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Network Working Group G. Klyne Request for Comments: 2912 Content Technologies Category: Standards Track September 2000

             Indicating Media Features for MIME Content

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2000).  All Rights Reserved.


 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 Multipurpose Internet Mail Extensions (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 Standards Track [Page 1] RFC 2912 Indicating Media Features for MIME Content September 2000

Table of Contents

 1. Introduction ...............................................  2
    1.1 Terminology and document conventions ...................  2
 2. Motivation and goals .......................................  3
 3. The 'Content-features:' MIME header ........................  4
    3.1 Whitespace and folding long headers ....................  4
    3.2 Usage considerations ...................................  4
      3.2.1 Simple message parts ...............................  4
      3.2.2 Multipart and other composites .....................  5
      3.2.3 Reference to external data .........................  5
 4. Examples ...................................................  5
    4.1 Simple message .........................................  5
    4.2 Fax message ............................................  6
    4.3 Multipart/alternative data .............................  6
    4.4 Reference to external message data .....................  8
    4.5 Compressed data ........................................  8
    4.6 Multipart/related data .................................  8
 5. Security Considerations ....................................  9
 6. Acknowledgements ........................................... 10
 7. References ................................................. 10
 8. Author's Address ........................................... 10
 Full Copyright Statement ...................................... 11

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.

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.

Klyne Standards Track [Page 2] RFC 2912 Indicating Media Features for MIME Content September 2000

 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.

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
 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.

Klyne Standards Track [Page 3] RFC 2912 Indicating Media Features for MIME Content September 2000

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 points where whitespace appears to avoid line length
 Therefore, it is recommended that whitespace is included as
 permitted, especially 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:'
       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.

Klyne Standards Track [Page 4] RFC 2912 Indicating Media Features for MIME Content September 2000

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.

3.2.3 Reference to external data

 Media feature information about data indirectly referenced by a MIME
 body part rather than contained within a 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) )

Klyne Standards Track [Page 5] RFC 2912 Indicating Media Features for MIME Content September 2000

4.2 Fax message

    Mime-Version: 1.0
    Content-Type: multipart/mixed; boundary="break"
      (& (Type="image/tiff")
         (image-coding=MH) (MRC-mode=0)
         (ua-media=stationery) )
  1. -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"
    (more data)
    Content-Type: image/tiff; name="document.tiff"
    Content-Transfer-Encoding: base64
    Content-Disposition: attachment; filename="document.tiff"
    (more data)

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.

Klyne Standards Track [Page 6] RFC 2912 Indicating Media Features for MIME Content September 2000

 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-features'
    headers and contained body parts.
    Mime-Version: 1.0
    Content-Type: multipart/alternative; boundary="break"
    Content-features: (& (Type="text/plain") (charset=US-ASCII) )
      (& (Type="text/html") (charset=ISO-8859-1) (color=limited) )
      (& (Type="text/html") (charset=ISO-8859-1) (color=binary) )
  1. -break

Content-type: "text/plain";charset=US-ASCII

    Content-features: (color=binary)
    Content-type: "text/plain";charset=US-ASCII
    Content-features: (color=limited)
    Content-type: text/html;charset=iso-8859-1
    Content-features: (color=binary)
    Content-type: text/html;charset=iso-8859-1
    Content-features: (color=limited)

Klyne Standards Track [Page 7] RFC 2912 Indicating Media Features for MIME Content September 2000

4.4 Reference to external message data

    Mime-Version: 1.0
    Content-type: message/external-body; access-type=URL;
    Content-type: Multipart/mixed
    Content-features: (& (Type="text/plain") (charset=US-ASCII) )
    Content-features: (& (Type="image/tiff") (color=limited) )

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

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="<>"
    Content-features: (& (type="text/html") (charset=US-ASCII) )
    Content-features: (type="image/gif")
  1. -boundary-example

Content-Type: text/html;charset=US-ASCII

    Content-ID: <>
    referencing a resource in another body part, for example
    through a statement such as:
    <IMG SRC=""
     ALT="IETF logo">

Klyne Standards Track [Page 8] RFC 2912 Indicating Media Features for MIME Content September 2000

  1. -boundary-example

    Content-Type: IMAGE/GIF
    Content-Transfer-Encoding: BASE64
  1. -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
 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.)

Klyne Standards Track [Page 9] RFC 2912 Indicating Media Features for MIME Content September 2000

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 [7].
 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]  Klyne, G., "A Syntax for Describing Media Feature Sets", RFC
      2533, March 1999.
 [2]  Holtman, K., Mutz, A. and T. Hardie, "Media Feature Tag
      Registration Procedure", RFC 2506, March 1999.
 [3]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
      Specifications: ABNF", RFC 2234, November 1997.
 [4]  Crocker, D., "Standard for the format of ARPA Internet text
      messages", STD 11, RFC 822, August 1982.
 [5]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
      Extensions (MIME) Part 1: Format of Internet message bodies",
      RFC 2045, November 1996.
 [6]  Levinson, E., "The MIME Multipart/Related Content-type", RFC
      2387, August 1998.
 [7]  Palme, J., Hopmann, A. and N. Shelness, "MIME Encapsulation of
      Aggregate Documents, such as HTML (MHTML)", RFC 2557, March

8. Author's Address

 Graham Klyne
 Content Technologies Ltd.
 1220 Parkview,
 Arlington Business Park
 Reading, RG7 4SA
 United Kingdom
 Phone: +44 118 930 1300
 Fax:   +44 118 930 1301

Klyne Standards Track [Page 10] RFC 2912 Indicating Media Features for MIME Content September 2000

9. Full Copyright Statement

 Copyright (C) The Internet Society (2000).  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
 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


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Klyne Standards Track [Page 11]

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