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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                                                Yahoo! Inc
Obsoletes: 3066 (if approved)                              M. Davis, Ed.
Expires: August 27, 2006                                          Google
                                                       February 23, 2006


                       Matching of Language Tags
                      draft-ietf-ltru-matching-10

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document describes different mechanisms for comparing, matching,
   and evaluating language tags.  Possible algorithms for language
   negotiation or content selection, filtering, and lookup are
   described.  This document, in combination with RFC 3066bis (Ed.:
   replace "3066bis" with the RFC number assigned to
   draft-ietf-ltru-registry-14), replaces RFC 3066, which replaced RFC
   1766.



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  The Language Range . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Basic Language Range . . . . . . . . . . . . . . . . . . .  4
     2.2.  Extended Language Range  . . . . . . . . . . . . . . . . .  5
     2.3.  The Language Priority List . . . . . . . . . . . . . . . .  5
   3.  Types of Matching  . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  Choosing a Type of Matching  . . . . . . . . . . . . . . .  7
     3.2.  Filtering  . . . . . . . . . . . . . . . . . . . . . . . .  8
       3.2.1.  Basic Filtering  . . . . . . . . . . . . . . . . . . .  9
       3.2.2.  Extended Filtering . . . . . . . . . . . . . . . . . . 10
     3.3.  Lookup . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.  Other Considerations . . . . . . . . . . . . . . . . . . . . . 14
     4.1.  Choosing Language Ranges . . . . . . . . . . . . . . . . . 14
     4.2.  Meaning of Language Tags and Ranges  . . . . . . . . . . . 15
     4.3.  Considerations for Private Use Subtags . . . . . . . . . . 15
     4.4.  Length Considerations in Matching  . . . . . . . . . . . . 15
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   6.  Changes  . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   8.  Character Set Considerations . . . . . . . . . . . . . . . . . 20
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 21
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 21
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 22
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
   Intellectual Property and Copyright Statements . . . . . . . . . . 24























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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 or in some
   specific set of information items or "content".

   One use for language identifiers, such as those defined in
   [RFC3066bis], is to select content by matching the associated
   language tags to a user's language preferences.

   This document defines a syntax (called a language range (Section 2))
   for specifying items in the user's language preferences (called a
   language priority list (Section 2.3)), as well as several schemes for
   selecting or filtering sets of content by comparing the content's
   language tags to the user's preferences.  Applications, protocols, or
   specifications will have varying needs and requirements that affect
   the choice of a suitable matching scheme.  Depending on the choice of
   scheme, there are various options left to the implementation.
   Protocols that implement a matching scheme either need to specify
   each particular choice or indicate the options that are left to the
   implementation to decide.

   This document is divided into three main sections.  One describes how
   to indicate a user's preferences using language ranges.  Then a
   section describes various schemes for matching these ranges to a set
   of language tags.  There is also a section that deals with various
   practical considerations that apply to implementing and using these
   schemes.

   This document, in combination with [RFC3066bis] (Ed.: replace
   "3066bis" globally in this document with the RFC number assigned to
   draft-ietf-ltru-registry-14), replaces [RFC3066], which replaced
   [RFC1766].

   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 [RFC3066bis] are used to identify the language of some
   information item or content.  Applications or protocols 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 one such mechanism in its discussion of
   the Accept-Language header (Section 14.4), which is used when
   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".

   There are different types of language range, whose specific
   attributes vary to match their application.  Language ranges are
   similar in content to language tags: they consist of a sequence of
   subtags separated by hyphens.  In a language range, each subtag MUST
   either be a sequence of ASCII alphanumeric characters or the single
   character '*' (%2A, ASTERISK).  The character '*' is a "wildcard"
   that matches any sequence of subtags.  Restrictions on the meaning
   and use of wildcards vary according to the type of language range.

   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.

2.1.  Basic Language Range

   A "basic language range" identifies the set of language tags that all
   begin with the same sequence of subtags.  Each range consists of a
   sequence of alphanumeric subtags separated by hyphens.  The basic
   language range is defined by the following ABNF [RFC4234]:

   language-range   = (1*8ALPHA *("-" 1*8alphanum)) / "*"
   alphanum         = ALPHA / DIGIT

   Basic language ranges (originally described by HTTP/1.1 [RFC2616] and
   later [RFC3066]) have the same syntax as an [RFC3066] language tag or
   are the single character "*".  They differ from the language tags
   defined in [RFC3066bis] only in that there is no requirement that
   they be "well-formed" or be validated against the IANA Language
   Subtag Registry (although such ill-formed ranges will probably not
   match anything).  (Note that the ABNF [RFC4234] in [RFC2616] is



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   incorrect, since it disallows the use of digits anywhere in the
   'language-range': this is mentioned in the errata)

   Use of a basic language range seems to imply that there is a semantic
   relationship between language tags that share the same 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 represent mutually intelligible languages.

