One document matched: draft-ietf-mhtml-cid-01.txt

Differences from draft-ietf-mhtml-cid-00.txt

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



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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.




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     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.






















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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 ...







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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>








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