Network Working Group K. Zeilenga

Network Working Group K. Zeilenga
Request for Comments: 4521 OpenLDAP Foundation
BCP: 118 June 2006
Category: Best Current Practice

Considerations for
Lightweight Directory Access Protocol (LDAP) Extensions

Status of This Memo

This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2006).

Abstract

The Lightweight Directory Access Protocol (LDAP) is extensible. It
provides mechanisms for adding new operations, extending existing
operations, and expanding user and system schemas. This document
discusses considerations for designers of LDAP extensions.

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Table of Contents

1. Introduction ....................................................3
1.1. Terminology ................................................3
2. General Considerations ..........................................4
2.1. Scope of Extension .........................................4
2.2. Interaction between extensions .............................4
2.3. Discovery Mechanism ........................................4
2.4. Internationalization Considerations ........................5
2.5. Use of the Basic Encoding Rules ............................5
2.6. Use of Formal Languages ....................................5
2.7. Examples ...................................................5
2.8. Registration of Protocol Values ............................5
3. LDAP Operation Extensions .......................................6
3.1. Controls ...................................................6
3.1.1. Extending Bind Operation with Controls ..............6
3.1.2. Extending the Start TLS Operation with Controls .....7
3.1.3. Extending the Search Operation with Controls ........7
3.1.4. Extending the Update Operations with Controls .......8
3.1.5. Extending the Responseless Operations with Controls..8
3.2. Extended Operations ........................................8
3.3. Intermediate Responses .....................................8
3.4. Unsolicited Notifications ..................................9
4. Extending the LDAP ASN.1 Definition .............................9
4.1. Result Codes ...............................................9
4.2. LDAP Message Types .........................................9
4.3. Authentication Methods ....................................10
4.4. General ASN.1 Extensibility ...............................10
5. Schema Extensions ..............................................10
5.1. LDAP Syntaxes .............................................11
5.2. Matching Rules ............................................11
5.3. Attribute Types ...........................................12
5.4. Object Classes ............................................12
6. Other Extension Mechanisms .....................................12
6.1. Attribute Description Options .............................12
6.2. Authorization Identities ..................................12
6.3. LDAP URL Extensions .......................................12
7. Security Considerations ........................................12
8. Acknowledgements ...............................................13
9. References .....................................................13
9.1. Normative References ......................................13
9.2. Informative References ....................................15

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1. Introduction

The Lightweight Directory Access Protocol (LDAP) [RFC4510] is an
extensible protocol.

LDAP allows for new operations to be added and for existing
operations to be enhanced [RFC4511].

LDAP allows additional schema to be defined [RFC4512][RFC4517]. This
can include additional object classes, attribute types, matching
rules, additional syntaxes, and other elements of schema. LDAP
provides an ability to extend attribute types with options [RFC4512].

LDAP supports a Simple Authentication and Security Layer (SASL)
authentication method [RFC4511][RFC4513]. SASL [RFC4422] is
extensible. LDAP may be extended to support additional
authentication methods [RFC4511].

LDAP supports establishment of Transport Layer Security (TLS)
[RFC4511][RFC4513]. TLS [RFC4346] is extensible.

LDAP has an extensible Uniform Resource Locator (URL) format
[RFC4516].

Lastly, LDAP allows for certain extensions to the protocol's Abstract
Syntax Notation - One (ASN.1) [X.680] definition to be made. This
facilitates a wide range of protocol enhancements, for example, new
result codes needed to support extensions to be added through
extension of the protocol's ASN.1 definition.

This document describes practices that engineers should consider when
designing extensions to LDAP.

1.1. Terminology

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]. In
this document, "the specification", as used by BCP 14, RFC 2119,
refers to the engineering of LDAP extensions.

The term "Request Control" refers to a control attached to a client-
generated message sent to a server. The term "Response Control"
refers to a control attached to a server-generated message sent to a
client.

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DIT stands for Directory Information Tree.
DSA stands for Directory System Agent, a server.
DSE stands for DSA-Specific Entry.
DUA stands for Directory User Agent, a client.
DN stands for Distinguished Name.

