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Network Working Group                                           G. KlyneRequest for Comments: 2913                          Content TechnologiesCategory: Standards Track                                 September 2000MIME Content Types in Media Feature ExpressionsStatus 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 media feature tag whose value is a Multipurpose   Internet Mail Extensions (MIME) content type.  This allows the   construction of feature expressions that take account of the MIME   content type of the corresponding data.Table of Contents1. Introduction ..................................................21.1 Terminology and document conventions ......................22. Motivation and goals ..........................................33. MIME content type feature tag .................................34. Examples ......................................................44.1 Simple text ...............................................44.2 Fax image .................................................44.3 Voice message .............................................44.4 Web browser capabilities ..................................55. IANA Considerations ...........................................56. Security Considerations .......................................57. Acknowledgements ..............................................58. References ....................................................69. Author's Address ..............................................6Appendix A: 'Type' feature tag registration ......................7   Full Copyright Statement .........................................9Klyne                       Standards Track                     [Page 1]

RFC 2913       MIME Content in Media Feature Expressions  September 20001. 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 media feature tag whose value is a MIME content   type.  This allows the construction of feature expressions that take   account of the MIME content type of the corresponding data.   Note that a content type feature value may contain parameters, but   this is discouraged.  Seesection 3 andappendix A, "Summary of the   media features indicated" for discussion of this point.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 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.Klyne                       Standards Track                     [Page 2]

RFC 2913       MIME Content in Media Feature Expressions  September 20002. Motivation and goals   The media feature expression syntax [1] and feature tags [2] were   designed with a view to providing content media information that   augments basic MIME content type information.  There are some   situations where it is useful to be able include that content type   information in a media feature expression:   o  Media feature details may depend upon the content type being used.      The media feature combining algebra and syntax [1] cannot apply to      content type information unless it appears in the feature      expression.      For example, in HTTP 1.1 [4] with Transparent Content Negotiation      (TCN) [5] acceptable content types and other media features are      indicated in different request headers, with no clear way to      indicate that they may be acceptable only in certain combinations.   o  It is sometimes useful for all media capability information to be      included in a single expression.  For example, DSN and MDN      extensions [6] that allow a recipient to indicate media      capabilities provide a single field for conveying this      information.   o  When media features are used to describe a message content, they      may refer to inner parts of a MIME composite; e.g. the component      parts of a 'multipart', files in a compressed archive, or      encrypted message data.3. MIME content type feature tag   Feature tag name    Legal values   ----------------    ------------   type                <string>                       containing a MIME content-type value.   Reference: this document,appendix A.   The 'type' feature tag indicates a MIME media content type (i.e.   that appears in a 'Content-type:' header of the corresponding MIME-   formatted data).  It must be a string of the form "type/subtype",   where 'type' and 'subtype' are defined by the MIME specification [7].   Only lower-case letters should be used.   The content type must be given without any content-type parameter   values.Klyne                       Standards Track                     [Page 3]

RFC 2913       MIME Content in Media Feature Expressions  September 2000   To include information in media feature expressions that is otherwise   conveyed in a MIME content-type parameter, a separate media feature   tag should be registered [2] and used in the media feature   expression.  This is illustrated by the use of 'charset' in the   example at 4.1 below -- the 'charset' tag is defined by a separate   registration [10].      NOTE:  Allowing content-type parameters to be part of a type tag      value was considered, but rejected because of concerns about      canonicalization, ordering, case sensitivity, etc.  Only exact,      case-sensitive, character matching is defined for media feature      expressions [1].4. Examples4.1 Simple text      (& (type="text/plain") (charset=US-ASCII)         (color=binary) (paper-size=A4) )4.2 Fax image      (& (type="image/tiff")         (color=binary)         (image-file-structure=TIFF-S)         (dpi=200)         (dpi-xyratio=[200/100,200/200])         (paper-size=A4)         (image-coding=MH) (MRC-mode=0)         (ua-media=stationery) )4.3 Voice message      (& (type="multipart/voice-message")         (VPIM-version="3.0")         (audio-codec=[G726-32,GSM-610])         (audio-file-structure=[None,WAV])         (ua-terminal=mobile-handset)         (audio-channels=1) )         NOTE:  in this case, some media features apply to MIME parts         contained within the declared 'multipart/voice- message'         content type.  The goal here is not so much to mirror the MIME         structure as to convey useful information about the (possible)         message content.Klyne                       Standards Track                     [Page 4]

