Lightweight Directory Access Protocol (LDAP) Modify-Increment Extension
Add one to a directory attribute without reading it first.
Zeilenga, K., "Lightweight Directory Access Protocol (LDAP) Modify-Increment Extension", RFC 4525, DOI 10.17487/RFC4525, June 2006, <https://www.rfc-editor.org/info/rfc4525>.
@misc{rfc4525,
series = {Request for Comments},
number = 4525,
howpublished = {RFC 4525},
publisher = {RFC Editor},
doi = {10.17487/RFC4525},
url = {https://www.rfc-editor.org/info/rfc4525},
author = {K. Zeilenga},
title = {{Lightweight Directory Access Protocol (LDAP) Modify-Increment Extension}},
pagetotal = 6,
year = 2006,
month = jun,
} RFC text
Contents
Network Working Group K. Zeilenga
Request for Comments: 4525 OpenLDAP Foundation
Category: Informational June 2006
Lightweight Directory Access Protocol (LDAP)
Modify-Increment Extension
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes an extension to the Lightweight Directory
Access Protocol (LDAP) Modify operation to support an increment
capability. This extension is useful in provisioning applications,
especially when combined with the assertion control and/or the pre-
read or post-read control extension.
Table of Contents
1. Background and Intended Use .....................................1
2. The Modify-Increment Extension ..................................2
3. LDIF Support ....................................................2
4. Security Considerations .........................................3
5. IANA Considerations .............................................3
5.1. Object Identifier ..........................................3
5.2. LDAP Protocol Mechanism ....................................3
5.3. LDAP Protocol Mechanism ....................................4
6. References ......................................................4
6.1. Normative References .......................................4
6.2. Informative References .....................................5
1. Background and Intended Use
The Lightweight Directory Access Protocol (LDAP) [RFC4510] does not
currently provide an operation to increment values of an attribute.
A client must read the values of the attribute and then modify those
values to increment them by the desired amount. As the values may be
updated by other clients between this add and modify, the client must
be careful to construct the modify request so that it fails in this
case, and upon failure, to re-read the values and construct a new
modify request.
This document extends the LDAP Modify Operation [RFC4511] to support
an increment values capability. This feature is intended to be used
with either the LDAP pre-read or post-read control extensions
[RFC4527]. This feature may also be used with the LDAP assertion
control extension [RFC4528] to provide test-and-increment
functionality.
In this document key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" are to be interpreted as described in BCP 14 [RFC2119].
2. The Modify-Increment Extension
This document extends the LDAP Modify request to support a increment
values capability. Implementations of this extension SHALL support
an additional ModifyRequest operation enumeration value increment
(3), as described herein. Implementations not supporting this
extension will treat this value as they would an unlisted value,
e.g., as a protocol error.
The increment (3) operation value specifies that an increment values
modification is requested. All existing values of the modification
attribute are to be incremented by the listed value. The
modification attribute must be appropriate for the request (e.g., it
must have INTEGER or other increment-able values), and the
modification must provide one and only one value. If the attribute
is not appropriate for the request, a constraintViolation or other
appropriate error is to be returned. If multiple values are
provided, a protocolError is to be returned.
Servers supporting this feature SHOULD publish the object identifier
(OID) 1.3.6.1.1.14 as a value of the 'supportedFeatures' [RFC4512]
attribute in the root DSE. Clients supporting this feature SHOULD
NOT use the feature unless they know the server supports it.
3. LDIF Support
To represent Modify-Increment requests in LDAP Data Interchange
Format [RFC2849], the ABNF [RFC4234] production <mod-spec> is
extended as follows:
mod-spec =/ "increment:" FILL AttributeDescription SEP
attrval-spec "-" SEP
For example,
# Increment uidNumber
dn: cn=max-assigned uidNumber,dc=example,dc=com
changetype: modify
increment: uidNumber
uidNumber: 1
-
This LDIF fragment represents a Modify request to increment the
value(s) of uidNumber by 1.
4. Security Considerations
General LDAP security considerations [RFC4510], as well as those
specific to the LDAP Modify [RFC4511], apply to this Modify-Increment
extension. Beyond these considerations, it is noted that
introduction of this extension should reduce application complexity
(by providing one operation for what presently requires multiple
operations) and, hence, it may aid in the production of correct and
secure implementations.
5. IANA Considerations
Registration of the following values [RFC4520] have been completed.
5.1. Object Identifier
The IANA has assigned an LDAP Object Identifier to identify the LDAP
Modify-Increment feature, as defined in this document.
