As per Relevance of the word features, we have this rfc below:
Network Working Group G.
Request for Comments: 2912 Content
Category: Standards Track September 2000
Indicating Media Features for MIME
Status of this
This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited
Copyright
Copyright (C) The Internet Society (2000). All Rights Reserved
In "A Syntax for Describing Media Feature Sets", an expression
is presented for describing media feature capabilities using
media feature tags
This memo defines a Multipurpose Internet Mail Extensions (MIME
'Content-features:' header that can be used to annotate a
message part using this expression format, and indicates some ways
might be used
Klyne Standards Track [Page 1]
RFC 2912 Indicating Media Features for MIME Content September 2000
Table of
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.
In "A Syntax for Describing Media Feature Sets" [1], an
format is presented for describing media feature capabilities as
combination of simple media feature tags, registered according
"Media Feature Tag Registration Procedure" [2]. This provides
format for message handling agents to describe the media
content of messages that they can handle
This memo defines a MIME 'Content-features:' header that can be
to annotate a MIME message part using these feature expressions
This header provides a means to indicate media-related features
message content that go beyond the MIME content type
Consideration is also given to how it may be used to present
media content information that is problematic to express within
basic MIME framework
1.1 Terminology and document
This section defines a number of terms and other
conventions, which are used with specific meaning in this memo
Klyne Standards Track [Page 2]
RFC 2912 Indicating Media Features for MIME Content September 2000
media
information that indicates facilities assumed to be
for the message content to be properly rendered or
presented. Media features are not intended to
information that affects message transmission
feature
some set of media features described by a media
assertion, as described in "A Syntax for Describing
Feature Sets" [1]. (See that memo for a more formal
of this term.)
feature set
a string that describes some feature set, formulated
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
RFC 2234 "Augmented BNF for Syntax Specifications: ABNF" [3].
NOTE: Comments like this provide additional
information about the rationale behind this document.
information is not needed for building a
implementation, but may help those who wish to understand
design in greater depth
2. Motivation and
It is envisaged that media feature labelling of message parts may
used in the following ways
o to supply more detailed media feature information about a
content than can be provided by the 'Content-type:' header
o to provide summary media feature information (possibly
MIME content types) about the content of a composite MIME
part (e.g. 'multipart' or 'message'), without having to open
the inner content of the message
o to supply media feature information about external data
by a message part (e.g. 'message/external-body' MIME type).
information would not be available by examination of the
content
o to describe the content of a message that is encrypted or
using some application-specific file structure that hides
content from a MIME processor. This information also would not
generally available by examination of the message content
Klyne Standards Track [Page 3]
RFC 2912 Indicating Media Features for MIME Content September 2000
3. The 'Content-features:' MIME
A new header field is defined that extends the allowable formats
'optional-field' [4] with the following syntax
optional-field =/ "Content-features" ":" Feature-
Feature-expr = filter ; See [1], section 4.1
where 'filter' is the media feature expression format defined by "
Syntax for Describing Media Feature Sets" [1].