2. General Considerations

2.1. Scope of Extension

Mutually agreeing peers may, within the confines of an extension,
agree to significant changes in protocol semantics. However,
designers MUST consider the impact of an extension upon protocol
peers that have not agreed to implement or otherwise recognize and
support the extension. Extensions MUST be "truly optional"
[RFC2119].

2.2. Interaction between extensions

Designers SHOULD consider how extensions they engineer interact with
other extensions.

Designers SHOULD consider the extensibility of extensions they
specify. Extensions to LDAP SHOULD themselves be extensible.

Except where it is stated otherwise, extensibility is implied.

2.3. Discovery Mechanism

Extensions SHOULD provide adequate discovery mechanisms.

As LDAP design is based upon the client-request/server-response
paradigm, the general discovery approach is for the client to
discover the capabilities of the server before utilizing a particular
extension. Commonly, this discovery involves querying the root DSE
and/or other DSEs for operational information associated with the
extension. LDAP provides no mechanism for a server to discover the
capabilities of a client.

The 'supportedControl' attribute [RFC4512] is used to advertise
supported controls. The 'supportedExtension' attribute [RFC4512] is
used to advertise supported extended operations. The
'supportedFeatures' attribute [RFC4512] is used to advertise
features. Other root DSE attributes MAY be defined to advertise
other capabilities.

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2.4. Internationalization Considerations

LDAP is designed to support the full Unicode [Unicode] repertory of
characters. Extensions SHOULD avoid unnecessarily restricting
applications to subsets of Unicode (e.g., Basic Multilingual Plane,
ISO 8859-1, ASCII, Printable String).

LDAP Language Tag options [RFC3866] provide a mechanism for tagging
text (and other) values with language information. Extensions that
define attribute types SHOULD allow use of language tags with these
attributes.

2.5. Use of the Basic Encoding Rules

Numerous elements of LDAP are described using ASN.1 [X.680] and are
encoded using a particular subset [Protocol, Section 5.2] of the
Basic Encoding Rules (BER) [X.690]. To allow reuse of
parsers/generators used in implementing the LDAP "core" technical
specification [RFC4510], it is RECOMMENDED that extension elements
(e.g., extension specific contents of controlValue, requestValue,
responseValue fields) described by ASN.1 and encoded using BER be
subjected to the restrictions of [Protocol, Section 5.2].

2.6. Use of Formal Languages

Formal languages SHOULD be used in specifications in accordance with
IESG guidelines [FORMAL].

2.7. Examples

Example DN strings SHOULD conform to the syntax defined in [RFC4518].
Example LDAP filter strings SHOULD conform to the syntax defined in
[RFC4515]. Example LDAP URLs SHOULD conform to the syntax defined in
[RFC4516]. Entries SHOULD be represented using LDIF [RFC2849].

2.8. Registration of Protocol Values

Designers SHALL register protocol values of their LDAP extensions in
accordance with BCP 64, RFC 4520 [RFC4520]. Specifications that
create new extensible protocol elements SHALL extend existing
registries or establish new registries for values of these elements
in accordance with BCP 64, RFC 4520 [RFC4520] and BCP 26, RFC 2434
[RFC2434].

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3. LDAP Operation Extensions

Extensions SHOULD use controls in defining extensions that complement
existing operations. Where the extension to be defined does not
complement an existing operation, designers SHOULD consider defining
an extended operation instead.

For example, a subtree delete operation could be designed as either
an extension of the delete operation or as a new operation. As the
feature complements the existing delete operation, use of the control
mechanism to extend the delete operation is likely more appropriate.

As a counter (and contrived) example, a locate services operation (an
operation that would return for a DN a set of LDAP URLs to services
that may hold the entry named by this DN) could be designed as either
a search operation or a new operation. As the feature doesn't
complement the search operation (e.g., the operation is not contrived
to search for entries held in the Directory Information Tree), it is
likely more appropriate to define a new operation using the extended
operation mechanism.

