Network Working Group                                   A. Phillips, Ed.
Internet-Draft                                            Quest Software
Expires: November 14, 2005                                 M. Davis, Ed.
                                                            May 13, 2005

                     Matching Language Identifiers

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document describes different mechanisms for comparing and
   matching the tags for the identification of languages defined by [RFC
   3066bis] [1].  Possible algorithms for language negotiation and
   content selection are described.  Portions of this document obsolete
   [RFC 3066] [19].

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  The Language Range . . . . . . . . . . . . . . . . . . . . . .  4
     2.1   Basic Language Range . . . . . . . . . . . . . . . . . . .  4
       2.1.1   Matching . . . . . . . . . . . . . . . . . . . . . . .  5
       2.1.2   Lookup . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.2   Extended Language Range  . . . . . . . . . . . . . . . . .  6
       2.2.1   Extended Range Matching  . . . . . . . . . . . . . . .  7
       2.2.2   Extended Range Lookup  . . . . . . . . . . . . . . . .  8
       2.2.3   Scored Matching  . . . . . . . . . . . . . . . . . . .  9
     2.3   Meaning of Language Tags and Ranges  . . . . . . . . . . . 10
     2.4   Choosing Between Alternate Matching Schemes  . . . . . . . 11
     2.5   Considerations for Private Use Subtags . . . . . . . . . . 11
   3.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   4.  Changes  . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   6.  Character Set Considerations . . . . . . . . . . . . . . . . . 16
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     7.1   Normative References . . . . . . . . . . . . . . . . . . . 17
     7.2   Informative References . . . . . . . . . . . . . . . . . . 18
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 18
   A.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
       Intellectual Property and Copyright Statements . . . . . . . . 20

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1.  Introduction

   Human beings on our planet have, past and present, used a number of
   languages.  There are many reasons why one would want to identify the
   language used when presenting or requesting information.

   Information about a user's language preferences commonly needs to be
   identified so that appropriate processing can be applied.  For
   example, the user's language preferences in a browser can be used to
   select web pages appropriately.  A choice of language preference can
   also be used to select among tools (such as dictionaries) to assist
   in the processing or understanding of content in different languages.

   Given a set of language identifiers, such as those defined in
   RFC3066bis, various mechanisms can be envisioned for performing
   language negotiation and tag matching.  The suitability of a
   particular mechanism to a particular application depends on the needs
   of that application.

   This document defines language ranges and syntax for specifying user
   preferences in a request for language content.  It also specifies a
   default algorithm for matching language ranges to content (language
   tags), as well as alternate mechanisms suitable for certain

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC 2119] [5].

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2.  The Language Range

   Language Tags are used to identify the language of some information
   item or content.  Applications that use language tags are often faced
   with the problem of identifying sets of content that share certain
   language attributes.  For example, HTTP 1.1 [10] describes language
   ranges in its discussion of the Accept-Language header (Section
   14.4), which is used for selecting content from servers based on the
   language of that content.

   When selecting content according to its language, it is useful to
   have a mechanism for identifying sets of language tags that share
   specific attributes.  This allows users to select or filter content
   based on specific requirements.  Such an identifier is called a
   "Language Range".

2.1  Basic Language Range

   A basic language range (such as described in RFC 3066 [19] and HTTP
   1.1 [10]) is a set of languages whose tags all begin with the same
   sequence of subtags.  A basic language range can be represented by a
   'language-range' tag, by using the definition from HTTP/1.1 [10] :
   language-range = language-tag / "*"

   That is, a language-range has the same syntax as a language-tag or is
   the single character "*".  This definition of language-range implies
   that there is a semantic relationship between tags that share the
   same prefix.

   In particular, the set of language tags that match a specific
   language-range may not all be mutually intelligible.  The use of a
   prefix when matching tags to language ranges does not imply that
   language tags are assigned to languages in such a way that it is
   always true that if a user understands a language with a certain tag,
   then this user will also understand all languages with tags for which
   this tag is a prefix.  The prefix rule simply allows the use of
   prefix tags if this is the case.

   When working with tags and ranges you should also note the following:

   1.  Private-use and Extension subtags are normally orthogonal to
       language tag fallback.  Implementations should ignore
       unrecognized private-use and extension subtags when performing
       language tag fallback.  Since these subtags are always at the end
       of the sequence of subtags, they naturally fall out of the
       default fallback pattern (above).  Thus a request to match the
       tag "en-US-boont-x-1943" would produce exactly the same
       information content as the example above.

