ISO/IEC 9594-4:2014

Information technology - Open Systems Interconnection - The Directory - Part 4: Procedures for distributed operation

ISO/IEC 9594 has been produced to facilitate the interconnection of information processing systems to provide directory services. A set of such systems, together with the directory information that they hold, can be viewed as an integrated whole, called the Directory. The information held by the Directory, collectively known as the Directory Information Base (DIB), is typically used to facilitate communication between, with or about objects such as application entities, people, terminals and distribution lists. ISO/IEC 9594-4:2014 specifies the procedures required for a distributed directory consisting of a mix of Directory System Agents (DSAs) and lightweight directory access protocol (LDAP) servers acting together to provide a consistent service to its users, independent of the point of access. It also describes procedures for protocol conversion between the directory access protocol/directory system protocol (DAP/DSP) protocols and the LDAP protocol.

Technologies de l'information — Interconnexion de systèmes ouverts (OSI) — L'annuaire — Partie 4: Procédures pour le fonctionnement réparti

General Information

• Status: Withdrawn
• Publication Date: 24-Feb-2014
• Withdrawal Date: 24-Feb-2014
• ICS: 35.100.70 - Application layer
• Technical Committee: ISO/IEC JTC 1/SC 6 - Telecommunications and information exchange between systems
• Drafting Committee: ISO/IEC JTC 1/SC 6 - Telecommunications and information exchange between systems
• Current Stage: 9599 - Withdrawal of International Standard
• Start Date: 05-May-2017
• Completion Date: 30-Oct-2025

Relations

• Revised: ISO/IEC 9594-4:2017 - Information technology - Open Systems Interconnection - The Directory - Part 4: Procedures for distributed operation
• Effective Date: 10-Dec-2016
• Revises: ISO/IEC 9594-4:2008 - Information technology - Open Systems Interconnection - The Directory: Procedures for distributed operation - Part 4:
• Effective Date: 31-Aug-2013

Standards Content (Sample)

INTERNATIONAL ISO/IEC
STANDARD 9594-4
Seventh edition
2014-03-01
Information technology — Open Systems
Interconnection — The Directory —
Part 4:
Procedures for distributed operation
Technologies de l'information — Interconnexion de systèmes ouverts
(OSI) — L'annuaire
Partie 4: Procédures pour le fonctionnement réparti

Reference number
©
ISO/IEC 2014
©  ISO/IEC 2014
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ii © ISO/IEC 2014 – All rights reserved

Foreword
ISO (the International Organization for Standardization) and IEC (the International Electrotechnical
Commission) form the specialized system for worldwide standardization. National bodies that are members of
ISO or IEC participate in the development of International Standards through technical committees
established by the respective organization to deal with particular fields of technical activity. ISO and IEC
technical committees collaborate in fields of mutual interest. Other international organizations, governmental
and non-governmental, in liaison with ISO and IEC, also take part in the work. In the field of information
technology, ISO and IEC have established a joint technical committee, ISO/IEC JTC 1.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of the joint technical committee is to prepare International Standards. Draft International
Standards adopted by the joint technical committee are circulated to national bodies for voting. Publication as
an International Standard requires approval by at least 75 % of the national bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO and IEC shall not be held responsible for identifying any or all such patent rights.
ISO/IEC 9594-4 was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology,
Subcommittee SC 6, Telecommunications and information exchange between systems, in collaboration with
ITU-T. The identical text is published as Rec. ITU-T X.518 (10/2012).
This seventh edition cancels and replaces the sixth edition (ISO/IEC 9594-4:2008), which has been
technically revised. It also incorporates the Technical Corrigendum ISO/IEC 9594-4:2008/Cor.1:2011.
ISO/IEC 9594 consists of the following parts, under the general title Information technology — Open Systems
Interconnection — The Directory:
— Part 1: Overview of concepts, models and services
— Part 2: Models
— Part 3: Abstract service definition
— Part 4: Procedures for distributed operation
— Part 5: Protocol specifications
— Part 6: Selected attribute types
— Part 7: Selected object classes
— Part 8: Public-key and attribute certificate frameworks
— Part 9: Replication
© ISO/IEC 2014 – All rights reserved

