openldap/doc/drafts/draft-zeilenga-ldap-rfc2596-xx.txt

INTERNET-DRAFT                           Editor: Kurt D. Zeilenga
Intended Category: Standard Track             OpenLDAP Foundation
Expires in six months                                1 March 2002
Obsoletes: RFC 2596


                     Language Tags and Ranges in LDAP
                    draft-zeilenga-ldap-rfc2596-01.txt


Status of Memo

  This document is an Internet-Draft and is in full conformance with all
  provisions of Section 10 of RFC2026.

  This document is intended to be, after appropriate review and
  revision, submitted to the RFC Editor as a Standard Track document.
  Distribution of this memo is unlimited.  Technical discussion of this
  document will take place on the IETF LDAP Extensions Working Group
  (LDAPext) mailing list <ietf-ldapext@netscape.com>.  Please send
  editorial comments directly to the document editor
  <Kurt@OpenLDAP.org>.

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  Copyright 2002, The Internet Society.  All Rights Reserved.

  Please see the Copyright section near the end of this document for
  more information.

Abstract

  It is often desirable to to be able to indicate the natural language
  associated with values held in a directory and to be able to query the
  directory for values which fulfill the user's language needs.  This
  document details the use of Language Tags and Ranges in the
  Lightweight Directory Access Protocol (LDAP).



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Conventions

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
  document are to be interpreted as described in BCP 14 [RFC2119].


1. Background and Intended Use

  The Lightweight Directory Access Protocol (LDAP) [Roadmap] provides a
  means for clients to interrogate and modify information stored in a
  distributed directory system.  The information in the directory is
  maintained as attributes of entries.  Most of these attributes have
  syntaxes which are human-readable strings, and it is desirable to be
  able to indicate the natural language associated with attribute
  values.

  This document describes how language tags and ranges [RFC3066] are
  carried in LDAP and are to be interpreted by LDAP implementations.
  All implementations MUST be prepared to accept language tags and
  ranges in the LDAP protocol.

  This document replaces RFC 2596.  Appendix A summaries changes made
  since RFC 2596.

  Appendix B discusses differences from X.500(1997) "contexts"
  mechanism.

  Appendix A and B are provided for information purposes and are not a
  normative part of this specification.

  The remainder of this section provides a summary of Language Tags,
  Language Ranges, and Attribute Descriptions.


1.1. Language Tags

  Section 2 of BCP 47 [RFC3066] describes the language tag format which
  is used in LDAP.  Briefly, it is a string of ASCII alphabetic
  characters and hyphens.  Examples include "fr", "en-US" and "ja-JP".
  Language tags are case insensitive.  For example, the language tag
  "en-us" is the same as "EN-US".

  Section 2 of this document details use of language tags in LDAP.


1.2. Language Ranges




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  Section 2.5 of BCP 47 [RFC3066] describes the language ranges.
  Language ranges are used to specify sets of language tags.

  A language range matches a language tag if it exactly equals the tag,
  or if it exactly equals a prefix of the tag such that the first
  character following the prefix is "-".  The special tag "*" matches
  all tags.

  Due to restrictions upon option naming in LDAP, this document uses a
  different language range syntax.  However, the semantics of language
  ranges in LDAP is consistent with BCP 47.

  Section 3 of this document details use of language ranges in LDAP.


1.3. Attribute Descriptions

  This section provides an overview of attributes in LDAP.  LDAP
  attributes are defined in [Models].

  An attribute consists of a type, a set of zero or more associated
  tagging options, and a set of one or more values.  The type and the
  options are combined into the AttributeDescription.
  AttributeDescriptions can also contain options which are not part of
  the attribute, but indicate some other function such as the transfer
  encoding.

  An attribute with one or more tagging options is a direct subtype of
  each attribute of the same with all but one of the tagging options.
  If the attribute's type is a direct subtype of some other type, then
  the attribute is also a direct subtype of the attribute whose
  description consists of the the supertype and all of the tagging
  options.  That is, CN;x-bar;x-foo is a direct subtype of CN;x-bar,
  CN;x-foo, and name;x-bar;x-foo.  Note that CN is a subtype of name.

  If the attribute description contains an unrecognized option, the
  attribute description is treated as an unrecognized attribute type.

  As language tags are intended to stored with the attribute, they are
  to treated as tagging options as described in Section 2.  Language
  range are used only to match against language ranges and are not
  stored with the attribute.  They are not treated tagging options (nor
  as transfer options), but as described in Section 3.


