One document matched: draft-ietf-conneg-content-features-00.txt
IETF conneg working group Graham Klyne
Internet draft 5GM/Content Technologies
Category: Work-in-progress 15 February 1999
Expires: August 1999
Indicating media features for MIME content
<draft-ietf-conneg-content-features-00.txt>
Status of this memo
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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.
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Table of contents
1. Introduction ............................................2
1.1 Terminology and document conventions 3
2. Motivation and goals ....................................3
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 4
3.1.3 Reference to external data 5
4. Examples ................................................5
4.1 Simple message 5
4.2 Fax message 6
4.3 Reference to external message data 6
4.4 Compressed data 6
5. Security considerations .................................7
6. Full copyright statement ................................7
7. Acknowledgements ........................................8
8. References ..............................................8
9. Author's address ........................................9
Appendix A: Revision history ...............................9
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.
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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.
The terms are listed in alphabetical order.
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).
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.
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 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.
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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.
[[[I am assuming that a new feature tag will be registered to carry
MIME content-type information in a media feature expression]]].
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
suggest 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.
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.
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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 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)
:
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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=[200/100,200/200])
(paper-size=A4)
(image-coding=MH) (MRC-mode=0)
(ua-media=stationery) )
--break
Content-Type: image/tiff; fax-coverpage=yes;name="coverpage.tiff"
Content-Transfer-Encoding: base64
Content-Description: This part is a coverpage
Content-Disposition: attachment; filename="coverpage.tiff"
0M8R4KGxGuEAAAAAAAAAAAAAAAAAAAAAPgADAP7/CQAGAAAAAAAAAAAAAAAB
AAAAAQAAAAAAAAAAEAAAZAAAAAEAAAD+////AAAAAAAAAAD/////////////
:
(more data)
:
--break
Content-Type: image/tiff; name="document.tiff"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="document.tiff"
AAAADgAAAA8AAAAQAAAAEQAAABIAAAATAAAAFAAAABUAAAAWAAAAFwAAABgA
AAAZAAAAGgAAABsAAAAcAAAAHQAAAB4AAAAfAAAAIAAAACEAAAAiAAAAIwAA
:
(more data)
:
--break--
4.3 Reference to external message data
[[[TODO]]]
4.4 Compressed data
[[[TODO]]]
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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.
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7. Acknowledgements
This proposal draws from discussions with Dan Wing. The fax
message example was taken from a proposal by Mike Ruhl.
8. References
[1] "A syntax for describing media feature sets"
Graham Klyne, 5GM/Content Technologies
Internet draft: <draft-ietf-conneg-feature-syntax-00.txt>"
Work in progress, September 1998.
[2] "Media Feature Tag Registration Procedure"
Koen Holtman, TUE
Andrew Mutz, Hewlett-Packard
Ted Hardie, NASA
Internet draft: <draft-ietf-conneg-feature-reg-03.txt>
Work in progress, July 1998.
[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: <draft-ietf-drums-msg-fmt-07.txt>
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.
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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
00a 10-Feb-1999 Initial draft.
TODO:
o Complete examples
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