Network Working Group A. Phillips, Ed. Internet-Draft Quest Software Obsoletes: 3066 (if approved) M. Davis, Ed. Expires:
April 10,May 20, 2006 IBM October 7,November 16, 2005 Matching Tags for the Identification of Languages draft-ietf-ltru-matching-05draft-ietf-ltru-matching-06 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 10,May 20, 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. 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. 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 .2.3. Extended Language Range . . . . . . . . . . . . . . . . . 5 3. Types of Matching . . . . . 5 2.2.2. Lookup. . . . . . . . . . . . . . . . . 8 3.1. Choosing a Type of Matching . . . . . . . 6 2.3. Extended Language Range. . . . . . . . 8 3.2. Filtering . . . . . . . . . 7 2.3.1. Extended Range Matching. . . . . . . . . . . . . . . 9 2.3.2. Extended Range Lookup3.2.1. Filtering with Basic Language Ranges . . . . . . . . . 10 3.2.2. Filtering with Extended Language Ranges . . . . . . . 10 220.127.116.11.2.3. Distance Metric SchemeFiltering . . . . . . . . . . . . . . 11 3.3. Lookup . . . . . . . . . . . . . . . 11 2.4. Meaning of Language Tags and Ranges. . . . . . . . . . . 13 2.5. Choosing Between Alternate Matching Schemes4. Other Considerations . . . . . . . . . . . . . . . . . . . . . 16 4.1. Meaning of Language Tags and Ranges . . . . . . . . . . 14 2.6.. 16 4.2. Considerations for Private Use Subtags . . . . . . . . . . 15 2.7.17 4.3. Length Considerations in Matching . . . . . . . . . . . . 16 3.17 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 4.20 6. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 5.21 7. Security Considerations . . . . . . . . . . . . . . . . . . . 20 6.22 8. Character Set Considerations . . . . . . . . . . . . . . . . . 21 7.23 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22 7.1.24 9.1. Normative References . . . . . . . . . . . . . . . . . . . 22 7.2.24 9.2. Informative References . . . . . . . . . . . . . . . . . . 2324 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 2425 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 2526 Intellectual Property and Copyright Statements . . . . . . . . . . 2627 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 preferenceLanguage 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 [draft- registry],[RFC3066bis], various mechanisms can be envisioned for performing language negotiation and tag matching. The suitabilityApplications, protocols, or specifications will have varying needs and requirements that will affect the choice of a suitable mechanism. Protocols and specifications SHOULD clearly indicate the particular mechanism to a particular application depends on the needs of that application.used in selecting or matching language tags. This document defines several mechanisms for matching andmatching, selecting, or filtering content whose natural language contentis identified using Language Tags [draft- registry]. It also defines[RFC3066bis], as well as the syntax (called a "language range") associated with each of these mechanisms for specifying userthe user's language preferences. This document, in combination with [RFC3066bis] (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]. 2. The Language Range Language Tags [draft-registry][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 language ranges in its discussion of the Accept- LanguageAccept-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]ABNF[RFC4234] 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 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 forin most applicationscases (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 usingwhen working with basic language rangesranges, no attempt is made 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. TheThus the choice of subtags in both the language tag and language range may affect the results produced as a result. 2.2.1. Matching2.3. Extended Language tag matching is used to select all content that matches a given prefix. In matching, the language range representsRange A Basic Language Range does not always provide the least specific tag whichmost appropriate way to specify a user's preferences. Sometimes it is an acceptable match and every piece of contentbeneficial to define a more granular matching scheme that matches is returned. Iftakes advantage of the internal structure of language priority list contains more than one range,tags, by allowing the matches returned are typically ordered in descending level of preference. For example, if an application is applying a styleuser to all content inspecify, for example, the value of a documentspecific field in a particular language, it might uselanguage tag matching to select the contentor to indicate which the style is applied. A language-range matches a language-tag if it exactly equals the tag, or if it exactly equals a prefixvalues are of interest in filtering or selecting the tag such that the first character followingcontent. In an extended language range, the prefix is "-". (That is,identifier takes the language-range "de-de" matchesform of a series of subtags which must consist of well-formed subtags or the language tag "de-DE-1996", but notspecial subtag "*". For example, the language tag "de-Deva".) The specialrange "*" matches any tag. A protocol which uses"en-*-US" specifies a primary language ranges MAY specify additional rules about the semanticsof "*"; for instance, HTTP/1.1 specifies that the range "*" matches only languages not matched'en', followed 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 matchesscript subtag, followed by the