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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 4647

Network Working Group                                   A. Phillips, Ed.
Internet-Draft                                            Quest Software
Obsoletes: 3066 (if approved)                              M. Davis, Ed.
Expires: April 10, 2006                                              IBM
                                                         October 7, 2005


           Matching Tags for the Identification of Languages
                      draft-ietf-ltru-matching-05

Status of this Memo

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   This Internet-Draft will expire on April 10, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document describes different mechanisms for comparing, matching,
   and evaluating language tags.  Possible algorithms for language
   negotiation and content selection are described.







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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  The Language Range . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Lists of Language Ranges . . . . . . . . . . . . . . . . .  4
     2.2.  Basic Language Range . . . . . . . . . . . . . . . . . . .  4
       2.2.1.  Matching . . . . . . . . . . . . . . . . . . . . . . .  5
       2.2.2.  Lookup . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.3.  Extended Language Range  . . . . . . . . . . . . . . . . .  7
       2.3.1.  Extended Range Matching  . . . . . . . . . . . . . . .  9
       2.3.2.  Extended Range Lookup  . . . . . . . . . . . . . . . . 10
       2.3.3.  Distance Metric Scheme . . . . . . . . . . . . . . . . 11
     2.4.  Meaning of Language Tags and Ranges  . . . . . . . . . . . 13
     2.5.  Choosing Between Alternate Matching Schemes  . . . . . . . 14
     2.6.  Considerations for Private Use Subtags . . . . . . . . . . 15
     2.7.  Length Considerations in Matching  . . . . . . . . . . . . 16
   3.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   4.  Changes  . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   6.  Character Set Considerations . . . . . . . . . . . . . . . . . 21
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 22
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 23
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 24
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
   Intellectual Property and Copyright Statements . . . . . . . . . . 26

























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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 [draft-
   registry], 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 several mechanisms for matching and filtering
   natural language content identified using Language Tags [draft-
   registry].  It also defines the syntax (called a "language range")
   associated with each of these mechanisms for specifying user language
   preferences.

   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].























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2.  The Language Range

   Language Tags [draft-registry] 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
   [RFC2616] 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.  Lists of Language Ranges

   When users specify a language preference they often need to specify a
   prioritized list of language ranges in order to best reflect their
   language requirements for the matching operation.  This is especially
   true for speakers of minority languages.  A speaker of Breton in
   France, for example, may specify "be" followed by "fr", meaning that
   if Breton is available, it is preferred, but otherwise French is the
   best alternative.  It can get more complex: a speaker may wish to
   fallback from Skolt Sami to Northern Sami to Finnish.

   A "Language Priority List" consists of a prioritized or weighted list
   of language ranges.  One well known example of such a list is the
   "Accept-Language" header defined in RFC 2616 [RFC2616] (see Section
   14.4) and RFC 3282 [RFC3282].  The various matching operations
   described in this document include considerations for using a
   language priority list.

2.2.  Basic Language Range

   A "Basic Language Range" identifies the set of content whose language
   tags begin with the same sequence of subtags.  A basic language range
   is identified by its 'language-range' tag, by adapting the
   ABNF[RFC2234bis] from HTTP/1.1 [RFC2616] :

   language-range = language-tag / "*"
   language-tag   = 1*8[alphanum] *["-" 1*8alphanum]
   alphanum       = ALPHA / DIGIT

   That is, a language-range has the same syntax as a language-tag or is
   the single character "*".  Basic Language Ranges imply that there is
   a semantic relationship between language tags that share the same



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   prefix.  While this is often the case, it is not always true and
   users should note that the set of language tags that match a specific
   language-range may not be mutually intelligible.

   Basic language ranges were originally described in [RFC3066] and HTTP
   1.1 [RFC2616] (where they are referred to as simply a "language
   range").