CONTENTS
Page
1 Scope . 1
2 Normative references . 1
2.1 Identical Recommendations | International Standards . 1
2.2 Other references . 2
3 Definitions . 2
3.1 Basic Directory definitions . 2
3.2 Directory model definitions . 2
3.3 DSA information model definitions . 2
3.4 Abstract service definitions . 3
3.5 Protocol definitions . 3
3.6 Directory replication definitions . 3
3.7 Distributed operation definitions . 3
4 Abbreviations . 5
5 Conventions . 5
6 Overview . 7
7 Distributed Directory system model . 8
8 DSA interactions model . 9
8.1 Decomposition of a request . 9
8.2 Uni-chaining . 9
8.3 Multi-chaining . 10
8.4 Referral. 11
8.5 Mode determination . 12
9 Overview of DSA abstract service . 13
10 Information types . 13
10.1 Introduction . 13
10.2 Information types defined elsewhere . 13
10.3 Chaining arguments . 14
10.4 Chaining results . 17
10.5 Operation progress . 17
10.6 Trace information . 18
10.7 Reference type . 18
10.8 Access point information . 19
10.9 DIT bridge knowledge . 20
10.10 Exclusions . 20
10.11 Continuation reference . 21
11 Bind and Unbind . 22
11.1 DSA Bind . 22
11.2 DSA Unbind . 23
12 Chained operations . 23
12.1 Chained operations . 23
12.2 Chained Abandon operation . 24
12.3 Chained operations and protocol version . 24
13 Chained errors . 24
13.1 Introduction . 24
13.2 DSA referral . 24
14 Introduction . 26
14.1 Scope and limits . 26
14.2 Conformance . 26
14.3 Conceptual model. 26
14.4 Individual and cooperative operation of DSAs . 26
Rec. ITU-T X.518 (10/2012) iii

Page
14.5 Cooperative agreements between DSAs . 27
15 Distributed Directory behaviour . 27
15.1 Cooperative fulfilment of operations . 27
15.2 Phases of operation processing . 27
15.3 Managing Distributed Operations . 28
15.4 Loop handling . 29
15.5 Other considerations for distributed operation . 30
15.6 Authentication of Distributed operations . 31
16 The Operation Dispatch er . 32
16.1 General concepts . 32
16.2 Procedures of the Operation Dispatcher . 37
16.3 Overview of procedures . 38
17 Request Validation procedure . 39
17.1 Introduction . 39
17.2 Procedure parameters . 40
17.3 Procedure definition . 41
18 Name Resolution procedure . 44
18.1 Introduction . 44
18.2 Find DSE procedure parameters. 44
18.3 Procedures . 45
19 Operation evaluation . 54
19.1 Modification procedures . 55
19.2 Single entry interrogation procedure . 62
19.3 Multiple entry interrogation procedure . 62
20 Continuation Reference procedures . 77
20.1 Chaining strategy in the presence of shadowing . 77
20.2 Issuing chained subrequests to a remote DSA or LDAP server . 79
20.3 Procedures' parameters . 80
20.4 Definition of the procedures . 80
20.5 Abandon procedures . 89
20.6 DAP request to LDAP request procedure . 91
20.7 LDAP result to DAP reply procedure . 95
21 Results Merging procedure . 97
22 Procedures for distributed authentication . 99
22.1 Requester authentication . 100
22.2 Results authentication . 100
23 Knowledge administration overview . 101
23.1 Maintenance of knowledge references . 101
23.2 Requesting cross reference . 102
23.3 Knowledge inconsistencies . 103
24 Hierarchical operational bindings . 104
24.1 Operational binding type characteristics . 104
24.2 Operational binding information object Class definition . 107
24.3 DSA procedures for hierarchical operational binding management . 107
24.4 Procedures for operations . 111
24.5 Use of application contexts . 111
25 Non-specific hierarchical operational binding . 111
25.1 Operational binding type characteristics . 111
25.2 Operational binding information object class definition . 112
25.3 DSA procedures for non-specific hierarchical operational binding management . 113
25.4 Procedures for operations . 114
25.5 Use of application contexts . 114
iv Rec. ITU-T X.518 (10/2012)
Page
Annex A – ASN.1 for Distributed Operations . 115
Annex B – Specification of hierarchical and non-specific hierarchical operational binding types . 119
Annex C – Example of distributed name resolution . 121
Annex D – Distributed use of authentication . 123
D.1 Summary . 123
D.2 Distributed protection model . 123
D.3 Signed chained operations . 123
Annex E – Knowledge maintenance example . 125
Annex F – Amendments and corrigenda . 128

