resource, even within some limited set of URNs, is hard. If a namespace
is partitioned, and names are unambiguous in the partitions, then the names
are unambiguous in the whole namespace. The partitioning can be reorganised at
will without causing any names to be homonyms (i.e. ambiguously naming more
than one resource). A property of the form "the only name for the resource in
this partition" does not generally correspond to any similar property in a
different partitioning.

Responsibility for a property must rest with an authority, so a per-scheme
property will require an authority responsible for managing all the URNs in
that scheme. A property preserved under partitioning can be delegated to
sub-authorities for the partitions, but this cannot be done otherwise.

The Functional Requirements for URN use "unique" to mean what I have called
"unambiguous" here, and avoid any "only name for resource" requirement. I
think that this is the only feasible position, but the use of the word
"unique" may be causing some misunderstanding.

Regards,
Owen Rees <rtor@ansa.co.uk>
Information about ANSA is at <URL:http://www.ansa.co.uk/>.

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