2.2.  Extended Language Range

   Basic language ranges allow users to specify a set of language tags
   that share the same initial subtags.  Occasionally users will wish to
   select a set of language tags based on the presence of specific
   subtags.  For example, a user might wish to select all language tags
   that contains the region subtag 'CH'.  Extended language ranges are
   useful in specifying a particular sequence of subtags that appear in
   the set of matching tags without having to specify all of the
   intervening subtags.

   An extended language range can be represented by the following ABNF:

   extended-language-range = (1*8ALPHA / "*")
                             *("-" (1*8alphanum / "*"))

   Figure 2: Extended Language Range

   The wildcard subtag '*' MAY occur in any position in the extended
   language range, where it matches any sequence of subtags that might
   occur in that position in a language tag.  However wildcards outside
   the first position in an extended language range are ignored by most
   matching schemes.  Use of multiple wildcards SHOULD NOT be taken to
   imply that a certain number of subtags will appear in the matching
   set of language tags.

   Implementations that specify basic ranges MAY map extended language
   ranges to basic language ranges: if the first subtag is a "*" then
   the entire range is treated as "*" (which matches the default
   content), otherwise each wildcard subtag is removed.  For example, if
   the language range were "en-*-US", then the range would be mapped to
   "en-US".

2.3.  The Language Priority List

   When users specify a language preference they often need to specify a
   prioritized list of language ranges in order to best reflect their
   language preferences.  This is especially true for speakers of
   minority languages.  A speaker of Breton in France, for example, may



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   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 fall back from Skolt Sami
   to Northern Sami to Finnish.

   A "Language Priority List" is 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].  A simple list of ranges, i.e. one that
   contains no weighting information, is considered to be in descending
   order of priority.

   The various matching operations described in this document include
   considerations for using a language priority list.  This document
   does not define any syntax for a language priority list; defining
   such a syntax is the responsibility of the protocol, application, or
   implementation that uses it.  When given as examples in this
   document, language priority lists will be shown as a quoted sequence
   of ranges separated by semicolons, like this: "en; fr; zh-Hant"
   (which would be read as "English before French before Chinese as
   written in the Traditional script").

   Where a language priority list provides "quality weights" for the
   language ranges, such as the use of Q weights in the syntax of the
   "Accept-Language" header (defined in [RFC2616], Section 14.4, and
   [RFC3282]), language ranges without a weight are given values equal
   to the value of the previous language range (processing from first to
   last).  If the first language range has no weight, it is given a
   value of 1.0.  Then language ranges with zero weights are removed.
   For example, "fr, en;q=0.5, de, it" becomes "fr;q=1.0, en;q=0.5,
   de;q=0.5, it;q=0.5".  The language priority list is then sorted from
   highest priority to lowest, with language ranges that share the same
   weights remain in the same order as in the original language priority
   list.

















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3.  Types of Matching

   Matching language ranges to language tags can be done in a number of
   different ways.  This section describes several different matching
   schemes, as well as the considerations for choosing between them.
   Protocols and specifications SHOULD clearly indicate the particular
   mechanism used in selecting or matching language tags.

   There are several types of matching scheme.  This document presents
   two types: those that produce zero or more information items (called
   "filtering") and those that produce a single information item for a
   given request (called "lookup").

   Implementations or protocols MAY use different matching schemes than
   the ones described in this document, as long as those mechanisms are
   clearly specified.

3.1.  Choosing a Type of Matching

   Applications, protocols, and specifications are faced with the
   decision of what type of matching to use.  Sometimes, different
   styles of matching might be suited for different kinds of processing
   within a particular application or protocol.

   Language tag matching is a tool, and does not by itself specify a
   complete procedure for the use of language tags.  Such procedures are
   intimately tied to the application protocol in which they occur.
   When specifying a protocol operation using matching, the protocol
   MUST specify:

   o  Which type(s) of language tag matching it uses

   o  Whether the operation returns a single result (lookup) or a
      possibly empty set of results (filtering)

   o  For lookup, what the result is when no matching tag is found.  For
      instance, a protocol might define the result as failure of the
      operation, an empty value, returning some protocol defined or
      implementation defined default, or returning i-default [RFC2277].

   This document describes three types of matching:

   1.  Basic Filtering (Section 3.2.1) matches a language priority list
       consisting of basic language ranges (Section 2.1) to sets of
       language tags.

