draft-ietf-conneg-feature-type-02.txt   rfc2913.txt 
IETF conneg working group Graham Klyne
Internet draft Content Technologies
Category: Work-in-progress 30 November 1999
Expires: May 2000
MIME content types in media feature expressions Network Working Group G. Klyne
<draft-ietf-conneg-feature-type-02.txt> Request for Comments: 2913 Content Technologies
Category: Standards Track September 2000
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six MIME Content Types in Media Feature Expressions
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet- Drafts
as reference material or to cite them other than as "work in
progress."
The list of current Internet-Drafts can be accessed at Status of this Memo
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at This document specifies an Internet standards track protocol for the
http://www.ietf.org/shadow.html. 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 Notice
Copyright (C) The Internet Society 1999. All Rights Reserved. Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract Abstract
In RFC 2533, "A syntax for describing media feature sets", an In "A Syntax for Describing Media Feature Sets", an expression format
expression format is presented for describing media feature is presented for describing media feature capabilities using simple
capabilities using simple media feature tags. media feature tags.
This memo defines a media feature tag whose value is a MIME content
type. This allows the construction of feature expressions that
take account of the MIME content type of the corresponding data.
Internet draft MIME content types in media feature expressions This memo defines a media feature tag whose value is a Multipurpose
30 November 1999 Internet Mail Extensions (MIME) content type. This allows the
construction of feature expressions that take account of the MIME
content type of the corresponding data.
Table of contents Table of Contents
1. Introduction ............................................2 1. Introduction .................................................. 2
1.1 Terminology and document conventions 2 1.1 Terminology and document conventions ...................... 2
1.2 Discussion of this document 3 2. Motivation and goals .......................................... 3
2. Motivation and goals ....................................4 3. MIME content type feature tag ................................. 3
3. MIME content type feature tag ...........................4 4. Examples ...................................................... 4
4. Examples ................................................5 4.1 Simple text ............................................... 4
4.1 Simple text 5 4.2 Fax image ................................................. 4
4.2 Fax image 5 4.3 Voice message ............................................. 4
4.3 Voice message 5 4.4 Web browser capabilities .................................. 5
4.4 Web browser capabilities 6 5. IANA Considerations ........................................... 5
5. IANA considerations .....................................6 6. Security Considerations ....................................... 5
6. Security considerations .................................6 7. Acknowledgements .............................................. 5
7. Acknowledgements ........................................6 8. References .................................................... 6
8. References ..............................................6 9. Author's Address .............................................. 6
9. Author's address ........................................8 Appendix A: 'Type' feature tag registration ...................... 7
Appendix A: 'Type' feature tag registration ................8 Full Copyright Statement ......................................... 9
Full copyright statement ...................................10
Revision history ...........................................11
1. Introduction 1. Introduction
In "A syntax for describing media feature sets" [1], an expression In "A Syntax for Describing Media Feature Sets" [1], an expression
format is presented for describing media feature capabilities as a format is presented for describing media feature capabilities as a
combination of simple media feature tags, registered according to combination of simple media feature tags, registered according to
"Media Feature Tag Registration Procedure" [2]. This provides a "Media Feature Tag Registration Procedure" [2]. This provides a
format for message handling agents to describe the media feature format for message handling agents to describe the media feature
content of messages that they can handle. content of messages that they can handle.
This memo defines a media feature tag whose value is a MIME content This memo defines a media feature tag whose value is a MIME content
type. This allows the construction of feature expressions that type. This allows the construction of feature expressions that take
take account of the MIME content type of the corresponding data. account of the MIME content type of the corresponding data.
Note that a content type feature value may contain parameters, but Note that a content type feature value may contain parameters, but
this is discouraged. See section 3 and appendix A "Summary of the this is discouraged. See section 3 and appendix A, "Summary of the
media features indicated" for discussion of this point. media features indicated" for discussion of this point.
1.1 Terminology and document conventions 1.1 Terminology and document conventions
This section defines a number of terms and other document This section defines a number of terms and other document
conventions, which are used with specific meaning in this memo. 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.
Internet draft MIME content types in media feature expressions
30 November 1999
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 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.
To subscribe to this list, send a message with the body 'subscribe' feature set
to "ietf-medfree-request@imc.org". 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.)