This header provides additional information about the message
directly contained or indirectly referenced in the corresponding
message part
3.1 Whitespace and folding long
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
expression. Further, RFC822/MIME [4,5] allows folding of
headers at points where whitespace appears to avoid line
restrictions
Therefore, it is recommended that whitespace is included
permitted, especially in long media feature expressions,
facilitate the folding of headers by agents that do not
understand the syntax of this field
3.2 Usage
3.2.1 Simple message
When applied to a simple MIME message part, the header should
just once and is used to convey additional information about
message part content that goes beyond that provided by the
'Content-type:' header field. The 'Content-features:' header
indicate a content type that is different than that given by the
'Content-type:' header. This is possible but not recommended
applied to a non-composite body part: in any case, MIME content
processing must be performed in accordance with the 'Content-type:'
header
NOTE: Once the message content has been delivered to
application, it is possible that subsequent processing may
affected by content type information indicated by the
feature expression. See example 4.5 below
Klyne Standards Track [Page 4]
RFC 2912 Indicating Media Features for MIME Content September 2000
3.2.2 Multipart and other
'Content-features:' headers may be applied to a MIME
indicating information about the inner content of the multipart
Implementations must not assume a one-to-one relationship
'Content-features' headers and contained body parts. Headers
appear on a containing multipart wrapper in a different order
the body parts to which they refer; a single header may refer
more than one contained body part; several headers may refer to
same contained body part
If it is important to relate specific media features to
contained MIME body parts, then the 'Content-features:' header
be applied directly to the body part concerned, rather than
surrounding composite
NOTE: The intent here is to allow summary media
information to be provided without having to open up
examine the inner content of the MIME message
Similar usage may apply when the message format is a non-MIME
opaque composite; e.g. 'application/zip', or an encrypted message
In these cases, the option of examining the message content
discover media feature information is not available
3.2.3 Reference to external
Media feature information about data indirectly referenced by a
body part rather than contained within a message can be
using one or more 'Content-features:' headers
For example, media information --including contained MIME
type(s)-- about the data referenced by a MIME 'Message/external-body
may be conveyed
4.
4.1 Simple
Mime-Version: 1.0
Content-type: text/plain;charset=US-
Content-features: (& (paper-size=A4) (ua-media=stationery) )
:
(data
:
Klyne Standards Track [Page 5]
RFC 2912 Indicating Media Features for MIME Content September 2000
4.2 Fax
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) )
--
Content-Type: image/tiff; name="coverpage.tiff
Content-Transfer-Encoding: base64
Content-Description: This part is a
Content-Disposition: attachment; filename="coverpage.tiff
0M8R4KGxGuEAAAAAAAAAAAAAAAAAAAAAPgADAP7/
AAAAAAAAEAAAZAAAAAEAAAD+////AAAAAAAAAAD////////////////////
:
(more data
:
--
Content-Type: image/tiff; name="document.tiff
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="document.tiff
AAAADgAAAA8
GgAAABsAAAAcAAAAHQAAAB4
:
(more data
:
--break--
4.3 Multipart/alternative
This example illustrates three points
o Information about the various parts in a multipart/alternative
be made available before the alternative body parts are processed
This may facilitiate optimum one-pass processing
multipart/alternative data
Klyne Standards Track [Page 6]
RFC 2912 Indicating Media Features for MIME Content September 2000
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) )
--
Content-type: "text/plain";charset=US-
Content-features: (color=binary
:
(data
:
--
Content-type: "text/plain";charset=US-
Content-features: (color=limited
:
(data
:
--
Content-type: text/html;charset=iso-8859-1
Content-features: (color=binary
:
(data
:
--
Content-type: text/html;charset=iso-8859-1
Content-features: (color=limited
:
(data
:
--break--
Klyne Standards Track [Page 7]
RFC 2912 Indicating Media Features for MIME Content September 2000
4.4 Reference to external message
Mime-Version: 1.0
Content-type: message/external-body; access-type=URL
URL="http://www.foo.com/file1.html
Content-type: Multipart/
Content-features: (& (Type="text/plain") (charset=US-ASCII) )
Content-features: (& (Type="image/tiff") (color=limited) )
4.5 Compressed
This example shows how the 'Content-features' header can be used
overcome the problem noted in the MIME registration
'Application/zip' regarding information about the data content
Mime-Version: 1.0
Content-type: application/
Content-features: (& (Type="text/plain") (charset=US-ASCII) )
Content-features: (& (Type="image/tiff") (color=limited) )
Content-transfer-encoding: base64
:
(data
:
4.6 Multipart/related
(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=""
Content-features: (& (type="text/html") (charset=US-ASCII) )
Content-features: (type="image/gif")
--boundary-
Content-Type: text/html;charset=US-
Content-ID:
referencing a resource in another body part, for
through a statement such as
Klyne Standards Track [Page 8]
RFC 2912 Indicating Media Features for MIME Content September 2000
--boundary-
Content-Location
http://www.ietf.cnri.reston.va.us/images/ietflogo.
