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IETF conneg working group
Internet draft
Category: Work-in-progress
Graham Klyne
Content Technologies
4 April 2000
Expires: October 2000

Indicating media features for MIME content
  <draft-ietf-conneg-content-features-03.txt>

Status of this memo

This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its 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 and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress."

To view the list Internet-Draft Shadow Directories, see http://www.ietf.org/shadow.html.

Copyright Notice

Copyright (C) The Internet Society 1999. All Rights Reserved.

Abstract

In "A syntax for describing media feature sets", an expression format is presented for describing media feature capabilities using simple media feature tags.

This memo defines a MIME 'Content-features:' header that can be used to annotate a MIME message part using this expression format, and indicates some ways it might be used.

Internet draft              Indicating media features for MIME content
4 April 2000

Table of contents

  1. Introduction ............................................2
     1.1 Terminology and document conventions                 3
     1.2 Discussion of this document                          3
  2. Motivation and goals ....................................4
  3. The 'Content-features:' MIME header .....................4
     3.1 Whitespace and folding long headers                  4
     3.2 Usage considerations                                 5
       3.2.1 Simple message parts                             5
       3.2.2 Multipart and other composites                   5
       3.2.3 Reference to external data                       6
  4. Examples ................................................6
     4.1 Simple message                                       6
     4.2 Fax message                                          6
     4.3 Multipart/alternative data                           7
     4.4 Reference to external message data                   8
     4.5 Compressed data                                      8
     4.6 Multipart/related data                               9
  5. Security considerations .................................9
  6. Acknowledgements ........................................10
  7. References ..............................................10
  8. Author's address ........................................12
  Full copyright statement ...................................12
  Revision history ...........................................13

1. Introduction

In "A syntax for describing media feature sets" [1], an expression format is presented for describing media feature capabilities as a combination of simple media feature tags, registered according to "Media Feature Tag Registration Procedure" [2]. This provides a format for message handling agents to describe the media feature content of messages that they can handle.

This memo defines a MIME 'Content-features:' header that can be used to annotate a MIME message part using these feature expressions. This header provides a means to indicate media- related features of message content that go beyond the MIME content type.

Consideration is also given to how it may be used to present message media content information that is problematic to express within the basic MIME framework.

Internet draft              Indicating media features for MIME content
4 April 2000

1.1 Terminology and document conventions

This section defines a number of terms and other document conventions, which are used with specific meaning in this memo.
  media feature
            information that indicates facilities assumed to be
            available for the message content to be properly rendered
            or otherwise presented.  Media features are not intended
            to include information that affects message transmission.
  feature set
            some set of media features described by a media feature
            assertion, as described in "A syntax for describing media
            feature sets" [1].  (See that memo for a more formal
            definition of this term.)
  feature set expression
            a string that describes some feature set, formulated
            according to the rules in "A syntax for describing media
            feature sets" [1] (and possibly extended by other
            specifications).

This specification uses syntax notation and conventions described in RFC 2234 "Augmented BNF for Syntax Specifications: ABNF" [3].

       NOTE:  Comments like this provide additional nonessential
       information about the rationale behind this document.
       Such information is not needed for building a conformant
       implementation, but may help those who wish to understand
       the design in greater depth.

1.2 Discussion of this document

Discussion of this document should take place on the content negotiation and media feature registration mailing list hosted by the Internet Mail Consortium (IMC):

Please send comments regarding this document to:

      ietf-medfree@imc.org

To subscribe to this list, send a message with the body 'subscribe' to "ietf-medfree-request@imc.org".

To see what has gone on before you subscribed, please see the mailing list archive at:

      http://www.imc.org/ietf-medfree/
Internet draft              Indicating media features for MIME content
4 April 2000

2. Motivation and goals

It is envisaged that media feature labelling of message parts may be used in the following ways:
  o  to supply more detailed media feature information about a message
     content than can be provided by the 'Content-type:' header.
  o  to provide summary media feature information (possibly including
     MIME content types) about the content of a composite MIME message
     part (e.g. 'multipart' or 'message'), without having to open up
     the inner content of the message.
  o  to supply media feature information about external data
     referenced by a message part (e.g. 'message/external-body' MIME
     type).  This information would not be available by examination of
     the message content.
  o  to describe the content of a message that is encrypted or encoded
     using some application-specific file structure that hides the
     content from a MIME processor.  This information also would not
     be generally available by examination of the message content.