Rec. ITU-T X.518 (10/2012) v
Introduction
This Recommendation | International Standard, together with other Recommendations | International Standards, have
been produced to facilitate the interconnection of information processing systems to provide directory services. A set of
such systems, together with the directory information that they hold, can be viewed as an integrated whole, called the
Directory. The information held by the Directory, collectively known as the Directory information base (DIB), is
typically used to facilitate communication between, with or about objects such as application entities, people, terminals
and distribution lists.
The Directory plays a significant role in Open Systems Interconnection, whose aim is to allow, with a minimum of
technical agreement outside of the interconnection standards themselves, the interconnection of information processing
systems:
– from different manufacturers;
– under different managements;
– of different levels of complexity; and
– of different ages.
This Recommendation | International Standard specifies the procedures by which the distributed components of the
Directory interwork in order to provide a consistent service to its users.
This Recommendation | International Standard provides the foundation frameworks upon which industry profiles can be
defined by other standards groups and industry forums. Many of the features defined as optional in these frameworks
may be mandated for use in certain environments through profiles. This seventh edition technically revises and
enhances the sixth edition of this Recommendation | International Standard.
This seventh edition specifies versions 1 and 2 of the Directory protocols.
The first and second editions specified only version 1. Most of the services and protocols specified in this edition are
designed to function under version 1. However, some enhanced services and protocols, e.g., signed errors, will not
function unless all Directory entities involved in the operation have negotiated version 2. Whichever version has been
negotiated, differences between the services and between the protocols defined in the seven editions, except for those
specifically assigned to version 2, are accommodated using the rules of extensibility defined in Rec. ITU-T X.519 |
ISO/IEC 9594-5.
Annex A, which is an integral part of this Recommendation | International Standard, provides the ASN.1 module for
directory distributed operations.
Annex B, which is an integral part of this Recommendation | International Standard, provides the ASN.1 module
providing definitions for hierarchical operational bindings.
Annex C, which is not an integral part of this Recommendation | International Standard, describes an example of
distributed name resolution.
Annex D, which is not an integral part of this Recommendation | International Standard, describes authentication in the
distributed operations environment.
Annex E, which is not an integral part of this Recommendation | International Standard, illustrates knowledge
maintenance.
Annex F, which is not an integral part of this Recommendation | International Standard, lists the amendments and defect
reports that have been incorporated to form this edition of this Recommendation | International Standard.
vi Rec. ITU-T X.518 (10/2012)
INTERNATIONAL STANDARD ISO/IEC 9594-4
RECOMMENDATION ITU-T X.518
Information technology – Open Systems Interconnection –
The Directory: Procedures for distributed operation
SECTION 1 – GENERAL
1 Scope
This Recommendation | International Standard specifies the behaviour of DSAs taking part in a distributed directory
consisting of multiple Directory systems agents (DSAs) and/or LDAP servers with at least one DSA. The allowed
behaviour has been designed to ensure a consistent service given a wide distribution of the DIB across a distributed
directory. Only the behaviour of DSAs taking part in a distributed directory is specified. The behaviour of LDAP
servers are specified in relevant LDAP specifications. There are no special requirements on an LDAP server beyond
those given by the LDAP specifications.
The Directory is not intended to be a general purpose database system, although it may be built on such systems. It is
assumed that there is a considerably higher frequency of queries than of updates.
2 Normative references
The following Recommendations and International Standards contain provisions which, through reference in this text,
constitute provisions of this Recommendation | International Standard. At the time of publication, the editions indicated
