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.


























Zeilenga                 Best Current Practice                  [Page 1]

RFC 4521                    LDAP Extensions                    June 2006


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









Zeilenga                 Best Current Practice                  [Page 2]

RFC 4521                    LDAP Extensions                    June 2006


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.






Zeilenga                 Best Current Practice                  [Page 3]

RFC 4521                    LDAP Extensions                    June 2006


   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.






Zeilenga                 Best Current Practice                  [Page 4]

RFC 4521                    LDAP Extensions                    June 2006


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].








Zeilenga                 Best Current Practice                  [Page 5]

RFC 4521                    LDAP Extensions                    June 2006


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.




Zeilenga                 Best Current Practice                  [Page 6]

RFC 4521                    LDAP Extensions                    June 2006


   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.




Zeilenga                 Best Current Practice                  [Page 7]

RFC 4521                    LDAP Extensions                    June 2006


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.






Zeilenga                 Best Current Practice                  [Page 8]

RFC 4521                    LDAP Extensions                    June 2006


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.



Zeilenga                 Best Current Practice                  [Page 9]

RFC 4521                    LDAP Extensions                    June 2006


   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



Zeilenga                 Best Current Practice                 [Page 10]

RFC 4521                    LDAP Extensions                    June 2006


   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.



Zeilenga                 Best Current Practice                 [Page 11]

RFC 4521                    LDAP Extensions                    June 2006


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





Zeilenga                 Best Current Practice                 [Page 12]

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.





Zeilenga                 Best Current Practice                 [Page 13]

RFC 4521                    LDAP Extensions                    June 2006


   [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.







Zeilenga                 Best Current Practice                 [Page 14]

RFC 4521                    LDAP Extensions                    June 2006


   [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: Kurt@OpenLDAP.org




Zeilenga                 Best Current Practice                 [Page 15]

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
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM 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.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.

Acknowledgement

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).







Zeilenga                 Best Current Practice                 [Page 16]