language priority list for a given request. In lookup, eachregion subtag 'US'. An extended 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,can be represented by the languagefollowing ABNF: extended-language-range = range is progressively truncated from the end until; a matching piece of content is located. For example, starting with therange "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/ privateuse ; private use tag granularity or is sparsely populated (than if the default/ grandfathered ; grandfathered registrations range = (language ["-" script] ["-" region] *("-" variant) *("-" extension) ["-" privateuse]) language = (2*3ALPHA [ extlang ]) ; shortest ISO 639 code / 4ALPHA ; reserved for the system or content were used). When performing lookup using a language priority list, the progressive search MUST proceed to consider eachfuture use / 5*8ALPHA ; registered 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 / "*" ; ...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 = %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 InA field not present in the middle of an extended language range, the identifier takesrange 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 or specifications that use a lookup (Section 3.3) matching scheme 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. 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 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 [RFC3066bis] generally fall into this category. There are several matching algorithms or schemes which can be applied when matching extended language ranges to language tags. 3. Types of Matching Matching language ranges to language tags can be done in a number of different ways. This section describes the different types of matching scheme, as well as the considerations for choosing between them. There are two basic types of matching scheme: those that produce an open-ended set of content (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 range for filtering operations is always the _least_ specific tag one will accept as a match, while for lookup operations the language range is always the _most_ specific tag. 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. 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 some processing that is language sensitive 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. This document describes three types of filtering: 1. Basic Filtering (Section 3.2.1) is used to match content using basic language rangesSection 2.2. It is compatible with implementations that do not produce extended language ranges. 2. Extended Range Filtering (Section 3.2.2) is used to match content using extended language rangesSection 2.3. Newer implementations SHOULD use this form of filtering in preference to basic filtering. 3. Scored Filtering (Section 3.2.3) produces an ordered set of content using either basic or 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. Lookup (Section 3.3) is used when each request MUST produce exactly one piece of content. For example, a Web server might use the Accept-Language HTTP header to choose which language to return a custom 404 page in: since it can return only one page, it must choose a single item and it must return some item, even if no content matches the language ranges supplied by the user. Most types of matching in this document are designed so that implementations do not have to examine the values of the subtags supplied and, except for scored filtering, they do not need access to the Language Subtag Registry nor do they require the use of valid subtags in either language tags or language 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 implementation might infer that content labeled "zh-CN" is morely 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 prefix. It is called "filtering" because this set of content may contain no items at all or it may return an arbitrary number of matching items--as many as match the language range used to specify the items, thus filtering out the non-matching content. In filtering, the language range represents the _least_ specific 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 range is "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 2.1) 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 language range. o Displaying the formset of documents containing a seriesparticular set of subtags which must consistkeywords written in a specific language. o Selecting all email items written in specific range of well-formed subtagslanguages. Filtering can produce either ordered or the special subtag "*".unordered set of results. For example, the language range "en-*-US" specifiesapplying formatting to a primarydocument based on the language of 'en', followed by any script subtag, followed by the region subtag 'US'. A field not present in the middlespecific pieces of an extended language range MAY be treated as if the field contained a "*". For example,content does not require the range "en-US" MAY be considered to be equivalentcontent to the range "en-*-US". This also means that multiple wildcards canbe collapsed (so that "en-*-*-US"ordered. It is equivalentsufficient to "en-*-US"). When working with tags and ranges users SHOULD noteknow whether a specific piece of content matches or does not match. A search application, on 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 atother hand, probably would put the end ofresults into a priority order. If an ordered set is desired, as described above, then the sequence of subtags, they don't normally interfere withapplication or protocol needs to determine the userelative "quality" of prefixes for matching inthe schemes described below. 