3.1. Controls

Controls [Protocol, Section 4.1.11] are the RECOMMENDED mechanism for
extending existing operations. The existing operation can be a base
operation defined in [RFC4511] (e.g., search, modify) , an extended
operation (e.g., Start TLS [RFC4511], Password Modify [RFC3062]), or
an operation defined as an extension to a base or extended operation.

Extensions SHOULD NOT return Response controls unless the server has
specific knowledge that the client can make use of the control.
Generally, the client requests the return of a particular response
control by providing a related request control.

An existing operation MAY be extended to return IntermediateResponse
messages [Protocol, Section 4.13].

Specifications of controls SHALL NOT attach additional semantics to
the criticality of controls beyond those defined in [Protocol,
Section 4.1.11]. A specification MAY mandate the criticality take on
a particular value (e.g., TRUE or FALSE), where appropriate.

3.1.1. Extending Bind Operation with Controls

Controls attached to the request and response messages of a Bind
Operation [RFC4511] are not protected by any security layers
established by that Bind operation.

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Specifications detailing controls extending the Bind operation SHALL
detail that the Bind negotiated security layers do not protect the
information contained in these controls and SHALL detail how the
information in these controls is protected or why the information
does not need protection.

It is RECOMMENDED that designers consider alternative mechanisms for
providing the function. For example, an extended operation issued
subsequent to the Bind operation (hence, protected by the security
layers negotiated by the Bind operation) might be used to provide the
desired function.

Additionally, designers of Bind control extensions MUST also consider
how the controls' semantics interact with individual steps of a
multi-step Bind operation. Note that some steps are optional and
thus may require special attention in the design.

3.1.2. Extending the Start TLS Operation with Controls

Controls attached to the request and response messages of a Start TLS
Operation [RFC4511] are not protected by the security layers
established by the Start TLS operation.

Specifications detailing controls extending the Start TLS operation
SHALL detail that the Start TLS negotiated security layers do not
protect the information contained in these controls and SHALL detail
how the information in these controls is protected or why the
information does not need protection.

It is RECOMMENDED that designers consider alternative mechanisms for
providing the function. For example, an extended operation issued
subsequent to the Start TLS operation (hence, protected by the
security layers negotiated by the Start TLS operation) might be used
to provided the desired function.

3.1.3. Extending the Search Operation with Controls

The Search operation processing has two distinct phases:

- finding the base object; and

- searching for objects at or under that base object.

Specifications of controls extending the Search Operation should
clearly state in which phase(s) the control's semantics apply.
Semantics of controls that are not specific to the Search Operation
SHOULD apply in the finding phase.

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3.1.4. Extending the Update Operations with Controls

Update operations have properties of atomicity, consistency,
isolation, and durability ([ACID]).

- atomicity: All or none of the DIT changes requested are made.

- consistency: The resulting DIT state must be conform to schema
and other constraints.

- isolation: Intermediate states are not exposed.

- durability: The resulting DIT state is preserved until
subsequently updated.

When defining a control that requests additional (or other) DIT
changes be made to the DIT, these additional changes SHOULD NOT be
treated as part of a separate transaction. The specification MUST be
clear as to whether the additional DIT changes are part of the same
or a separate transaction as the DIT changes expressed in the request
of the base operation.

When defining a control that requests additional (or other) DIT
changes be made to the DIT, the specification MUST be clear as to the
order in which these and the base changes are to be applied to the
DIT.

3.1.5. Extending the Responseless Operations with Controls

The Abandon and Unbind operations do not include a response message.
For this reason, specifications for controls designed to be attached
to Abandon and Unbind requests SHOULD mandate that the control's
criticality be FALSE.

3.2. Extended Operations

Extended Operations [Protocol, Section 4.12] are the RECOMMENDED
mechanism for defining new operations. An extended operation
consists of an ExtendedRequest message, zero or more
IntermediateResponse messages, and an ExtendedResponse message.

