Network Working Group                                        E. Levinson
Request for Comments: 2392                                   August 1998
Obsoletes: 2111
Category: Standards Track


          Content-ID and Message-ID Uniform Resource Locators

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

Abstract

   The Uniform Resource Locator (URL) schemes, "cid:" and "mid:" allow
   references to messages and the body parts of messages.  For example,
   within a single multipart message, one HTML body part might include
   embedded references to other parts of the same message.

Changes from (RFC 2111)

   Clarified the example on page 3 on of converting cid URLs to
   Content-IDs.  The example now uses a cid URL instead of an mid.

   Corrected the example messages to have the correct Content-ID form;
   they now use the angle brackets.  Added a Message-ID header to the
   second example.

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 or other data 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 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.  The "mid" scheme uses
   (a part of) the message-id of an email message to refer to a specific
   message.  The "cid" scheme refers to a specific body part of a
   message; its use is generally limited to references to other body
   parts in the same message as the referring body part.  The "mid"
   scheme may also refer to a specific body part within a designated
   message, by including the content-ID's address.

   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.

2. The MID and CID URL Schemes

   RFC 1738 [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 together.


   The URLs take the form

     content-id    = url-addr-spec

     message-id    = url-addr-spec

     url-addr-spec = addr-spec  ; URL encoding of RFC 822 addr-spec

     cid-url       = "cid" ":" content-id

     mid-url       = "mid" ":" message-id [ "/" content-id ]

     Notes: In Internet mail messages, the addr-spec in a Content-ID
     [MIME] or Message-ID [822] header is enclosed in angle brackets
     (<>).  Since addr-spec in a Message-ID or Content-ID might contain
     characters not allowed within a URL; any such character (including
     "/", which is reserved within the "mid" scheme) must be hex-encoded
     using the %hh escape mechanism in [URL].

   A "mid" URL with only a "message-id" refers to an entire message.
   With the appended "content-id", it refers to a body part within a
   message, as does a "cid" URL.  The Content-ID of a MIME body part is
   required to be globally unique.  However, in many systems that store
   messages, body parts are not indexed independently their context
   (message).  The "mid" URL long form was designed to supply the
   context needed to support interoperability with such systems.

   A implementation conforming to this specification is required to
   support the "mid" URL long form (message-id/content-id).  Conforming
   implementations can choose to, but are not required to, take
   advantage of the content-id's uniqueness and interpret a "cid" URL to
   refer to any body part within the message store.

   In limited circumstances (e.g., within multipart/alternate), a single
   message may contain several body parts that have the same Content-ID.
   That occurs, for example, when identical data can be accessed through
   different methods.  In those cases, conforming implementations are
   required to use the rules of the containing MIME entity (e.g.,
   multipart/alternate) to select the body part to which the Content-ID
   refers.

   A "cid" URL is converted to the corresponding Content-ID message
   header [MIME] by removing the "cid:" prefix, converting the % encoded
   character to their equivalent US-ASCII characters, and enclosing the
   remaining parts with an angle bracket pair, "<" and ">".  For
   example, "cid:foo4%25foo1@bar.net" corresponds to

     Content-ID: <foo4%25foo1@bar.net>

   Reversing the process and converting URL special characters to their
   % encodings produces the original cid.

   A "mid" URL is converted to a Message-ID or Message-ID/Content-ID
   pair in a similar fashion.

   Both message-id and content-id are required to be globally unique.
   That is, no two different messages will ever have the same Message-ID
   addr-spec; no different body parts will ever have the same Content-ID
   addr-spec.  A common technique used by many message systems is to use
   a time and date stamp along with the local host's domain name, e.g.,
   950124.162336@XIson.com.

   Some Examples

   The following message contains an HTML body part that refers to an
   image contained in another body part.  Both body parts are contained
   in a Multipart/Related MIME entity.  The HTML IMG tag contains a
   cidurl which points to the image.

     From: foo1@bar.net
     To: foo2@bar.net
     Subject: A simple example
     Mime-Version: 1.0
     Content-Type: multipart/related; boundary="boundary-example-1";
                   type=Text/HTML

     --boundary-example 1
     Content-Type: Text/HTML; charset=US-ASCII

     to the other body part, for example through a statement such as:
     <IMG SRC="cid:foo4*foo1@bar.net" ALT="IETF logo">

     --boundary-example-1

     Content-ID: <foo4*foo1@bar.net>
     Content-Type: IMAGE/GIF
     Content-Transfer-Encoding: BASE64

     R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk5
     NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4A
     etc...

     --boundary-example-1--

   The following message points to another message (hopefully still in
   the recipient's message store).

     From: bar@none.com
     To: phooey@all.com
     Subject: Here's how to do it
     Message-ID: <970701.32784@VIers.none.com>
     Content-type: text/html; charset=usascii

     <A HREF= "mid:960830.1639@XIson.com/partA.960830.1639@XIson.com">
     previous message</A>, shows how the approach you propose can be
     used to accomplish ...

3. Security Considerations

   The URLs defined here provide an addressing or referencing mechanism.
   The values of these URLs disclose no more about the originators
   environment 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

   [822]     Crocker, D., "Standard for the Format of ARPA Internet Text
             Messages", August 1982, STD 11, RFC 822, August 1982.

   [MIME]    Borenstein, N., and N. Freed, "Multipurpose Internet Mail
             Extensions (MIME) Part One: Format of Internet Message
             Bodies", RFC 2045, November 1996.

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

   [MULREL]  Levinson, E., "The MIME Multipart/Related Content-type",
             RFC 2387, August 1998.

5. Acknowledgments

   The original concept of "mid" and "cid" URLs were part of the Tim
   Berners-Lee's original vision of the World Wide Web. The ideas and
   design have benefited greatly by discussions with Harald Alvestrand,
   Dan Connolly, Roy Fielding, Larry Masinter, Jacob Palme, and others
   in the MHTML working group.

6. Author's Address

   Edward Levinson
   47 Clive Street
   Metuchen, NJ  08840-1060
   USA

   Phone: +1 908 549 3716
   EMail: XIson@cnj.digex.net

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