were valid. All Recommendations and Standards are subject to revision, and parties to agreements based on this
Recommendation | International Standard are encouraged to investigate the possibility of applying the most recent
edition of the Recommendations and Standards listed below. Members of IEC and ISO maintain registers of currently
valid International Standards. The Telecommunication Standardization Bureau of the ITU maintains a list of currently
valid ITU-T Recommendations.
2.1 Identical Recommendations | International Standards
– Recommendation ITU-T X.200 (1994) | ISO/IEC 7498-1:1994, Information technology – Open Systems
Interconnection – Basic Reference Model: The basic model.
– Recommendation ITU-T X.500 (2012 | ISO/IEC 9594-1:2014, Information technology – Open Systems
Interconnection – The Directory: Overview of concepts, models and services.
– Recommendation ITU-T X.501 (2012) | ISO/IEC 9594-2:2014, Information technology – Open Systems
Interconnection – The Directory: Models.
– Recommendation ITU-T X.509 (2012) | ISO/IEC 9594-8:2014, Information technology – Open Systems
Interconnection – The Directory: Public-key and attribute certificate frameworks.
– Recommendation ITU-T X.511 (2012) | ISO/IEC 9594-3:2014, Information technology – Open Systems
Interconnection – The Directory: Abstract service definition.
– Recommendation ITU-T X.519 (2012) | ISO/IEC 9594-5:2014, Information technology – Open Systems
Interconnection – The Directory: Protocol specifications.
– Recommendation ITU-T X.520 (2012) | ISO/IEC 9594-6:2014, Information technology – Open Systems
Interconnection – The Directory: Selected attribute types.
– Recommendation ITU-T X.521 (2012) | ISO/IEC 9594-7:2014, Information technology – Open Systems
Interconnection – The Directory: Selected object classes.
– Recommendation ITU-T X.525 (2012) | ISO/IEC 9594-9:2014, Information technology – Open Systems
Interconnection – The Directory: Replication.
– Recommendation ITU-T X.680 (2008) | ISO/IEC 8824-1:2008, Information technology – Abstract
Syntax Notation One (ASN.1): Specification of basic notation.
– Recommendation ITU-T X.681 (2008) | ISO/IEC 8824-2:2008, Information technology – Abstract
Syntax Notation One (ASN.1): Information object specification.
– Recommendation ITU-T X.682 (2008) | ISO/IEC 8824-3:2008, Information technology – Abstract
Syntax Notation One (ASN.1): Constraint specification.
Rec. ITU-T X.518 (10/2012) 1
– Recommendation ITU-T X.683 (2008) | ISO/IEC 8824-4:2008, Information technology – Abstract
Syntax Notation One (ASN.1): Parameterization of ASN.1 specifications.
2.2 Other references
– IETF RFC 3672 (2003), Subentries in the Lightweight Directory Access Protocol (LDAP).
– IETF RFC 4510 (2006), Lightweight Directory Access Protocol (LDAP): Technical Specification Road
Map.
– IETF RFC 4511 (2006), Lightweight Directory Access Protocol (LDAP): The Protocol.
– IETF RFC 4514 (2006), Lightweight Directory Access Protocol (LDAP): String Representation of
Distinguished Names.
3 Definitions
For the purposes of this Recommendation | International Standard, the following definitions apply:
3.1 Basic Directory definitions
The following terms are defined in Rec. ITU-T X.500 | ISO/IEC 9594-1:
a) (the) Directory;
b) Directory Information Base.
3.2 Directory model definitions
The following terms are defined in Rec. ITU-T X.501 | ISO/IEC 9594-2:
a) access point;
b) alias;
c) Directory Information Tree ;
d) Directory System Agent (DSA);
e) Directory User Agent (DUA);
f) distinguished name;
g) relative distinguished name.
3.3 DSA information model definitions
The following terms are defined in Rec. ITU-T X.501 | ISO/IEC 9594-2:
a) category;
b) commonly usable;
c) context prefix;
d) cross reference;
e) DIB fragment;
f) DSA information tree;
g) DSA-Specific Entry (DSE);
h) DSE type;
i) immediate superior reference;
j) knowledge information;
k) knowledge reference category;
l) knowledge reference type;
m) naming context;
n) non-specific knowledge;
o) non-specific subordinate reference;
2 Rec. ITU-T X.518 (10/2012)
p) operational attribute;
q) reference path;
r) specific knowledge;
s) subordinate reference;
t) superior reference.
3.4 Abstract service definitions
The following terms are defined in Rec. ITU-T X.511 | ISO/IEC 9594-3:
a) reply;
b) request;
c) requester.
3.5 Protocol definitions
The following terms are defined in Rec. ITU-T X.519 | ISO/IEC 9594-5:
a) application-association;
b) application-entity-title.
3.6 Directory replication definitions
The following terms are defined in Rec. ITU-T X.525 | ISO/IEC 9594-9:
