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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: June 10, 2006                                               IBM
                                                        December 7, 2005


                       Matching of Language Tags
                      draft-ietf-ltru-matching-08

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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 or content selection, filtering, and lookup are
   described.  This document, in combination with RFC 3066bis (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 . . . . . . . . . . . . . . . .  7
   3.  Types of Matching  . . . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Choosing a Type of Matching  . . . . . . . . . . . . . . .  8
     3.2.  Filtering  . . . . . . . . . . . . . . . . . . . . . . . .  9
       3.2.1.  Filtering with Basic Language Ranges . . . . . . . . . 10
       3.2.2.  Filtering with Extended Language Ranges  . . . . . . . 11
       3.2.3.  Scored Filtering . . . . . . . . . . . . . . . . . . . 11
     3.3.  Lookup . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   4.  Other Considerations . . . . . . . . . . . . . . . . . . . . . 18
     4.1.  Choosing Language Ranges . . . . . . . . . . . . . . . . . 18
     4.2.  Meaning of Language Tags and Ranges  . . . . . . . . . . . 19
     4.3.  Considerations for Private Use Subtags . . . . . . . . . . 20
     4.4.  Length Considerations in Matching  . . . . . . . . . . . . 21
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
   6.  Changes  . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   8.  Character Set Considerations . . . . . . . . . . . . . . . . . 26
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 27
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 27
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 28
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
   Intellectual Property and Copyright Statements . . . . . . . . . . 30






















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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.  Language preferences 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.

   This document defines a syntax (called a language range (Section 2))
   for specifying a user's language preferences, as well as several
   schemes for selecting or filtering content by comparing language
   ranges to the language tags [RFC3066bis] used to identify the natural
   language of that content.  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 choose a particular option
   or indicate that the particular options is left to the specific
   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 in order to select specific content.  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".

   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 as well.

2.1.  Basic Language Range

   A "basic language range" identifies the set of content whose language
   tags 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 the ABNF[RFC4234]:

   language-range = language-tag / "*"
   language-tag   = 1*8[alphanum] *["-" 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).

   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 be mutually intelligible.





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2.2.  Extended Language Range

   A Basic Language Range does not always provide the most appropriate
   way to specify a user's preferences.  Sometimes it is beneficial to
   use a more fine-grained matching scheme that takes advantage of the
   internal structure of language tags.  This allows the user to
   specify, for example, the value of a specific field in a language tag
   or to indicate which values are of interest in filtering or selecting
   the content.

   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'.

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


































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   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
                 / 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     = "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



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   A field not present in the middle of an extended language range is
   treated as if the field contained a "*".  Implementations that
   normalize extended language ranges SHOULD expand missing fields to be
   "*" so that the semantic meaning of the language range is clear to
   the user.  At the same time, multiple wildcards in a row are
   redundant and implementations SHOULD collapse these to a single
   wildcard when normalizing the range (for brevity).  For example, both
   the range "sl-nedis" and the range "sl-*-*-nedis" are equivalent to
   and should be normalized as "sl-*-nedis".

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
   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 semi-colons, like this: "en; fr; zh-Hant"
   (which would be read as "English before French before Chinese as
   written in the Traditional script").














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

   A key difference between these two types of matching scheme is that
   the language ranges in the language priority list represent the
   _least_ specific content one will accept as a match, while for lookup
   operations the language ranges represent the _most_ specific content.

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 result in failure of the operation, an
      empty value, returning some protocol defined or implementation
      defined default, or returning i-default [RFC2277].

   Filtering can be used to produce a set of results (such as a
   collection of documents).  For example, if using a search engine, one
   might use filtering to limit the results to documents written in
   French.  It can 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.




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   This document describes four types of matching (three types of
   filtering, plus the lookup scheme):

   1.  Basic Filtering (Section 3.2.1) is used to match content using
       basic language ranges (Section 2.1).

   2.  Extended Range Filtering (Section 3.2.2) is used to match content
       using extended language ranges (Section 2.2).

   3.  Scored Filtering (Section 3.2.3) produces an ordered set of
       content using extended language ranges.  It SHOULD be used when
       the quality of the match within a specific language range is
       important, as when presenting a list of documents resulting from
       a search.

   4.  Lookup (Section 3.3) is used when each request needs to produce
       _exactly_ one piece of content.  For example, if process were to
       insert a human readable error message into a protocol header, it
       might select the text based on the user's language preference.
       Since it can return only one item, it must choose a single item
       and it must return some item, even if no content matches the
       language priority list supplied by the user.

   Most types of matching in this document are designed so that
   implementations are not required to validate or understand any of the
   semantics of the subtags supplied and, except for scored filtering,
   they do not need access to the IANA Language Subtag Registry (see
   Section 3 in [RFC3066bis]).  This simplifies and speeds the
   performance of implementations.

