Network Working Group                                        E. Levinson
Request for Comments: 2111                                   XIson, Inc.
Category: Standards Track                                     March 1997

          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.


   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.

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.

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RFC 2111                    CID and MID URLs                  March 1997

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 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 ]

      Note: in Internet mail messages, the addr-spec in a Content-ID
      [MIME] or Message-ID [822] header are 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 [MIME, sect. 7.2.3].  In those cases, conforming
   implementations are required to use the rules of the containing MIME
   entity (e.g., multi-part/alternate) to select the body part to which
   the Content-ID refers.

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RFC 2111                    CID and MID URLs                  March 1997

   A "cid" URL is converted to the corresponding Content-ID message
   header [MIME] by removing the "cid:" prefix, converting %hh hex-
   escaped characters to their ASCII equivalents and enclosing the
   remaining parts with an angle bracket pair, "<" and ">".  For
   example, "" corresponds to

        Message-ID: <>

   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.,

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.

     Subject: A simple example
     Mime-Version: 1.0
     Content-Type: multipart/related; boundary="boundary-example-1";

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

     ... text of the HTML document, which might contain a hyperlink
     to the other body part, for example through a statement such as:
     <IMG SRC="cid:foo4*" ALT="IETF logo">

     Content-ID: foo4*
     Content-Type: IMAGE/GIF
     Content-Transfer-Encoding: BASE64

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RFC 2111                    CID and MID URLs                  March 1997



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

     Subject: Here's how to do it
     Content-type: text/html; charset=usascii

     ...  The items in my
     <A HREF= "">
     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

4. References

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

[MIME]    N. Borenstein, N. Freed, "MIME (Multipurpose Internet Mail
          Extensions) Part One:  Mechanisms for Specifying and
          Describing the Format of Internet Message Bodies,"
          September 1993, RFC 1521.

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

[MULREL]  E. Levinson, "The MIME Multipart/Related Content-type,"
          December 1995, RFC 1874.

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RFC 2111                    CID and MID URLs                  March 1997

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
   +1 908 549 3716

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