Internet-Draft H. Alvestrand draft-alvestrand-content-language-00.txt Cisco Systems Target Category: Standards Track August 2000 Obsoletes: RFC 1766 Expires: February 2001 Content Language Headers Status of this Memo The file name of this memo is draft-alvestrand-content-language- 00.txt 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 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 http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Comments on this draft should be sent to the mailing list <ietf- languages@iana.org> Abstract This document defines a "Content-language:" header, for use in the case where one desires to indicate the language of something that has RFC- 822-like headers, like MIME body parts or Web documents. It also preserves (but does not standardize) an extension to multipart/alternative for use when multiple language variants of a document are transmitted. Content Language Headers Harald Alvestrand draft-alvestrand-contentùlanguage-00.txt Expires December 2000 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 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 Or in RFC 2234 ABNF: Content-Language = "Content-Language" CFWS ":" Language-List Language-List = Language-Tag [ CFWS "," CFWS Language-List ] 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 draft-alvestrand-lang-tags-v2-01.txt [Page 2]
Content Language Headers Harald Alvestrand draft-alvestrand-contentùlanguage-00.txt Expires December 2000 continuation lines. A formal definition is given in the update to the RFC 822 grammar, currently a work in progress. 2.1 Examples of Content-language values Norwegian official document, with parallel text in both official versions of Norwegian. (Both versions are readable by all Norwegians). Content-Type: multipart/alternative; differences=content-language Content-Language: no-nyn, no-bok 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 An 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 (All the tags used in these examples were registered with IANA after the publication of RFC 1766) 3. The Accept-Language header The "Accept-Language" header is intended for use in the case where a user or a process desires to indentify the language(s) he prefers 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: draft-alvestrand-lang-tags-v2-01.txt [Page 3]
Content Language Headers Harald Alvestrand draft-alvestrand-contentùlanguage-00.txt Expires December 2000 Accept-Language = "Accept-Language" ":" 1#( language-range [ ";" "q" "=" qvalue ] ) The syntax and semantics of language-range is defined in [TAGS]. (Note that RFC-822 EBNF rather than ABNF is used here, in order to ensure that the syntax is identical with that specified in [RFC 2616]). 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 surveilllance. 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 Codes may always be expressed using the US-ASCII character repertoire (a-z), which is present in most character sets. The issue of deciding upon the rendering of a character set based on the language tag is not addressed in this memo; however, it is thought impossible to make such a decision correctly for all cases unless means of switching language in the middle of a text are defined (for example, a rendering engine that decides font based on Japanese or Chinese language may fail to work when a mixed Japanese-Chinese text is encountered) 6. Acknowledgements This document has benefited from many rounds of review and comments in various fora of the IETF and the Internet working groups. draft-alvestrand-lang-tags-v2-01.txt [Page 4]
Content Language Headers Harald Alvestrand draft-alvestrand-contentùlanguage-00.txt Expires December 2000 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, Jim Conklin, John Cowan, Dave Crocker, Martin Duerst, Michael Everson, Ned Freed, Tim Goodwin, Dirk-Willem van Gulik, Paul Hoffman, Olle Jarnefors, John Klensin, Keith Moore, 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 complete period since the publication of RFC 1766, and has provided a great deal of input to this revision. 7. Author's Address Harald Tveit Alvestrand Cisco Systems Weidemanns vei 27 7043 Trondheim NORWAY EMail: Harald@Alvestrand.no Phone: +47 73 50 33 52 8. References [TAGS] "Tags for the identification of languages", work in progress [draft-alvestrand-lang-tag-v2] [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] draft-alvestrand-lang-tags-v2-01.txt [Page 5]
Content Language Headers Harald Alvestrand draft-alvestrand-contentùlanguage-00.txt Expires December 2000 ISO/DIS 15924 - Codes for the representation of names of scripts (under development by ISO TC46/SC2) [RFC 1327] Kille, S., "Mapping between X.400(1988) / ISO 10021 and RFC 822", RFC 1327, University College London, May 1992. [RFC 1521] Borenstein, N., and N. Freed, "MIME Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies", RFC 1521, Bellcore, Innosoft, September 1993. [RFC 2119] Key words for use in RFCs to Indicate Requirement Levels. S. Bradner. March 1997. [RFC 2234] Augmented BNF for Syntax Specifications: ABNF. D. Crocker, Ed., P. Overell, November 1997. [RFC 2616] Hypertext Transfer Protocol -- HTTP/1.1. R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P. Leach, T. Berners-Lee. June 1999. Appendix A: Use of Content-Language with Multipart/Alternative NOTE: This appendix details an idea that was proposed in RFC 1766 to deal with a particular kind of alternative content. However, this has not found use in practice, and is therefore not suitable for the IETF standards track. It is being preserved here as a non-normative appendix only. When using the Multipart/Alternative body part of MIME, it is possible to have the body parts giving the same information content in different languages. In this case, one should put a Content-Language header on each of the body parts, and a summary Content-Language header onto the Multipart/Alternative itself. The differences parameter to multipart/alternative As defined in RFC 1541, "Multipart/Alternative" only has one parameter: boundary. The common usage of "Multipart/Alternative" is to have more than one format of the same message (f.ex. PostScript and ASCII). The use of language tags to differentiate between different alternatives will certainly not lead all MIME UAs to present the most meaningful, understandable or significant body part as default. draft-alvestrand-lang-tags-v2-01.txt [Page 6]
Content Language Headers Harald Alvestrand draft-alvestrand-contentùlanguage-00.txt Expires December 2000 Therefore, a new parameter is defined, to allow the configuration of MIME readers to handle language differences in a sensible manner. Name: Differences Value: One or more of Content-Type Content-Language Further values can be registered with IANA; these shall refer to the name of a header for which a definition exists in a published RFC. If not present, "Differences=Content-Type" is assumed. The intent is that the MIME reader can look at these headers of the message component to make an intelligent choice of what to present to the user, based on knowledge about the user preferences and capabilities. (The intent of having registration with IANA of the fields used in this context is to maintain a list of usages that a mail UA may expect to encounter, not to reject usages.) (NOTE: The MIME specification [RFC 1521], section 7.2, states that headers not beginning with "Content-" are generally to be ignored in body parts. People defining a header for use with "differences=" should take note of this.) The mechanism for deciding which body part to present is outside the scope of this document. MIME EXAMPLE: Content-Type: multipart/alternative; differences=Content-Language; boundary="limit" Content-Language: en, fr, de --limit Content-Language: fr Le renard brun et agile saute par dessus le chien paresseux --limit Content-Language: de Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-encoding: quoted-printable Der schnelle braune Fuchs h=FCpft =FCber den faulen Hund --limit Content-Language: en The quick brown fox jumps over the lazy dog --limit-- When composing a message, the choice of sequence may be arbitrary. However, non-MIME mail readers will show the first body part first, draft-alvestrand-lang-tags-v2-01.txt [Page 7]
Content Language Headers Harald Alvestrand draft-alvestrand-contentùlanguage-00.txt Expires December 2000 meaning that this should most likely be the language understood by most of the recipients. Appendix X: Work-in-progress notes This appendix is to be deleted by the RFC Editor before publication as RFC. This document was split out from the revised RFC 1766 after the publication of draft-alvestrand-rfc1766bis-02.txt. For the list of changes between versions, see draft-alvestrand- rfc1766bis-03.txt [TAGS]. draft-alvestrand-lang-tags-v2-01.txt [Page 8]
