MIMESGML Working Group E. Levinson Internet Draft: CID and MID URLs 30 August 1996 <draft-ietf-mhtml-cid-01.txt> Content-ID and Message-ID Uniform Resource Locators This draft document is being circulated for comment. Please send your comments to the authors or to the mhtml mail list <mhtml@segate.sunet.se>. If consensus is reached, this Access Type may be registered with IANA and this document may be submitted to the RFC editor as an Informational protocol specification. Status of this Memo This document is an Internet Draft; Internet Drafts are working documents of the Internet Engineering Task Force (IETF) its Areas, and Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months. They may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a "working draft" or "work in progress". Please check the abstract listing in each Internet Draft directory for the current status of this or any other Internet Draft. Abstract The Uniform Resource Locator (URL) schemes, "cid:" and "mid:" allow the content of Text/HTML or other MIME media types to contain references to other body parts in the same or a different message. Change log "images" -> "images or other data". Corrected typos: "body body", "to to", and "URLs takes". Added examples to section 2 and cleaned up references. 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 Levinson Expires February 15, 1997 [Page 1]
Internet Draft Message- & Content-ID URLs 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 identi- fiers for messages and their body parts. In the case of a MID the mes- sage must exist inside of the user's mail storage. A CID URL refers to a body part within the same message as a referring body part. A CID may occur as part of a MID in which case the CID refers to a body part in the message identified by the MID URL portion. 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 Multi- part message. 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 cidurl = "cid" ":" addr-spec midurl = msgmid / msgcid msgmid = "mid" ":" addr-spec msgcid = msgmid "/" addr-spec where "addr-spec" is defined in [822]. The midurl and cidurl "scheme- parts" must consist of an "xchar" sequence [URL]. Addr-spec, however, admits a wider range of characters. Consequently some characters in addr-spec must be represented within a midurl or cidurl using the escape mechanism in [URL]. NOTE: Various separators have been suggested, based on an analogous use in other URLs, for the msgcid rule: slash, "/"; question mark, "?"; and number sign, "#". The slash suggests a hierarchical rela- tionship between the cidurl and midurl, analogous to file system directories; the questions mark, that the cidurl is a search argu- ment; and the number sign, that the cidurl is a label, analogous an anchor identifier. The slash was chosen to avoid encoding the num- ber sign and avoid suggesting that a search was required. Levinson Expires February 15, 1997 [Page 2]
Internet Draft Message- & Content-ID URLs The analogy to a hierarchy should not be taken strictly. A midurl reference may be a message with a MIME entity and corresponding Content-ID that contains another message (Content-Type: Mes- sage/822) with its own Message-ID and that enclosed message may have entities labeled with Content-ID. The midurl syntax, to avoid any need for recursion, intentionally does not support that situa- tion. To be referenced by a midurl, an enclosed message must be extracted and be directly represented in the users message system. A msgmid refers to the entire message and the msgcid refers to a single body part within the referenced message. A cidurl refers to another body part within the message that contains the cidurl. A message may contain, usually in a Multipart/Alternate, several bodies with the same Content-ID. A cidurl (msgcid) reference may thus be ambiguous; the Multipart/Alternate [MIME] selection rules shall apply to disambiguate the referenced body part. A msgmid (cidurl) can be converted to its corresponding Message-ID (Con- tent-ID) by removing the "mid:" ("cid:) prefix, converting escaped char- acters to their ASCII equivalent, and enclosing the remaining part with an angle bracket pair, "<" and ">". Ignoring the escape mechanism, "mid:_addr-spec_" has the message-id "<_addr-spec_>". Similarly, a msg- cid can be converted to a message-id, content-id pair. Cidurl and midurls (content-ids and message-ids) are globally unique [MIME, p.19]. A common technique for generating a globally unique cidurl and midurl uses a time and date stamp with the local host's domain name, e.g., 950124.162336@XIson.com. Levinson Expires February 15, 1997 [Page 3]
Internet Draft Message- & Content-ID URLs 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 Mul- tipart/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: <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 in the recipient's mes- sage store. From: bar@none.com To: phooey@all.com Subject: Here's how to do it Content-type: text/plain; charset=usascii The items in my previous message, mid:960830.1639@XIson.com/partA.960830.1639@XIson.com shows how the approach you propose can be used to accomplish ... Levinson Expires February 15, 1997 [Page 4]
Internet Draft Message- & Content-ID URLs 3. Security The URLs defined here provide an addressing or referencing mechanism. The values of these URLs disclose no more about the originators environ- ment 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 Describ- ing 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," Decem- ber 1995, RFC 1874. 5. Acknowledgments This work reflects the ideas freely provided to the author by Harald T. Alvestrand, UNINETT, including Tim Berners-Lee, W3O, who pointed me at the idea of using a URL "scheme" in the SGML encapsulation proposal, Daniel W. Connolly, W3O, and Roy T. Fielding, UCI. 6. Author's Address Edward Levinson 47 Clive Street Metuchen, NJ 08840-1060 USA +1 908 549 3716 <XIson@cnj.digex.net> Levinson Expires February 15, 1997 [Page 5]
