Re: URN single or multiplevariants

Tim Berners-Lee (timbl)
Fri, 24 Sep 93 08:24:38 MET DST

Date: Thu, 23 Sep 1993 15:18:36 -0500
From: Tony Sanders <>

> If you could write out what you mean it will be a lot clearer (for me).
> Give a couple of real-world examples.

> I'm confused about what you want exactly.
>Is this it?
> GIMIE URN,color="gray",type="image/gif",res="100dpi"

Sorry, I will try an example. It's going to be HTTP, which
I had avoided as John wanted low level deatils kept out of it.

I have a document which has an interesting reference
with HREF="urn1". To the relevant server (leaving out
any as yet undefined URN/L indirection lookup stage)
I ask

GET urn1 HTTP/1.0
Accept: text/html; application/postscript, text/docbook, vr/iso6598
Accept-Encoding: compress
Accept-Language: eng; fr q=.7; d q=.2

and get back

HTTP/1.0 200 document follows
MIME-Version 1.2
Content-type: text/html
Content-Encoding: 8bit
Language: eng
URI: urn1 vary=language, content-type, version
URI: urn2 vary=content-type
URI: urn3

Now I know that I can use urn1 (which coincidentally to be
the one I started with) to refer to this document in general,
which may exists in multiple translations, and may evolve in
time. I canuse urn2 to refer to this thing I am reading
in exactlythe same translation and exactlythe same version,
plus or minus on the conversion from one representation to
another (like rendering in postscript). I can use urn3 to
refer to the bitstream I got back, which I will cache.

I will probably cache all thes urns, and the software may
decide to give me back the same bitstream whenever I ask
for urn1 or urn2 or urn3 again today.

When I make a link, I have the optionof which way
I link. Maybe different menu options appear,
"Link to doc", "link to version","Link to image".
Which one I chose depends on whether I am quoting
a good textbook, or pointing out a spelling mistake,
or pointing out a legally binding document.

Does that make it any clearer what I meant?