INTERNET-DRAFT Kurt D. Zeilenga
Intended Category: BCP OpenLDAP Foundation
Expires in six months 4 November 2002
Considerations for LDAP Extensions
<draft-zeilenga-ldap-ext-02.txt>
Status of this 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 Best Current Practices
document. Distribution of this memo is unlimited. Technical
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Copyright 2002, The Internet Society. All Rights Reserved.
Please see the Copyright section near the end of this document for
more information.
Abstract
The Lightweight Directory Access Protocol (LDAP) is extensible. It
provides mechanisms for adding new operations, extending existing
operations, and expanding system schema. This document discusses
considerations for designers of LDAP extensions.
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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]. In
this case, "the specification" as used by BCP 14 refers to the
processing of protocols being submitted to the IETF standards process.
The term "Request Control" refers to a control attached to client
generated message sent to a server. The term "Response Control"
refers to a control attached to server generated message sent to a
client.
1. Background and Intended Use
The Lightweight Directory Access Protocol (LDAP) [RFC3377] is an
extensible protocol.
LDAP allows for new operations to be added and existing operations to
be enhanced [RFC2251].
LDAP allows additional schema to be defined [RFC2252]. This can
include additional object classes, attribute types, matching rules,
additional syntaxes, and other elements of schema.
LDAP supports a Simple Authentication and Security Layer (SASL)
authentication method [RFC2829]. SASL [RFC2222] is extensible.
LDAP supports establishment of Transport Layer Security (TLS)
[RFC2830]. TLS [RFC2246] is extensible.
LDAP provides an ability to extend attribute types with options
[RFC2251]. LDAP defines one option, ;binary). Other options can be
defined in other documents.
LDAP has an extensible URL format [RFC2255].
Lastly, LDAP allows for certain extensions to the protocol's ASN.1
[X.680] definition to be made. This allows a wide range of changes.
For example, new result codes needed to support extensions to be
added.
This document describes practices which engineers MUST consider when
designing extensions to LDAP.
2. General Considerations
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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
which have not agreed to implement, or otherwise recognize and
support, the extension.
2.2. Discovery Mechanism
Extensions SHOULD provide an adequate discover mechanism.
Generally, a client discovers the capabilities of the server by
querying the root DSE and/or other DSEs containing operational
information (such as subschema subentries).
The 'supportedControl' attribute is used to advertised supported
controls. The 'supportedExtension' attribute is used to advertised
supported extended operations. The 'supportedFeatures' is used to
advertise features [FEATURES]. Other root DSE attributes MAY be
defined to advertise other capabilities.
As LDAP design is based upon the client-request/server-response
paradigm, no mechanism is provided for a server to discover the
capabilities of a client.
2.3. Interaction between extensions
Designers SHOULD consider how extensions they engineer interaction
with other extensions.
Designers SHOULD consider extensibility of extensions they specify.
Extensions to LDAP SHOULD themselves be extensible.
2.4. Internationalization Considerations
LDAP is designed to support the full ISO 10646-1 (a superset of
Unicode) Universal Character Set (UCS) [ISOUCS]. Extensions SHOULD
avoid unnecessarily restricting applications to subsets of UCS (e.g.,
Basic Multilingual Plane, ISO 8859-1, ASCII, Printable String).
LDAP Language Tag Options [RFC2596] provides a mechanism for tagging
text (and other) values with language information. Extensions which
define attribute types SHOULD allow use of language tags with these
attributes.
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2.5. IANA Considerations
Designers SHALL register protocol values of their LDAP extensions in
accordance with BCP 64 [RFC3383]. Specifications which create new
extensible protocol elements SHALL establish registries for values of
these elements in accordance with BCP 26 [RFC2434].
2.6. Use of Formal Languages
LDAP is described in terms of ASN.1 [X.680] and encoded using a
particular subset [RFC2251, Section 5.1] of the Basic Encoding Rules
[X.690]. To allow reuse of parsers/generators used in implementing
the LDAP "core" technical specification [RFC3377], it is generally
RECOMMENDED that protocol elements (e.g. extension specific contains
of controlValue, requestValue, response fields, and value syntax)
specified by extensions be described and encoded similarly.
Formal languages SHOULD be used in specifications in accordance with
IESG guidelines [FORMAL].
3. LDAP Operation Extensions
Extensions SHOULD use controls to define extensions which complement
existing operations. Where the extension to be defined does not
complement the 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 [LOCATE]
operation (an operation which would return for a DN a set of LDAP URLs
which hold this DN) could be designed as either a search operation or
a new operation. As the feature doesn't complement the search
operation (the operation is not searching for information in the DIT,
but for information in the Domain Name System), use of the extended
operation mechanism to define a new operation is likely more
appropriate.