   Users SHOULD avoid subtags that add no distinguishing value to a
   language range.  For example, script subtags SHOULD NOT be used to
   form a language range with language subtags which have a matching
   Suppress-Script field in their registry record.  Thus the language
   range "en-Latn" is probably inappropriate for most applications
   (because the vast majority English documents are written in the Latin
   script and thus the 'en' language subtag has a Suppress-Script field
   for 'Latn' in the registry).

   Language tags and thus language ranges are to be treated as case
   insensitive: there exist conventions for the capitalization of some
   of the subtags, but these MUST NOT be taken to carry meaning.
   Matching of language tags to language ranges MUST be done in a case
   insensitive manner.

   When working with tags and ranges, note that extensions and most
   private use subtags are generally orthogonal to language tag fallback
   and users SHOULD avoid using these subtags in language ranges, since
   they will often interfere with the selection of available language
   content.  Since these subtags are always at the end of the sequence
   of subtags, they don't normally interfere with the use of prefixes
   for matching in the schemes described below.

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

   Note that neither matching nor lookup using basic language ranges
   attempt to process the semantics of the tags or ranges in any way.
   The language tag and language range are compared in a case
   insensitive manner using basic string processing.  The choice of
   subtags in both the language tag and language range may affect the
   results produced as a result.

2.2.1.  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.  If the language priority list contains
   more than one range, the matches returned are typically ordered in



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   descending level of preference.

   For example, if an application is applying a style to all content in
   a document in a particular language, it might use language tag
   matching to select the content to which the style is applied.

   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
   "de-de" matches the language tag "de-DE-1996", but not the language
   tag "de-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"
   header.

2.2.2.  Lookup

   Content lookup is used to select the single information item that
   best matches the language priority list for a given request.  In
   lookup, each language range in the language priority list represents
   the most specific tag which is an acceptable match; only the closest
   matching item according the user's priority is returned.

   For example, if an application inserts some dynamic content into a
   document, returning an empty string if there is no exact match is not
   an option.  Instead, the application "falls back" until it finds a
   suitable piece of content to insert.

   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



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   the default language for the system or content were used).

   When performing lookup using a language priority list, the
   progressive search MUST proceed to consider each language range
   before finding the default content or empty tag.  For example, for
   the list "fr-FR; zh-Hant" would search for content as follows:
   1. fr-FR
   2. fr
   3. zh-Hant // next language
   4. zh
   5. (default content or the empty tag)

   Figure 3: Lookup Using a Language Priority List

2.3.  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 might 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  = range ; a range
                 / privateuse              ; private use tag
                 / grandfathered           ; grandfathered registrations

   range         = (language
                    ["-" script]
                    ["-" region]
                    *("-" variant)
                    *("-" extension)
                    ["-" privateuse])

   language      = (2*3ALPHA [ extlang ]) ; shortest ISO 639 code
                 / 4ALPHA                 ; reserved for future use
                 / 5*8ALPHA               ; registered language subtag
                 / "*"                    ; ... or wildcard

   extlang       = *2("-" 3ALPHA) ("-" ( 3ALPHA / "*"))
                                          ; reserved for future use
                                          ; wildcard can only appear
                                          ;   at the end

   script        = 4ALPHA                 ; ISO 15924 code
                 / "*"                    ; or wildcard

   region        = 2ALPHA                 ; ISO 3166 code



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                 / 3DIGIT                 ; UN M.49 code
                 / "*"                    ; ... or wildcard

   variant       = 5*8alphanum            ; registered variants
                 / (DIGIT 3alphanum)      ;
                 / "*"                    ; ... or wildcard

   extension     = singleton *("-" (2*8alphanum)) [ "-*" ]
                                          ; extension subtags
                                          ; wildcard can only appear
                                          ;   at the end

   singleton     = %x41-57 / %x59-5A / %x61-77 / %x79-7A / DIGIT
                 ; "a"-"w" / "y"-"z" / "A"-"W" / "Y"-"Z" / "0"-"9"
                 ; Single letters: x/X is reserved for private use

   privateuse    = ("x"/"X") 1*("-" (1*8alphanum))

   grandfathered = 1*3ALPHA 1*2("-" (2*8alphanum))
                   ; grandfathered registration
                   ; Note: i is the only singleton
                   ; that starts a grandfathered tag

   alphanum      = (ALPHA / DIGIT)       ; letters and numbers

   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".
   This also means that multiple wildcards can be collapsed (so that
   "en-*-*-US" is equivalent to "en-*-US").

