Re: [URN] Re: URI documents

Michael Mealling (michael@bailey.dscga.com)
Fri, 2 Jan 1998 12:03:23 -0500 (EST)


From: Michael Mealling <michael@bailey.dscga.com>
Message-Id: <199801021703.MAA09409@bailey.dscga.com>
Subject: Re: [URN] Re: URI documents
In-Reply-To: <34A5F57A.F02C06D7@parc.xerox.com> from Larry Masinter at "Dec 27, 97 10:45:15 pm"
To: masinter@parc.xerox.com (Larry Masinter)
Date: Fri, 2 Jan 1998 12:03:23 -0500 (EST)
Cc: paf@swip.net, fielding@kiwi.ics.uci.edu, harald.t.alvestrand@uninett.no,

(Can you tell I just got back from the holidays?)

Larry Masinter said this:
> I've been wondering if we might progress with a different
> perspective, which I will lay out briefly:

I have to say I like this approach with some minor wordsmithing.
It falls into my desire for an abstract definition instead of
a protocol/markup language specific one.

> The term "URI" denotes a concept: some kind of uniform
> space of identifiers for resources. The terms "URL", and "URN",
> however, denote explicit protocol elements, with specific

I'd suggest removing the "protocol" in the last sentence. It
should be up to each design to determine whether or not it
is intended for a real wire protocol or not.

> syntax and semantics associated with that syntax. The URL
> syntax is well defined and widely implemented; the URN syntax
> is newer, but well underway.
> 
> It is possible, and convenient, to allow for mutual embedding
> of these protocol elements. That is, the space of URNs can
> be embedded in the space of URLs by prepending the URN with
> the string "urn:". This means that "all URNs are URLs" only
> in the trivial sense that it is possible to write a URN as
> a URL. The simple embedding is possible because the URN syntax
> was chosen (wisely) to fit within the URL syntax.

This is inline with Dan's cut-n-paste of Tim's Axiom 3 on non-uniqueness
of URL namespaces (the phone: example).

> Conversely, one could imagine embedding the URL name space
> in the URN space. Imagine, for example, a URN which consisted
> of a date and a URL. The resource named would be 'the
> resource that was available at the specified location
> at the specified time. This would be perfectly good URN;
> perhaps one could say that 'dated-url' was the naming authority,
> and write <durl:199712272238:http://www.parc.xerox.com/masinter>.
> as the undecorated URN.

Sure. Someone suggested this before. Actually, I think you did. 

> With this notion of 'URI is a concept', we don't need or want
> a general URI syntax. There's a URL syntax and a URN syntax.
> There might be a completely separate URC syntax at some point.
> While it is convenient to embed URNs into the URL name space,
> it is just an embedding; the semantics associated with URLs
> are not inherited by URNs than they are inherited by email
> addresses merely because there is a 'mailto:' URL scheme.

Exactly! (I'll have to remember your mailto example).

-MM

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