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   2.  Implementations that choose not to interpret one or more private-
       use or extension subtags should not remove or modify these
       extensions in content that they are processing.  When a language
       tag instance is to be used in a specific, known protocol, and is
       not being passed through to other protocols, language tags may be
       filtered to remove subtags and extensions that are not supported
       by that protocol.  This should be done with caution, since it it
       is removing information that may be relevant if services on the
       other end of the protocol would make use of that information.

   3.  Some applications of language tags may want or need to consider
       extensions and private-use subtags when matching tags.  If
       extensions and private-use subtags are included in a matching
       process that utilizes the default fallback mechanism, then the
       implementation should canonicalize the language tags and/or
       ranges before performing the matching.  Note that language tag
       processors that claim to be "well-formed" processors as defined
       in [1] generally fall into this category.

   There are two matching schemes  that are commonly associated with
   basic language ranges:  matching and  lookup.

2.1.1  Matching

   Language tag matching is used to select all content that matches a
   given prefix.  In matching, the language range represents the least
   specific tag which is an acceptable match and every piece of content
   that matches is returned.

   For example, if an application is applying a style to all content in
   a web page in a particular language, it might use language tag
   matching to perform the matching.

   A language-range matches a language-tag if it exactly equals the tag,
   or if it exactly equals a prefix of the tag such that the first
   character following the prefix is "-".  (That is, the language-range
   "en-de" matches the language tag "en-DE-boont", but not the language
   tag "en-Deva".)

   The special range "*" matches any tag.  A protocol which uses
   language ranges may specify additional rules about the semantics of
   "*"; for instance, HTTP/1.1 specifies that the range "*" matches only
   languages not matched by any other range within an "Accept-Language:"

2.1.2  Lookup

   Content lookup is used to select the single information item that

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   best matches the language range for a given request.  In lookup, the
   language range represents the most specific tag which is an
   acceptable match and only the closest matching item is returned.

   For example, if an application inserts some dynamic content into a
   web page, returning an empty string if there is no exact match is not
   an option.  Instead, the application "falls back".

   When performing lookup, the language range is progressively truncated
   from the end until a matching piece of content is located.  For
   example, starting with the range "zh-Hant-CN-x-wadegile", the lookup
   would progressively search for content as shown below:

   Range to match: zh-Hant-CN-x-wadegile
   1. zh-Hant-CN-x-wadegile
   2. zh-Hant-CN
   3. zh-Hant
   4. zh
   5. (default content or the empty tag)

                Figure 2: Default Fallback Pattern Example

   This scheme allows some flexibility in finding content.  It also
   typically provides better results when data is not available at a
   specific level of tag granularity or is sparsely populated (than if
   the default language for the system or content were used).

2.2  Extended Language Range

   Prefix matching using a Basic Language Range, as described above, is
   not always the most appropriate way to access the information
   contained in language tags when selecting or filtering content.  Some
   applications may wish to define a more granular matching scheme and
   such a matching scheme requires the ability to specify the various
   attributes of a language tag in the language range.  An extended
   language range can be represented by the following ABNF:
   extended-language-range = grandfathered / privateuse / range
   range   = ( lang [ "-" script ] [ "-" region ] *( "-" variant )
                [ "-" privateuse ] )
   lang    = ( 2*8ALPHA *[ *( "-" extlang ] ) ) / "*"
   extlang = 3ALPHA / "*"
   script  = 4ALPHA / "*"
   region  = 2ALPHA / 3DIGIT / "*"
   variant = 5*8alphanum / ( DIGIT 3alphanum ) / "*"
   privateuse    = ( "x" / "X" ) 1*( "-" ( 1*8alphanum ) )
   grandfathered = 1*3ALPHA 1*2( "-" ( 2*8alphanum ) )
   alphanum      = ( ALPHA / DIGIT )

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   In an extended language range, the identifier takes the form of a
   series of subtags which must consist of well-formed subtags or the
   special subtag "*".  For example, the language range "en-*-US"
   specifies a primary language of 'en', followed by any script subtag,
   followed by the region subtag 'US'.

   A field not present in the middle of an extended language range MAY
   be treated as if the field contained a "*".  For example, the range
   "en-US" MAY be considered to be equivalent to the range "en-*-US".

   There are several matching algorithms or schemes which may be applied
   when matching extended language ranges to language tags.

2.2.1  Extended Range Matching

   In extended range matching, the subtags in a language tag are
   compared to the corresponding subtags in the extended language range.
   A subtag is considered to match if it exactly matches the
   corresponding subtag in the range or the range contains a subtag with
   the value "*" (which matches all subtags, including the empty
   subtag).  Extended Range Matching is an extension of basic matching
   (Section 2.1.1): the language range represents the least specific tag
   which is an acceptable match.