a) attribute completeness;
b) shadowing operational binding;
c) subordinate completeness;
d) unit of replication.
3.7 Distributed operation definitions
The following terms are defined in this Recommendation | International Standard:
3.7.1 base object: The object or alias entry that is the target for an operation as issued by the requester.
3.7.2 bound DSA: The DSA to which the requesting DUA or LDAP client has bound, by having performed a Bind
operation with that DSA.
3.7.3 bound-DSA paged results: The paging is performed entirely by the DSA to which the DUA is bound.
NOTE – This is the only mode of paging supported by systems conforming to editions prior to the fifth edition.
3.7.4 chaining: The generic term for uni-chaining or multi-chaining.
3.7.5 context prefix information: Operational and user information supplied by the superior DSA to the
subordinate DSA in an RHOB regarding DIT vertices superior to the subordinate context prefix.
3.7.6 directory server: A DSA or an LDAP server.
3.7.7 distributed directory: An interconnected set of directory servers where at least one directory server shall be
a DSA.
3.7.8 distributed name resolution: The process by which name resolution starts in a DSA and continues in one or
more Directory servers.
3.7.9 DSP paged results: The DSP protocol provisions when a performing DSA is different from a bound DSA,
whereby paged results by the initial performer are accomplished.
3.7.10 error: Information sent from the performer to the requester conveying a negative outcome of a previously
received request.
3.7.11 hard error: A definite error which indicates that the operation cannot currently be performed without
external intervention.
Rec. ITU-T X.518 (10/2012) 3
3.7.12 hierarchical operational binding (HOB): Relationship between two master DSAs holding naming contexts,
one of which is immediately subordinate to the other, in which the superior DSA holds a subordinate reference to the
subordinate DSA.
3.7.13 initial performer: The first DSA or LDAP server to start performing on an operation, i.e., the first DSA or
LDAP server to enter the evaluation phase of the operation.
3.7.14 LDAP requester: A DSA that has the ability to access an LDAP server by using the LDAP protocol.
3.7.15 modification operations: These are the Directory Modify operations, i.e., Modify Entry, Add Entry, Remove
Entry, Modify DN, Change Password and Administer Password operations.
3.7.16 multi-chaining: A mode of interaction in which a DSA processing a request itself sends multiple requests
either in parallel or sequentially to a set of other DSAs.
3.7.17 multiple entry interrogation operations: These are the Directory Search operations, i.e., List and Search
operations.
3.7.18 name resolution: The process of locating an entry by sequentially matching each RDN in a purported name
to a vertex of the DIT.
3.7.19 non-specific hierarchical operational binding (NHOB): Relationship between two master DSAs holding
naming contexts, one of which is immediately subordinate to the other, in which the superior DSA holds a non-specific
subordinate reference to the subordinate DSA.
3.7.20 NSSR decomposition: Decomposition of non-specific knowledge references into subrequests for other DSAs
to pursue; these subrequests may be either chained to these DSAs by the DSA performing the decomposition, or a
continuation reference identifying the DSAs may be returned to the requester for it to pursue, or the decomposing DSA
may pursue some of the subrequests, leaving others unexplored for the requester to pursue.
3.7.21 operation progress: A set of values which denotes the extent to which name resolution has taken place.
3.7.22 paging: A search or list result is returned piecewise in the form of one or more pages that are comprised
by a limited number of entries.
3.7.23 performer: DSA receiving a request (i.e., to perform an operation).
NOTE – The performer is also the initial performer except possibly for operations that involve more than one DSA for their
evaluation.
3.7.24 procedure: An (informal) specification of how a DSA maps a given set of input arguments and its DSA
information tree into a result.
NOTE – Input arguments and results may correspond to information received in a requested operation and information sent in a
reply, or they may represent intermediate stages in the computation of a reply from a requested operation. In clause 14.2, the
former variety of input arguments and results are termed external.
3.7.25 relevant hierarchical operational binding (RHOB): Either an HOB or an NHOB, depending on the context.