3. The 'Content-features:' MIME header

A new header field is defined that extends the allowable formats for 'optional-field' [4] with the following syntax:
     optional-field =/ "Content-features" ":" Feature-expr
     Feature-expr   =  filter      ; See [1], section 4.1

where 'filter' is the media feature expression format defined by "A syntax for describing media feature sets" [1].

This header provides additional information about the message content directly contained or indirectly referenced in the corresponding MIME message part.

3.1 Whitespace and folding long headers

In some circumstances, media feature expressions can be very long.

According to "A syntax for describing media feature sets" [1], whitespace is allowed between lexical elements of a media feature expression. Further, RFC822/MIME [4,5] allows folding of long headers at oints where whitespace appears to avoid lione length restrictions.

Internet draft              Indicating media features for MIME content
4 April 2000

Therefore, it is recommended that whitespace is included as permitted, especialy in long media feature expressions, to facilitate the folding of headers by agents that do not otherwise understand the syntax of this field.

3.2 Usage considerations

3.2.1 Simple message parts

When applied to a simple MIME message part, the header should appear just once and is used to convey additional information about the message part content that goes beyond that provided by the MIME 'Content-type:' header field. The 'Content-features:' header may indicate a content type that is different than that given by the MIME 'Content-type:' header. This is possible but not recommended when applied to a non-composite body part: in any case, MIME content type processing must be performed in accordance with the 'Content-type:' header.
       NOTE:  Once the message content has been delivered to an
       application, it is possible that subsequent processing
       may be affected by content type information indicated by
       the media feature expression.  See example 4.5 below.

3.2.2 Multipart and other composites

'Content-features:' headers may be applied to a MIME multipart indicating information about the inner content of the multipart.

Implementations MUST NOT assume a one-to-one relationship between 'Content-features' headers and contained body parts. Headers may appear on a containing multipart wrapper in a different order than the body parts to which they refer; a single header may refer to more than one contained body part; several headers may refer to the same contained body part.

If it is important to relate specific media features to specific contained MIME body parts, then the 'Content-features:' header should be applied directly to the body part concerned, rather than the surrounding composite.

       NOTE:  The intent here is to allow summary media feature
       information to be provided without having to open up and
       examine the inner content of the MIME message.

Similar usage may apply when the message format is a non-MIME or opaque composite; e.g. 'application/zip', or an encrypted message. In these cases, the option of examining the message content to discover media feature information is not available.

Internet draft              Indicating media features for MIME content
4 April 2000

3.2.3 Reference to external data

Media feature information about data indirectly referenced by a MIME body part rather than contained within message can be conveyed using one or more 'Content-features:' headers.

For example, media information --including contained MIME content type(s)-- about the data referenced by a MIME 'Message/external- body' may be conveyed.

4. Examples

4.1 Simple message

     Mime-Version: 1.0
     Content-type: text/plain;charset=US-ASCII
     Content-features: (& (paper-size=A4) (ua-media=stationery) )
      :
     (data)
      :

4.2 Fax message

     Mime-Version: 1.0
     Content-Type: multipart/mixed; boundary="break"
     Content-features:
       (& (Type="image/tiff")
          (color=Binary)
          (image-file-structure=TIFF-S)
          (dpi=200)
          (dpi-xyratio=200/100)
          (paper-size=A4)
          (image-coding=MH) (MRC-mode=0)
          (ua-media=stationery) )
     --break
     Content-Type: image/tiff; name="coverpage.tiff"
     Content-Transfer-Encoding: base64
     Content-Description: This part is a coverpage
     Content-Disposition: attachment; filename="coverpage.tiff"
     0M8R4KGxGuEAAAAAAAAAAAAAAAAAAAAAPgADAP7/CQAGAAAAAAAAAAAAAAA
     AAAAAAAAEAAAZAAAAAEAAAD+////AAAAAAAAAAD////////////////////
      :
     (more data)
      :
     --break
Internet draft              Indicating media features for MIME content
4 April 2000
     Content-Type: image/tiff; name="document.tiff"
     Content-Transfer-Encoding: base64
     Content-Disposition: attachment; filename="document.tiff"
     AAAADgAAAA8AAAAQAAAAEQAAABIAAAATAAAAFAAAABUAAAAWAAAAFwAAABg
     GgAAABsAAAAcAAAAHQAAAB4AAAAfAAAAIAAAACEAAAAiAAAAIwAAACQAAAA
      :
     (more data)
      :
     --break--

