IETF conneg working group Graham Klyne
Internet draft Content Technologies
Category: Work-in-progress 4 April 2000
Expires: October 2000
Indicating media features for MIME content
<draft-ietf-conneg-content-features-03.txt>
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
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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
1.2 Discussion of this document 3
2. Motivation and goals ....................................4
3. The 'Content-features:' MIME header .....................4
3.1 Whitespace and folding long headers 4
3.2 Usage considerations 5
3.2.1 Simple message parts 5
3.2.2 Multipart and other composites 5
3.2.3 Reference to external data 6
4. Examples ................................................6
4.1 Simple message 6
4.2 Fax message 6
4.3 Multipart/alternative data 7
4.4 Reference to external message data 8
4.5 Compressed data 8
4.6 Multipart/related data 9
5. Security considerations .................................9
6. Acknowledgements ........................................10
7. References ..............................................10
8. Author's address ........................................12
Full copyright statement ...................................12
Revision history ...........................................13
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.
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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.
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/
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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 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 oints where whitespace appears to avoid lione length
restrictions.
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Therefore, it is recommended that whitespace is included as
permitted, especialy 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.
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.
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3.2.3 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;charset=US-ASCII
Content-features: (& (paper-size=A4) (ua-media=stationery) )
:
(data)
:
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
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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.
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-feature
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
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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--
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>
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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>
... text of the HTML document, which might contain a URI
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">
--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.
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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.)
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.
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] 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.
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[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.
[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
(Non-normative)
[8] RFC 2387, "The MIME Multipart/Related Content-type"
E. Levinson
August 1998
(Non-normative)
[9] "Registration of Charset and Languages Media Features Tags"
Paul Hoffman, IMC
Internet draft: <draft-hoffman-char-lang-media-01.txt>
Work in progress, July 1999.
(Non-normative)
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8. Author's address
Graham Klyne
Content Technologies Ltd.
1220 Parkview,
Arlington Business Park
Theale
Reading, RG7 4SA
United Kingdom.
Telephone: +44 118 930 1300
Facsimile: +44 118 930 1301
E-mail: GK@ACM.ORG
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
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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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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.
02a 20-Jul-1999 Incorporated review comments -- editorial changes.
02b 29-Nov-1999 Added (charset=...) to (type=text/*) examples.
Added citation to charset and language feature
registration document.
02c 29-Nov-1999 Indicated motivation for multipart/alternative
example. Moved copyright section to end of text.
03a 04-Apr-2000 Clarification of Content-feaures on outer wrapper
of a multipart. Recommend use of whitespace in
feature expressions to facilitate MIME header
wrapping. Add discussion of Content-features on
multipart/signed data.
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