   By default all extensions and their subtags are ignored for extended
   language range matching.

   Private use subtags may be specified in the language range and MUST
   NOT be ignored when matching.

   Subtags not specified, included those at the end of the language
   range, are assigned the value "*".  This makes each range into a
   prefix much like that used in basic language range matching.  For
   example, the extended language range "zh-*-CN" matches all of the
   following tags because the unspecified variant field is expanded to






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2.2.2  Extended Range Lookup

   In extended range lookup, the subtags in a language tag are compared
   to the corresponding subtags in the extended language range.  The
   subtag is considered to match if it exactly matches the corresponding
   subtag in the range or the range contains a subtag with the value "*"
   (which matches all subtags, including the empty subtag).  Extended
   language range lookup is an extension of basic lookup
   (Section 2.1.2): the language range represents the most specific tag
   which will form an acceptable match.

   Subtags not specified are assigned the value "*" prior to performing
   tag matching.  Unlike in extended range matching, however, fields at
   the end of the range MUST NOT be expanded in this manner.  For
   example, "en-US" must not be considered to be the same as the range
   "en-US-*".  This allows ranges to be specific.  The "*" wildcard MUST
   be used at the end of the range to indicate that all tags with the
   range as a prefix are allowable matches.  That is, the range "zh-*"
   matches the tags "zh-Hant" and "zh-Hant-CN", while the range "zh"
   matches neither of those tags.

   The wildcard "*" at the end of a range SHOULD be considered to match
   any private use subtag sequences (making extended language range
   lookup function exactly like extended range matching Section 2.2.1).

   By default all extensions and their subtags SHOULD be ignored for
   extended language range lookup.  Private use subtags may be specified
   in the language range and MUST NOT be ignored when performing lookup.
   The wildcard "*" at the end of a range SHOULD be considered to match
   any private use subtag sequences in addition to variants.

   For example, the range "*-US" matches all of the following tags:



      en-US-r-extends (extensions are ignored)


   For example, the range "en-*-US" matches _none_ of the following


      en (missing region US)

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      en-Latn (missing region US)

      en-Latn-US-scouse (variant field is present)

   For example, the range "en-*" matches all of the following tags:






   It should be noted that the ability to be specific in extended range
   lookup may make this matching scheme a more appropriate replacement
   for basic matching than the extended range matching scheme.

2.2.3  Scored Matching

   In the "scored matching" scheme, the extended language range and the
   language tags are pre-normalized by mapping grandfathered and
   obsolete tags into modern equivalents.

   The language range and the language tags are normalized into
   quadruples of the form (language, script, country, variant), where
   extended language is considered part of language and x-private-codes
   are considered part of the language if they are initial and part of
   the variant if not initial.  Missing components are set to "*".  An
   "*" pattern becomes the quadruple ("*", "*", "*", "*").

   Each language tag  being matched or filtered is assigned a "quality
   value" such that higher values indicate better matches and lower
   values indicate worse ones.  If the language matches, add 8 to the
   quality value.  If the script matches, add 4 to the quality value.
   If the region matches, add 2 to the quality value.  If the variant
   matches, add 1 to the quality value.  Elements of the quadruples are
   considered to match if they are the same or if one of them is "*".

   A value of 15 is a perfect match; 0 is no match at all.  Different
   values may be more or less appropriate for different applications and
   implementations should probably allow users to choose the most
   appropriate selection value.

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2.3  Meaning of Language Tags and Ranges

   A language tag defines a language as spoken (or written, signed or
   otherwise signaled) by human beings for communication of information
   to other human beings.

   If a language tag B contains language tag A as a prefix, then B is
   typically "narrower" or "more specific" than A. For example, "zh-
   Hant-TW" is more specific than "zh-Hant".

   This relationship is not guaranteed in all cases: specifically,
   languages that begin with the same sequence of subtags are NOT
   guaranteed to be mutually intelligible, although they may be.  For
   example, the tag "az" shares a prefix with both "az-Latn"
   (Azerbaijani written using the Latin script) and "az-Cyrl"
   (Azerbaijani written using the Cyrillic script).  A person fluent in
   one script may not be able to read the other, even though the text
   might be otherwise identical.  Content tagged as "az" most probably
   is written in just one script and thus might not be intelligible to a
   reader familiar with the other script.

