Indicating Media Features for MIME Content
draft-ietf-conneg-content-features-03

IETF conneg working group Graham Klyne
Internet draft Content Technologies
Category: Work-in-progress 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.
Klyne Work-in-progress [Page 1]
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.
Klyne Work-in-progress [Page 2]
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/
Klyne Work-in-progress [Page 3]
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.
Klyne Work-in-progress [Page 4]
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.
Klyne Work-in-progress [Page 5]
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
Klyne Work-in-progress [Page 6]
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
Klyne Work-in-progress [Page 7]
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>
Klyne Work-in-progress [Page 8]
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.
Klyne Work-in-progress [Page 9]
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.
Klyne Work-in-progress [Page 10]
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)
Klyne Work-in-progress [Page 11]
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.
Klyne Work-in-progress [Page 12]
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.
Klyne Work-in-progress [Page 13]