To see what has gone on before you subscribed, please see the feature set expression
mailing list archive at: 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).
http://www.imc.org/ietf-medfree/ This specification uses syntax notation and conventions described in
RFC 2234, "Augmented BNF for Syntax Specifications: ABNF" [3].
Internet draft MIME content types in media feature expressions NOTE: Comments like this provide additional nonessential
30 November 1999 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 2. Motivation and goals
The media feature expression syntax [1] and feature tags [2] were The media feature expression syntax [1] and feature tags [2] were
designed with a view to providing content media information that designed with a view to providing content media information that
augments basic MIME content type information. There are some augments basic MIME content type information. There are some
situations where it is useful to be able include that content type situations where it is useful to be able include that content type
information in a media feature expression: information in a media feature expression:
o Media feature details may depend upon the content type being o Media feature details may depend upon the content type being used.
used. The media feature combining algebra and syntax [1] cannot The media feature combining algebra and syntax [1] cannot apply to
apply to content type information unless it appears in the content type information unless it appears in the feature
feature expression. expression.
For example, in HTTP 1.1 [4] with Transparent Content Negotiation For example, in HTTP 1.1 [4] with Transparent Content Negotiation
(TCN) [5] acceptable content types and other media features are (TCN) [5] acceptable content types and other media features are
indicated in different request headers, with no clear way to indicated in different request headers, with no clear way to
indicate that they may be acceptable only in certain indicate that they may be acceptable only in certain combinations.
combinations.
o It is sometimes useful for all media capability information to be o It is sometimes useful for all media capability information to be
included in a single expression. For example, DSN and MDN included in a single expression. For example, DSN and MDN
extensions [6] that allow a recipient to indicate media extensions [6] that allow a recipient to indicate media
capabilities provide a single field for conveying this capabilities provide a single field for conveying this
information. information.
o When media features are used to describe a message content, they o When media features are used to describe a message content, they
may refer to inner parts of a MIME composite; e.g. the component may refer to inner parts of a MIME composite; e.g. the component
parts of a 'multipart', files in a compressed archive, or parts of a 'multipart', files in a compressed archive, or
encrypted message data. encrypted message data.
3. MIME content type feature tag 3. MIME content type feature tag
Feature tag name Legal values Feature tag name Legal values
---------------- ------------ ---------------- ------------
type <string> type <string>
containing any MIME content type value. containing a MIME content-type value.
Reference: this document, appendix A. Reference: this document, appendix A.
The 'type' feature tag indicates a MIME media content type (i.e. The 'type' feature tag indicates a MIME media content type (i.e.
that appears in a 'Content-type:' header in the corresponding MIME- that appears in a 'Content-type:' header of the corresponding MIME-
formatted data). formatted data). It must be a string of the form "type/subtype",
where 'type' and 'subtype' are defined by the MIME specification [7].
Only lower-case letters should be used.
The media type should be given without any parameter values. The The content type must be given without any content-type parameter
intention here is that information that is conveyed in MIME content values.
type parameters is more usefully handled in a media feature
expression by separate feature tags.
Internet draft MIME content types in media feature expressions To include information in media feature expressions that is otherwise
30 November 1999 conveyed in a MIME content-type parameter, a separate media feature
tag should be registered [2] and used in the media feature
expression. This is illustrated by the use of 'charset' in the
example at 4.1 below -- the 'charset' tag is defined by a separate
registration [10].
NOTE: content type parameters in a 'type' value are NOTE: Allowing content-type parameters to be part of a type tag
strongly discouraged, but not prohibited. In deciding value was considered, but rejected because of concerns about
whether or not to include a parameter value, implementers canonicalization, ordering, case sensitivity, etc. Only exact,
should bear in mind the feature set matching rules [1]. case-sensitive, character matching is defined for media feature
These would cause 'type' values with and without expressions [1].
parameters to be treated as completely distinct values,
which may lead to unexpected results.