   The relationship between the tag and the information it relates to is
   defined by the standard describing the context in which it appears.
   Accordingly, this section can only give possible examples of its

   o  For a single information object, the associated language tags
      might be interpreted as the set of languages that is required for
      a complete comprehension of the complete object.  Example: Plain
      text documents.

   o  For an aggregation of information objects, the associated language
      tags could be taken as the set of languages used inside components
      of that aggregation.  Examples: Document stores and libraries.

   o  For information objects whose purpose is to provide alternatives,
      the associated language tags could be regarded as a hint that the
      content is provided in several languages, and that one has to
      inspect each of the alternatives in order to find its language or
      languages.  In this case, the presence of multiple tags might not
      mean that one needs to be multi-lingual to get complete
      understanding of the document.  Example: MIME multipart/

   o  In markup languages, such as HTML and XML, language information
      can be added to each part of the document identified by the markup
      structure (including the whole document itself).  For example, one
      could write <span lang="FR">C'est la vie.</span> inside a

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      Norwegian document; the Norwegian-speaking user could then access
      a French-Norwegian dictionary to find out what the marked section
      meant.  If the user were listening to that document through a
      speech synthesis interface, this formation could be used to signal
      the synthesizer to appropriately apply French text-to-speech
      pronunciation rules to that span of text, instead of misapplying
      the Norwegian rules.

2.4  Choosing Between Alternate Matching Schemes

   Implementations MAY choose to implement different styles of matching
   for different kinds of processing.  For example, an implementation
   could treat an absent script subtag as a "wildcard" field; thus
   "az-AZ" would match "az-AZ", "az-Cyrl-AZ", "az-Latn-AZ", etc. but not
   "az" (this is extended range lookup).  If one item is to be chosen,
   the implementation could pick among those matches based on other
   information, such as the most likely script used in the language/
   region in question or the script used by other content selected.

   Because the primary language subtag cannot be absent in a language
   tag, the 'UND' subtag may sometimes be used as a 'wildcard' in basic
   matching.  For example, in a query where you want to select all
   language tags that contain 'Latn' as the script code and 'AZ' as the
   region code, you could use the range "und-Latn-AZ".  This requires an
   implementation to examine the actual values of the subtags, though.
   The matching schemes described elsewhere in this document do not
   require implementations to examine the values supplied and, except
   for scored matching, they do not require access to the Language
   Subtag Registry nor the use of valid subtags in language tags or
   ranges.  This has great benefit for speed and simplicity of

   Implementations may also wish to use semantic information external to
   the langauge tags when performing fallback.  For example, the primary
   language subtags 'nn' (Nynorsk Norwegian) and 'nb' (Bokmal Norwegian)
   might both be usefully matched to the more general subtag 'no'
   (Norwegian).  Or an application might infer that content labeled
   "zh-CN" is morely likely to match the range "zh-Hans" than equivalent
   content labeled "zh-TW".

2.5  Considerations for Private Use Subtags

   Private-use subtags require private agreement between the parties
   that intend to use or exchange language tags that use them and great
   caution should be used in employing them in content or protocols
   intended for general use.  Private-use subtags are simply useless for
   information exchange without prior arrangement.

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   The value and semantic meaning of private-use tags and of the subtags
   used within such a language tag are not defined.  Matching private
   use tags using language ranges or extended language ranges may result
   in unpredictable content being returned.

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3.  IANA Considerations

   This document presents no new or existing considerations for IANA.

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4.  Changes

   This is the first version of this document.  Changes from the
   reference work (draft-phillips-matching-00) are too numerious to

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5.  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 evaluation of the exact magnitude of the threat, and any possible
   countermeasures, is left to each application protocol.

   Although the specification of valid subtags for an extension MUST be
   available over the Internet, implementations SHOULD NOT mechanically
   depend on it being always accessible, to prevent denial-of-service

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6.  Character Set Considerations

   The syntax in this document requires that language ranges use only
   the characters A-Z, a-z, 0-9, and HYPHEN-MINUS legal in language
   tags.  These characters are present in most character sets, so
   presentation of language tags should not have any character set

   Rendering of characters based on the content of a language tag is not
   addressed in this memo.  Historically, some languages have relied on
   the use of specific character sets or other information in order to
   infer how a specific character should be rendered (notably this
   applies to language and culture specific variations of Han ideographs
   as used in Japanese, Chinese, and Korean).  When language tags are
   applied to spans of text, rendering engines may use that information
   in deciding which font to use in the absence of other information,
   particularly where languages with distinct writing traditions use the
   same characters.

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7.  References

7.1  Normative References

   [1]   Phillips, A., Ed. and M. Davis, Ed., "Tags for the
         Identification of Languages (Internet-Draft)", February 2005, <

   [2]   Hardcastle-Kille, S., "Mapping between X.400(1988) / ISO 10021
         and RFC 822", RFC 1327, May 1992.