3.1. Controls
Controls [RFC2251, Section 4.1.12] is the RECOMMENDED mechanism for
extending existing operations. The existing operation can be a base
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operation defined in RFC 2251 (e.g., search, modify, etc.) or an
extended operation (e.g. Start TLS, Password Modify), or a operation
defined as an extension to a base or extended operation.
Extensions SHOULD NOT return Response controls unless 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 [LDAPIMR].
Specifications of controls SHALL NOT attach additional semantics to
criticality of controls beyond those defined in RFC 2251, Section
4.1.12. A specification MAY mandate the criticality take on a
particular value (e.g., TRUE or FALSE) where appropriate.
3.2. Extended Operations
Extended Operations [RFC2251, Section 4.12] is the RECOMMENDED
mechanism for defining new operations. An extended operation consists
of an ExtendedRequest message, zero or more IntermediateResponse
messages [LDAPIMR], and an ExtendedResponse message.
3.3. Intermediate Responses
Extensions SHALL use Intermediate Responses [LDAPIMR] instead of
ExtendedResponse [RFC2251] messages to return partial results.
3.4. Unsolicited Notifications
Unsolicited notifications [RFC2251, Section 4.4] offer a capability
for the server to notify the client of events not associated with
operation currently being processed.
Extensions SHOULD be designed such that unsolicited notifications are
not be returned unless the server has specific knowledge that the
client can make use of the notification. Generally, the client
requests the return of 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 which were enabled by
an extended operation (which possibly specified the desired interval).
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4. Extending LDAP ASN.1 definition
LDAP allows limited extension of the LDAPv3 ASN.1 definition [RFC2251,
Appendix A] to made.
4.1. Result Codes
Extensions which 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
which are consistent with each codes intended meaning
[RFC2251][X.511]. Extensions MAY introduce new result codes where no
existing result code provides an adequate indication of the nature of
the result.
Extensions SHALL NOT disallow nor otherwise restrict the return of
general service result codes, especially those reporting a protocol,
service, or security problem, indicating 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 and unsolicited notifications)
SHOULD be used instead. However, there may be cases where an
extension does not fit well into these mechanisms. In such cases, a
new extension mechanism SHOULD be defined which can be used by
multiple extensions which have similar needs.
4.3. Authentication Methods
The Bind operation currently supports two authentication methods,
simple and SASL. SASL [RFC2222] is an extensible authentication
framework used by multiple application level protocols (e.g. BEEP,
SMTP, IMAP). 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 primary function is to establish the LDAP
association [RFC2829]. No other operation SHALL be defined (or
extended) to establish the LDAP association [RFC2829]. However, other
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operations MAY be defined to establish other security associations
(e.g., IPSEC).
4.4. Elements of SEQUENCE extensions
Section 4 of [RFC2251] states:
In order to support future extensions to this protocol, clients
and servers MUST ignore elements of SEQUENCE encodings whose tags
they do not recognize.
This mechanism is NOT RECOMMENDED. That is, designers SHOULD consider
other mechanisms before resorting inserting new elements in existing
SEQUENCE encodings.
5. Schema Extensions
Extensions defining LDAP schema elements SHALL provide schema
definitions conformant with syntaxes defined in [RFC2252].
Definitions provided MAY be reformatted (line wrapped) for readability
but this SHALL be noted in the extension specification.
For definitions 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
SHALL NOT contain information imperative to the implementor. That is,
the interpretation of the specification SHALL NOT be different if all
DESC fields were removed.
The extension SHALL NOT mandate that implementations provide the same
DESC field in schema they publish. Implementators 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.
5.1. LDAP Syntaxes
Each LDAP syntax 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).
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Each LDAP syntax SHOULD have a string encoding defined for it. It is
RECOMMENDED that this string encoding be restricted to a UTF-8 encoded
UCS characters. Use of general purpose string encoding rules (such as
Generic String Encoding Rules [GSER]) 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]). 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.
5.3. Attribute Types
Designers SHOULD carefully consider how the structure of values is to
restricted. Designers SHOULD consider that servers will only enforce
constraints of the attribute's syntax. That is, an attribute intended
to hold URIs, but which has directoryString syntax, is not restricted
to values which 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 attributes of an
object class is required ("MUST") or allowed ("MAY").
Extensions specifying object classes which allow (or require)
operational attributes MUST specify how servers are to maintain and/or
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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 hyphen.
This string SHOULD be short.
7. Other Considerations
7.1. Examples
Example DN strings SHOULD conform to the syntax defined in [RFC2253].
Example LDAP filter strings SHOULD conform to the syntax defined in
[RFC2254]. Example LDAP URLs SHOULD conform to the syntax defined in
[RFC2255]. Entries SHOULD be represented using LDIF [RFC2849].
7.2. Authorization Identities
Extensions interacting with authorization identities SHALL support the
LDAP authzId format [RFC2829, Section 9].
