MIME message with mail

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.

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.

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, "mid:foo4%25foo1@bar.net" 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.,
  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

    ... text of the HTML document, which might contain a hyperlink
    to the other body part, for example through a statement such as:
    

    --boundary-example-1
    Content-ID: foo4*foo1@bar.net
    Content-Type: IMAGE/GIF
    Content-Transfer-Encoding: BASE64








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RFC 2111                    CID and MID URLs                  March 1997


    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
    Content-type: text/html; charset=usascii

    ...  The items in my
    
    previous message, 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, University of Delaware, STD 11, RFC
         822.

[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
  USA
  +1 908 549 3716