2. Implementations that choose not to interpret one or more private- use or extension subtags SHOULD NOT remove or modify these extensions in content that they are processing. When amatch between different language tag instance is to be used in a specific, known protocol,tags and the language range. This measurment is not being passed throughcalled a "distance metric". A distance metric assigns a numeric value to other protocols,the comparison of each language tags MAY be filteredtag to remove subtagsa language range 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 onrepresents the other end of'distance' between the protocol would make usetwo. A distance of zero means that information. 3. Some applications of language tags might want or need to consider extensions and private-use subtags when matching tags. If extensionsthey are identical, a small distance indicates that they are very similar, and private-use subtagsa large distance indicated that they are included invery different. Using a matchingdistance metric, implementations can, for example, allow users to select a threshold distance for a match to be "successful" while filtering or it can use the numeric value to order the results. 3.2.1. Filtering with Basic Language Ranges When filtering process that utilizesusing a basic language range, the onelanguage range matches a language tag if it exactly equals the tag, or if it exactly equals a prefix of the schemes described in this document, thentag such that the implementation SHOULD canonicalizefirst character following the language tags and/or ranges before performingprefix is "-". (That is, the language-range "de-de" matches the matching. Note thatlanguage 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"de-DE-1996", but not the language tag "de-Deva".) The special range "*" matches any tag. A protocol which can be applied when matching extendeduses language ranges to language tags. 2.3.1.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. 3.2.2. Filtering with Extended Language Ranges In the Extended Range Matching In extended range matching,scheme, 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):3.2.1): 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 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 eachzh-cmn-Hans-CN-x-private 3.2.3. Distance Metric Filtering Both basic and extended language range are compared to the corresponding subtags in the language tag being examined. A subtag is considered to match iffiltering produce simple boolean matches. Sometimes 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 ismay be beneficial to provide an extensionarray 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 contentresults 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 enddifferent levels of the range MUST NOT be expanded in this manner. Formatching, 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 atsorting results based on the endoverall "quality" of the range to indicate that all tags with the range asmatch. Distance metric filtering provides a prefix are allowable matches. That is,way to generate these quality values. First both the extended language range "zh-*" matchesand the language tags "zh-Hant"to be matched to it must be canonicalized by mapping grandfathered and "zh-Hant-CN", while the range "zh" matches neither of those tags.obsolete tags into modern equivalents. The wildcard "*" atlanguage range and the endlanguage tags are then transformed into quintuples of a range SHOULD beelements of the form (language, script, country, variant, extension). Any extended language subtags are considered to match anypart of the language element; private use subtag sequences (making extendedare considered part of the language range lookup function exactly like extended range matching Section 2.3.1). By default all extensionselement if in the initial position in the tag and theirpart of the variant element if not. Language subtags SHOULD be ignored for'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 languagerange lookup. Private uselookup: missing internal subtags MAY be specified inare expanded to "*". Missing end subtags are expanded as the language rangeempty string. Thus a pattern "en-US" becomes the quintuple ("en","*","US","",""). Here are some examples of language-tags and MUST NOT be ignored when performing lookup.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 compared is assigned a distance value, whereby small values indicate better matches and large values indicate worse ones. The wildcard "*" atdistance between the end of a range SHOULD be considered to match any private use subtag sequences in addition to variants. For example,pair is the range "*-US" matches allsum of the following tags: en-US en-Latn-US en-US-r-extends (extensions are ignored) fr-US For example,distances for each of the range "en-*-US" matches _none_corresponding elements of the following tags: fr-US en (missing region US) en-Latn (missing region US) en-Latn-US-scouse (variant fieldquintuple. If the elements are identical or one is present) For example,'*', then the range "en-*" matches all ofdistance value between them is zero. Otherwise, it is given by 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 schemetable: 256 language mismatch 128 script mismatch 32 region mismatch 4 variant mismatch 1 extension mismatch A value of 0 is a moreperfect match; 421 is no match at all. Different threshold values might be appropriate replacementfor 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 withdifferent levels of matching, for