   When working with tags and ranges users SHOULD 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 don't normally interfere with
       the use of prefixes for matching in the schemes described below.

   2.  Implementations that choose not to interpret one or more private-
       use or extension subtags SHOULD NOT remove or modify these



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       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.  Such filtering SHOULD be avoided, if possible,
       since it removes information that might be relevant if services
       on the other end of the protocol would make use of that
       information.

   3.  Some applications of language tags might want or need to consider
       extensions and private-use subtags when matching tags.  If
       extensions and private-use subtags are included in a matching or
       filtering process that utilizes the one of the schemes described
       in this document, 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 [draft-registry] generally fall into
       this category.

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

2.3.1.  Extended Range Matching

   In extended range matching, each extended language range in the
   language priority list is considered in turn, according to priority.
   The subtags in each extended language range are compared to the
   corresponding subtags in the language tag being examined.  The subtag
   from the range is considered to match if it exactly matches the
   corresponding subtag in the tag or the range's subtag has the value
   "*" (which matches all subtags, including the empty subtag).
   Extended Range Matching is an extension of basic matching
   (Section 2.2.1): the language range represents the least specific tag
   which is an acceptable match.

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

   Subtags not specified, including 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
   "*":

      zh-Hant-CN





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      zh-CN

      zh-Hans-CN

      zh-CN-x-wadegile

      zh-Latn-CN-boont

      zh-cmn-Hans-CN-x-wadegile

2.3.2.  Extended Range Lookup

   In extended range lookup, each extended language range in the
   language priority list is considered in turn.  The subtags in each
   extended language range are compared to the corresponding subtags in
   the language tag being examined.  A subtag is considered to match if
   it exactly matches the corresponding subtag in the tag or the range's
   subtag has the value "*" (which matches all subtags, including the
   empty subtag).  Extended language range lookup is an extension of
   basic lookup (Section 2.2.2): each language range represents the most
   specific tag which will form an acceptable match.  If no match is
   found, the default content or content with the empty language tag is
   usually returned (or the search can be considered to have failed).

   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 2.3.1).

   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




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      en-Latn-US

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

      fr-US

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

      fr-US

      en (missing region US)

      en-Latn (missing region US)

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

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

      en-Latn

      en-Latn-US

      en-Latn-US-scouse

      en-US

      en-scouse

   Note that the ability to be specific in extended range lookup can
   make this matching scheme a more appropriate replacement for basic
   matching than the extended range matching scheme.

2.3.3.  Distance Metric Scheme

   Both Basic and Extended Language Ranges produce simple boolean
   matches.  Some applications may benefit by providing an array of
   results with different levels of matching, for example, sorting
   results based on the overall "quality" of the match.

   This type of matching is sometimes called a "distance metric".  A
   distance metric assigns a pair of language tags a numeric value
   representing the 'distance' between the two.  A distance of zero
   means that they are identical, a small distance indicates that they
   are very similar, and a large distance indicated that they are very
   different.  Using a distance metric, implementations can, for
   example, allow users to select a threshold distance for a match to be
   successful or a filter to be applied.



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   The first step in the process is to normalize the extended language
   range and the language tags to be matched to it by canonicalizing
   them, mapping grandfathered and obsolete tags into modern
   equivalents.

   The language range and the language tags are then transformed into
   quintuples of elements of the form (language, script, country,
   variant, extension).  Any extended language subtags are considered
   part of the language element; private use subtag sequences are
   considered part of the language element if in the initial position in
   the tag and part of the variant element if not.  Language subtags
   'und', 'mul', and the script subtag 'Zyyy' are converted to "*".