4. Examples 4. Examples
4.1 Simple text 4.1 Simple text
(& (type="text/plain") (charset=US-ASCII) (& (type="text/plain") (charset=US-ASCII)
(color=binary) (paper-size=A4) ) (color=binary) (paper-size=A4) )
4.2 Fax image 4.2 Fax image
(& (type="image/tiff") (& (type="image/tiff")
(color=binary) (color=binary)
(image-file-structure=TIFF-S) (image-file-structure=TIFF-S)
(dpi=200) (dpi=200)
(dpi-xyratio=[200/100,200/200]) (dpi-xyratio=[200/100,200/200])
(paper-size=A4) (paper-size=A4)
(image-coding=MH) (MRC-mode=0) (image-coding=MH) (MRC-mode=0)
(ua-media=stationery) ) (ua-media=stationery) )
4.3 Voice message 4.3 Voice message
(& (type="multipart/voice-message") (& (type="multipart/voice-message")
(VPIM-version="3.0") (VPIM-version="3.0")
(audio-codec=[G726-32,GSM-610]) (audio-codec=[G726-32,GSM-610])
(audio-file-structure=[None,WAV]) (audio-file-structure=[None,WAV])
(ua-terminal=mobile-handset) (ua-terminal=mobile-handset)
(audio-channels=1) ) (audio-channels=1) )
NOTE: in this case, some media features apply to MIME
parts contained within the declared 'multipart/voice-
message' content type. The goal here is not so much to
mirror the MIME structure as to convey useful information
about the (possible) message content.
Internet draft MIME content types in media feature expressions NOTE: in this case, some media features apply to MIME parts
30 November 1999 contained within the declared 'multipart/voice- message'
content type. The goal here is not so much to mirror the MIME
structure as to convey useful information about the (possible)
message content.
4.4 Web browser capabilities 4.4 Web browser capabilities
(& (pix-x<=800) (pix-y<=600) (& (pix-x<=800) (pix-y<=600)
(| (& (type="text/html") (charset=iso-8859-1) (color=limited)) (| (& (type="text/html") (charset=iso-8859-1)
(& (type="text/plain") (charset=US-ASCII) ) (color=limited) )
(& (type="image/gif") (color=mapped)) (& (type="text/plain") (charset=US-ASCII) )
(& (type="image/jpeg") (color=full) ) ) ) (& (type="image/gif") (color=mapped))
(& (type="image/jpeg") (color=full) ) ) )
This example describes an HTML viewer that can deal with a limited This example describes an HTML viewer that can deal with a limited
number of color text tags, a gif viewer that supports mapped color, number of color text tags, a gif viewer that supports mapped color,
and a jpeg viewer that supports color. and a jpeg viewer that supports color.
5. IANA considerations 5. IANA Considerations
Appendix A of this document calls for registration of a feature tag Appendix A of this document calls for registration of a feature tag
in the "IETF tree", as defined in section 3.1.1 of "Media Feature in the "IETF tree", as defined in section 3.1.1 of "Media Feature Tag
Tag Registration Procedure" [2] (i.e. these feature tags are Registration Procedure" [2] (i.e. these feature tags are subject to
subject to the "IETF Consensus" policies described in RFC 2434 the "IETF Consensus" policies described in RFC 2434 [9]).
[9]).
An ASN.1 identifier should be assigned for this registered feature ASN.1 identifier 1.3.6.1.8.1.30 has been assigned by the IANA for
tag and replaced in the body of the registration. this registered feature tag and has been placed in the body of the
registration.
6. Security considerations 6. Security Considerations
This memo is not believed to introduce any security considerations This memo is not believed to introduce any security considerations
that are not already inherent in the use of media feature tags and that are not already inherent in the use of media feature tags and
expressions [1,2]. expressions [1,2].
7. Acknowledgements 7. Acknowledgements
This proposal draws from discussions in the IETF 'conneg' working This proposal draws from discussions in the IETF 'conneg' working
group. The voice message example is based on some ideas by Glen group. The voice message example is based on some ideas by Glen
Parsons. Parsons.
The author would like to thank the following people who offered The author would like to thank the following people who offered
comments that led to significant improvements: Ted Hardie, Larry comments that led to significant improvements: Ted Hardie, Larry
Masinter, Paul Hoffman, Jacob Palme. Masinter, Paul Hoffman, Jacob Palme, Ned Freed.