Subject: Request for LDAP Object Identifier Registration
Person & email address to contact for further information:
Kurt Zeilenga <kurt@OpenLDAP.org>
Specification: RFC 4525
Author/Change Controller: Author
Comments:
Identifies the LDAP Modify-Increment feature
5.2. LDAP Protocol Mechanism
The following LDAP Protocol Mechanism has been registered.
Subject: Request for LDAP Protocol Mechanism Registration
Object Identifier: 1.3.6.1.1.14
Description: Modify-Increment
Person & email address to contact for further information:
Kurt Zeilenga <kurt@openldap.org>
Usage: Feature
Specification: RFC 4525
Author/Change Controller: Kurt Zeilenga <kurt@openldap.org>
Comments: none
5.3. LDAP Protocol Mechanism
The IANA has assigned an LDAP ModifyRequest Operation Type (3)
[RFC4520] for use in this document.
Subject: Request for LDAP Protocol Mechanism Registration
ModifyRequest Operation Name: increment
Description: Modify-Increment
Person & email address to contact for further information:
Kurt Zeilenga <kurt@openldap.org>
Usage: Feature
Specification: RFC 4525
Author/Change Controller: Kurt Zeilenga <kurt@openldap.org>
Comments: none
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[RFC2849] Good, G., "The LDAP Data Interchange Format (LDIF) -
Technical Specification", RFC 2849, June 2000.
[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.
6.2. Informative References
[RFC4520] Zeilenga, K., "Internet Assigned Numbers Authority
(IANA) Considerations for the Lightweight Directory
Access Protocol (LDAP)", BCP 64, RFC 4520, June 2006.
[RFC4527] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP) Read Entry Controls", RFC 4527, June 2006.
[RFC4528] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP) Assertion Control", RFC 4528, June 2006.
Author's Address
Kurt D. Zeilenga
OpenLDAP Foundation
EMail: Kurt@OpenLDAP.org
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AI ELI5
On this page
§1The Problem With Incrementing
The Lightweight Directory Access Protocol (LDAP) is how applications read and write a directory of people, machines and objects. It has a Modify operation to change values, but no built-in way to increment a numeric attribute.
Today, if you want to increment a counter (like the next ID to assign), you must read the old value, add one to it, and write it back. But between reading and writing, another client might do the same thing, and both will write the same new value. The RFC calls this the read-modify-write problem.
Imagine a notebook with a counter. To increment it, you read it (say it says 10), write 11, and close the book. But another person might do the same at the same moment, and you both end up writing 11 instead of getting 10 and 11.
§2The Increment Operation
RFC 4525 extends the LDAP Modify request with a new operation type: increment (value 3). When a client sends an increment request, the server adds the supplied number to all existing values of that attribute.
The attribute must hold integer values (or other increment-able types). The client supplies exactly one value to add. If the attribute does not exist or holds multiple values, or if the value is not a number, the server returns an error. The whole operation is atomic—it either succeeds completely or fails completely, with no partial state.
You tell the directory: 'Take the uidNumber and add 1.' The directory does it all at once. If anything goes wrong (like the number is a name, not a number), it rejects the whole thing.
§3Server Support and Signaling
Servers that support this feature publish an object identifier (OID) 1.3.6.1.1.14 in their 'supportedFeatures' attribute, which clients can read from the root directory entry. Clients that support the feature should not use it unless they know the server supports it.
Servers that do not support it will treat the increment operation as an unknown value and return a protocol error. This ensures forward compatibility.
Servers that can do this announce it, so clients know it's safe to ask. If a server doesn't support it, it says 'I don't know that operation' and you have to do it the old way.
§4LDIF Representation
The LDAP Data Interchange Format (LDIF) is a text format for bulk directory changes. RFC 4525 extends LDIF to represent increment requests using the keyword 'increment', followed by the attribute name and the value to add. For example: 'increment: uidNumber' followed by 'uidNumber: 1' tells the directory to add 1 to the uidNumber of an entry.
The text format for directory changes gets a new command: 'increment'. You write the field name and what to add, and the directory handles it.
§5Status and Use Cases
RFC 4525 is informational, meaning it describes a feature but does not define an Internet standard. The RFC works best when combined with the LDAP pre-read or post-read control extensions (RFC 4527) to let the client see the value before or after the increment, and with the LDAP assertion control (RFC 4528) to test a condition and increment only if it is true. These combinations are useful in provisioning applications where you want to generate unique IDs or track quota usage atomically.
This is a helpful feature that works especially well when paired with other LDAP tools that let you read the result and check conditions. It is useful for systems that hand out ID numbers and need to track how many have been given out.