Re: mid and cid URLs

Al Gilman (
Wed, 22 Nov 1995 15:57:45 -0500 (EST)

From: (Al Gilman)
Message-Id: <>
Subject: Re: mid and cid URLs
To: elevinso@Accurate.COM (Ed Levinson)
Date: Wed, 22 Nov 1995 15:57:45 -0500 (EST)
In-Reply-To: <9511221709.AA08740@Accurate.COM> from "Ed Levinson" at Nov 22, 95 12:08:45 pm

To follow up on what Ed Levinson said ...
  The context I intended for cid: is the current message, for mid: the 
  user's mail hierarchy (i.e., all mail folders, not just an inbox).
  Ned pointed out some cases where one might want to use mid: to refer
  to an encapsulated message but that could as easily be done with a c-ID
  header for the Msg/822 body part.  Thus mid: means a message the user
  owns, a cid: means a MIME entity in the current message, and a
  mid: w/a cid: refers to a MIME entity in a message the user owns.
That is what MIME may use it for, but the Content-ID URI has a much
larger domain of application as a uniform way of inserting machine-
processable cross-references among all manner of mail and News posts.

This is a simple drag and drop from folder to message or message
to message.  The question is what do you drop.  The goal of the CID 
citation scheme is to be the winner in that contest.

  Under that definition, a MIME entity of type Msg/822 has no externally
  viewable structure.  To do otherwise creates a much more difficult
If the receiver has some method of asking the sender for a retry
if a referenced part fails to arrive, then the sender must be
caching it or some object-building script for it.  That makes it
a persistent object somebody else could ask for, and the scope of
applicability is anybody querying the sender for an object
matching its [Message-ID and] Content-ID.

Otherwise, as I suggested to Keith, if there is no use of
Content-ID outside the scope of the current multipart, then it
should be unknown to the dictionary of Uniform Resource
Identifiers as it lacks the breadth of use to need "Uniform"
interpretation.  It is a MIME-private construct.  There is no
need to burden MIME with URI strings attached to its private parts.

But the Message-ID is a real gem and an Internet public standard
encoding of a URI based on the Message-ID as practiced in mail
and News is an important agenda item.