   If an implementation canonicalizes either ranges or tags, then the
   implementation will require the IANA Language Subtag Registry
   information for that purpose.  Implementations MAY 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 content that matches a given
   language priority list.  It is called "filtering" because this set of
   content may contain no items at all or it may return an arbitrarily
   large number of matching items--as many as match the language range
   used to specify the items, thus filtering out the non-matching
   content.



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   In filtering, the language range represents the _least_ specific
   (that is, the fewest number of subtags) language tag which is an
   acceptable match.  That is, all of the language tags in the set of
   filtered content will have an equal or greater number of subtags than
   the language range.  For example, if the language priority list
   consists of the range "de-CH", one might see matching content with
   the tag "de-CH-1996" but one will never see a match with the tag
   "de".

   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 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.

   Filtering can produce either an ordered or an 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 is selected by the language priority list (or not).  A search
   application, on the other hand, probably would want to order the
   results.

   If an ordered set is desired, as described above, then the
   application or protocol needs to determine the relative "quality" of
   the match between different language tags and the language range.

   This measurement is called a "distance metric".  A distance metric
   assigns a numeric value to the comparison of a language tag to a
   language range that represents 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 indicates
   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" while filtering, or they
   might use the numeric values to order the results.

3.2.1.  Filtering with Basic Language Ranges

   When filtering using basic language ranges, each basic language range



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   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 "-".  (That is,
   the language-range "de-de" matches the language tag "de-DE-1996", but
   not the language tag "de-Deva".)

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

3.2.2.  Filtering with Extended Language Ranges

   When filtering using extended language ranges, 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).

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

      de-DE

      de-Latn-DE

      de-Latf-DE

      de-DE-x-goethe

      de-Latn-DE-1996

3.2.3.  Scored Filtering

   Both basic and extended language range filtering produce simple
   boolean matches between a language range and a language tag.
   Sometimes it may be useful to provide an array of results with
   different levels of matching, for example, sorting results based on
   the overall "quality" of the match.  Scored (or "distance metric")



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   filtering provides a way to generate these quality values.

   As with the other forms of filtering, the process considers each
   language range in the language priority list in order of priority.

   Each extended language range and language tag MUST first be
   canonicalized by mapping grandfathered and obsolete tags into modern
   equivalents.  This requires the information in the IANA Language
   Subtag Registry (see Section 3 of [RFC3066bis]).

   The language range and each language tag it is to be compared to are
   then transformed into a "quintuple" consisting of five "elements" in
   the form (language, script, country, variant, extension).

   Any extended language subtags are considered part of the language
   "element".  For example, the language element for the tag "zh-cmn-
   Hans" would be "zh-cmn".

   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.  The different handling of private-use
   sequences prevents a range such as "x-twain" from matching all
   possible tags, while a range such as "en-US-x-twain" would closely
   match nearly all tags for English as used in the United States.

   Language subtags 'und', 'mul', and the script subtag 'Zyyy' are
   converted to "*": these subtag values represent undetermined,
   multiple, or private-use values which are consistent with the use of
   the wildcard.

   For language tags that have no script subtag but whose language
   subtag's record in the IANA Language Subtag Registry contains the
   field "Suppress-Script", the script element in the quintuple MUST be
   set to the script subtag in the Suppress-Script field.  This is
   necessary because [RFC3066bis] strongly recommends that users not use
   this subtag to form language tags and this document recommends that
   users not use them to form ranges.  For example, if the script were
   not expanded in this manner, a range such as "de-DE" would produce a
   more-distant score for content that happened to be labeled
   "de-Latn-DE" than users would expect that it should.  Note that
   languages which have a "Suppress-Script" field in the registry are
   predominantly written in a single script.

   Any remaining missing components in the language tag are set to "*";
   thus an empty language tag becomes the quintuple ("*", "*", "*", "*",
   "*").  Missing components in the 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



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   pattern "en-US" becomes the quintuple ("en","*","US","","").

   Here are some examples of language tags, showing their quintuples as
   both language tags and language ranges:

   en-US
      Tag:   (en, *, US, *, *)
      Range: (en, *, US, "", "")

   sr-Latn
      Tag:   (sr, Latn, *, *, *)
      Range: (sr, Latn, "", "", "")

   zh-cmn-Hant
      Tag:   (zh-cmn, Hant, *, *, *)
      Range: (zh-cmn, Hant, "", "", "")

   x-foo
      Tag:   (x-foo, *, *, *, *)
      Range: (x-foo, "", "", "", "")

   en-x-foo
      Tag:   (en, *, *, x-foo, *)
      Range: (en, *, *, x-foo, "")

   i-default
      Tag:   (i-default, *, *, *, *)
      Range: (i-default, "", "", "", "")

   sl-Latn-IT-rozaj
      Tag:   (sl, Latn, IT, rozaj, *)
      Range: (sl, Latn, IT, rozaj, "")

   zh-r-wadegile (hypothetical)
      Tag:   (zh, *, *, *, r-wadegile)
      Range: (zh, *, *, *, r-wadegile)

   Figure 3: Examples of Distance Metric Quintuples

   Each pair of quintuples being compared is assigned a distance value,
   in which 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 or
   protocols.  Implementations will usually allow users to choose the
   most appropriate selection value, ranking the matched items based on
   score.