3.7.26 referral: An outcome which can be returned by a DSA or LDAP server which cannot perform an operation
itself, and which identifies one or more other DSAs or LDAP servers more able to perform the operation.
3.7.27 request decomposition: Decomposition by a DSA of a request into subrequests for other Directory servers to
pursue; these subrequests may be either chained to these Directory servers by the DSA performing the decomposition,
or continuation references identifying the Directory servers may be returned to the requester for it to pursue, or the
decomposing DSA may pursue some of the subrequests, leaving others unexplored for the requester to pursue.
3.7.28 single entry interrogation operations: These are the Directory Read operations, i.e., Read and Compare
operations.
3.7.29 soft error: An error which may be transient, or which may indicate a localized problem, in which case the use
of a different knowledge reference or access point may enable a result or hard error to be obtained.
3.7.30 subordinate DSA: Of the two DSAs sharing an HOB or an NHOB, the DSA holding the subordinate naming
context.
3.7.31 subrequest: A request generated by request decomposition.
3.7.32 superior DSA: Of the two DSAs sharing an HOB or an NHOB, the DSA holding the superior naming
context.
4 Rec. ITU-T X.518 (10/2012)
3.7.33 superior, subordinate DSA: Two master DSAs holding naming contexts, one of which is immediately
subordinate to the other; the relationship between the two DSAs is managed explicitly via an HOB (or NHOB), or exists
implicitly by virtue of the superior DSA holding a subordinate (or non-specific subordinate) reference to the
subordinate DSA.
3.7.34 target object name: The name of an entry either to which the operation is to be directed at a particular stage
of name resolution, or which is involved in the evaluation of the operation.
3.7.35 uni-chaining: A mode of interaction optionally used by a DSA which cannot perform an operation itself. The
DSA chains by invoking an operation of another DSA or LDAP server and then relaying the outcome to the original
requester.
4 Abbreviations
For the purposes of this Recommendation | International Standard, the following abbreviations apply:
ASN.1 Abstract Syntax Notation One
DAP Directory Access Protocol
DIB Directory Information Base
DISP Directory Information Shadowing Protocol
DMD Directory Management Domain
DOP Directory Operational Binding Management Protocol
DSA Directory System Agent
DSE DSA-Specific Entry
DSP Directory System Protocol
DUA Directory User Agent
HOB Hierarchical Operational Binding
LDAP Lightweight Directory Access Protocol
NHOB Non-specific Hierarchical Operational Binding
NSSR Non-Specific Subordinate Reference
RHOB Relevant Hierarchical Operational Binding
5 Conventions
The term "Directory Specification" (as in "this Directory Specification") shall be taken to mean Rec. ITU-T X.518 |
ISO/IEC 9594-4. The term "Directory Specifications" shall be taken to mean the X.500-series Recommendations and all
parts of ISO/IEC 9594.
This Directory Specification uses the term first edition systems to refer to systems conforming to the first edition of the
Directory Specifications, i.e., the 1988 edition of the CCITT X.500-series Recommendations and the
ISO/IEC 9594:1990 edition.
This Directory Specification uses the term second edition systems to refer to systems conforming to the second edition
of the Directory Specifications, i.e., the 1993 edition of the ITU-T X.500-series Recommendations and the
ISO/IEC 9594:1995 edition.
This Directory Specification uses the term third edition systems to refer to systems conforming to the third edition of the
Directory Specifications, i.e., the 1997 edition of the ITU-T X.500-series Recommendations and the
ISO/IEC 9594:1998 edition.
This Directory Specification uses the term fourth edition systems to refer to systems conforming to the fourth edition of
the Directory Specifications, i.e., the 2001 editions of Rec. ITU-T X.500, X.501, X.511, X.518, X.519, X.520, X.521,
X.525, and X.530, the 2000 edition of Rec. ITU-T X.509, and parts 1-10 of the ISO/IEC 9594:2001 edition.
This Directory Specification uses the term fifth edition systems to refer to systems conforming to the fifth edition of the
Directory Specifications, i.e., the 2005 edition of the ITU-T X.500-series Recommendations and the
ISO/IEC 9594:2005 edition.
Rec. ITU-T X.518 (10/2012) 5
This Directory Specification uses the term sixth edition systems to refer to systems conforming to the sixth edition of the
