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.

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 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.






















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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.





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   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
      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.



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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
   restrictions.

   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:'
   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.



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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) )

       :
      (data)
       :




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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

      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.





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   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) )
      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


      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--






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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>

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>

      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">




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      --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.

   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.)










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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
        1999.

8. Author's Address

   Graham Klyne
   Content Technologies Ltd.
   1220 Parkview,
   Arlington Business Park
   Theale
   Reading, RG7 4SA
   United Kingdom

   Phone: +44 118 930 1300
   Fax:   +44 118 930 1301
   EMail: GK@ACM.ORG



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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
   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.

Acknowledgement

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



















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