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

   "fr-FR"          384 (language & region mismatch)
   "fr"             256 (language mismatch, region match)
   "en-GB"           32 (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)

   Note: A variation of this algorithm might vary the scoring used
   overall or for specific values.  For example, sometimes it might make
   sense to use more sophisticated weighting that depends on the values
   of the corresponding elements.  Thus, depending on the domain, an
   implementation might assign a smaller distance to the difference
   between closely related subtags (or treat certain values as equal).
   Some examples of closely related subtags might be:

   Language:
     no (Norwegian)
     nb (Bokmal Norwegian)
     nn (Nynorsk Norwegian)

   Script:
     Kata (katakana)
     Hira (hiragana)

   Region:
     US (United States of America)
     UM (United States Minor Outlying Islands)

   Figure 6: Examples of Closely Related Subtags







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3.3.  Lookup

   Lookup is used to select the single information item that best
   matches the language priority list for a given request.  When
   performing lookup, each language range in the language priority list
   is considered in turn, according to priority.  By contrast with
   filtering, each language ranges represents the _most_ specific tag
   which is an acceptable match.  The first information item found with
   a matching tag, according the user's priority, is considered the
   closest match and is the item returned.  For example, if the language
   range is "de-CH", one might expect to receive an information item
   with the tag "de" but never one with the tag "de-CH-1996".  Usually
   if no content matches the request, a "default" item 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.  Other 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 or the empty tag)

   Figure 7: Example of a Lookup Fallback Pattern

   This scheme allows some flexibility in finding content.  For example,
   it provides better results for cases in which data 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



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   subtag sequence provides more opportunity to find a match between
   available content and the user's request.

   The default content is implementation defined.  It might be content
   with no language tag; might have an empty value (the built-in
   attribute xml:lang in [XML10] permits the empty value); might be a
   particular language designated for that bit of content; or it might
   be content that is labeled with 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
   default content 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 8: 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



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   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
   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 content that
   is first in ASCII-order.  For example, if the language range were
   "*-CH" and the set of content included "de-CH", "fr-CH", and "it-CH",
   then the content labeled "de-CH" would be returned.

   Another way an implementation could address extended language ranges
   would be to map them to basic language ranges: if the first subtag is
   a "*" then the entire range is treated as "*" (which matches the
   default content), otherwise the wildcard subtag is removed.  For
   example, if the language range were "en-*-US", then the range would
   be mapped to "en-US".






























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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 interefer with filtering
   (Section 3), since they appear at the end of the tag and will match
   all prefixes.

   When working with language tags and language ranges note that:

   o  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



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      filtering (Section 3.2) matching schemes described below.
      However, they do interfere with filtering when used in language
      ranges and SHOULD be avoided in ranges as a result.

   o  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.

   o  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.  A language tag or range
   identifies 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-Arab"
   (Azerbaijani written using the Arabic 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.  They also seem to
   represent relatively low divisions with a high chance of at least
   limited understanding, although this depends on the specific variant
   in question.

   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.

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



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   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

   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.

   [RFC3066bis] 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.

   An application or protocol 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.

   Applications or protocols 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 2.2) 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



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   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 [RFC3066bis]):
   these fields provide guidance on when specific additional subtags
   SHOULD (and SHOULD NOT) be used.

   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-Latn-CN-variant1-a-extend1-x-wadegile-private1
   1. zh-Latn-CN-variant1-a-extend1-x-wadegile
   2. zh-Latn-CN-variant1-a-extend1
   3. zh-Latn-CN-variant1
   4. zh-Latn-CN
   5. zh-Latn
   6. zh

   Figure 9: Example of Tag Truncation
















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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.

   The following changes were put into this document since draft-07:

      Added a mention of "*" to the Character Set Considerations section
      (D.Ewell)











































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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.

   [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, 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 3.2.3.  Mark Davis originated the scheme
   described in the Section 3.3.


























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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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Intellectual Property Statement

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Disclaimer of Validity

   This document and the information contained herein are provided on an
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   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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Copyright Statement

   Copyright (C) The Internet Society (2005).  This document is subject
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Acknowledgment

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   Internet Society.




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