   Missing components in the language-tag are set to "*"; thus a "*"
   pattern becomes the quintuple ("*", "*", "*", "*", "*").  Missing
   components in the extended language-range are handled similarly to
   extended range lookup: missing internal subtags are expanded to "*".
   Missing end subtags are expanded as the empty string.  Thus a pattern
   "en-US" becomes the quintuple ("en","*","US","","").

   Here are some examples of language-tags and their quintuples:

      en-US ("en","*","US","*","*")

      sr-Latn ("sr,"Latn","*","*","*")

      zh-cmn-Hant ("zh-cmn","Hant","*","*","*")

      x-foo ("x-foo","*","*","*","*")

      en-x-foo ("en","*","*","x-foo","*")

      i-default ("i-default","*","*","*","*")

      sl-Latn-IT-roazj ("sl","Latn","IT","rozaj","*")

      zh-r-wadegile ("zh","*","*","*","r-wadegile") // hypothetical

   Each language-range/language-tag pair being matched or filtered is
   assigned a distance value, whereby small values indicate better
   matches and large values indicate worse ones.  The distance between
   the pair is the sum of the distances for each of the corresponding
   elements of the quintuple.  If the elements are identical or one is
   '*', then the distance value between them is zero.  Otherwise, it is
   given by the following table:






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     256    language mismatch
     128    script mismatch
      32    region mismatch
       4    variant mismatch
       1    extension mismatch

   A value of 0 is a perfect match; 421 is no match at all.  Different
   threshold values might be appropriate for different applications and
   implementations will probably allow users to choose the most
   appropriate selection value, ranking the selections based on score.

   Examples of various tag's distances from the range "en-US":

   "fr"             256 (language mismatch, region match)
   "en-GB"          384 (language, region mismatch)
   "en-Latn-US"       0 (all fields match)
   "en-Brai"         32 (region mismatch)
   "en-US-x-foo"      4 (variant mismatch: range is the empty string)
   "en-US-r-wadegile" 1 (extension mismatch: range is the empty string)

   Implementations may want to use more sophisticated weights that
   depend on the values of the corresponding elements.  For example,
   depending on the domain, an implemenation might give a small distance
   to the difference between the language subtag 'no' and the closely
   related language subtags 'nb' or 'nn'; or between the script subtags
   'Kata' and 'Hira'; or between the region subtags 'US' and 'UM'.

2.4.  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 might 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 might 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.



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   Variant subtags in particular seem to represent specific divisions in
   mutual understanding, since they often encode dialects or other
   idiosyncratic variations within a language.

   The relationship between the language 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 usage.

   o  For a single information object, the associated language tags
      might be interpreted as the set of languages that are necessary
      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/
      alternative.

   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
      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.

2.5.  Choosing Between Alternate Matching Schemes

   Implementers are faced with the decision of what form of matching to
   use in a specific application.  An application can choose to
   implement different styles of matching for different kinds of
   processing.

   The most basic choice is between schemes that produce an open-ended
   set of content (a "matching" application) and those that usually



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   produce a single information item (a "lookup" application).  Note
   that lookup applications can produce multiple items, but usually only
   a single item for any given piece of content, and they can be used to
   order content (the later in the overall fallback that the content
   appears to match, the more distant the match).

   Matching applications can produce an ordered or unordered set of
   results.  For example, applying formatting to a document based on the
   language of specific pieces of content does not require the content
   to be ordered.  It is sufficient to know whether a specific piece of
   content matches or does not match.  A search application, on the
   other hand, probably would put the results into a priority order.

   If single item is to be chosen, it may sometimes be useful to apply
   additional information, such as the most likely script used in the
   language or region in question or the script used by other content
   selected, in order to make a more "informed" choice.

   The matching schemes in this document are designed so that
   implementations do not have to examine the values of the subtags
   supplied and, except for scored matching, they do not need access to
   the Language Subtag Registry nor do they require the use of valid
   subtags in language tags or ranges.  This has great benefit for speed
   and simplicity of implementation.

