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Re: non-hierarchical URIs and square brackets

From: Al Gilman <asgilman@iamdigex.net>
Date: Mon, 23 Jun 2003 09:20:42 -0400
Message-Id: <>
To: Graham Klyne <GK@ninebynine.org>, uri@w3.org

At 08:00 AM 2003-06-23, Graham Klyne wrote:
>At 14:35 19/06/03 -0400, Al Gilman wrote:
>>At 05:45 AM 2003-06-19, Graham Klyne wrote:
>>>or mid:, defined by RFC 2392, which is clearly non-hierarchical, but:
>>>      mid-url       = "mid" ":" message-id [ "/" content-id ]
>>>-- http://www.faqs.org/rfcs/rfc2392.html
>>Please explain in what terms you find the 'mid' scheme to be "clearly 
>I meant in the sense that it does not conform to the normal rules for 
>hierarchical URIs;  e.g.
>given a base URI:
>   mid:m@example.ord/c@example.org
>and a URI-reference:
>   m2@example.ord/c2@example.org
>making an absolute URI from these parts using normal hierarchical URI 
>rules would yield:
>   mid:m@example.ord/m2@example.ord/c2@example.org
>which isn't a valid mid: URI.

But there is no use for the relative URI that you cooked up.

Just because there are syntactically well-formed but semantically garbage
examples that can be constructed doesn't mean that there is no hierarchy
in the scheme.  Just that the hierarchy isn't everything that
you expect.

If the second multipart was sent bundled in the same message
as the first, the relative-URI calculus works fine.







It's true that you can't root the relative URI with the '/' character, but
it's also true that if that is what you want to do the alternative of
starting fresh with mid: and having a fully qualified URI is not a major
cost in characters.

Because the Message-ID space pools all email messages regardless of
authority, there is no point in using the relative URI you posited.

In that sense these URIs are not hierarchical, in that the Message-ID and
Content-ID namespaces are flat and not hierarchical by DNS domain.  The
hierarchical long form of mid: scheme reference to an object with an
assigned Content-ID is redundant.  The associated Message-ID is not required
for identification but aids in recovery nonetheless.

Within the limited confines of the hierarchical relationship between a MIME
part and its enclosing MIME multipart bundle, the mechanisms of relative
URI reconstitution into a fully-qualified 'absolute' URI work just fine.

Beyond that, there is no *further* hierarchy in the scheme.

To the extent that relative URIs are useful for references into MIME 
the existing syntax and algorithm work.  'Tain't broke.  Don't fix it.  But 
talk as though it isn't there.


>Graham Klyne
>PGP: 0FAA 69FF C083 000B A2E9  A131 01B9 1C7A DBCA CB5E
Received on Monday, 23 June 2003 09:36:04 UTC

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