2. Use of Language Tags in LDAP

  This section describes how LDAP implementations MUST interpret



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  language tags in performing operations.

  Servers which support storing attributes with language tag options in
  the Directory Information Tree (DIT) SHOULD allow any attribute type
  it recognizes that has the Directory String, IA5 String, or other
  textual string syntax to have language tag options associated with it.
  Servers MAY allow language options to be associated with other
  attributes types.

  Clients SHOULD NOT assume servers are capable of storing attributes
  with language tags in the directory.

  Implementations MUST NOT otherwise interpret the structure of the tag
  when comparing two tag, and MUST treat them simply as strings of
  characters.  Implementations MUST allow any arbitrary string which
  conforms to the syntax defined in BCP 47 [RFC3066] to be used as a
  language tag.


2.1. Language Tag Options

  A language tag option associates a natural language with values of an
  attribute.  An attribute description MAY contain multiple language tag
  options.  An entry MAY contain multiple attributes with same attribute
  type but different combinations of language tag (and other) options.

  A language tag option conforms to the following ABNF [RFC2234]:

      language-tag-option = "lang-" Language-Tag

  where the Language-Tag production is as defined in BCP 47 [RFC3066].
  This production and those it imports from [RFC2234] are provided here
  for convenience:

      Language-Tag = Primary-subtag *( "-" Subtag )

      Primary-subtag = 1*8ALPHA

      Subtag = 1*8(ALPHA / DIGIT)

      ALPHA = %x41-5A / %x61-7A   ; A-Z / a-z

      DIGIT = %x30-39             ; 0-9

  A language tag option is a tagging option [Models].  A language tag
  option has no effect on the syntax of the attribute's values nor their
  transfer encoding.




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  Examples of valid AttributeDescription:

    givenName;lang-en-US
    CN;lang-ja
    SN;lang-de;lang-gem-PFL
    O;lang-i-klingon;x-foobar
    description;x-foobar
    CN

  Notes: The last two have no language tag options.  The x-foobar option
         is fictious and used for example purposes.


2.2. Search Filter

  If language tag options are present in an AttributeDescription in an
  assertion, then for each entry within scope, the values of each
  attribute whose AttributeDescription consists of the same attribute
  type or its subtypes and contains each of the presented (and possibly
  other) options is to be matched.

  Thus for example a filter of an equality match of type
  "name;lang-en-US" and assertion value "Billy Ray", against the
  following directory entry

    dn: SN=Ray,DC=example,DC=com
    objectclass: top                    DOES NOT MATCH (wrong type)
    objectclass: person                 DOES NOT MATCH (wrong type)
    name;lang-en-US: Billy Ray          MATCHES
    name;lang-en-US: Billy Bob          DOES NOT MATCH (wrong value)
    CN;lang-en-US: Billy Ray            MATCHES
    CN;lang-en-US;x-foobar: Billy Ray   MATCHES
    CN;lang-en;x-foobar: Billy Ray      DOES NOT MATCH (differing lang-)
    CN;x-foobar: Billy Ray              DOES NOT MATCH (no lang-)
    name: Billy Ray                     DOES NOT MATCH (no lang-)
    SN;lang-en-GB;lang-en-US: Billy Ray MATCHES
    SN: Ray                             DOES NOT MATCH (no lang-,
                                            wrong value)

  (Note that "CN" and "SN" are subtypes of "name".)

  It is noted that providing a language tag option in a search filter
  AttributeDescription will filter out desirable values where the tag
  does not match exactly.  For example, the filter (name;lang-en=Billy
  Ray) does NOT match the attribute "name;lang-en-US:  Billy Ray".

  If the server does not support storing attributes with language tag
  options in the DIT, then any assertion which includes a language tag



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  option will not match as such it is an unrecognized attribute type.
  No error would be returned because of this; a presence assertion would
  evaluate to FALSE and all other assertions to Undefined.

  If no options are specified in the assertion, then only the base
  attribute type and the assertion value need match the value in the
  directory.

