# Re: mid and cid URLs

Keith Moore (moore@cs.utk.edu)
Tue, 21 Nov 1995 15:00:31 -0500

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    Message-Id: <199511212000.PAA01484@wilma.cs.utk.edu>

    From: Keith Moore <moore@cs.utk.edu>

    To: asg@severn.wash.inmet.com (Al Gilman)

    Subject: Re: mid and cid URLs

    In-Reply-To: Your message of "Tue, 21 Nov 1995 12:19:33 EST."

                 <9511211719.AA04753@severn.wash.inmet.com>

    Date: Tue, 21 Nov 1995 15:00:31 -0500

> 1. By construction, these two nominal schemes are one scheme and we
> should only use one name for them. MID or MIDCID are possibles.

No they're not the same, and as long as we're talking about URLs,
we shouldn't try to force the two into a single scheme.

a) A message is not the same as a body part. A message has senders,
recipients, a subject, and so on, in addition to content. A body part
just has typing information and content.

b) The likely use of CID is to reference bodyparts within a
multipart/related. The likely use of MID is to refer to messages in a
message store (like an IMAP server). Some mail readers will support
one and not the other. The two need to be able to succeed or fail on
their own merits, rather than trying to load each one with the other's
burdens.

c) You don't want to refer to a body part as a message-id plus an
offset. The same body part (with the same CID) can occur in multiple
messages.

Either MIDs or CIDs could probably be implemented fairly easily, and
(at least as importantly) consistently between differnet platforms.
But for a combined MID/CID, you don't know where to find the object.

If you want a generalized object name, it should be a URN, not a URL.
A URL should be just a shorthand notation for how to get a resource;
nothing more.

It's a Bad Idea to try to impose URN functionality on URLs. In order
to implement URNs, we need a URL substrate that works simply and
rationally.

Keith

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