Network Working Group G. Klyne
Request for Comments: 2912 Content Technologies
Category: Standards Track September 2000
Indicating Media Features for MIME Content
Status 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.
Table of Contents
1. IntrodUCtion ............................................... 2
1.1 Terminology and document conventions ................... 2
2. Motivation and goals ....................................... 3
3. The "Content-features:" MIME header ........................ 4
3.1 Whitespace and folding long headers .................... 4
3.2 Usage considerations ................................... 4
3.2.1 Simple message parts ............................... 4
3.2.2 Multipart and other composites ..................... 5
3.2.3 Reference to external data ......................... 5
4. Examples ................................................... 5
4.1 Simple message ......................................... 5
4.2 Fax message ............................................ 6
4.3 Multipart/alternative data ............................. 6
4.4 Reference to external message data ..................... 8
4.5 Compressed data ........................................ 8
4.6 Multipart/related data ................................. 8
5. Security Considerations .................................... 9
6. Acknowledgements ........................................... 10
7. References ................................................. 10
8. Author"s Address ........................................... 10
Full Copyright Statement ...................................... 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.
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.
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
RFC2234 "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.
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 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 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.
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. 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
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-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--
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>
4.6 Multipart/related data
(See also: RFC2387, "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">
--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.)
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 RFC2557 [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", RFC
2533, March 1999.
[2] Holtman, K., Mutz, A. and T. Hardie, "Media Feature Tag
Registration Procedure", RFC2506, March 1999.
[3] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC2234, November 1997.
[4] Crocker, D., "Standard for the format of ARPA Internet text
messages", STD 11, RFC822, August 1982.
[5] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part 1: Format of Internet message bodies",
RFC2045, November 1996.
[6] Levinson, E., "The MIME Multipart/Related Content-type", RFC
2387, August 1998.
[7] Palme, J., Hopmann, A. and N. Shelness, "MIME Encapsulation of
Aggregate Documents, such as HTML (MHTML)", RFC2557, 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.ORG
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