Re: URI-protocol mapping

touch@isi.edu
Tue, 25 Feb 1997 10:48:09 -0800


From: touch@isi.edu
Date: Tue, 25 Feb 1997 10:48:09 -0800
Message-Id: <199702251848.AA13026@ash.isi.edu>
To: touch@isi.edu, liberte@ncsa.uiuc.edu
Subject: Re: URI-protocol mapping
Cc: uri@bunyip.com

> From liberte@sdgmail.ncsa.uiuc.edu Fri Feb 21 08:16:57 1997
> 
> touch@isi.edu writes:
>  > At some point you must know
>  > 	who you're talking with
>  > 	what protocol to use
> 
> If you are resolving a URN, you need to do the same thing, as you also
> pointed out (i.e. you need a protocol to find out what the protocol is).
> And when resolving a URL, you might be given that same information as
> a redirection.  Perhaps what you meant to say is:
> 
>   At some point, you must know when you have the thing you requested
>   rather than an indirection to something else.

Nope - I was thinking the other way around. That to get to the 
first item you have to have a protocol and a fixed name. More
of a bootstrapping issue, with the observation that the overhead
of changing protocols is of little benefit thereafter.
 
>  > A URL is a context-independent absolute identifier.
> 
> Consider all the ways I listed for how URLs can, in fact, be resolved
> that make them context dependent and relative.  What is wrong with any
> of them?

Precisely that there are many different ways. There should be exactly one.

> Here are several ways in which a URN can resolve directly to the object.
> 
> The first looks almost like what people imagine, but there is an
> important distinction from the perspective of the client.  The client
> hands off a URN to be resolved by a proxy.  The proxy gets a URL back
> and resolves the URL and returns the result to the client.  Note that
> the client never saw the URL.

That level, and several others, is equivalent to application layer translation.
There is nothing I can do to prevent that; if it's invisible, I need
never know it, and I should not consider it part of *my* protocol, in
that case.

> Now let's get rid of the URL altogether.  The client sends a URN to a
> URN resolver.  The URN resolver maps the URN to some
... 
> Now let's get rid of the necessary indirection in the URN resolver.

Where do you send the request now?
With what protocol?

THis is the basis of my objection.

Joe
----------------------------------------------------------------------
Joe Touch - touch@isi.edu		    http://www.isi.edu/~touch/
ISI / Project Leader, ATOMIC-2, LSAM       http://www.isi.edu/atomic2/
USC / Research Assistant Prof.                http://www.isi.edu/lsam/