   2.  Extended Filtering (Section 3.2.2) matches a language priority
       list consisting of extended language ranges (Section 2.2) to sets



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       of language tags.

   3.  Lookup (Section 3.3) matches a language priority list consisting
       of basic language ranges to sets of language tags find the
       _exactly_ one language tag that best matches the range.

   Both types of filtering can be used to produce a set of results (such
   as a collection of documents) by comparing the user's preferences to
   language tags associated with the set of content.  For example, when
   performing a search, one might use filtering to limit the results to
   documents tagged as being written in French.  They might also be used
   when deciding whether to perform a language-sensitive process on some
   content.  For example, a process might cause paragraphs whose
   language tag matched the language range "nl" to be displayed in
   italics within a document.

   Lookup produces the single result that best matches a given set of
   user preferences, so it is useful in cases in which only a single
   item can be returned.  For example, if a process were to insert a
   human readable error message into a protocol header, it might select
   the text based on the user's language priority list.  Since the
   process can return only one item, it must choose a single item and it
   must return some item, even if no content's language tag matches the
   language priority list supplied by the user.

   The types of matching in this document are designed so that
   implementations are not required to validate or understand any of the
   semantics of the language tags or ranges or of the subtags in them.
   None of them require access to the IANA Language Subtag Registry (see
   Section 3 in [RFC3066bis]).  This simplifies and speeds the
   performance of implementations.

   Regardless of the matching scheme chosen, protocols and
   implementations MAY canonicalize language tags and ranges by mapping
   grandfathered and obsolete tags or subtags into modern equivalents.
   If an implementation canonicalizes either ranges or tags, then the
   implementation will require the IANA Language Subtag Registry
   information for that purpose.  Implementations MAY also use semantic
   information external to the registry when matching tags.  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 implementation might infer
   that content labeled "zh-CN" is more likely to match the range "zh-
   Hans" than equivalent content labeled "zh-TW".

3.2.  Filtering

   Filtering is used to select the set of language tags that matches a



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   given language priority list and return the associated content.  It
   is called "filtering" because this set might contain no items at all
   or it might return an arbitrarily large number of matching items: as
   many items as match the language priority list, thus "filtering out"
   the non-matching items.

   In filtering, the language range represents the _least_ specific
   (that is, the fewest number of subtags) language tag which is an
   acceptable match.  All of the language tags in the matching set of
   tags will have an equal or greater number of subtags than the
   language range.  Every non-wildcard subtag in the language range will
   appear in every one of the matching language tags.  For example, if
   the language priority list consists of the range "de-CH", one might
   see tags such as "de-CH-1996" but one will never see a tag such as
   "de" (because the 'CH' subtag is missing).

   If the language priority list (see Section 2.3) contains more than
   one range, the content returned is typically ordered in descending
   level of preference.

   Some examples of applications where filtering might be appropriate
   include:

   o  Applying a style to sections of a document in a particular set of
      languages.

   o  Displaying the set of documents containing a particular set of
      keywords written in a specific set of languages.

   o  Selecting all email items written in a specific set of languages.

   The content returned MAY either be ordered or unordered according to
   the priority in the language priority list (and other criteria),
   according to the needs of the application or protocol.

3.2.1.  Basic Filtering

   When filtering using basic language ranges, each basic language range
   in the language priority list is considered in turn, according to
   priority.  A particular language tag matches a language range 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 "-".  For
   example, the language-range "de-de" matches the language tag "de-DE-
   1996", but not the language tags "de-Deva" or "de-Latn-DE".

   The special range "*" in a language priority list matches any tag.  A
   protocol which uses language ranges MAY specify additional rules
   about the semantics of "*"; for instance, HTTP/1.1 [RFC2616]



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   specifies that the range "*" matches only languages not matched by
   any other range within an "Accept-Language" header.

3.2.2.  Extended Filtering

   When filtering using extended language ranges, each extended language
   range in the language priority list is considered in turn, according
   to priority.  A particular language range is compared to each
   language tag using the following process:

   Compare the first subtag in the extended language tag to the first
   subtag in the language tag in a case insensitive manner.  If the
   first subtag in the range is "*", it matches any value.  Otherwise
   the two values must match or the overall match fails.

   Take each non-wildcard subtag in the language range and compare it to
   the next subtag in the language tag in turn until a matching subtag
   is found or the langauge tag is exhausted.  If the end of the
   language tag is found first, the match fails.  If a match is found,
   this step is repeated with the next non-wildcard subtag in the
   language range (and beginning with the next subtag in the language
   tag) until the list of subtags in the language range is exhausted or
   the match fails.