8. Security Considerations
LDAP is designed not to limit the kinds of extensions which can be
implemented. This section attempts to outline some specific issues
which designers need to consider, it is not (and cannot be) all
encompassing. Designers MUST do their own evaluation of the security
considerations applicable to their extension.
Designers MUST NOT the assume that the LDAP "core" technical
specification [RFC3377] adequately addresses the specific concerns
surrounding their extension or that their extension has no specific
concerns.
8.1 Extending Bind Operation with Controls
Controls attached to the request and response messages of a Bind
Operation [RFC2251][RFC2829] are not protected by the security layers
established by the Bind operation.
Specifications detailing controls extending the Bind operation SHALL
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detail that the Bind negotiated security layers do not protect the
information contained in these controls and SHALL detail why this
protection is not needed.
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 provided the
desired function.
8.2 Extending the Start TLS Operation with Controls
Controls attached to the request and response messages of a Start TLS
Operation [RFC2830] 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
why this protection is not needed.
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.
9. Acknowledgment
The author would like thank the thoughtful comments of members of the
IETF LDAPbis and LDAPext working groups.
This work builds upon "LDAP Extension Style Guide" [GUIDE] by Bruce
Greenblatt.
Some text was borrowed from "Guidelines for Writing an IANA
Considerations Section in RFCs" [RFC2434] by Thomas Narten and Harald
Alvestrand.
10. Author's Address
Kurt D. Zeilenga
OpenLDAP Foundation
<Kurt@OpenLDAP.org>
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11. References
11.1. Normative References
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14 (also RFC 2119), March 1997.
[RFC2222] J. Myers, "Simple Authentication and Security Layer
(SASL)", RFC 2222, October 1997.
[RFC2234] D. Crocker, P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[RFC2251] M. Wahl, T. Howes, S. Kille, "Lightweight Directory Access
Protocol (v3)", RFC 2251, December 1997.
[RFC2252] M. Wahl, A. Coulbeck, T. Howes, S. Kille, "Lightweight
Directory Access Protocol (v3): Attribute Syntax
Definitions", RFC 2252, December 1997.
[RFC2253] M. Wahl, S. Kille, T. Howes, "Lightweight Directory Access
Protocol (v3): UTF-8 String Representation of Distinguished
Names", RFC 2253, December 1997.
[RFC2254] T. Howes, "A String Representation of LDAP Search Filters",
RFC 2254, December 1997.
[RFC2255] T. Howes, M. Smith, "The LDAP URL Format", RFC 2255,
December, 1997.
[RFC2256] Wahl, M., "A Summary of the X.500(96) User Schema for use
with LDAPv3", RFC 2256, December 1997.
[RFC2434] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26 (also RFC 2434),
October 1998.
[RFC2596] M. Wahl, T. Howes, "Use of Language Codes in LDAP", RFC
2596, May 1999.
[RFC2829] M. Wahl, H. Alvestrand, J. Hodges, R. Morgan,
"Authentication Methods for LDAP", RFC 2829, May 2000.
[RFC2830] J. Hodges, R. Morgan, and M. Wahl, "Lightweight Directory
Access Protocol (v3): Extension for Transport Layer
Security", RFC 2830, May 2000.
[RFC2849] G. Good, "The LDAP Data Interchange Format (LDIF) -
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Technical Specification", RFC 2849, June 2000.
[ISOUCS] "Universal Multiple-Octet Coded Character Set (UCS) -
Architecture and Basic Multilingual Plane", ISO/IEC 10646-1
: 1993.
[RFC3377] J. Hodges, R. Morgan, "Lightweight Directory Access
Protocol (v3): Technical Specification", RFC3377, September
2002.
[RFC3383] K. Zeilenga, "IANA Considerations for LDAP", BCP 64 (also
RFC 3383), September 2002.
[LDAPIMR] R. Harrison, K. Zeilenga, "LDAP Intermediate Response",
draft-rharrison-ldap-intermediate-resp-00.txt (a work in
progress).
[FEATURES] K. Zeilenga, "Feature Discovery in LDAP",
draft-zeilenga-ldap-features-xx.txt (a work in progress).
[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.680] ITU-T, "Abstract Syntax Notation One (ASN.1) -
Specification of Basic Notation", X.680, 1994.
[X.690] ITU-T, "Specification of ASN.1 encoding rules: Basic,
Canonical, and Distinguished Encoding Rules", X.690, 1994.
11.2. Informative References
[RFC2246] T. Dierks, C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
[GSER] S. Legg, "Generic String Encoding Rules for ASN.1 Types",
draft-legg-ldapext-gser-xx.txt, a work in progress.
[LOCATE] IETF LDAPext WG, "Discovering LDAP Services with DNS",
draft-ietf-ldapext-locate-xx.txt (work in progress).
[GUIDE] B. Greenblatt, "LDAP Extension Style Guide",
draft-greenblatt-ldapext-style-xx.txt (work in progress).
Copyright 2002, The Internet Society. All Rights Reserved.
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