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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>.
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>.
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).
Zeilenga Language Tags and Ranges in LDAP [Page 1]
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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
Zeilenga Language Tags and Ranges in LDAP [Page 3]
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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
Zeilenga Language Tags and Ranges in LDAP [Page 5]
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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
Zeilenga Language Tags and Ranges in LDAP [Page 7]
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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
Zeilenga Language Tags and Ranges in LDAP [Page 8]
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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"
Zeilenga Language Tags and Ranges in LDAP [Page 11]
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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
Zeilenga Language Tags and Ranges in LDAP [Page 12]
INTERNET-DRAFT draft-zeilenga-ldap-rfc2596-01.txt 1 March 2002
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,
provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be followed,
or as required to translate it into languages other than English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE AUTHORS, THE INTERNET SOCIETY, AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Zeilenga Language Tags and Ranges in LDAP [Page 14]
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