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Network Working Group                                           G. KlyneRequest for Comments: 2912                          Content TechnologiesCategory: Standards Track                                 September 2000Indicating Media Features for MIME ContentStatus 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.Klyne                       Standards Track                     [Page 1]

RFC 2912       Indicating Media Features for MIME Content September 2000Table of Contents1. Introduction ...............................................21.1 Terminology and document conventions ...................22. Motivation and goals .......................................33. The 'Content-features:' MIME header ........................43.1 Whitespace and folding long headers ....................43.2 Usage considerations ...................................43.2.1 Simple message parts ...............................43.2.2 Multipart and other composites .....................53.2.3 Reference to external data .........................54. Examples ...................................................54.1 Simple message .........................................54.2 Fax message ............................................64.3 Multipart/alternative data .............................64.4 Reference to external message data .....................84.5 Compressed data ........................................84.6 Multipart/related data .................................85. Security Considerations ....................................96. Acknowledgements ...........................................107. References .................................................108. Author's Address ...........................................10   Full Copyright Statement ......................................111. 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 inRFC 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.Klyne                       Standards Track                     [Page 3]

RFC 2912       Indicating Media Features for MIME Content September 20003. 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 considerations3.2.1 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.Klyne                       Standards Track                     [Page 4]

RFC 2912       Indicating Media Features for MIME Content September 20003.2.2 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. Examples4.1 Simple message      Mime-Version: 1.0      Content-type: text/plain;charset=US-ASCII      Content-features: (& (paper-size=A4) (ua-media=stationery) )       :      (data)       :Klyne                       Standards Track                     [Page 5]

RFC 2912       Indicating Media Features for MIME Content September 20004.2 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.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) )      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--Klyne                       Standards Track                     [Page 7]

RFC 2912       Indicating Media Features for MIME Content September 20004.4 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">Klyne                       Standards Track                     [Page 8]

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

RFC 2912       Indicating Media Features for MIME Content September 20006. 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 fromRFC 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",RFC2533, 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",RFC2387, 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.ORGKlyne                       Standards Track                    [Page 10]

RFC 2912       Indicating Media Features for MIME Content September 20009. 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.Klyne                       Standards Track                    [Page 11]