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DRAFT STANDARD
Network Working Group H. Alvestrand
Request for Comments: 3282 Cisco Systems
Obsoletes: 1766 May 2002
Category: Standards Track
Content Language Headers
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 (2002). All Rights Reserved.
Abstract
This document defines a "Content-language:" header, for use in cases
where one desires to indicate the language of something that has RFC
822-like headers, like MIME body parts or Web documents, and an
"Accept-Language:" header for use in cases where one wishes to
indicate one's preferences with regard to language.
1. Introduction
There are a number of languages presently or previously used by human
beings in this world.
A great number of these people would prefer to have information
presented in a language which they understand.
In some contexts, it is possible to have information available in
more than one language, or it might be possible to provide tools
(such as dictionaries) to assist in the understanding of a language.
In other cases, it may be desirable to use a computer program to
convert information from one format (such as plaintext) into another
(such as computer-synthesized speech, or Braille, or high-quality
print renderings).
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RFC 3282 Content Language Headers May 2002
A prerequisite for any such function is a means of labelling the
information content with an identifier for the language that is used
in this information content, such as is defined by [TAGS]. This
document specifies a protocol element for use with protocols that use
RFC 822-like headers for carrying language tags as defined in [TAGS].
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC 2119].
2. The Content-language header
The "Content-Language" header is intended for use in the case where
one desires to indicate the language(s) of something that has RFC
822-like headers, such as MIME body parts or Web documents.
The RFC 822 EBNF of the Content-Language header is:
Content-Language = "Content-Language" ":" 1#Language-tag
In the more strict RFC 2234 ABNF:
Content-Language = "Content-Language" ":" [CFWS] Language-List
Language-List = Language-Tag [CFWS]
*("," [CFWS] Language-Tag [CFWS])
The Content-Language header may list several languages in a comma-
separated list.
The CFWS construct is intended to function like the whitespace
convention in RFC 822, which means also that one can place
parenthesized comments anywhere in the language sequence, or use
continuation lines. A formal definition is given in RFC 2822
[RFC2822].
In keeping with the tradition of RFC 2822, a more liberal "obsolete"
grammar is also given:
obs-content-language = "Content-Language" *WSP ":"
[CFWS] Language-List
Like RFC 2822, this specification says that conforming
implementations MUST accept the obs-content-language syntax, but MUST
NOT generate it; all generated headers MUST conform to the Content-
Language syntax.
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2.1 Examples of Content-language values
Voice recording from Liverpool downtown
Content-type: audio/basic
Content-Language: en-scouse
Document in Mingo, an American Indian language which does not have an
ISO 639 code:
Content-type: text/plain
Content-Language: i-mingo
A English-French dictionary
Content-type: application/dictionary
Content-Language: en, fr (This is a dictionary)
An official European Commission document (in a few of its official
languages):
Content-type: multipart/alternative
Content-Language: da, de, el, en, fr, it
An excerpt from Star Trek
Content-type: video/mpeg
Content-Language: i-klingon
3. The Accept-Language header
The "Accept-Language" header is intended for use in cases where a
user or a process desires to identify the preferred language(s) when
RFC 822-like headers, such as MIME body parts or Web documents, are
used.
The RFC 822 EBNF of the Accept-Language header is:
Accept-Language = "Accept-Language" ":"
1#( language-range [ ";" "q" "=" qvalue ] )
A slightly more restrictive RFC 2234 ABNF definition is:
Accept-Language = "Accept-Language:" [CFWS] language-q
*( "," [CFWS] language-q )
language-q = language-range [";" [CFWS] "q=" qvalue ] [CFWS]
qvalue = ( "0" [ "." 0*3DIGIT ] )
/ ( "1" [ "." 0*3("0") ] )
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A more liberal RFC 2234 ABNF definition is:
Obs-accept-language = "Accept-Language" *WSP ":" [CFWS]
obs-language-q *( "," [CFWS] obs-language-q ) [CFWS]
obs-language-q = language-range
[ [CFWS] ";" [CFWS] "q" [CFWS] "=" qvalue ]
Like RFC 2822, this specification says that conforming
implementations MUST accept the obs-accept-language syntax, but MUST
NOT generate it; all generated messages MUST conform to the Accept-
Language syntax.
