IETF conneg working group Graham Klyne Internet draft 5GM/Content Technologies Category: Work-in-progress 13 April 1999 Expires: October 1999 Indicating media features for MIME content 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 13 April 1999 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 Usage considerations 4 3.1.1 Simple message parts 4 3.1.2 Multipart and other composites 5 3.1.3 Reference to external data 5 4. Examples ................................................5 4.1 Simple message 5 4.2 Fax message 6 4.3 Multipart/alternative data 7 4.4 Reference to external message data 7 4.5 Compressed data 8 4.6 Multipart/related data 8 5. Security considerations .................................9 6. Full copyright statement ................................9 7. Acknowledgements ........................................10 8. References ..............................................10 9. Author's address ........................................11 Appendix A: Revision history ...............................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. Along with the new MIME header definition, 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 13 April 1999 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 13 April 1999 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 Usage considerations 3.1.1 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: In such cases, MIME content type processing must be performed in accordance with the MIME 'Content-type:' header. Klyne Work-in-progress [Page 4] Internet draft Indicating media features for MIME content 13 April 1999 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. 3.1.2 Multipart and other composites 'Content-features:' headers may be applied to a MIME multipart indicating information about the inner content of the multipart. No one-to-one relationship between headers and contained body parts is assumed. If it is important to relate specific media features to specific 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. 3.1.3 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 Content-features: (& (paper-size=A4) (ua-media=stationery) ) : (data) : Klyne Work-in-progress [Page 5] Internet draft Indicating media features for MIME content 13 April 1999 4.2 Fax message Mime-Version: 1.0 Content-Type: multipart/fax-message; 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-Coverpage: yes 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-- Klyne Work-in-progress [Page 6] Internet draft Indicating media features for MIME content 13 April 1999 4.3 Multipart/alternative data Mime-Version: 1.0 Content-Type: multipart/alternative; bounday="break" Content-features: (Type="text/plain") Content-features: (& (Type="text/html") (color=binary) ) Content-features: (& (Type="text/html") (color=limited) ) --break Content-type: (Type="text/plain") : (data) : --break Content-type: text/html Content-features: (color=binary) : (data) : --break Content-type: text/html 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") Content-features: (& (Type="image/tiff") (color=limited) ) Klyne Work-in-progress [Page 7] Internet draft Indicating media features for MIME content 13 April 1999 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") Content-features: (& (Type="image/tiff") (color=limited) ) Content-transfer-encoding: base64 : (data) : 4.6 Multipart/related data Mime-Version: 1.0 Content-Type: multipart/related; boundary="boundary-example"; type="text/html"; start="" Content-features: (type="text/html") Content-features: (type="image/gif") --boundary-example Content-Type: text/html;charset="US-ASCII" Content-ID: ... text of the HTML document, which might contain a URI referencing a resource in another body part, for example through a statement such as: 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-- Klyne Work-in-progress [Page 8] Internet draft Indicating media features for MIME content 13 April 1999 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. 6. 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 9] Internet draft Indicating media features for MIME content 13 April 1999 7. 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. 8. 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. [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: 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. Klyne Work-in-progress [Page 10] Internet draft Indicating media features for MIME content 13 April 1999 [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 9. Author's address Graham Klyne 5th Generation Messaging Ltd. Content Technologies Ltd. 5 Watlington Street Forum 1, Station Road Nettlebed Theale Henley-on-Thames, RG9 5AB Reading, RG7 4RA United Kingdom United Kingdom. Telephone: +44 1491 641 641 +44 118 930 1300 Facsimile: +44 1491 641 611 +44 118 930 1301 E-mail: GK@ACM.ORG Appendix A: 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. Klyne Work-in-progress [Page 11]