Content-Type: IMAGE/
Content-Transfer-Encoding: BASE64
R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk
NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4
etc...
--boundary-example--
5. Security
When applied to simple or multipart MIME formatted data, a
feature expression provides summary information about the
data, which in many cases can be determined by examination of
message content. Under these circumstances, no additional
considerations appear to be raised
When applied to other message composites, especially
message content, feature expressions may disclose information that
otherwise unavailable. In these cases, some security
associated with media content negotiation [1,2] may have
relevance
It is suggested here that media feature descriptions may be
employed with encrypted message content. In doing this, take care
ensure that the purpose of encryption is not compromised (e.g
encryption might be intended to conceal the fact that a
application data format is being used, which fact might be
by an injudiciously applied Content-features header).
If a 'Content-features' header is applied to a multipart/
object (or indeed outside any other form of signed data) the
feature information is not protected. This unprotected
could be tampered with, possibly fooling implementations into
inappropriate things with the contained material. (Putting the
feature information inside the signed information would
this, at the cost of requiring implementations to parse the
structure to find it.)
Klyne Standards Track [Page 9]
RFC 2912 Indicating Media Features for MIME Content September 2000
6.
This proposal draws from discussions with Dan Wing. The fax
example was taken from a proposal by Mike Ruhl.
multipart/related example is developed from RFC 2557 [7].
The author would like to thank the following people who
comments that led to significant improvements: Mr Hiroshi Tamura
Ted Hardie, Maurizio Codogno, Jacob Palme, Ned Freed
7.
[1] Klyne, G., "A Syntax for Describing Media Feature Sets",
2533, March 1999.
[2] Holtman, K., Mutz, A. and T. Hardie, "Media Feature
Registration Procedure", RFC 2506, March 1999.
[3] Crocker, D. and P. Overell, "Augmented BNF for
Specifications: ABNF", RFC 2234, November 1997.
[4] Crocker, D., "Standard for the format of ARPA Internet
messages", STD 11, RFC 822, August 1982.
[5] Freed, N. and N. Borenstein, "Multipurpose Internet
Extensions (MIME) Part 1: Format of Internet message bodies",
RFC 2045, November 1996.
[6] Levinson, E., "The MIME Multipart/Related Content-type",
2387, August 1998.
[7] Palme, J., Hopmann, A. and N. Shelness, "MIME Encapsulation
Aggregate Documents, such as HTML (MHTML)", RFC 2557,
1999.
8. Author's
Graham
Content Technologies Ltd
1220 Parkview
Arlington Business
Reading, RG7 4
United
Phone: +44 118 930 1300
Fax: +44 118 930 1301
EMail: GK@ACM.
Klyne Standards Track [Page 10]
RFC 2912 Indicating Media Features for MIME Content September 2000
9. Full Copyright
Copyright (C) The Internet Society (2000). All Rights Reserved
This document and translations of it may be copied and furnished
others, and derivative works that comment on or otherwise explain
or assist in its implementation may be prepared, copied,
and distributed, in whole or in part, without restriction of
kind, provided that the above copyright notice and this paragraph
included on all such copies and derivative works. However,
document itself may not be modified in any way, such as by
the copyright notice or references to the Internet Society or
Internet organizations, except as needed for the purpose
developing Internet standards in which case the procedures
copyrights defined in the Internet Standards process must
followed, or as required to translate it into languages other
English
The limited permissions granted above are perpetual and will not
revoked by the Internet Society or its successors or assigns
This document and the information contained herein is provided on
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED,
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE
Funding for the RFC Editor function is currently provided by
Internet Society
Klyne Standards Track [Page 11]
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