GENWiki

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".

Levinson Standards Track [Page 1] 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.

Levinson Standards Track [Page 2] 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%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

Levinson Standards Track [Page 3] 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*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.

Levinson Standards Track [Page 4] 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: XIson@cnj.digex.net

Levinson Standards Track [Page 5] RFC 2392 Message- & Content-ID URLs August 1998

7. Full Copyright Statement

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

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 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
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 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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Levinson Standards Track [Page 6]