   Subtags not specified, including those at the end of the language
   range, are thus treated as if assigned the wildcard value "*".
   Extended filtering works, therefore, much like basic filtering.  For
   example, the extended language range "de-*-DE" matches all of the
   following tags:

      de-DE

      de-Latn-DE

      de-Latf-DE

      de-DE-x-goethe

      de-Latn-DE-1996

3.3.  Lookup

   Lookup is used to select the single language tag that best matches
   the language priority list for a given request and return the
   associated content.  When performing lookup, each language range in
   the language priority list is considered in turn, according to
   priority.  By contrast with filtering, each language range represents
   the _most_ specific tag which is an acceptable match.  The first



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   content found with a matching tag, according to the user's priority,
   is considered the closest match and is the content returned.  For
   example, if the language range is "de-ch", a lookup operation might
   produce content with the tags "de" or "de-CH" but never one with the
   tag "de-CH-1996".  Usually if no content matches the request, the
   "default" content 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
   matching language tag associated with a suitable piece of content to
   insert.  Examples of lookup might include:

   o  Selection of a template containing the text for an automated email
      response.

   o  Selection of a item containing some text for inclusion in a
      particular Web page.

   o  Selection of a string of text for inclusion in an error log.

   In the lookup scheme, 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-private", the lookup
   progressively searches for content as shown below:

   Range to match: zh-Hant-CN-x-private
   1. zh-Hant-CN-x-private
   2. zh-Hant-CN
   3. zh-Hant
   4. zh
   5. (default content)

   Figure 3: Example of a Lookup Fallback Pattern

   This scheme allows some flexibility in finding a match.  For example,
   lookup provides better results for cases in which content is not
   available that exactly matches the user request than if the default
   language for the system or content were returned immediately.  Not
   every specific level of tag granularity is usually available or
   language content may be sparsely populated, so "falling back" through
   the subtag sequence provides more opportunity to find a match between
   available language tags and the user's request.

   The default behavior when no tag matches the language priority list
   is implementation defined.  An implementation might, for example,
   return content with no language tag; might supply content with an
   empty language tag value (the built-in attribute xml:lang in [XML10]



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   permits the empty value); might be a particular language designated
   for the bit of content being selected; or it might select the tag
   "i-default" (see [RFC2277]).  When performing lookup using a language
   priority list, the progressive search MUST proceed to consider each
   language range in the list before finding the default content or
   empty tag.

   One common way for an application or implementation to provide for a
   default is to allow a specific language range to be set as the
   default for a specific type of request.  This language range is then
   treated as if it were appended to the end of the language priority
   list as a whole, rather than after each item in the language priority
   list.

   For example, if a particular user's language priority list were
   "fr-FR; zh-Hant" and the program doing the matching had a default
   language range of "ja-JP", the program would search for content as
   follows:
   1. fr-FR
   2. fr
   3. zh-Hant // next language
   4. zh
   5. (search for the default content)
      a. ja-JP
      b. ja
      c. (implementation defined default)

   Figure 4: Lookup Using a Language Priority List

   Implementations SHOULD ignore extensions and unrecognized private-use
   subtags when performing lookup, since these subtags are usually
   orthogonal to the user's request.

   The special language range "*" matches any language tag.  In the
   lookup scheme, this range does not convey enough information by
   itself to determine which content is most appropriate, since it
   matches everything.  If the language range "*" is the only one in the
   language priority list, it matches the default content.  If the
   language range "*" is followed by other language ranges, it should be
   skipped.

   In some cases, the language priority list might contain one or more
   extended language ranges (as, for example, when the same language
   priority list is used as input for both lookup and filtering
   operations).  Wildcard values in an extended language range normally
   match any value that occurs in that position in a language tag.
   Since only one item can be returned for any given lookup request,
   wildcards in a language range have to be processed in a consistent



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   manner or the same request will produce widely varying results.
   Implementations that accept extended language ranges MUST define
   which content is returned when more than one item matches the
   extended language range.

   For example, an implementation could return the matching tag that is
   first in ASCII-order.  If the language range were "*-CH" and the set
   of tags included "de-CH", "fr-CH", and "it-CH", then the tag "de-CH"
   would be returned.  Another example would be for an implementation to
   map the extended language ranges to basic ranges.









































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4.  Other Considerations

   When working with language ranges and matching schemes, there are
   some additional points that may influence the choice of either.

4.1.  Choosing Language Ranges

   Users indicate their language preferences via the choice of a
   language range or the list of language ranges in a language priority
   list.  The type of matching affects what the best choice is for a
   given user.