8. References 8. References
[1] RFC 2533, "A syntax for describing media feature sets" [1] Klyne, G., "A Syntax for Describing Media Feature Sets", RFC
Graham Klyne, 5GM/Content Technologies 2533, March 1999.
March 1999.
[2] RFC 2506, "Media Feature Tag Registration Procedure"
Koen Holtman, TUE
Andrew Mutz, Hewlett-Packard
Ted Hardie, NASA
March 1999.
Internet draft MIME content types in media feature expressions [2] Holtman, K., Mutz, A. and T. Hardie, "Media Feature Tag
30 November 1999 Registration Procedure", RFC 2506, March 1999.
[3] RFC 2234, "Augmented BNF for Syntax Specifications: ABNF" [3] Crocker, D. and P. Overell, "Augmented BNF for Syntax
D. Crocker (editor), Internet Mail Consortium Specifications: ABNF", RFC 2234, November 1997.
P. Overell, Demon Internet Ltd.
November 1997.
[4] RFC 2068, "Hyptertext Transfer Protocol -- HTTP/1.1" [4] Fielding, R., Gettys, J., Mogul, J., Frystyk, H. and T.
R. Fielding, UC Irvine Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC
J. Gettys, 2068, January 1997.
J. Mogul, DEC
H. Frytyk,
T. Berners-Lee, MIT/LCS
January 1997.
[5] RFC 2295, "Transparent Content Negotiation in HTTP" [5] Holtman, K. and A. Mutz, "Transparent Content Negotiation in
Koen Holtman, TUE HTTP", RFC 2295, March 1998.
Andrew Mutz, Hewlett Packard
March 1998.
[6] RFC 2530, "Indicating Supported Media Features Using Extensions [6] Wing, D., "Indicating Supported Media Features Using Extensions
to DSN and MDN" to DSN and MDN", RFC 2530, March 1999.
Dan Wing, Cisco Systems
March 1999.
[7] RFC 2045, "Multipurpose Internet Mail Extensions (MIME) [7] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Part 1: Format of Internet message bodies" Extensions (MIME) Part One: Format of Internet Message Bodies",
N. Freed, Innosoft RFC 2045, November 1996.
N. Borenstein, First Virtual
November 1996.
[8] RFC 2046, "Multipurpose Internet Mail Extensions (MIME) [8] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Part 2: Media types" Extensions (MIME) Part Two: Media Types", RFC 2046, November
N. Freed, Innosoft 1996.
N. Borenstein, First Virtual
November 1996.
[9] RFC 2434, "Guidelines for Writing an IANA Considerations Section [9] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
in RFCs" Considerations Section in RFCs", RFC 2434, October 1998.
T. Narten, IBM
H. Alvestrand, Maxware
October 1998.
[10] "Registration of Charset and Languages Media Features Tags" [10] Hoffman, P., "Registration of Charset and Languages Media
Paul Hoffman, IMC Features Tags", Work in Progress.
Internet draft: <draft-hoffman-char-lang-media-01.txt>
Work in progress, July 1999.
Internet draft MIME content types in media feature expressions 9. Author's Address
30 November 1999
9. Author's address Graham Klyne
Content Technologies Ltd.
1220 Parkview,
Arlington Business Park
Theale
Reading, RG7 4SA
United Kingdom
Graham Klyne Phone: +44 118 930 1300
Content Technologies Ltd. Fax: +44 118 930 1301
1220 Parkview, EMail: GK@ACM.ORG
Arlington Business Park
Theale
Reading, RG7 4SA
United Kingdom.
Telephone: +44 118 930 1300
Facsimile: +44 118 930 1301
E-mail: GK@ACM.ORG
Appendix A: 'Type' feature tag registration Appendix A: 'Type' feature tag registration
- Media Feature tag name(s): - Media Feature tag name(s):
Type
- ASN.1 identifiers associated with this feature tag:
[[[New assignments by IANA]]]
- Summary of the media features indicated:
This feature tag indicates a MIME content type that a message
agent is capable of handling, or contained within some message
data.
The content type consists of the MIME media type and subtype,
presented using all lower case letters and with any whitespace
characters removed.
In exceptional cases, content type parameters may be included, in
which case the parameter name is also presented in lower case
letters, with all whitespace surrounding the ';' and '=' removed.
