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".





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RFC 2392               Message- & Content-ID URLs            August 1998


  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.



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RFC 2392               Message- & Content-ID URLs            August 1998


  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%[email protected]" corresponds to

    Content-ID: <foo4%[email protected]>

  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.,
  [email protected].

  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: [email protected]
    To: [email protected]
    Subject: A simple example
    Mime-Version: 1.0
    Content-Type: multipart/related; boundary="boundary-example-1";
                  type=Text/HTML



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RFC 2392               Message- & Content-ID URLs            August 1998


    --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*[email protected]" ALT="IETF logo">

    --boundary-example-1

    Content-ID: <foo4*[email protected]>
    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: [email protected]
    To: [email protected]
    Subject: Here's how to do it
    Message-ID: <[email protected]>
    Content-type: text/html; charset=usascii

    <A HREF= "mid:[email protected]/[email protected]">
    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.










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RFC 2392               Message- & Content-ID URLs            August 1998


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: [email protected]



















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RFC 2392               Message- & Content-ID URLs            August 1998


7.  Full Copyright Statement

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

  This document and translations of it may be copied and furnished to
  others, and derivative works that comment on or otherwise explain it
  or assist in its implementation may be prepared, copied, published
  and distributed, in whole or in part, without restriction of any
  kind, provided that the above copyright notice and this paragraph are
  included on all such copies and derivative works.  However, this
  document itself may not be modified in any way, such as by removing
  the copyright notice or references to the Internet Society or other
  Internet organizations, except as needed for the purpose of
  developing Internet standards in which case the procedures for
  copyrights defined in the Internet Standards process must be
  followed, or as required to translate it into languages other than
  English.

  The limited permissions granted above are perpetual and will not be
  revoked by the Internet Society or its successors or assigns.

  This document and the information contained herein is provided on an
  "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
  TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
  BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
  HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
  MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
























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