MIMESGML Working Group                                       E. Levinson
Internet Draft: CID and MID URLs                          13 August 1996

          Content-ID and Message-ID Uniform Resource Locators

This draft document is being circulated for comment.  Please send your
comments to the authors or to the mhtml mail list
<mhtml@segate.sunet.se>.  If consensus is reached, this Access Type may
be registered with IANA and this document may be submitted to the RFC
editor as an Informational protocol specification.

Status of this Memo

This document is an Internet Draft; Internet Drafts are working
documents of the Internet Engineering Task Force (IETF) its Areas, and
Working Groups.  Note that other groups may also distribute working
documents as Internet Drafts.

Internet Drafts are draft documents valid for a maximum of six months.
They may be updated, replaced, or obsoleted by other documents at any
time.  It is not appropriate to use Internet Drafts as reference
material or to cite them other than as a "working draft" or  "work in

Please check the abstract listing in each Internet Draft directory for
the current status of this or any other Internet Draft.


The Uniform Resource Locator (URL) schemes, "cid:" and "mid:" allow the
content of Text/HTML or other MIME media types to contain references to
other body parts in the same or a different message.

1. Introduction

The use of [MIME] within email to convey Web pages and their associated
images requires a URL scheme to permit the HTML to refer to the images
included in the message.  The Content-ID Uniform Resource Locator,
"cid:", serves that purpose.

Similarly Net News readers use Message-IDs to link related messages
together.  The Message-ID URL provides a scheme, "mid:", to to refer to
such messages as a "resource".

The MID (Message-ID) and CID (Content-ID) URL schemes provide
identifiers for messages and their body parts.  In the case of a MID the
message must exist inside of the user's mail storage.  A CID URL refers

Levinson                Expires February 15, 1997               [Page 1]

Internet Draft                                Message- & Content-ID URLs

to a body part within the same message as a referring body body.  A CID
may occur as part of a MID in which case the CID refers to a body part
in the message identified by the MID URL portion.

A note on terminology.  The terms "body part" and "MIME entity" are used
interchangeably.  They refer to the headers and body of a MIME message,
either the message itself or one of the body parts contained in a
Multipart message.

     NOTE:  This draft corresponds to <draft-levinson-cid-02.txt> which has
     expired.  Changes have been made to the introductory and security
     sections.  The introductory changes provide relevance to the mhtml
     working groups efforts and the security section changes spell out the
     security elements.  Only typographic changes were made to sections 2
     and 3.
     The syntax and semantics
     specified in the cited Internet draft remain unchanged.

2. The MID and CID URL Schemes

RFC1738 [URL] reserves the "mid" and "cid" schemes for Message-ID and
Content-ID respectively.  This memorandum defines the syntax for those
URLs.  Because they use the same syntactic elements they are presented

The URLs takes the form

     cidurl     = "cid" ":" addr-spec

     midurl     = msgmid / msgcid

     msgmid     = "mid" ":" addr-spec

     msgcid     = msgmid "/" addr-spec

where "addr-spec" is defined in [822].  The midurl and cidurl "scheme-
parts" must consist of an "xchar" sequence [URL].  Addr-spec, however,
admits a wider range of characters.  Consequently some characters in
addr-spec must be represented within a midurl or cidurl using the escape
mechanism in [URL].

     NOTE: Various separators have been suggested, based on an analogous
     use in other URLs, for the msgcid rule: slash, "/"; question mark,
     "?"; and number sign, "#".  The slash suggests a hierarchical rela-
     tionship between the cidurl and midurl, analogous to file system
     directories; the questions mark, that the cidurl is a search argu-
     ment; and the number sign, that the cidurl is a label, analogous an

Levinson                Expires February 15, 1997               [Page 2]

Internet Draft                                Message- & Content-ID URLs

     anchor identifier.  The slash was chosen to avoid encoding the num-
     ber sign and avoid suggesting that a search was required.

     The analogy to a hierarchy should not be taken strictly.  A midurl
     reference may be a message with a MIME entity and corresponding
     Content-ID that contains another message (Content-Type: Mes-
     sage/822) with its own Message-ID and that enclosed message may
     have entities labeled with Content-ID.  The midurl syntax, to avoid
     any need for recursion, intentionally does not support that situa-
     tion.  To be referenced by a midurl, an enclosed message must be
     extracted and be directly represented in the users message system.

A msgmid refers to the entire message and the msgcid refers to a single
body part within the referenced message.  A cidurl refers to another
body part within the message that contains the cidurl.

A message may contain, usually in a Multipart/Alternate, several bodies
with the same Content-ID.  A cidurl (msgcid) reference may thus be
ambiguous; the Multipart/Alternate [MIME] selection rules shall apply to
disambiguate the referenced body part.

A msgmid (cidurl) can be converted to its corresponding Message-ID (Con-
tent-ID) by removing the "mid:" ("cid:) prefix, converting escaped char-
acters to their ASCII equivalent, and enclosing the remaining part with
an angle bracket pair, "<" and ">".  Ignoring the escape mechanism,
"mid:_addr-spec_" has the message-id "<_addr-spec_>".  Similarly, a msg-
cid can be converted to a message-id, content-id pair.

Cidurl and midurls (content-ids and message-ids) are globally unique
[MIME, p.19].  A common technique for generating a globally unique
cidurl and midurl uses a time and date stamp with the local host's
domain name, e.g., 950124.162336@XIson.com.

3. Security

The URLs defined here provide an addressing or referencing mechanism.
The values of these URLs disclose no more about the originators environ-
ment than the corresponding Message-ID and Content-ID values.  Where
concern exists about such disclosures the originator of a message using
mid and cid URLs must take precautions to insure that confidential
information is not disclosed.  Those precautions should already be in
place to handle existing mail use of the Message-ID and Content-ID.

4. References

     Crocker, D., Standard for the Format of ARPA Internet Text

Levinson                Expires February 15, 1997               [Page 3]

Internet Draft                                Message- & Content-ID URLs

     Messages, August 1982, University of Delaware, RFC 822, STD 11.

     N. Borenstein, N. Freed, "MIME (Multipurpose Internet Mail Exten-
     sions) Part One:  Mechanisms for Specifying and Describing the For-
     mat of Internet Message Bodies", 09/23/1993, RFC 1521.

     Berners-Lee, T., Masinter, L., and McCahill, M., Uniform Resource
     Locators (URL), December 1994, RFC 1738.

5. Acknowledgments

This work reflects the ideas freely provided to the author by Harald T.
Alvestrand, UNINETT, including Tim Berners-Lee, W3O, who pointed me at
the idea of using a URL "scheme" in the SGML encapsulation proposal,
Daniel W. Connolly, HAL, and Roy T. Fielding, UCI.

6. Author's Address

Edward Levinson
47 Clive Street
Metuchen, NJ  08840-1060
+1 908 549 3716

Levinson                Expires February 15, 1997               [Page 4]