   Implementations might 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".

2.6.  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.

   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 can result
   in unpredictable content being returned.






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2.7.  Length Considerations in Matching

   RFC 3066 [RFC3066] did not provide an upper limit on the size of
   language tags or ranges.  RFC 3066 did define the semantics of
   particular subtags in such a way that most language tags or ranges
   consisted of language and region subtags with a combined total length
   of up to six characters.  Larger tags and ranges (in terms of both
   subtags and characters) did exist, however.

   [draft-registry] also does not impose a fixed upper limit on the
   number of subtags in a language tag or range (and thus an upper bound
   on the size of either).  The syntax in that document suggests that,
   depending on the specific language or range of languages, more
   subtags (and thus characters) are sometimes necessary as a result.
   Length considerations and their impact on the selection and
   processing of tags are described in Section 2.1.1 of that document.

   A matching implementation MAY choose to limit the length of the
   language tags or ranges used in matching.  Any such limitation SHOULD
   be clearly documented, and such documentation SHOULD include the
   disposition of any longer tags or ranges (for example, whether an
   error value is generated or the language tag or range is truncated).
   If truncation is permitted it MUST NOT permit a subtag to be divided,
   since this changes the semantics of the subtag being matched and can
   result in false positives or negatives.

   Implementations that restrict storage SHOULD consider the impact of
   tag or range truncation on the resulting matches.  For example,
   removing the "*" from the end of an extended language range (see
   Section 2.3) can greatly modify the set of returned matches.  A
   protocol that allows tags or ranges to be truncated at an arbitrary
   limit, without giving any indication of what that limit is, has the
   potential for causing harm by changing the meaning of values in
   substantial ways.

   In practice, most tags do not require additional subtags or
   substantially more characters.  Additional subtags sometimes add
   useful distinguishing information, but extraneous subtags interfere
   with the meaning, understanding, and especially matching of language
   tags.  Since language tags or ranges MAY be truncated by an
   application or protocol that limits storage, when choosing language
   tags or ranges users and applications SHOULD avoid adding subtags
   that add no distinguishing value.  In particular, users and
   implementations SHOULD follow the 'Prefix' and 'Suppress-Script'
   fields in the registry (defined in Section 3.6 of [draft-registry]):
   these fields provide guidance on when specific additional subtags
   SHOULD (and SHOULD NOT) be used.




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   Implementations MUST support a limit of at least 33 characters.  This
   limit includes at least one subtag of each non-extension, non-private
   use type.  When choosing a buffer limit, a length of at least 42
   characters is strongly RECOMMENDED.

   The practical limit on tags or ranges derived solely from registered
   values is 42 characters.  Implementations MUST be able to handle tags
   and ranges of this length.  Support for tags and ranges of at least
   62 characters in length is RECOMMENDED.  Implementations MAY support
   longer values, including matching extensive sets of private use or
   extension subtags.

   Applications or protocols which have to truncate a tag MUST do so by
   progressively removing subtags along with their preceding "-" from
   the right side of the language tag until the tag is short enough for
   the given buffer.  If the resulting tag ends with a single-character
   subtag, that subtag and its preceding "-" MUST also be removed.  For
   example:

   Tag to truncate: zh-Hant-CN-variant1-a-extend1-x-wadegile-private1
   1. zh-Hant-CN-variant1-a-extend1-x-wadegile
   2. zh-Hant-CN-variant1-a-extend1
   3. zh-Hant-CN-variant1
   4. zh-Hant-CN
   5. zh-Hant
   6. zh

   Figure 7: Example of Tag Truncation























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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.

   The following changes were put into this document since draft-03:

      Modified the ABNF to match changes in [draft-registry]
      (K.Karlsson)

      Matched the references and reference formats to [draft-registry]
      (K.Karlsson)

      Various edits, additions, and emendations to deal with changes in
      the Last Call of draft-registry as well as cleaning up the text.





