  Thus for example a filter of an equality match of type "name" and
  assertion value "Billy Ray", against the following directory entry

    dn: SN=Ray,DC=example,DC=net
    objectclass: top                    DOES NOT MATCH (wrong type)
    objectclass: person                 DOES NOT MATCH (wrong type)
    name;lang-en-US: Billy Ray          MATCHES
    name;lang-en-US: Billy Bob          DOES NOT MATCH (wrong value)
    CN;lang-en-US;x-foobar: Billy Ray   MATCHES
    CN;lang-en;x-foobar: Billy Ray      MATCHES
    CN;x-foobar: Billy Ray              MATCHES
    name: Billy Ray                     MATCHES
    SN;lang-en-GB;lang-en-US: Billy Ray MATCHES
    SN: Ray                             DOES NOT MATCH (wrong value)


2.3. Requested Attributes in Search

  Clients can provide language tag options in AttributeDescription in
  the requested attribute list in a search request.

  If language tag options are provided in an attribute description, then
  only attributes in a directory entry whose attribute descriptions have
  the same attribute type or its subtype and the provided language tags
  options are to be returned.  Thus if a client requests just the
  attribute "name;lang-en", the server would return "name;lang-en" and
  "CN;lang-en;lang-ja" but not "SN" nor "name;lang-fr".

  Clients can provide in the attribute list multiple
  AttributeDescription which have the same base attribute type but
  different options. For example a client could provide both
  "name;lang-en" and "name;lang-fr", and this would permit an attribute
  with either language tag option to be returned.  Note there would be
  no need to provide both "name" and "name;lang-en" since all subtypes
  of name would match "name".

  If a server does not support storing attributes with language tag
  options in the DIT, then any attribute descriptions in the list which
  include language tag options are to be ignored, just as if they were
  unknown attribute types.



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  If a request is made specifying all attributes or an attribute is
  requested without providing a language tag option, then all attribute
  values regardless of their language tag option are returned.

  For example, if the client requests a "description" attribute, and a
  matching entry contains the following attributes:

    objectclass: top
    objectclass: organization
    O: Software GmbH
    description: software
    description;lang-en: software products
    description;lang-de: Softwareprodukte
    postalAddress: Berlin 8001 Germany
    postalAddress;lang-de: Berlin 8001 Deutschland

  The server would return:

    description: software
    description;lang-en: software products
    description;lang-de: Softwareprodukte


2.4. Compare

  Language tag options can be present in an AttributeDescription used in
  a compare request AttributeValueAssertion.  This is to be treated by
  servers the same as the use of language tag options in a search filter
  with an equality match, as described in Section 2.2.  If there is no
  attribute in the entry with the same subtype and language tag options,
  the noSuchAttributeType error will be returned.

  Thus for example a compare request of type "name" and assertion value
  "Johann", against an entry containing the following attributes:

    objectclass: top
    objectclass: person
    givenName;lang-de-DE: Johann
    CN: Johann Sibelius
    SN: Sibelius

  would cause the server to return compareTrue.

  However, if the client issued a compare request of type "name;lang-de"
  and assertion value "Johann" against the above entry, the request
  would fail with the noSuchAttributeType error.

  If the server does not support storing attributes with language tag



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  options in the DIT, then any comparison which includes a language tag
  option will always fail to locate an attribute, and
  noSuchAttributeType will be returned.


  2.5. Add Operation

  Clients can provide language options in AttributeDescription in
  attributes of a new entry to be created.

  A client can provide multiple attributes with the same attribute type
  and value, so long as each attribute has a different set of language
  tag options.

  For example, the following is a legal request.

    dn: CN=John Smith,DC=example,DC=com
    objectclass: top
    objectclass: person
    objectclass: residentialPerson
    name: John Smith
    CN: John Smith
    CN;lang-en: John Smith
    SN: Smith
    SN;lang-en;lang-en-US: Smith
    streetAddress: 1 University Street
    streetAddress;lang-en: 1 University Street
    streetAddress;lang-fr: 1 rue Universite
    houseIdentifier;lang-fr: 9e etage

  If a server does not support storing language tag options with
  attribute values in the DIT, then it MUST treat an
  AttributeDescription with a language tag option as an unrecognized
  attribute.  If the server forbids the addition of unrecognized
  attributes then it MUST fail the add request with an appropriate
  result code.


2.6. Modify Operation

  A client can provide language tag options in an AttributeDescription
  as part of a modification element in the modify operation.

  Attribute types and language tag options MUST match exactly against
  values stored in the directory.  For example, if the modification is a
  "delete", then if the stored values to be deleted have language tag
  options, then those language tag options MUST be provided in the
  modify operation, and if the stored values to be deleted do not have



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  any language tag option, then no language tag option is to be
  provided.

