Network Working Group R. Harrison

Network Working Group R. Harrison
Request for Comments: 3771 Novell, Inc.
Updates: 2251 K. Zeilenga
Category: Standards Track OpenLDAP Foundation
April 2004

The Lightweight Directory Access Protocol (LDAP)
Intermediate Response Message

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 (2004). All Rights Reserved.

Abstract

This document defines and describes the IntermediateResponse message,
a general mechanism for defining single-request/multiple-response
operations in Lightweight Directory Access Protocol (LDAP). The
IntermediateResponse message is defined in such a way that the
protocol behavior of existing LDAP operations is maintained. This
message is intended to be used in conjunction with the LDAP
ExtendedRequest and ExtendedResponse to define new single-
request/multiple-response operations or in conjunction with a control
when extending existing LDAP operations in a way that requires them
to return intermediate response information.

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RFC 3771 LDAP Intermediate Response April 2004

1. Introduction

The Lightweight Directory Access Protocol (LDAP), version 3 [RFC3377]
is an extensible protocol. Extended operations ([RFC2251] Section
4.12) are defined to allow for the addition of operations to LDAP,
without requiring revisions of the protocol. Similarly, controls
([RFC2251] Section 4.1.12) are defined to extend or modify the
behavior of existing LDAP operations.

LDAP is a client-request/server-response based protocol. With the
exception of the search operation, the entire response to an
operation request is returned in a single protocol data unit (i.e.,
LDAP message). While this single-request/single-response paradigm is
sufficient for many operations (including all but one of those
currently defined by [RFC3377]), both intuition and practical
experience validate the notion that it is insufficient for others.

For example, the LDAP delete operation could be extended via a
subtree control to mean that an entire subtree is to be deleted. A
subtree delete operation needs to return continuation references
based upon subordinate knowledge information contained in the server
so that the client can complete the operation. Returning references
as they are found, instead of with the final result, allows the
client to perform the operation more efficiently because it does not
have to wait for the final result to get this continuation reference
information.

Similarly, an engineer might choose to design the subtree delete
operation as an extended operation of its own rather than using a
subtree control in conjunction with the delete operation. Once
again, the same continuation reference information is needed by the
client to complete the operation, and sending the continuation
references as they are found would allow the client to perform the
operation more efficiently.

Operations that are completed in stages or that progress through
various states as they are completed might want to send intermediate
responses to the client, thereby informing it of the status of the
operation. For example, an LDAP implementation might define an
extended operation to create a new replica of an administrative area
on a server, and the operation is completed in three stages: (1)
begin creation of replica, (2) send replica data to server, (3)
replica creation complete. Intermediate messages might be sent from
the server to the client at the beginning of each stage with the
final response for the extended operation being sent after stage (3)
is complete.

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RFC 3771 LDAP Intermediate Response April 2004

As LDAP [RFC3377] is currently defined, there is no general LDAP
message type that can be used to return intermediate results. A
single, reusable LDAP message for carrying intermediate response
information is desired to avoid repeated modification of the
protocol. Although the ExtendedResponse message is defined in LDAP,
it is defined to be the one and only response message to an
ExtendedRequest message ([RFC2251] Section 4.12), for unsolicited
notifications ([RFC2251] Section 4.4), and to return intermediate
responses for the search operation ([RFC3377] Section 4.5.2, also see
Section 5 below). The adaptation of ExtendedResponse as a general
intermediate response mechanism would be problematic. In particular,
existing APIs would likely have to be redesigned. It is believed
(based upon operational experience) that the addition of a new
message to carry intermediate result information is easier to
implement and is less likely to cause interoperability problems with
existing deployed implementations.

This document defines and describes the LDAP IntermediateResponse
message. This message is intended to be used in conjunction with
ExtendedRequest and ExtendedResponse to define new single-
request/multiple-response operations or in conjunction with a control
when extending existing LDAP operations in a way that requires them
to return intermediate response information.

It is intended that the definitions and descriptions of extended
operations and controls using the IntermediateResponse message will
define the circumstances in which an IntermediateResponse message can
be sent by a server and the associated meaning of the
IntermediateResponse message sent in a particular circumstance.
Similarly, it is intended that clients will explicitly solicit
IntermediateResponse messages by issuing operations that specifically
call for their return.

The LDAP Content Sync Operation [ZEILENGA] demonstrates one use of
LDAP Intermediate Response messages.

2. Conventions used in this document

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].

The term "request control" is used to describe a control that is
included in an LDAP request message sent from an LDAP client to an
LDAP server.

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RFC 3771 LDAP Intermediate Response April 2004

3. The IntermediateResponse Message

This document extends the protocolOp CHOICE of LDAPMessage ([RFC2251]
Section 4.1.1) to include the field:

intermediateResponse IntermediateResponse

where IntermediateResponse is defined as:

IntermediateResponse ::= [APPLICATION 25] SEQUENCE {
responseName [0] LDAPOID OPTIONAL,
responseValue [1] OCTET STRING OPTIONAL }

IntermediateResponse messages SHALL NOT be returned to the client
unless the client issues a request that specifically solicits their
return. This document defines two forms of solicitation: extended
operation and request control.