3.3. Intermediate Responses

Extensions SHALL use IntermediateResponse messages instead of
ExtendedResponse messages to return intermediate results.

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3.4. Unsolicited Notifications

Unsolicited notifications [Protocol, Section 4.4] offer a capability
for the server to notify the client of events not associated with the
operation currently being processed.

Extensions SHOULD be designed such that unsolicited notifications are
not returned unless the server has specific knowledge that the client
can make use of the notification. Generally, the client requests the
return of a particular unsolicited notification by performing a
related extended operation.

For example, a time hack extension could be designed to return
unsolicited notifications at regular intervals that were enabled by
an extended operation (which possibly specified the desired
interval).

4. Extending the LDAP ASN.1 Definition

LDAP allows limited extension [Protocol, Section 4] of the LDAP ASN.1
definition [Protocol, Appendix B] to be made.

4.1. Result Codes

Extensions that specify new operations or enhance existing operations
often need to define new result codes. The extension SHOULD be
designed such that a client has a reasonably clear indication of the
nature of the successful or non-successful result.

Extensions SHOULD use existing result codes to indicate conditions
that are consistent with the intended meaning [RFC4511][X.511] of
these codes. Extensions MAY introduce new result codes [RFC4520]
where no existing result code provides an adequate indication of the
nature of the result.

Extensions SHALL NOT disallow or otherwise restrict the return of
general service result codes, especially those reporting a protocol,
service, or security problem, or indicating that the server is unable
or unwilling to complete the operation.

4.2. LDAP Message Types

While extensions can specify new types of LDAP messages by extending
the protocolOp CHOICE of the LDAPMessage SEQUENCE, this is generally
unnecessary and inappropriate. Existing operation extension
mechanisms (e.g., extended operations, unsolicited notifications, and
intermediate responses) SHOULD be used instead. However, there may
be cases where an extension does not fit well into these mechanisms.

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In such cases, a new extension mechanism SHOULD be defined that can
be used by multiple extensions that have similar needs.

4.3. Authentication Methods

The Bind operation currently supports two authentication methods,
simple and SASL. SASL [RFC4422] is an extensible authentication
framework used by multiple application-level protocols (e.g., BEEP,
IMAP, SMTP). It is RECOMMENDED that new authentication processes be
defined as SASL mechanisms. New LDAP authentication methods MAY be
added to support new authentication frameworks.

The Bind operation's primary function is to establish the LDAP
association [RFC4513]. No other operation SHALL be defined (or
extended) to establish the LDAP association. However, other
operations MAY be defined to establish other security associations
(e.g., IPsec).

4.4. General ASN.1 Extensibility

Section 4 of [RFC4511] states the following:

In order to support future extensions to this protocol,
extensibility is implied where it is allowed per ASN.1 (i.e.,
sequence, set, choice, and enumerated types are extensible). In
addition, ellipses (...) have been supplied in ASN.1 types that
are explicitly extensible as discussed in [RFC4520]. Because of
the implied extensibility, clients and servers MUST (unless
otherwise specified) ignore trailing SEQUENCE components whose
tags they do not recognize.

Designers SHOULD avoid introducing extensions that rely on
unsuspecting implementations to ignore trailing components of
SEQUENCE whose tags they do not recognize.

5. Schema Extensions

Extensions defining LDAP schema elements SHALL provide schema
definitions conforming with syntaxes defined in [Models, Section
4.1]. While provided definitions MAY be reformatted (line wrapped)
for readability, this SHALL be noted in the specification.

For definitions that allow a NAME field, new schema elements SHOULD
provide one and only one name. The name SHOULD be short.

Each schema definition allows a DESC field. The DESC field, if
provided, SHOULD contain a short descriptive phrase. The DESC field
MUST be regarded as informational. That is, the specification MUST

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be written such that its interpretation is the same with and without
the provided DESC fields.

The extension SHALL NOT mandate that implementations provide the same
DESC field in the schema they publish. Implementors MAY replace or
remove the DESC field.