   [3]   Borenstein, N. and N. Freed, "MIME (Multipurpose Internet Mail
         Extensions) Part One: Mechanisms for Specifying and Describing
         the Format of Internet Message Bodies", RFC 1521,
         September 1993.

   [4]   Hovey, R. and S. Bradner, "The Organizations Involved in the
         IETF Standards Process", BCP 11, RFC 2028, October 1996.

   [5]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [6]   Freed, N. and K. Moore, "MIME Parameter Value and Encoded Word
         Extensions: Character Sets, Languages, and Continuations",
         RFC 2231, November 1997.

   [7]   Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", RFC 2234, November 1997.

   [8]   Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
         Resource Identifiers (URI): Generic Syntax", RFC 2396,
         August 1998.

   [9]   Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 2434,
         October 1998.

   [10]  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.

   [11]  Carpenter, B., Baker, F., and M. Roberts, "Memorandum of
         Understanding Concerning the Technical Work of the Internet
         Assigned Numbers Authority", RFC 2860, June 2000.

   [12]  Yergeau, F., "UTF-8, a transformation format of ISO 10646",
         STD 63, RFC 3629, November 2003.

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7.2  Informative References

   [13]  International Organization for Standardization, "ISO 639-
         1:2002, Codes for the representation of names of languages --
         Part 1: Alpha-2 code", ISO Standard 639, 2002.

   [14]  International Organization for Standardization, "ISO 639-2:1998
         - Codes for the representation of names of languages -- Part 2:
         Alpha-3 code - edition 1", August 1988.

   [15]  ISO TC46/WG3, "ISO 15924:2003 (E/F) - Codes for the
         representation of names of scripts", January 2004.

   [16]  International Organization for Standardization, "Codes for the
         representation of names of countries, 3rd edition",
         ISO Standard 3166, August 1988.

   [17]  Statistical Division, United Nations, "Standard Country or Area
         Codes for Statistical Use", UN Standard Country or Area Codes
         for Statistical Use, Revision 4 (United Nations publication,
         Sales No. 98.XVII.9, June 1999.

   [18]  Alvestrand, H., "Tags for the Identification of Languages",
         RFC 1766, March 1995.

   [19]  Alvestrand, H., "Tags for the Identification of Languages",
         BCP 47, RFC 3066, January 2001.

   [20]  Klyne, G. and C. Newman, "Date and Time on the Internet:
         Timestamps", RFC 3339, July 2002.

Authors' Addresses

   Addison Phillips (editor)
   Quest Software

   Email: addison dot phillips at quest dot com

   Mark Davis (editor)

   Email: mark dot davis at ibm dot com

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Appendix A.  Acknowledgements

   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.

   The contributors to RFC 3066 and RFC 1766, the precursors of this
   document, made enormous contributions directly or indirectly to this
   document and are generally responsible for the success of language

   The following people (in alphabetical order) contributed to this
   document or to RFCs 1766 and 3066:

   Glenn Adams, Harald Tveit Alvestrand, Tim Berners-Lee, Marc Blanchet,
   Nathaniel Borenstein, Eric Brunner, Sean M. Burke, Jeremy Carroll,
   John Clews, Jim Conklin, Peter Constable, John Cowan, Mark Crispin,
   Dave Crocker, Martin Duerst, Michael Everson, Doug Ewell, Ned Freed,
   Tim Goodwin, Dirk-Willem van Gulik, Marion Gunn, Joel Halpren,
   Elliotte Rusty Harold, Paul Hoffman, Richard Ishida, Olle Jarnefors,
   Kent Karlsson, John Klensin, Alain LaBonte, Eric Mader, Keith Moore,
   Chris Newman, Masataka Ohta, George Rhoten, Markus Scherer, Keld Jorn
   Simonsen, Thierry Sourbier, Otto Stolz, Tex Texin, Andrea Vine, Rhys
   Weatherley, Misha Wolf, Francois Yergeau and many, many others.

   Very special thanks must go to Harald Tveit Alvestrand, who
   originated RFCs 1766 and 3066, and without whom this document would
   not have been possible.  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.  Special thanks to Doug
   Ewell, for his production of the first complete subtag registry, and
   his work in producing a test parser for verifying language tags.

   For this particular document, John Cowan originated the scheme
   described in Section 2.2.3.  Mark Davis originated the scheme
   described in the Section 2.1.2.

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Internet-Draft                langMatching                      May 2005

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