Network Working Group K. Zeilenga, Ed.
Request for Comments: 3866 OpenLDAP Foundation
Obsoletes: 2596 July 2004
Category: Standards Track
Language Tags and Ranges in the
Lightweight Directory Access Protocol (LDAP)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
It is often desirable 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).
1. Background and Intended Use
The Lightweight Directory Access Protocol (LDAP) [RFC3377] 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 LDAP implementations MUST be prepared to accept language tags and
ranges.
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].
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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 informational purposes only.
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] letters and
hyphens. Examples include "fr", "en-US" and "ja-JP". Language tags
are case insensitive. That is, 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
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 is exactly equal to the
tag, or if it is exactly equal to a prefix of the tag such that the
first character following the prefix is "-". That is, the language
range "de" matches the language tags "de" and "de-CH" but not "den".
The special language range "*" matches all language tags.
Due to attribute description option naming restrictions in LDAP, this
document defines a different language range syntax. However, the
semantics of language ranges in LDAP are 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 attribute descriptions in LDAP.
LDAP attributes and attribute descriptions are defined in [RFC2251].
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.
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AttributeDescriptions can also contain options which are not part of
the attribute, but indicate some other function (such as range
assertion or transfer encoding).
An AttributeDescription with one or more tagging options is a direct
subtype of each AttributeDescription of the same type with all but
one of the tagging options. If the AttributeDescription's type is a
direct subtype of some other type, then the AttributeDescription is
also a direct subtype of the AttributeDescription which consists of
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".
2. Use of Language Tags in LDAP
This section describes how LDAP implementations MUST interpret
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 syntaxes 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 tags, 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
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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. A language tag option has
no effect on the syntax of the attribute's values nor their transfer
encoding.
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: person DOES NOT MATCH (wrong type)
objectClass: extensibleObject 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
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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
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=com
objectClass: person DOES NOT MATCH (wrong type)
objectClass: extensibleObject 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 each 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 contains
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each of the presented (and possibly other) language tag 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
AttributeDescriptions 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.
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 products
description;lang-en: software products
description;lang-de: Softwareprodukte
The server would return:
description: software products
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 attribute type or its
subtype and contains each of the presented (or possibly other)
language tag options, the noSuchAttributeType error will be returned.
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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
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 valid request:
dn: CN=John Smith,DC=example,DC=com
objectClass: residentialPerson
CN: John Smith
CN;lang-en: John Smith
SN: Smith
SN;lang-en: Smith
streetAddress: 1 University Street
streetAddress;lang-en-US: 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.
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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
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 in
Section 2.1 for convenience.
A language range option matches a language tag option if the 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.
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Examples of valid AttributeDescription containing language range
options:
givenName;lang-en-
CN;lang-
SN;lang-de-;lang-gem-
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
type which they allow to be 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: person DOES NOT MATCH (wrong type)
objectClass: extensibleObject 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".
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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.
3.2. Requested Attributes in Search
Clients can provide language range options in each
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
AttributeDescriptions 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:
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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" 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 1.3.6.1.4.1.4203.1.5.4
as a value of 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
1.3.6.1.4.1.4203.1.5.5 as a value of the "supportedFeatures"
[RFC3674] 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. Interoperability with Non-supporting Implementations
Implementators of this specification should take care that their use
of language tag options does not impede proper function of
implementations which do not support language tags.
Per RFC 2251, "an AttributeDescription with one or more options is
treated as a subtype of the attribute type without any options." A
non-supporting server will treat an AttributeDescription with any
language tag options as an unrecognized attribute type. A non-
supporting client will either do the same, or will treat the
AttributeDescription as it would any other unknown subtype.
Typically, non-supporting clients simply ignore unrecognized subtypes
(and unrecognized attribute types) of attributes they request.
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To ensure proper function of non-supporting clients, supporting
clients SHOULD ensure that entries they populate with tagged values
are also populated with non-tagged values.
Additionally, supporting clients SHOULD be prepared to handle entries
which are not populated with tagged values.
6. 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 [RFC3377].
7. IANA Considerations
Registration of these protocol mechanisms [RFC3383] has been
completed by the IANA.
Subject: Request for LDAP Protocol Mechanism Registration
Object Identifier: 1.3.6.1.4.1.4203.1.5.4
Description: Language Tag Options
Object Identifier: 1.3.6.1.4.1.4203.1.5.5
Description: Language Range Options
Person & email address to contact for further information:
Kurt Zeilenga
Usage: Feature
Specification: RFC 3866
Author/Change Controller: IESG
Comments: none
These OIDs were assigned [ASSIGN] by OpenLDAP Foundation, under its
IANA-assigned private enterprise allocation [PRIVATE], for use in
this specification.
8. 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 also borrows from a number of IETF documents including
BCP 47 by H. Alvestrand.
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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.
[RFC2234] Crocker, D., Ed. and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", RFC 2234, November 1997.
[RFC2251] Wahl, M., Howes, T. and S. Kille, "Lightweight
Directory Access Protocol (v3)", RFC 2251, December
1997.
[RFC3066] Alvestrand, H., "Tags for the Identification of
Languages", BCP 47, RFC 3066, January 2001.
[RFC3377] Hodges, J. and R. Morgan, "Lightweight Directory Access
Protocol (v3): Technical Specification", RFC 3377,
September 2002.
[RFC3674] Zeilenga, K., "Feature Discovery in Lightweight
Directory Access Protocol (LDAP)", RFC 3674, December
2003.
[ASCII] Coded Character Set--7-bit American Standard Code for
Information Interchange, ANSI X3.4-1986.
9.2. Informative References
[X.501] International Telecommunication Union -
Telecommunication Standardization Sector, "The
Directory -- Models," X.501(1997).
[RFC3383] Zeilenga, K., "Internet Assigned Numbers Authority
(IANA) Considerations for Lightweight Directory Access
Protocol (LDAP)", BCP 64, RFC 3383, September 2002.
[ASSIGN] OpenLDAP Foundation, "OpenLDAP OID Delegations",
http://www.openldap.org/foundation/oid-delegate.txt.
[PRIVATE] IANA, "Private Enterprise Numbers",
http://www.iana.org/assignments/enterprise-numbers.
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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 (new in this revision).
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.
Editor's Address
Kurt D. Zeilenga
OpenLDAP Foundation
EMail: Kurt@OpenLDAP.org
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Full Copyright Statement
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except as set forth therein, the authors retain all their rights.
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