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Network Working Group                                        E. LevinsonRequest for Comments: 2392                                   August 1998Obsoletes:2111Category: Standards TrackContent-ID and Message-ID Uniform Resource LocatorsStatus 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 SchemesRFC 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 ofRFC 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/HTMLLevinson                    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 19984. 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.netLevinson                    Standards Track                     [Page 5]

RFC 2392               Message- & Content-ID URLs            August 19987.  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.Levinson                    Standards Track                     [Page 6]