The syntax and semantics of language-range is defined in [TAGS]. The
Accept-Language header may list several language-ranges in a comma-
separated list, and each may include a quality value Q. If no Q
values are given, the language-ranges are given in priority order,
with the leftmost language-range being the most preferred language;
this is an extension to the HTTP/1.1 rules, but matches current
practice.
If Q values are given, refer to HTTP/1.1 [RFC 2616] for the details
on how to evaluate it.
4. Security Considerations
The only security issue that has been raised with language tags since
the publication of RFC 1766, which stated that "Security issues are
believed to be irrelevant to this memo", is a concern with language
ranges used in content negotiation - that they may be used to infer
the nationality of the sender, and thus identify potential targets
for surveillance.
This is a special case of the general problem that anything you send
is visible to the receiving party; it is useful to be aware that such
concerns can exist in some cases.
The exact magnitude of the threat, and any possible countermeasures,
is left to each application protocol.
5. Character set considerations
This document adds no new considerations beyond what is mentioned in
[TAGS].
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6. Acknowledgements
This document has benefited from many rounds of review and comments
in various fora of the IETF and the Internet working groups.
Any list of contributors is bound to be incomplete; please regard the
following as only a selection from the group of people who have
contributed to make this document what it is today.
In alphabetical order:
Tim Berners-Lee, Nathaniel Borenstein, Sean M. Burke, John Clews, Jim
Conklin, John Cowan, Dave Crocker, Martin Duerst, Michael Everson,
Ned Freed, Tim Goodwin, Dirk-Willem van Gulik, Marion Gunn, Paul
Hoffman, Olle Jarnefors, John Klensin, Bruce Lilly, Keith Moore,
Chris Newman, Masataka Ohta, Keld Jorn Simonsen, Rhys Weatherley,
Misha Wolf, Francois Yergeau and many, many others.
Special thanks must go to Michael Everson, who has served as language
tag reviewer for almost the entire period, since the publication of
RFC 1766, and has provided a great deal of input to this revision.
Bruce Lilly did a special job of reading and commenting on my ABNF
definitions.
7. References
[TAGS] Alvestrand, H., "Tags for the Identification of
Languages", BCP 47, RFC 3066
[ISO 639] ISO 639:1988 (E/F) - Code for the representation of names
of languages - The International Organization for
Standardization, 1st edition, 1988-04-01 Prepared by
ISO/TC 37 - Terminology (principles and coordination).
Note that a new version (ISO 639-1:2000) is in
preparation at the time of this writing.
[ISO 639-2] ISO 639-2:1998 - Codes for the representation of names of
languages -- Part 2: Alpha-3 code - edition 1, 1998-11-
01, 66 pages, prepared by ISO/TC 37/SC 2
[ISO 3166] ISO 3166:1988 (E/F) - Codes for the representation of
names of countries - The International Organization for
Standardization, 3rd edition, 1988-08-15.
[ISO 15924] ISO/DIS 15924 - Codes for the representation of names of
scripts (under development by ISO TC46/SC2)
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[RFC 2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC 2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[RFC 2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII
Text", RFC 2047, November 1996.
[RFC 2048] Freed, N., Klensin, J. and J. Postel, "Multipurpose
Internet Mail Extensions (MIME) Part Four: Registration
Procedures", RFC 2048, November 1996.
[RFC 2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and
Examples", RFC 2049, November 1996.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[RFC 2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC 2822] Resnick, P., "Internet Message Format", RFC 2822, April
2001.
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Appendix A: Changes from RFC 1766
The definition of the language tags has been split, and is now RFC
3066. The differences parameter to multipart/alternative is no
longer part of this standard, because no implementations of the
function were ever found. Consult RFC 1766 if you need the
information.
The ABNF for content-language has been updated to use the RFC 2234
ABNF.
Author's Address
Harald Tveit Alvestrand
Cisco Systems
Weidemanns vei 27
7043 Trondheim
NORWAY
EMail: Harald@Alvestrand.no
Phone: +47 73 50 33 52
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RFC 3282 Content Language Headers May 2002
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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