   Most matching schemes make no attempt to process the semantic meaning
   of the subtags.  The language range (or its subtags) is usually
   compared in a case-insensitive manner to each language tag being
   matched, using basic string processing.

   Users SHOULD avoid subtags that add no distinguishing value to a
   language range.  Generally, the fewer subtags that appear in the
   language range, the more content the range will match.

   Most notably, script subtags SHOULD NOT be used to form a language
   range in combination with language subtags that have a matching
   Suppress-Script field in their registry entry.  Thus the language
   range "en-Latn" is probably inappropriate in most cases (because the
   vast majority of English documents are written in the Latin script
   and thus the 'en' language subtag has a Suppress-Script field for
   'Latn' in the registry).

   When working with tags and ranges, note that extensions and most
   private-use subtags are orthogonal to language tag matching, in that
   they specify additional attributes of the text not related to the
   goals of most matching schemes.  Users SHOULD avoid using these
   subtags in language ranges, since they interfere with the selection
   of available content.  When used in language tags (as opposed to
   ranges), these subtags normally do not interfere with filtering
   (Section 3), since they appear at the end of the tag and will match
   all prefixes.

   Private-use and Extension subtags are normally orthogonal to language
   tag fallback.  Implementations or specifications that use a lookup
   (Section 3.3) matching scheme often ignore unrecognized private-use
   and extension subtags when performing language tag fallback.  In
   addition, since these subtags are always at the end of the sequence
   of subtags, their use in language tags normally doesn't interfere
   with the use of ranges that omit them in the filtering (Section 3.2)
   matching schemes described below.  However, they do interfere with
   filtering when used in language ranges and SHOULD be avoided in



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   ranges as a result.

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

   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 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
   [RFC3066bis] generally fall into this category.

4.2.  Meaning of Language Tags and Ranges

   Selecting content using language ranges requires some understanding
   by users of what they are selecting.  The meaning of the various
   subtags in a language range are identical to their meaning in a
   language tag (see Section 4.2 in [RFC3066bis]), with the addition
   that the wildcard "*" represents any matching sequence of values.

4.3.  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.

4.4.  Length Considerations in Matching

   Language ranges are very similar to language tags in terms of content
   and usage.  The same types of restrictions on length that apply to
   language tags could also apply to language ranges.  Implementation,



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   protocol, and specificiation authors SHOULD apply the considerations
   in [RFC3066bis] Section 4.3 (Length Considerations) where appropriate
   to language ranges and language priority lists.
















































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5.  IANA Considerations

   This document presents no new or existing considerations for IANA.
















































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6.  Changes

   This is the first version of this document.
















































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7.  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 a
   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 or protocol.





































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8.  Character Set Considerations

   Language tags permit only the characters A-Z, a-z, 0-9, and HYPHEN-
   MINUS (%x2D).  Language ranges also use the character ASTERISK
   (%x2A).  These characters are present in most character sets, so
   presentation or exchange of language tags or ranges should not be
   constrained by character set issues.












































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9.  References

9.1.  Normative References

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

   [RFC2277]  Alvestrand, H., "IETF Policy on Character Sets and
              Languages", BCP 18, RFC 2277, January 1998.

   [RFC3066bis]
              Phillips, A., Ed. and M. Davis, Ed., "Tags for the
              Identification of Languages", October 2005, <http://
              www.ietf.org/internet-drafts/
              draft-ietf-ltru-registry-14.txt>.

   [RFC4234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, October 2005.

9.2.  Informative References

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

   [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.

   [RFC2616errata]
              IETF, "HTTP/1.1 Specification Errata", 10 2004,
              <http://purl.org/NET/http-errata>.

   [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.

   [XML10]    Bray (et al), T., "Extensible Markup Language (XML) 1.0",
              02 2004.











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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 [RFC3066bis], [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:

   Harald Alvestrand, Jeremy Carroll, John Cowan, Martin Duerst, Frank
   Ellermann, Doug Ewell, Marion Gunn, Kent Karlsson, Ira McDonald, M.
   Patton, Randy Presuhn, Eric van der Poel, Markus Scherer, 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 scoring
   scheme.  Mark Davis originated the scheme described in Section 3.3.


























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Authors' Addresses

   Addison Phillips (editor)
   Yahoo! Inc

   Email: addison at inter dash locale dot com


   Mark Davis (editor)
   Google

   Email: mark dot davis at macchiato dot com







































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Intellectual Property Statement

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Disclaimer of Validity

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Copyright Statement

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Acknowledgment

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   Internet Society.




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