Published schema elements SHALL NOT be redefined. Replacement schema
elements (new OIDs, new NAMEs) SHOULD be defined as needed.

Schema designers SHOULD reuse existing schema elements, where
appropriate. However, any reuse MUST not alter the semantics of the
element.

5.1. LDAP Syntaxes

Each LDAP syntax [RFC4517] is defined in terms of ASN.1 [X.680].
Each extension detailing an LDAP syntax MUST specify the ASN.1 data
definition associated with the syntax. A distinct LDAP syntax SHOULD
be created for each distinct ASN.1 data definition (including
constraints).

Each LDAP syntax SHOULD have a string encoding defined for it. It is
RECOMMENDED that this string encoding be restricted to UTF-8
[RFC3629] encoded Unicode [Unicode] characters. Use of Generic
String Encoding Rules (GSER) [RFC3641][RFC3642] or other generic
string encoding rules to provide string encodings for complex ASN.1
data definitions is RECOMMENDED. Otherwise, it is RECOMMENDED that
the string encoding be described using a formal language (e.g., ABNF
[RFC4234]). Formal languages SHOULD be used in specifications in
accordance with IESG guidelines [FORMAL].

If no string encoding is defined, the extension SHALL specify how the
transfer encoding is to be indicated. Generally, the extension
SHOULD mandate use of binary or other transfer encoding option.

5.2. Matching Rules

Three basic kinds of matching rules (e.g., EQUALITY, ORDERING, and
SUBSTRING) may be associated with an attribute type. In addition,
LDAP provides an extensible matching rule mechanism.

The matching rule specification SHOULD detail which kind of matching
rule it is and SHOULD describe which kinds of values it can be used
with.

In addition to requirements stated in the LDAP technical
specification, equality matching rules SHOULD be commutative.

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5.3. Attribute Types

Designers SHOULD carefully consider how the structure of values is to
be restricted. Designers SHOULD consider that servers will only
enforce constraints of the attribute's syntax. That is, an attribute
intended to hold URIs, but that has directoryString syntax, is not
restricted to values that are URIs.

Designers SHOULD carefully consider which matching rules, if any, are
appropriate for the attribute type. Matching rules specified for an
attribute type MUST be compatible with the attribute type's syntax.

Extensions specifying operational attributes MUST detail how servers
are to maintain and/or utilize values of each operational attribute.

5.4. Object Classes

Designers SHOULD carefully consider whether each attribute of an
object class is required ("MUST") or allowed ("MAY").

Extensions specifying object classes that allow (or require)
operational attributes MUST specify how servers are to maintain
and/or utilize entries belonging to these object classes.

6. Other Extension Mechanisms

6.1. Attribute Description Options

Each option is identified by a string of letters, numbers, and
hyphens. This string SHOULD be short.

6.2. Authorization Identities

Extensions interacting with authorization identities SHALL support
the LDAP authzId format [RFC4513]. The authzId format is extensible.

6.3. LDAP URL Extensions

LDAP URL extensions are identified by a short string, a descriptor.
Like other descriptors, the string SHOULD be short.

7. Security Considerations

LDAP does not place undue restrictions on the kinds of extensions
that can be implemented. While this document attempts to outline
some specific issues that designers need to consider, it is not (and

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RFC 4521 LDAP Extensions June 2006

cannot be) all encompassing. Designers MUST do their own evaluations
of the security considerations applicable to their extensions.

Designers MUST NOT assume that the LDAP "core" technical
specification [RFC4510] adequately addresses the specific concerns
surrounding their extensions or assume that their extensions have no
specific concerns.

Extension specifications, however, SHOULD note whether security
considerations specific to the feature they are extending, as well as
general LDAP security considerations, apply to the extension.

8. Acknowledgements

The author thanks the IETF LDAP community for their thoughtful
comments.

This work builds upon "LDAP Extension Style Guide" [GUIDE] by Bruce
Greenblatt.

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.

[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.