  If the server does not support storing language tag options with
  attribute values in the DIT, then it MUST treat an
  AttributeDescription with a language tag option as an unrecognized
  attribute, and MUST fail the request with an appropriate result code.


3. Use of Language Ranges in LDAP

  Since the publication of RFC 2596, it has become apparent that there
  is a need to provide a mechanism for a client to request attributes
  based upon set of language tag options whose tags all begin with the
  same sequence of language sub-tags.

  AttributeDescriptions containing language range options are intended
  to be used in attribute value assertions, search attribute lists, and
  other places where the client desires to provide an attribute
  description matching of a range of language tags associated with
  attributes.

  A language range option conforms to the following ABNF [RFC2234]:

      language-range-option = "lang-" [ Language-Tag "-" ]

  where the Language-Tag production is as defined in BCP 47 [RFC3066].
  This production and those it imports from [RFC2234] are provided here
  for convenience:

      Language-Tag = Primary-subtag *( "-" Subtag )

      Primary-subtag = 1*8ALPHA

      Subtag = 1*8(ALPHA / DIGIT)

      ALPHA = %x41-5A / %x61-7A   ; A-Z / a-z

      DIGIT = %x30-39             ; 0-9

  A language range option matches a language tag option if language
  range option less the trailing "-" matches exactly the language tag or
  if the language range option (including the trailing "-") matches a
  prefix of the language tag option.  Note that the language range
  option "lang-" matches all language tag options.

  Examples of valid AttributeDescription containing language range
  options:



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    givenName;lang-en-
    CN;lang-
    O;lang-x-;x-foobar

  A language range option is not a tagging option.  Attributes cannot be
  stored with language range options.  Any attempt to add or update an
  attribute description with a language range option SHALL be treated as
  an undefined attribute type and result in an error.

  A language range option has no effect on the transfer encoding nor on
  the syntax of the attribute values.

  Servers SHOULD support assertion of language ranges for any attribute
  which they allow to stored with language tags.


3.1. Search Filter

  If a language range option is present in an AttributeDescription in an
  assertion, then for each entry within scope, the values of each
  attribute whose AttributeDescription consists of the same attribute
  type or its subtypes and contains a language tag option matching the
  language range option are to be returned.

  Thus for example a filter of an equality match of type "name;lang-en-"
  and assertion value "Billy Ray", against the following directory entry

    dn: SN=Ray,DC=example,DC=com
    objectclass: top                    DOES NOT MATCH (wrong type)
    objectclass: person                 DOES NOT MATCH (wrong type)
    name;lang-en-US: Billy Ray          MATCHES
    name;lang-en-US: Billy Bob          DOES NOT MATCH (wrong value)
    CN;lang-en-US: Billy Ray            MATCHES
    CN;lang-en-US;x-foobar: Billy Ray   MATCHES
    CN;lang-en;x-foobar: Billy Ray      MATCHES
    CN;x-foobar: Billy Ray              DOES NOT MATCH (no lang-)
    name: Billy Ray                     DOES NOT MATCH (no lang-)
    SN;lang-en-GB;lang-en-US: Billy Ray MATCHES
    SN: Ray                             DOES NOT MATCH (no lang-,
                                          wrong value)

  (Note that "CN" and "SN" are subtypes of "name".)

  If the server does not support storing attributes with language tag
  options in the DIT, then any assertion which includes a language range
  option will not match as it is an unrecognized attribute type.  No
  error would be returned because of this; a presence filter would
  evaluate to FALSE and all other assertions to Undefined.



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3.2. Requested Attributes in Search

  Clients can provide language range options in AttributeDescription in
  the requested attribute list in a search request.

  If a language range option is provided in an attribute description,
  then only attributes in a directory entry whose attribute descriptions
  have the same attribute type or its subtype and a language tag option
  matching the provided language range option are to be returned.  Thus
  if a client requests just the attribute "name;lang-en-", the server
  would return "name;lang-en-US" and "CN;lang-en;lang-ja" but not "SN"
  nor "name;lang-fr".

  Clients can provide in the attribute list multiple
  AttributeDescription which have the same base attribute type but
  different options.  For example a client could provide both
  "name;lang-en-" and "name;lang-fr-", and this would permit an
  attribute whose type was name or subtype of name and with a language
  tag option matching either language range option to be returned.

