Internet DRAFT - draft-alvestrand-content-language

draft-alvestrand-content-language




               Internet-Draft                                       H. Alvestrand 
               draft-alvestrand-content-language-03.txt             Cisco Systems 
               Target Category: Standards Track                     February 2002 
               Updates: RFC 1766                             Expires: August 2002 
                
                
                
                
                
                
                
                
                

               Content Language Headers 
                

               Status of this Memo 
                    The file name of this memo is draft-alvestrand-content-language-
                    03.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 
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                    Drafts. 
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                    in progress." 
                    The list of current Internet-Drafts can be accessed at 
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                    The list of Internet-Draft Shadow Directories can be accessed at 
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               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, and an 
               "Accept-Language:" header for use in the case where one wishes to 
               indicate one's preferences with regard to languages. 

               1. Introduction 
                
               Content Language Headers                         Harald Alvestrand 
               draft-alvestrand-content-language-03.txt       Expires August 2002 
                
                
               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 
                
               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 
                
                
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               Content Language Headers                         Harald Alvestrand 
               draft-alvestrand-content-language-03.txt       Expires August 2002 
                
                
               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. 
                
               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 
                
               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 
                

               3. The Accept-Language header 
               The "Accept-Language" header is intended for use in the case 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: 
                
                
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               Content Language Headers                         Harald Alvestrand 
               draft-alvestrand-content-language-03.txt       Expires August 2002 
                
                
               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") ] ) 
                
               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]. 
               (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 
               This document adds no new considerations beyond what is mentioned in 
               [TAGS]. 
                
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               Content Language Headers                         Harald Alvestrand 
               draft-alvestrand-content-language-03.txt       Expires August 2002 
                
                
               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 complete 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. 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]    Alvestrand, H., "Tags for the identification of languages", 
               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] 


                
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               Content Language Headers                         Harald Alvestrand 
               draft-alvestrand-content-language-03.txt       Expires August 2002 
                
                
                    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)  
               [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. 
               [RFC 2822] 
                    Internet Message Format. P. Resnick, Editor. April 2001. 

               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. 







                
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