4.3 Multipart/alternative data

This example illustrates three points:
  o  Information about the various parts in a multipart/alternative
     can be made available before the alternative body parts are
     processed.  This may facilitiate optimum one-pass processing of
     multipart/alternative data.
  o  There may be alternatives having the same basic MIME content-
     type, but differing in the content features that they use.
  o  There is NO defined correspondence between Content-feature
     headers and contained body parts.
     Mime-Version: 1.0
     Content-Type: multipart/alternative; boundary="break"
     Content-features: (& (Type="text/plain") (charset=US-ASCII) )
     Content-features:
       (& (Type="text/html") (charset=ISO-8859-1) (color=limited) )
     Content-features:
       (& (Type="text/html") (charset=ISO-8859-1) (color=binary) )
     --break
     Content-type: "text/plain";charset="US-ASCII"
     Content-features: (color=binary)
      :
     (data)
      :
     --break
     Content-type: "text/plain";charset="US-ASCII"
     Content-features: (color=limited)
      :
     (data)
      :
     --break
Internet draft              Indicating media features for MIME content
4 April 2000
     Content-type: text/html;charset="iso-8859-1"
     Content-features: (color=binary)
      :
     (data)
      :
     --break
     Content-type: text/html;charset="iso-8859-1"
     Content-features: (color=limited)
      :
     (data)
      :
     --break--

4.4 Reference to external message data

     Mime-Version: 1.0
     Content-type: message/external-body; access-type=URL;
                   URL="http://www.foo.com/file1.html"
     Content-type: Multipart/mixed
     Content-features: (& (Type="text/plain") (charset=US-ASCII) )
     Content-features: (& (Type="image/tiff") (color=limited) )
     <end>

4.5 Compressed data

This example shows how the "Content-features" header can be used to overcome the problem noted in the MIME registration for "Application/zip" regarding information about the data content.
     Mime-Version: 1.0
     Content-type: application/zip
     Content-features: (& (Type="text/plain") (charset=US-ASCII) )
     Content-features: (& (Type="image/tiff") (color=limited) )
     Content-transfer-encoding: base64
      :
     (data)
      :
     <end>
Internet draft              Indicating media features for MIME content
4 April 2000

4.6 Multipart/related data

(See also: RFC 2387, "The MIME Multipart/Related Content-type" [8])
     Mime-Version: 1.0
     Content-Type: multipart/related; boundary="boundary-example";
                   type="text/html"; start="<foo3@foo1@bar.net>"
     Content-features: (& (type="text/html") (charset=US-ASCII) )
     Content-features: (type="image/gif")
     --boundary-example
     Content-Type: text/html;charset="US-ASCII"
     Content-ID: <foo3@foo1@bar.net>
     ... text of the HTML document, which might contain a URI
     referencing a resource in another body part, for example
     through a statement such as:
     <IMG SRC="http://www.ietf.cnri.reston.va.us/images/ietflogo.gif"
      ALT="IETF logo">
     --boundary-example
     Content-Location:
        http://www.ietf.cnri.reston.va.us/images/ietflogo.gif
     Content-Type: IMAGE/GIF
     Content-Transfer-Encoding: BASE64
     R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk5
     NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4A
     etc...
     --boundary-example--

5. Security considerations

When applied to simple or multipart MIME formatted data, a media feature expression provides summary information about the message data, which in many cases can be determined by examination of the message content. Under these circumstances, no additional security considerations appear to be raised.

When applied to other message composites, especially encrypted message content, feature expressions may disclose information that is otherwise unavailable. In these cases, some security considerations associated with media content negotiation [1,2] may have greater relevance.

Internet draft              Indicating media features for MIME content
4 April 2000

It is suggested here that media feature descriptions may be usefully employed with encrypted message content. In doing this, take care to ensure that the purpose of encryption is not compromised (e.g. encryption might be intended to conceal the fact that a particular application data format is being used, which fact might be disclosed by an injudiciously applied Content-features header).