example, sorting resultsapplications or protocols. Implementations will usually allow users to choose the most appropriate selection value, ranking the matched items based on the overall "quality"score. Examples of various tag's distances from the match. This type of matchingrange "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 sometimes called a "distance metric". A distance metric assigns a pair of language tags a numeric value representingthe 'distance' betweenempty string) Implementations or protocols sometimes might wish to use more sophisticated weights that depend on the two. A distancevalues 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, forthe corresponding elements. For example, allow users to selectdepending on the domain, an implemenation might give a thresholdsmall distance for a match to be successful or a filterto be applied. The first step inthe process is to normalizedifference between the extendedlanguage rangesubtag 'no' and the closely related language tags to be matched to it by canonicalizing them, mapping grandfathered and obsolete tags into modern equivalents. The language range andsubtags 'nb' or 'nn'; or between the language tags are then transformed into quintuples of elements ofscript subtags 'Kata' and 'Hira'; or between the form (language, script, country, variant, extension). Any extended languageregion subtags are considered part of'US' and 'UM'. 3.3. Lookup Lookup is used to select the language element; private use subtag sequences are considered part ofsingle information item that best matches the language element ifpriority list for a given request. In lookup, each language range in the initial position inlanguage priority list represents the _most_ specific tag and part of the variant element if not. Language subtags 'und', 'mul', and the script subtag 'Zyyy' are converted to "*". Missing components inwhich is an acceptable match; only the language-tag are set to "*"; thus a "*" pattern becomesclosest matching item according the quintuple ("*", "*", "*", "*", "*"). Missing components inuser's priority is returned. For example, if the extended language-range are handled similarly to extendedlanguage range lookup: missing internal subtags are expandedis "de-CH", one might expect to "*". Missing end subtags are expanded asreceive an information item with the empty string. Thus a pattern "en-US" becomestag "de" but never one with 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 bettertag "de-CH-1996". Usually if no content matches and large values indicate worse ones. The distance betweenthe pairrequest, 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 sumapplication "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 distancestext for eachan automated email response. o Selection of the corresponding elementsa graphic containing text for inclusion in a particular Web page. o Selection of a string of text for inclusion in an error log. In the quintuple. IfLookup scheme, the elements are identical or onelanguage range is '*', thenprogressively truncated from the distance value between them is zero. Otherwise, itend until a matching piece of content is given bylocated. For example, starting with the following table: 256 language mismatch 128 script mismatch 32 region mismatch 4 variant mismatch 1 extension mismatch A valuerange "zh-Hant-CN-x-private", the lookup would progressively search 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 5: Example of 0 isa perfect match; 421Lookup Fallback Pattern This scheme allows some flexibility in finding content. It also typically provides better results when data is no matchnot available 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. Examplesa specific level of various tag's distances fromtag granularity or is sparsely populated (than if the default language for the system or content were used). The language 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:"*" matches any language tag. In the lookup scheme, this language range does not convey enough information to determine which content is the empty string) "en-US-r-wadegile" 1 (extension mismatch:most appropriate. If this language range is the empty string) Implementations may want to use more sophisticated weights that depend on the values ofonly one in the corresponding elements. For example, depending onlanguage priority list, it matches the domain, an implemenation might givedefault content. If this language range is followed by other language ranges, it should be skipped. When performing lookup using a small distance to the difference between thelanguage subtag 'no' andpriority list, the closely relatedprogressive search MUST proceed to consider each language subtags 'nb' or 'nn'; or betweenrange before finding the script subtags 'Kata' and 'Hira';default content or between the region subtags 'US' and 'UM'. 