[RFC2849] Good, G., "The LDAP Data Interchange Format (LDIF) -
Technical Specification", RFC 2849, June 2000.

[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.

[RFC3641] Legg, S., "Generic String Encoding Rules (GSER) for ASN.1
Types", RFC 3641, October 2003.

[RFC3642] Legg, S., "Common Elements of Generic String Encoding
Rules (GSER) Encodings", RFC 3642, October 2003.

[RFC4512] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): Directory Information Models", RFC 4512, June
2006.

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[RFC3866] Zeilenga, K., Ed., "Language Tags and Ranges in the
Lightweight Directory Access Protocol (LDAP)", RFC 3866,
July 2004.

[RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.

[RFC4510] Zeilenga, K., Ed., "Lightweight Directory Access Protocol
(LDAP): Technical Specification Road Map", RFC 4510, June
2006.

[RFC4511] Sermersheim, J., Ed., "Lightweight Directory Access
Protocol (LDAP): The Protocol", RFC 4511, June 2006.

[RFC4512] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): Directory Information Models", RFC 4512, June
2006.

[RFC4513] Harrison, R., Ed., "Lightweight Directory Access Protocol
(LDAP): Authentication Methods and Security Mechanisms",
RFC 4513, June 2006.

[RFC4515] Smith, M., Ed. and T. Howes, "Lightweight Directory Access
Protocol (LDAP): String Representation of Search Filters",
RFC 4515, June 2006.

[RFC4516] Smith, M., Ed. and T. Howes, "Lightweight Directory Access
Protocol (LDAP): Uniform Resource Locator", RFC 4516, June
2006.

[RFC4517] Legg, S., Ed., "Lightweight Directory Access Protocol
(LDAP): Syntaxes and Matching Rules", RFC 4517, June 2006.

[RFC4518] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): String Representation of Distinguished Names", RFC
4518, June 2006.

[RFC4520] Zeilenga, K., "Internet Assigned Numbers Authority (IANA)
Considerations for the Lightweight Directory Access
Protocol (LDAP)", BCP 64, RFC 4520, June 2006.

[RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422, June
2006.

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[Unicode] The Unicode Consortium, "The Unicode Standard, Version
3.2.0" is defined by "The Unicode Standard, Version 3.0"
(Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5),
as amended by the "Unicode Standard Annex #27: Unicode
3.1" (http://www.unicode.org/reports/tr27/) and by the
"Unicode Standard Annex #28: Unicode 3.2"
(http://www.unicode.org/reports/tr28/).

[FORMAL] IESG, "Guidelines for the use of formal languages in IETF
specifications",
<http://www.ietf.org/IESG/STATEMENTS/pseudo-code-in-
specs.txt>, 2001.

[X.511] International Telecommunication Union - Telecommunication
Standardization Sector, "The Directory: Abstract Service
Definition", X.511(1993) (also ISO/IEC 9594-3:1993).

[X.680] International Telecommunication Union - Telecommunication
Standardization Sector, "Abstract Syntax Notation One
(ASN.1) - Specification of Basic Notation", X.680(2002)
(also ISO/IEC 8824-1:2002).

[X.690] International Telecommunication Union - Telecommunication
Standardization Sector, "Specification of ASN.1 encoding
rules: Basic Encoding Rules (BER), Canonical Encoding
Rules (CER), and Distinguished Encoding Rules (DER)",
X.690(2002) (also ISO/IEC 8825-1:2002).

9.2. Informative References

[ACID] Section 4 of ISO/IEC 10026-1:1992.

[GUIDE] Greenblatt, B., "LDAP Extension Style Guide", Work in
Progress.

[RFC3062] Zeilenga, K., "LDAP Password Modify Extended Operation",
RFC 3062, February 2001.

[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.

Author's Address

Kurt D. Zeilenga
OpenLDAP Foundation

EMail: [email protected]

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RFC 4521 LDAP Extensions June 2006

Full Copyright Statement

Copyright (C) The Internet Society (2006).

This document is subject to the rights, licenses and restrictions
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