Although the responseName and responseValue are optional in some
circumstances, IntermediateResponse messages usually have a
predefined responseName and a responseValue. The value of the
responseName (if present), the syntax of the responseValue (if
present) and the semantics associated with a particular
IntermediateResponse message MUST be specified in documents
describing the extended operation or request control that uses them.
Sections 3.1 and 3.2 describe additional requirements for the
inclusion of responseName and responseValue in IntermediateResponse
messages.

3.1. Usage with LDAP ExtendedRequest and ExtendedResponse

A single-request/multiple-response operation may be defined using a
single ExtendedRequest message to solicit zero or more
IntermediateResponse messages, of one or more kinds, followed by an
ExtendedResponse message.

An extended operation that defines the return of multiple kinds of
IntermediateResponse messages MUST provide and document a mechanism
for the client to distinguish the kind of IntermediateResponse
message being sent. This SHALL be accomplished by using different
responseName values for each type of IntermediateResponse message
associated with the extended operation or by including identifying
information in the responseValue of each type of IntermediateResponse
message associated with the extended operation.

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RFC 3771 LDAP Intermediate Response April 2004

3.2. Usage with LDAP Request Controls

Any LDAP operation may be extended by the addition of one or more
controls ([RFC2251] Section 4.1.12). A control's semantics may
include the return of zero or more IntermediateResponse messages
prior to returning the final result code for the operation. One or
more kinds of IntermediateResponse messages may be sent in response
to a request control.

All IntermediateResponse messages associated with request controls
SHALL include a responseName. This requirement ensures that the
client can correctly identify the source of IntermediateResponse
messages when:

a) two or more controls using IntermediateResponse messages are
included in a request for any LDAP operation or

b) one or more controls using IntermediateResponse messages are
included in a request with an LDAP extended operation that uses
IntermediateResponse messages.

A request control that defines the return of multiple kinds of
IntermediateResponse messages MUST provide and document a mechanism
for the client to distinguish the kind of IntermediateResponse
message being sent. This SHALL be accomplished by using different
responseName values for each type of IntermediateResponse message
associated with the request control or by including identifying
information in the responseValue of each type of IntermediateResponse
message associated with the request control.

4. Advertising Support for IntermediateResponse Messages

Because IntermediateResponse messages are associated with extended
operations or controls and LDAP provides a means for advertising the
extended operations and controls supported by a server (using the
supportedExtension ([RFC2252] Section 5.2.3) and supportedControl
([RFC2252] Section 5.2.4) attributes of the root DSE), there is no
need for a separate means of advertising support for intermediate
response messages.

5. Use of IntermediateResponse and ExtendedResponse with Search

It is noted that ExtendedResponse messages may be sent in response to
LDAP search operations with controls ([RFC2251] Section 4.5.2). This
use of ExtendedResponse messages SHOULD be viewed as deprecated, in
favor of use of the IntermediateResponse messages.

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RFC 3771 LDAP Intermediate Response April 2004

6. Security Considerations

This document describes an enhancement to LDAP. All security
considerations of [RFC3377] apply to this document; however, it does
not introduce any new security considerations to LDAP.

Security considerations specific to each extension using this
protocol mechanism shall be discussed in the technical specification
detailing the extension.

7. IANA Considerations

Registration of the following value has been completed [RFC3383].

7.1. LDAP Message Type

The IANA has registered an LDAP Message Type (25) to identify the
LDAP IntermediateResponse message as defined in section 3 of this
document.

The following registration template is suggested:

Subject: Request for LDAP Message Type Registration
Person & email address to contact for further information:
Roger Harrison <[email protected]>
Specification: RFC3771
Author/Change Controller: IESG
Comments: Identifies the LDAP IntermediateResponse Message

8. Acknowledgments

The authors would like to acknowledge the members of the IETF LDAP
Extensions (ldapext) working group mail list who responded to the
suggestion that a multiple-response paradigm might be useful for LDAP
extended requests. Special thanks to two individuals: David Wilbur
who first introduced the idea on the working group list, and Thomas
Salter, who succinctly summarized the group's discussion.

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.

[RFC2251] Wahl, M., Howes, T. and S. Kille, "Lightweight Directory
Access Protocol (v3)", RFC 2251, December 1997.

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RFC 3771 LDAP Intermediate Response April 2004

[RFC2252] Wahl, M., Coulbeck, A., Howes, T. and S. Kille,
"Lightweight Directory Access Protocol (v3): Attribute
Syntax Definitions", RFC 2252, December 1997.

[RFC3377] Hodges, J. and R. Morgan, "Lightweight Directory Access
Protocol (v3): Technical Specification", RFC 3377,
September 2002.

[RFC3383] Zeilenga, K., "Internet Assigned Numbers Authority (IANA)
Considerations for the Lightweight Directory Access
Protocol (LDAP)", BCP 64, RFC 3383, September 2002.

9.2. Informative References

[ZEILENGA] Zeilenga, K., "LDAP Content Synchronization Operation",
Work in Progress, February 2004.

10. Authors' Addresses

Roger Harrison
Novell, Inc.
1800 S. Novell Place
Provo, UT 84606

Phone: +1 801 861 2642
EMail: [email protected]

Kurt D. Zeilenga
OpenLDAP Foundation

EMail: [email protected]

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RFC 3771 LDAP Intermediate Response April 2004

11. Full Copyright Statement

Copyright (C) The Internet Society (2004). 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
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INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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Acknowledgement

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Internet Society.

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