RFC2912
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.
Contents
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.
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.
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.
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.
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.
Usage considerations
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.
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.
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.
Examples
Simple message
Mime-Version: 1.0 Content-type: text/plain;charset=US-ASCII Content-features: (& (paper-size=A4) (ua-media=stationery) )
: (data) :
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--
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--
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>
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>
(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@[email protected]>"
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@[email protected]>
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--
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.)
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 [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.
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", RFC 2506, March 1999.
[3] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[4] Crocker, D., "Standard for the format of ARPA Internet text
messages", STD 11, RFC 822, August 1982.
[5] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part 1: Format of Internet message bodies",
RFC 2045, 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)", RFC 2557, March 1999.
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: [email protected]
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