If a 'Content-features' header is applied to a multipart/signed object (or indeed outside any other form of signed data) the media feature information is not protected. This unprotected information could be tampered with, possibly fooling implementations into doing inappropriate things with the contained material. (Putting the media feature information inside the signed information would overcome this, at the cost of requiring implementations to parse the inner structure to find it.)

6. Acknowledgements

This proposal draws from discussions with Dan Wing. The fax message example was taken from a proposal by Mike Ruhl. The multipart/related example is developed from RFC 2557.

The author would like to thank the following people who offered comments that led to significant improvements: Mr Hiroshi Tamura, Ted Hardie, Maurizio Codogno, Jacob Palme, Ned Freed.

7. References

[1]  RFC 2533, "A syntax for describing media feature sets"
     Graham Klyne, 5GM/Content Technologies
     March 1999.
[2]  RFC 2506, "Media Feature Tag Registration Procedure"
     Koen Holtman, TUE
     Andrew Mutz, Hewlett-Packard
     Ted Hardie, NASA
     March 1999.
[3]  RFC 2234, "Augmented BNF for Syntax Specifications: ABNF"
     D. Crocker (editor), Internet Mail Consortium
     P. Overell, Demon Internet Ltd.
     November 1997.
Internet draft              Indicating media features for MIME content
4 April 2000
[4]  RFC 822, "Standard for the format of ARPA Internet text messages"
     D. Crocker, Department of Electrical Engineering, University of
     Delaware
     August 1982.
     To be replaced by:
     "Internet Message Format Standard"
     P. Resnick (editor), QUALCOMM Incorporated
     Internet draft: <draft-ietf-drums-msg-fmt-07.txt>
     Work in progress, January 1999.
[5]  RFC 2045, "Multipurpose Internet Mail Extensions (MIME)
     Part 1: Format of Internet message bodies"
     N. Freed, Innosoft
     N. Borenstein, First Virtual
     November 1996.
[6]  RFC 2046, "Multipurpose Internet Mail Extensions (MIME)
     Part 2: Media types"
     N. Freed, Innosoft
     N. Borenstein, First Virtual
     November 1996.
[7]  RFC 2017, "Definition of the URL MIME External-Body Access-Type"
     N. Freed, Innosoft
     K. Moore, University of Tennessee
     A. Cargille, WG Chair
     October 1996
     (Non-normative)
[8]  RFC 2387, "The MIME Multipart/Related Content-type"
     E. Levinson
     August 1998
     (Non-normative)
[9]  "Registration of Charset and Languages Media Features Tags"
     Paul Hoffman, IMC
     Internet draft: <draft-hoffman-char-lang-media-01.txt>
     Work in progress, July 1999.
     (Non-normative)
Internet draft              Indicating media features for MIME content
4 April 2000

8. Author's address

  Graham Klyne
  Content Technologies Ltd.
  1220 Parkview,
  Arlington Business Park
  Theale
  Reading, RG7 4SA
  United Kingdom.
  Telephone: +44 118 930 1300
  Facsimile: +44 118 930 1301
  E-mail:    GK@ACM.ORG

Full copyright statement

Copyright (C) The Internet Society 1999. 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.

Internet draft              Indicating media features for MIME content
4 April 2000

Revision history

[[[RFC editor: please remove this section on publication]]]

00a 10-Feb-1999 Initial draft.

01a 16-Feb-1999 Added pointers to mailing list for discussion.

  01b  04-Mar-1999  Various editorial changes.  Added placeholder for
                    multipart/related example.
  01c  13-Apr-1999  Separated multipart/alternative and
                    message/external-body into separate examples.
                    Added example for compressed data.  Added example
                    for multipart/related data.  Updated references.

02a 20-Jul-1999 Incorporated review comments -- editorial changes.

  02b  29-Nov-1999  Added (charset=...) to (type=text/*) examples.
                    Added citation to charset and language feature
                    registration document.
  02c  29-Nov-1999  Indicated motivation for multipart/alternative
                    example.  Moved copyright section to end of text.
  03a  04-Apr-2000  Clarification of Content-feaures on outer wrapper
                    of a multipart.  Recommend use of whitespace in
                    feature expressions to facilitate MIME header
                    wrapping.  Add discussion of Content-features on
                    multipart/signed data.