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).
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 RFC822-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 [RFC2119].
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 RFC822 EBNF of the Content-Language header is:
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 RFC822, which means also that one can place parenthesized comments anywhere in the language sequence, or use continuation lines. A formal definition is given in RFC2822 [RFC2822].
In keeping with the tradition of RFC2822, a more liberal "obsolete" grammar is also given:
Like RFC2822, 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.
The "Accept-Language" header is intended for use in cases where a user or a process desires to identify the preferred language(s) when RFC822-like headers, such as MIME body parts or Web documents, are used.
Like RFC2822, 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 [RFC2616] 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 RFC1766, 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].
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 RFC1766, 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, RFC3066
[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)
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC2045, November 1996.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC2046, November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text", RFC2047, November 1996.
[RFC2048] Freed, N., Klensin, J. and J. Postel, "Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures", RFC2048, November 1996.
[RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples", RFC2049, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC2119, March 1997.
[RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC2234, November 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC2616, June 1999.
[RFC2822] Resnick, P., "Internet Message Format", RFC2822, April 2001.
Appendix A: Changes from RFC1766
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 RFC1766 if you need the information.
The ABNF for content-language has been updated to use the RFC2234 ABNF.
Author's Address
Harald Tveit Alvestrand Cisco Systems Weidemanns vei 27 7043 Trondheim NORWAY
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