2.4. Meaning of Language Tags and Ranges Aempty tag. The default content might be content with no language tag defines a language as spoken(or written, signedwith an empty value, as with xml:lang in the XML specification), or otherwise signaled) by human beingsit might be a particular language designated for communicationthat bit of informationcontent. One common way to other human beings. Ifprovide for default content is to allow a specific language tag B contains language tag Arange to be set as the default for a prefix, then B is typically "narrower" or "more specific" than A. For example, "zh- Hant-TW" is morespecific than "zh-Hant".type of request. This relationshiplanguage range is not guaranteed in all cases: specifically, languages that begin withthen treated as if it were appended to the same sequenceend of subtags are NOT guaranteed to be mutually intelligible, although they might be.the language priority list, rather than after each item in the language priority list. For example, the tag "az" sharesif a prefix with both "az-Latn" (Azerbaijani written using the Latin script)particular user's language priority list were "fr-FR; zh-Hant" and "az-Cyrl" (Azerbaijani written usingthe Cyrillic script). A person fluent in one script might not be able to readprogram doing the other, even thoughmatching had a default language range of "ja-JP", the text might be otherwise identical. Content taggedprogram would search for content as "az" most probablyfollows: 1. fr-FR 2. fr 3. zh-Hant // next language 4. zh 5. (return default content) a. ja-JP b. ja c. (empty tag or other default content) Figure 6: Lookup Using a Language Priority List 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 writtenused as input for both lookup and filtering operations). Wildcard values in an extended language range are supposed to match any value that occurs in that position in justa language tag. Since only one script and thus might notitem can be intelligible to a reader familiar withreturned for any given lookup request, the other script. Variant subtags in particular seem to represent specific divisionswildcards must be processed in mutual understanding, since they often encode dialects or other idiosyncratic variations withina language. The relationship betweenpredictable manner (or the same request might produce widely varying results). Thus, for each range in the language tag andpriority list, the information it relatesfollowing rules must be applied to is defined byproduce a basic language range for use in the standard describingfallback mechanism: 1. If the contextfirst subtag in which it appears. Accordingly, this section can only give possible examples of its usage. o For a single information object,the associatedextended language tags might be interpreted asrange is a "*" then entire range is converted to "*". 2. For each subsequent subtag, if the set of languagesvalue is a "*" then that subtag and its preceeding hyphen are necessary for a complete comprehension of the complete object. Example: Plain text documents. oremoved. For an aggregationexample: *-US becomes * en-*-US becomes en-US en-Latn-* becomes en-Latn Figure 7: Transformation of information objects,Extended Language Ranges For the associatedlanguage tags could be taken aspriority list "*-US; fr-*-FR; zh-Hant", the setfallback pattern would be: 1. * (skipped) 2. fr-FR 3. fr 4. zh-Hant 5. zh 6. (default content) Figure 8: Extended Language Range Fallback Example 4. Other Considerations When working with language ranges and matching schemes, there are some additional points that may influence the choice of languages used inside componentseither. 4.1. 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 that aggregation. Examples: Document stores and libraries. o Forinformation objects whose purpose isto provide alternatives, the associatedother human beings. If a language tags could be regardedtag B contains language tag A as a hint that the contentprefix, then B is providedtypically "narrower" or "more specific" than A. For example, "zh- Hant-TW" is more specific than "zh-Hant". This relationship is not guaranteed in several languages, andall cases: specifically, languages that one has to inspect each of the alternatives in order to find its language or languages. In this case,begin with the presencesame sequence of multiple tags might not mean that one needs to be multi-lingualsubtags are NOT guaranteed to get complete understanding of the document. Example: MIME multipart/ alternative. o In markup languages, such as HTML and XML, language information canbe added to each part of the document identified by the markup structure (including the whole document itself).mutually intelligible, although they might be. For example, one could write <span lang="FR">C'est la vie.</span> inside a Norwegian document;the Norwegian-speaking user could then accesstag "az" shares a French-Norwegian dictionary to find out whatprefix with both "az-Latn" (Azerbaijani written using the marked section meant. IfLatin script) and "az-Arab" (Azerbaijani written using the user were listeningArabic script). A person fluent in one script might not be able to that document through a speech synthesis interface, this formation couldread 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 used to signal the synthesizer to appropriately apply French text-to-speech pronunciation rulesintelligible to that span of text, instead of misapplying the Norwegian rules. 2.5. Choosing Between Alternate Matching Schemes Implementers are faceda reader familiar with the decision of what form of matchingother script. Variant subtags in particular seem to userepresent specific divisions in mutual understanding, since they often encode dialects or other idiosyncratic variations within a specific application. An application can choose to implement different styles of matching for different kinds of processing.language. The most basic choice isrelationship between schemes that produce an open-ended set of content (a "matching" application) and those that usually 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 inthe overall fallback thatlanguage tag and the content appearsinformation it relates to match,is defined by the more distantstandard describing the match). Matching applicationscontext in which it appears. Accordingly, this section can produce an ordered or unordered setonly give possible examples of