  If a server does not support storing attributes with language tag
  options in the DIT, then any attribute descriptions in the list which
  include language range options are to be ignored, just as if they were
  unknown attribute types.


3.3. Compare

  Language range options can be present in an AttributeDescription used
  in a compare request AttributeValueAssertion.  This is to be treated
  by servers the same as the use of language range options in a search
  filter with an equality match, as described in Section 3.1.  If there
  is no attribute in the entry with the same subtype and a matching
  language tag option, the noSuchAttributeType error will be returned.

  Thus for example a compare request of type "name;lang-" and assertion
  value "Johann", against the entry with the following attributes:

    objectclass: top
    objectclass: person
    givenName;lang-de-DE: Johann
    CN: Johann Sibelius
    SN: Sibelius

  will cause the server to return compareTrue.  (Note that the language
  range option "lang-" matches any language tag option.)

  However, if the client issued a compare request of type "name;lang-de"



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  and assertion value "Sibelius" against the above entry, the request
  would fail with the noSuchAttributeType error.

  If the server does not support storing attributes with language tag
  options in the DIT, then any comparison which includes a language
  range option will always fail to locate an attribute, and
  noSuchAttributeType will be returned.


4. Discovering Language Option Support

  A server SHOULD indicate that it supports storing attributes with
  language tag options in the DIT by publishing OID.TDB as a value of
  the supportedFeatures [FEATURES] attribute in the root DSE.

  A server SHOULD indicate that it supports language range matching of
  attributes with language tag options stored in the DIT by publishing
  OID.TDB as a value of the supportedFeatures [FEATURES] attribute in
  the root DSE.

  A server MAY restrict use of language tag options to a subset of the
  attribute types it recognizes.  This document does not define a
  mechanism for determining which subset of attribute types can be used
  with language tag options.


5. Security Considerations

  Language tags and range options are used solely to indicate the native
  language of values and in querying the directory for values which
  fulfill the user's language needed.  These options are not known to
  raise specific security considerations.  However, the reader should
  consider general directory security issues detailed in the LDAP
  technical specification [Roadmap].


6. Acknowledgments

  This document is a revision of RFC 2596 by Mark Wahl and Tim Howes.
  RFC 2596 was a product of the IETF ASID and LDAPEXT working groups.

  This document borrows from a number of IETF documents including BCP
  47.


7. Normative References

  [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate



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             Requirement Levels", BCP 14 (also RFC 2119), March 1997.

  [RFC2234] D. Crocker, P. Overell, "Augmented BNF for Syntax
             Specifications: ABNF", RFC 2234, November 1997.

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

  [Roadmap]  K. Zeilenga (editor), "LDAP: Technical Specification Road
             Map", draft-ietf-ldapbis-roadmap-xx.txt, a work in
             progress.

  [Models]   K. Zeilenga (editor), "LDAP: Directory Information Models",
             draft-ietf-ldapbis-models-xx.txt, a work in progress.

  [FEATURES] K. Zeilenga, "Feature Discovery in LDAP",
             draft-zeilenga-ldap-features-xx.txt (a work in progress).


8. Informative References

  [X.501]    "The Directory: Models", ITU-T Recommendation X.501, 1997.


Appendix A. Differences from RFC 2596

  This document adds support for language ranges, provides a mechanism
  that a client can use to discover whether a server supports language
  tags and ranges, and clarifies how attributes with multiple language
  tags are to be treated.  This document is a significant rewrite of RFC
  2596.


Appendix B. Differences from X.500(1997)

  X.500(1997) [X.501] defines a different mechanism, contexts, as the
  means of representing language tags (codes).  This section summarizes
  the major differences in approach.

  a) An X.500 operation which has specified a language code on a value
     matches a value in the directory without a language code.
  b) LDAP references BCP 47 [RFC3066], which allows for IANA
     registration of new tags as well as unregistered tags.
  c) LDAP supports language ranges.
  d) LDAP does not allow language tags (and ranges) in distinguished
     names.
  e) X.500 describes subschema administration procedures to allow
     language codes to be associated with particular attributes types.



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Copyright 2002, The Internet Society.  All Rights Reserved.

  This document and translations of it may be copied and furnished to
  others, and derivative works that comment on or otherwise explain it
  or assist in its implementation may be prepared, copied, published and
  distributed, in whole or in part, without restriction of any kind,
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  The limited permissions granted above are perpetual and will not be
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  ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED,
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