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5.  Security Considerations

   Language ranges used in content negotiation might be used to infer
   the nationality of the sender, and thus identify potential targets
   for surveillance.  In addition, unique or highly unusual language
   ranges or combinations of language ranges might be used to track
   specific individual's activities.

   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.





































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6.  Character Set Considerations

   The syntax of language tags and language ranges permit only the
   characters A-Z, a-z, 0-9, and HYPHEN-MINUS (%x2D).  These characters
   are present in most character sets, so presentation of language tags
   should not present any character set issues.













































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7.  References

7.1.  Normative References

   [ID.ietf-ltru-initial]
              Ewell, D., Ed., "Language Tags Initial Registry (work in
              progress)", August 2005, <http://www.ietf.org/
              internet-drafts/draft-ietf-ltru-initial-04.txt>.

   [RFC1327]  Hardcastle-Kille, S., "Mapping between X.400(1988) / ISO
              10021 and RFC 822", RFC 1327, May 1992.

   [RFC1521]  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.

   [RFC2028]  Hovey, R. and S. Bradner, "The Organizations Involved in
              the IETF Standards Process", BCP 11, RFC 2028,
              October 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2231]  Freed, N. and K. Moore, "MIME Parameter Value and Encoded
              Word Extensions: Character Sets, Languages, and
              Continuations", RFC 2231, November 1997.

   [RFC2234bis]
              Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", draft-crocker-abnf-rfc2234bis-00
              (work in progress), March 2005.

   [RFC2396]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifiers (URI): Generic Syntax", RFC 2396,
              August 1998.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [RFC2616]  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.

   [RFC2860]  Carpenter, B., Baker, F., and M. Roberts, "Memorandum of
              Understanding Concerning the Technical Work of the
              Internet Assigned Numbers Authority", RFC 2860, June 2000.



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   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [draft-registry]
              Phillips, A., Ed. and M. Davis, Ed., "Tags for the
              Identification of Languages (work in progress)",
              August 2005, <http://www.ietf.org/internet-drafts/
              draft-ietf-ltru-registry-12.txt>.

7.2.  Informative References

   [ISO15924]
              "ISO 15924:2004. Information and documentation -- Codes
              for the representation of names of scripts", January 2004.

   [ISO3166-1]
              "ISO 3166-1:1997. Codes for the representation of names of
              countries and their subdivisions -- Part 1: Country
              codes", 1997.

   [ISO639-1]
              "ISO 639-1:2002. Codes for the representation of names of
              languages -- Part 1: Alpha-2 code", 2002.

   [ISO639-2]
              "ISO 639-2:1998. Codes for the representation of names of
              languages -- Part 2: Alpha-3 code, first edition", 1998.

   [RFC1766]  Alvestrand, H., "Tags for the Identification of
              Languages", RFC 1766, March 1995.

   [RFC3066]  Alvestrand, H., "Tags for the Identification of
              Languages", BCP 47, RFC 3066, January 2001.

   [RFC3282]  Alvestrand, H., "Content Language Headers", RFC 3282,
              May 2002.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, July 2002.

   [UN_M.49]  Statistics 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.







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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 [draft-registry], [RFC3066] and [RFC1766], each
   of which is a precursor to this document, made enormous contributions
   directly or indirectly to this document and are generally responsible
   for the success of language tags.

   The following people (in alphabetical order by family name)
   contributed to this document:

   Jeremy Carroll, John Cowan, Frank Ellermann, Doug Ewell, Kent
   Karlsson, Ira McDonald, M. Patton, Randy Presuhn 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.

   For this particular document, John Cowan originated the scheme
   described in Section 2.3.3.  Mark Davis originated the scheme
   described in the Section 2.2.2.


























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Authors' Addresses

   Addison Phillips (editor)
   Quest Software

   Email: addison dot phillips at quest dot com


   Mark Davis (editor)
   IBM

   Email: mark dot davis at ibm dot com







































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