results.its usage: o For example, applying formatting toa document based onsingle information object, the associated language of specific pieces of content does not require the content totags might be ordered. It is sufficient to know whetherinterpreted as the set of languages that are necessary for a specific piececomplete comprehension of content matches or does not match. A search application, on the other hand, probably would putthe results into a priority order. If single item is to be chosen, it may sometimescomplete object. Example: Plain text documents. o For an aggregation of information objects, the associated language tags could be useful to apply additional information, suchtaken as the most likely scriptset of languages used ininside components of that aggregation. Examples: Document stores and libraries. o For information objects whose purpose is to provide alternatives, the associated language or region in question ortags could be regarded as a hint that the script used by othercontent selected, in order to make a more "informed" choice. The matching schemesis provided in this document are designed soseveral languages, and that implementations do not haveone has to examine the valuesinspect each 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 subtagsalternatives in order to find its language tagsor ranges. This has great benefit for speed and simplicitylanguages. In this case, the presence of implementation. Implementationsmultiple tags might also wishnot mean that one needs to use semanticbe multi-lingual to get complete understanding of the document. Example: MIME multipart/ alternative. o In markup languages, such as HTML and XML, language information externalcan be added to each part of the langauge tags when performing fallback.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 primary language subtags 'nn' (Nynorsk Norwegian) and 'nb' (Bokmal Norwegian) might bothNorwegian-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 usefully matchedused to signal the synthesizer to appropriately apply French text-to-speech pronunciation rules to the more general subtag 'no' (Norwegian). Or an application might inferthat content labeled "zh-CN" is morely likely to matchspan of text, instead of misapplying the range "zh-Hans" than equivalent content labeled "zh-TW". 2.6.Norwegian rules. 4.2. 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. 18.104.22.168. 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][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. A matching implementationAn 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. ImplementationsApplications 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 2.3) 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]):[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-Hant-CN-variant1-a-extend1-x-wadegile-private1zh-Latn-CN-variant1-a-extend1-x-wadegile-private1 1. zh-Hant-CN-variant1-a-extend1-x-wadegilezh-Latn-CN-variant1-a-extend1-x-wadegile 2. zh-Hant-CN-variant1-a-extend1zh-Latn-CN-variant1-a-extend1 3. zh-Hant-CN-variant1zh-Latn-CN-variant1 4. zh-Hant-CNzh-Latn-CN 5. zh-Hantzh-Latn 6. zh Figure 7:9: Example of Tag Truncation 3.5. IANA Considerations This document presents no new or existing considerations for IANA. 4.6. Changes This is the first version of this document. The following changes were put into this document since draft-03:draft-05: Modified the ABNF to match changes in [draft-registry][RFC3066bis] (K.Karlsson) Matched the references and reference formats to [draft-registry][RFC3066bis] (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. 5.Changed from 'defined' to 'identifies' in Section 4.1. (M.Gunn) Reorganized the text and broke it into sections (M.Duerst) Modified occurences of the word "application" to refer to "applications or protocols" or otherwise be specific (E. van der Poel) Removed "Extended Language Range Lookup", merging it with other text on lookup to form a single scheme. (M.Davis) Fixed or removed obsolete or dangling references (Ed.) Added an introduction to section 4 and added one sentence to make it flow better to the start of section 4.1. (Ed.) 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 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. 6.8. 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. 7.9. References 22.214.171.124. 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. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. [draft-registry][RFC3066bis] Phillips, A., Ed. and M. Davis, Ed., "Tags for the Identification of Languages (work in progress)", AugustLanguages", October 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<http:// www.ietf.org/internet-drafts/ draft-ietf-ltru-registry-14.txt>. [RFC4234] Crocker, D. 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. CodesP. Overell, "Augmented BNF for the representation of names of languages -- Part 2: Alpha-3 code, first edition", 1998.Syntax Specifications: ABNF", RFC 4234, October 2005. 9.2. Informative References [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.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],[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: Jeremy Carroll, John Cowan, Martin Duerst, Frank Ellermann, Doug Ewell, Marion Gunn, Kent Karlsson, Ira McDonald, M. Patton, Randy PresuhnPresuhn, 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 126.96.36.199.2.3. Mark Davis originated the scheme described in the Section 188.8.131.52.3. 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 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. 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