The parameter value should be presented in some canonical form.
- Values appropriate for use with this feature tag:
String
- The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation mechanisms:
Any application that wishes to convey MIME content type
information in a media feature expression.
Internet draft MIME content types in media feature expressions
30 November 1999
- Examples of typical use: Type
(type="text/plain") - ASN.1 identifier associated with this feature tag:
(type="text/plain;charset=iso-8859-1")
The second example is not a recommended form; the preferred form 1.3.6.1.8.1.30
is to use a 'charset' feature tag, per [10]. But note that all
spaces around the 'charset' parameter are removed, and the name
and value are presented in lower case.
- Related standards or documents: - Summary of the media features indicated:
MIME, RFC 2045 [7] This feature tag indicates a MIME content type that a message
agent is capable of handling, or that is contained within some
message data.
MIME, RFC 2046 [8] The content type consists of the MIME media type and subtype,
presented using all lower case letters and with any whitespace
characters removed.
Registration of Charset and Languages Media Features Tags [10] - Values appropriate for use with this feature tag:
- Considerations particular to use in individual applications, String
protocols, services, or negotiation mechanisms:
(N/A) - The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation mechanisms:
- Interoperability considerations: Any application that wishes to convey MIME content type
information in a media feature expression.
String feature matching is case sensitive, so consistent use of - Examples of typical use:
case for content type values and parameters is essential if
content type value matching is to be achieved in a fashion
consistent with MIME content type matching.
Similarly, white space must be used consistently. (type="image/tiff")
This registration specifies a canonical form to be used for (& (type="text/plain") (charset=US-ASCII) )
content type values (lower case letters and remove all
whitespace). If content type parameters are introduced, all
letters and whitespace that are not part of the parameter value
are treated similarly. The canonical form for parameter values
must be appropriate to the equivalence rules for that value.
- Related feature tags: - Related standards or documents:
(N/A) MIME, RFC 2045 [7]
- Intended usage: MIME, RFC 2046 [8]
Common Registration of Charset and Languages Media Features Tags [10]
- Author/Change controller: - Considerations particular to use in individual applications,
protocols, services, or negotiation mechanisms:
IETF (N/A)
Internet draft MIME content types in media feature expressions - Interoperability considerations:
30 November 1999
Full copyright statement String feature matching is case sensitive, so consistent use of
case for content type values and parameters is essential if
content type value matching is to be achieved in a fashion
consistent with MIME content type matching.
Copyright (C) The Internet Society 1999. All Rights Reserved. Similarly, white space must be used consistently.
This document and translations of it may be copied and furnished to This registration specifies a canonical form to be used for
others, and derivative works that comment on or otherwise explain content type values (lower case letters and remove all
it or assist in its implementation may be prepared, copied, whitespace).
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 - Related feature tags:
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on (N/A)
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.
Internet draft MIME content types in media feature expressions - Intended usage:
30 November 1999
Revision history Common
[[[RFC editor: please reove this section on publication]]] - Author/Change controller:
00a 16-Feb-1999 Initial draft. IETF
01a 16-Feb-1999 Added pointers to mailing list for discussion. Full Copyright Statement
01b 04-Mar-1999 Various editorial improvements. Copyright (C) The Internet Society (2000). All Rights Reserved.
01c 29-Apr-1999 Improved web browser example. 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.
02a 29-Apr-1999 Highlight and forward reference the content-type The limited permissions granted above are perpetual and will not be
parameter issue in the introduction. revoked by the Internet Society or its successors or assigns.
02b 20-Jul-1999 Incorporate review comments. Also cite charset This document and the information contained herein is provided on an
feature type registration work-in-progress. "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.
02c 14-Nov-1999 Fix RFC 2533 number in references. Acknowledgement
02d 30-Nov-1999 Add note about 'charset' feature tag to Funding for the RFC Editor function is currently provided by the
registration template. Moved copyright notice to Internet Society.
end of document text.
 End of changes. 93 change blocks. 
375 lines changed or deleted 263 lines changed or added

This html diff was produced by rfcdiff 1.41. The latest version is available from http://tools.ietf.org/tools/rfcdiff/