MIMESGML Working Group E. Levinson Internet Draft: CID and MID URLs Accurate Info. Sys., Inc. <draft-levinson-cid-02.txt> 7 February 1996 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 uri mail list <uri@bunyip.com>. 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. 0. Changes from 01 Corrected cid in example. Changed separator in cidmid to "/" Revised relationship between mid and cid urls and Message- and Content- IDs. Added rule for choosing among several body parts with the same Content- ID. 1. Introduction Levinson Expires August 15, 1996 [Page 1]
Internet Draft Message- & Content-ID URLs There is a desire to refer to email or NetNews messages and the body parts of such messages using Uniform Resource Locators (URLs). The MID and CID schemes described below permit such references. 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 included in the message. A Content-ID Uniform Resource Locator serves that purpose. Similarly NetNews readers use Message-IDs to link related messages together. The Message-ID URL provides a scheme to to refer to such messages as a "resource". The MID and CID URLs represent identifiers for messages and their body parts. In the case of a MID the message 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 body. 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 Multipart 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 takes the form cidurl = "cid" ":" addr-spec midurl = msgmid / cidmid msgmid = "mid" ":" addr-spec cidmid = msgmid "/" addr-spec where "addr-spec" is defined in [822]. The midurl and cidurl "schemeparts" must consist of an "xchar" sequence [URL]. Addr-spec, however, admits a wider range of characters. Consequently some charac- ters 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 Levinson Expires August 15, 1996 [Page 2]
Internet Draft Message- & Content-ID URLs use in other URLs, for the cidmid 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. 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 but be extracted and be directly represented in the users message system. A msgmid refers to the entire message and the cidmid 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 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 cid- mid can be converted to a message-id, content-id pair. Cidurl and midurls (message-ids and content-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. 3. Security Security issues are not addressed in this memorandum. 4. References [822] Crocker, D., Standard for the Format of ARPA Internet Text Mes- sages, August 1982, University of Delaware, RFC 822, STD 11. Levinson Expires August 15, 1996 [Page 3]
Internet Draft Message- & Content-ID URLs [MIME] N. Borenstein, N. Freed, "MIME (Multipurpose Internet Mail Exten- sions) Part One: Mechanisms for Specifying and Describing the For- mat of Internet Message Bodies", 09/23/1993, RFC 1521. [URL] Berners-Lee, T., Masinter, L., and McCahill, M., Uniform Resource Locators (URL), December 1994, RFC 1738. 5. Acknowledgements 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, HAL, and Roy T. Fielding, UCI. 6. Author's Address Edward Levinson XIson, Inc. 47 Clive Street Metuchen, NJ 08840-1060 USA +1 908 549 3716 <XIson@cnj.digex.net> Levinson Expires August 15, 1996 [Page 4]