RFC 2913       MIME Content in Media Feature Expressions  September 20004.4 Web browser capabilities      (& (pix-x<=800) (pix-y<=600)         (| (& (type="text/html") (charset=iso-8859-1)               (color=limited) )            (& (type="text/plain") (charset=US-ASCII) )            (& (type="image/gif") (color=mapped))            (& (type="image/jpeg") (color=full) ) ) )   This example describes an HTML viewer that can deal with a limited   number of color text tags, a gif viewer that supports mapped color,   and a jpeg viewer that supports color.5. IANA ConsiderationsAppendix A of this document calls for registration of a feature tag   in the "IETF tree", as defined insection 3.1.1 of "Media Feature Tag   Registration Procedure" [2] (i.e. these feature tags are subject to   the "IETF Consensus" policies described inRFC 2434 [9]).   ASN.1 identifier 1.3.6.1.8.1.30 has been assigned by the IANA for   this registered feature tag and has been placed in the body of the   registration.6. Security Considerations   This memo is not believed to introduce any security considerations   that are not already inherent in the use of media feature tags and   expressions [1,2].7. Acknowledgements   This proposal draws from discussions in the IETF 'conneg' working   group.  The voice message example is based on some ideas by Glen   Parsons.   The author would like to thank the following people who offered   comments that led to significant improvements:  Ted Hardie, Larry   Masinter, Paul Hoffman, Jacob Palme, Ned Freed.Klyne                       Standards Track                     [Page 5]

RFC 2913       MIME Content in Media Feature Expressions  September 20008. 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]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H. and T.        Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",RFC2068, January 1997.   [5]  Holtman, K. and A. Mutz, "Transparent Content Negotiation in        HTTP",RFC 2295, March 1998.   [6]  Wing, D., "Indicating Supported Media Features Using Extensions        to DSN and MDN",RFC 2530, March 1999.   [7]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail        Extensions (MIME) Part One: Format of Internet Message Bodies",RFC 2045, November 1996.   [8]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail        Extensions (MIME) Part Two: Media Types",RFC 2046, November        1996.   [9]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA        Considerations Section in RFCs",RFC 2434, October 1998.  [10]  Hoffman, P., "Registration of Charset and Languages Media        Features Tags", Work in Progress.9. 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 6]

RFC 2913       MIME Content in Media Feature Expressions  September 2000Appendix A: 'Type' feature tag registration   -  Media Feature tag name(s):         Type   -  ASN.1 identifier associated with this feature tag:         1.3.6.1.8.1.30   -  Summary of the media features indicated:         This feature tag indicates a MIME content type that a message         agent is capable of handling, or that is contained within some         message data.         The content type consists of the MIME media type and subtype,         presented using all lower case letters and with any whitespace         characters removed.   -  Values appropriate for use with this feature tag:         String   -  The feature tag is intended primarily for use in the following      applications, protocols, services, or negotiation mechanisms:         Any application that wishes to convey MIME content type         information in a media feature expression.   -  Examples of typical use:         (type="image/tiff")         (& (type="text/plain") (charset=US-ASCII) )   -  Related standards or documents:         MIME,RFC 2045 [7]         MIME,RFC 2046 [8]         Registration of Charset and Languages Media Features Tags [10]   -  Considerations particular to use in individual applications,      protocols, services, or negotiation mechanisms:         (N/A)Klyne                       Standards Track                     [Page 7]

RFC 2913       MIME Content in Media Feature Expressions  September 2000   -  Interoperability considerations:         String feature matching is case sensitive, so consistent use of         case for content type values and parameters is essential if         content type value matching is to be achieved in a fashion         consistent with MIME content type matching.         Similarly, white space must be used consistently.         This registration specifies a canonical form to be used for         content type values (lower case letters and remove all         whitespace).   -  Related feature tags:         (N/A)   -  Intended usage:         Common   -  Author/Change controller:         IETFKlyne                       Standards Track                     [Page 8]

RFC 2913       MIME Content in Media Feature Expressions  September 2000Full 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 9]