RFC2596 - Use of Language Codes in LDAP

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　　Network Working Group M. Wahl
Request for Comments: 2596 Innosoft International, Inc.
Category: Standards Track T. Howes
Netscape Communications Corp.
May 1999

Use of Language Codes in 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 (1999). All Rights Reserved.

1. Abstract

The Lightweight Directory Access Protocol [1] 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 [2] 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 codes [3] are carried in LDAP
and are to be interpreted by LDAP servers. All implementations MUST
be prepared to accept language codes in the LDAP protocols. Servers
may or may not be capable of storing attributes with language codes
in the directory. This document does not specify how to determine
whether particular attributes can or cannot have language codes.

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 RFC2119 [4].

2. Language Codes

Section 2 of RFC1766 [3] describes the language code 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 codes are case insensitive. For example, the language code
"en-us" is the same as "EN-US" and "en-US".

Implementations MUST NOT otherwise interpret the structure of the
code when comparing two codes, and MUST treat them as simply strings
of characters. Client and server implementations MUST allow any
arbitrary string which follows the patterns given in RFC1766 to be
used as a language code.

3. Use of Language Codes in LDAP

This section describes how LDAP implementations MUST interpret
language codes in performing operations.

In general, an attribute with a language code is to be treated as a
subtype of the attribute without a language code. If a server does
not support storing language codes with attribute values in the DIT,
then it MUST always treat an attribute with a language code as an
unrecognized attribute.

3.1. Attribute Description

An attribute consists of a type, a list of options for that type, and
a set of one or more values. In LDAP, the type and the options are
combined into the AttributeDescription, defined in section 4.1.5 of
[1]. This is represented as an attribute type name and a possibly-
empty list of options. One of these options associates a natural
language with values for that attribute.

language-option = "lang-" lang-code

lang-code = printable-ascii ; a code as defined in RFC1766

Multiple language options may be present on a particular value.

The language code has no effect on the character set encoding for
string representations of DirectoryString syntax values; the UTF-8
representation of UniversalString (ISO 10646) is always used.

Examples of valid AttributeDescription:
givenName;lang-en-US
CN;lang-ja

In LDAP and in examples in this document, a directory attribute is
represented as an AttributeDescription with a list of values. Note
that the data could be stored in the LDAP server in a different
representation.

3.2. Distinguished Names and Relative Distinguished Names

No attribute description options are permitted in Distinguished Names
or Relative Distinguished Names. Thus language codes MUST NOT be
used in forming DNs.

3.3. Search Filter

If a language code is present in an AttributeDescription in a search
filter, then only attribute values in the directory which match the
base attribute type or its subtype, the language code and the
assertion value match this filter.

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

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;dynamic: Billy Ray MATCHES
CN;lang-en;dynamic: Billy Ray DOES NOT MATCH (differing lang-)
name: Billy Ray DOES NOT MATCH (no lang-)
SN: Ray DOES NOT MATCH (wrong value)

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

Client implementors should however note that providing a language
code in a search filter AttributeDescription will often filter out
desirable values where the language code 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 language codes with attribute
values in the DIT, then any filter which includes a language code
will always fail to 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 forms to Undefined.

If no language code is specified in the search filter, 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

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;dynamic: Billy Ray MATCHES
CN;lang-en;dynamic: Billy Ray MATCHES
name: Billy Ray MATCHES
SN: Ray DOES NOT MATCH (wrong value)

Thus in general, clients SHOULD NOT use the language code option in
AttributeDescription fields in search filters.

3.4. Compare

A language code 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 codes in a search filter with
an equality match, as described in the previous section. If there is
no attribute in the entry with the same subtype and language code,
the noSuchAttributeType error will be returned.

Thus for example a compare request of type "name" and assertion value
"Johann", against an entry with all the following directory entry

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

will 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 language codes with attribute
values in the DIT, then any comparison which includes a language code
will always fail to locate an attribute type, and noSuchAttributeType
will be returned.

Thus in general, clients SHOULD NOT use the language code option in
AttributeDescription fields in the compare request.

3.5. Requested Attributes in Search

Clients MAY provide language codes in AttributeDescription in the
requested attribute list in a search request.

If a language code is provided in an attribute description, then only
attribute values in a directory entry which have the same language
code as that provided are to be returned. Thus if a client requests
an attribute "description;lang-en", the server MUST NOT return values
of an attribute "description" or "description;lang-fr".

Clients MAY provide in the attribute list multiple
AttributeDescription which have the same base attribute type but
different options. For example a client MAY provide both "name;lang-
en" and "name;lang-fr", and this would permit an attribute with
either language code 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 language codes with attribute
values in the DIT, then any attribute descriptions in the list which
include language codes 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 code, then all attribute
values regardless of their language code are returned.

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

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 will return:

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

3.6. Add Operation

Clients MAY provide language codes in AttributeDescription in
attributes of a new entry to be created, subject to the limitation
that the client MUST NOT use language codes in the attribute value or
values which form the RDN of the entry.

A client MAY provide multiple attributes with the same attribute type
and value, so long as each attribute has a different language code,
and at most one attribute does not have a language code option.

Servers which support storing language codes in the DIT MUST allow
any attribute it recognizes that has the Directory String syntax to
have a language option associated with it. Servers SHOULD allow
language options to be associated with other attributes.

For example, the following is a legal request.

objectclass: top
objectclass: person
objectclass: residentialPerson
name: John Smith
CN: John Smith
CN;lang-en: John Smith
SN: 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 codes with attribute
values in the DIT, then it MUST treat an AttributeDescription with a
language code as an unrecognized attribute. If the server forbids the
addition of unrecognized attributes then it MUST fail the add request
with the appropriate result code.

3.7. Modify Operation

A client MAY provide a language code in an AttributeDescription as
part of a modification element in the modify operation.

Attribute types and language codes 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 a language
code, the language code MUST be provided in the modify operation, and
if the stored values to be deleted do not have a language code, then
no language code is to be provided.

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

3.8. Diagnostic Messages

Servers SHOULD use only printable ASCII characters in the
errorMessage field, as not all clients will be able to display the
full range of Unicode.

4. Differences from X.500(1997)

X.500(1997) defines a different mechanism, contexts, as the means of
representing language tags. 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 RFC1766, which allows for IANA registration of
new tags.
c) LDAP does not allow language codes in distinguished names.
d) X.500 describes subschema administration procedures to allow
language codes to be associated with particular attributes types.

5. Security Considerations

There are no known security considerations for this document. See
the security considerations sections of [1] and [2] for security
considerations of LDAP in general.

6. Acknowledgements

This document is a product of the IETF ASID and LDAPEXT working
groups. Martin Duerst provided many valuable comments on an earlier
version of this document.

7. Bibliography

[1] Wahl, M., Howes, T. and S. Kille, "Lightweight Directory Access
Protocol (v3)", RFC2251, December 1997.

[2] Wahl, M., Coulbeck, A., Howes, T. and S. Kille, "Lightweight
X.500 Directory Access Protocol Attribute Syntax Definitions",
RFC2252, December 1997.

[3] Alvestrand, H.,"Tags for the Identification of Languages", RFC
1766, March 1995.

[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.

8. Authors' Addresses

Mark Wahl
Innosoft International, Inc.
8911 Capital of Texas Hwy Suite 4140
Austin, TX 78759 USA

EMail: M.Wahl@innosoft.com

Tim Howes
Netscape Communications Corp.
501 E. Middlefield Rd
Mountain View, CA 94043 USA

Phone: +1 650 937-3419
EMail: howes@netscape.com

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