Directory Specifications, i.e., the 2008 edition of the ITU-T X.500-series Recommendations and the
ISO/IEC 9594:2008 edition.
This Directory Specification uses the term seventh edition systems to refer to systems conforming to the seventh edition
of the Directory Specifications, i.e., the 2012 edition of the ITU-T X.500-series Recommendations and the
ISO/IEC 9594:2014 edition.
This Directory Specification presents ASN.1 notation in the bold Courier New typeface. When ASN.1 types and values
are referenced in normal text, they are differentiated from normal text by presenting them in the bold Courier New
typeface. The names of procedures, typically referenced when specifying the semantics of processing, are differentiated
from normal text by displaying them in bold Times New Roman. Access control permissions are presented in italicized
Times New Roman.
If the items in a list are numbered (as opposed to using "–" or letters), then the items shall be considered steps in a
procedure.
6 Rec. ITU-T X.518 (10/2012)
SECTION 2 – OVERVIEW
6 Overview
The Directory abstract service allows the interrogation, retrieval and modification of Directory information in the DIB.
This service is specified in in Rec. ITU-T X.511 | ISO/IEC 9594-3. Similarly, the lightweight directory access protocol
(LDAP) allows the interrogation, retrieval and modification of Directory information in the DIB. This protocol and the
services it enables are specified in IETF RFC 4511.
The abstract service as specified in Rec. ITU-T X.511 | ISO/IEC 9594-3 does not address the specification of Directory
system agents (DSA) within which the DIB is stored and managed, and through which the service is provided.
Furthermore, it does not consider whether the DIB is centralized, i.e., contained within a single DSA, or distributed over
a DSA and a number of additional DSAs and/or LDAP servers. Directory server is the common name for a DSA or an
LDAP server. Consequently, the requirements for DSAs to have knowledge of, navigate to and cooperate with other
DSAs and or LDAP servers, in order to support the abstract service in a distributed environment is also not covered by
the abstract service specification.
This Directory Specification specifies how a set of one or more DSAs and zero or more LDAP servers collectively
constitute the distributed directory service.
In addition, this Directory Specification specifies the permissible ways in which the DIB may be distributed over one or
more DSAs and zero or more LDAP servers. For the limiting case where the DIB is contained within a single DSA, the
Directory is in fact centralized; for the case where the DIB is distributed over two or more DSAs, knowledge and
navigation mechanisms are specified which ensure that the whole of the DIB is potentially accessible from all DSAs
that hold constituent entries.
Portions of the DIB may also be replicated in multiple DSAs. The protocols described in this Directory Specification
allow the use of replicated information to improve the availability, performance and efficiency of the distributed
directory service. The use of replicated information is, to some extent, under the user's control, through the use of
service control options. The procedures described in this Directory Specification also indicate some of the opportunities
for design optimizations when using the replicated information.
Additionally, request handling interactions are specified that enable particular operational characteristics of the
Directory to be controlled by its users. In particular, the user has control over whether a DSA, responding to a directory
inquiry pertaining to information held in other directory server(s), has the option of interrogating the other DSA(s)
directly (chaining) or, whether it should respond with information about other directory server(s) which could further
progress the inquiry (referral).
Generally, the decision by a DSA to chain or refer is determined by the service controls set by the user, and by the
DSA's own administrative, operational or technical circumstances.
Recognizing that, in general, the Directory will be distributed, and that directory inquiries will be satisfied by an
arbitrary number of cooperating DSAs which may arbitrarily chain or refer according to the above criteria, this
Directory Specification specifies the appropriate procedures to be effected by DSAs in responding to distributed
directory inquiries. These procedures will ensure that users of the distributed Directory service perceive it to be both
user-friendly and consistent.
NOTE – Although an LDAP server may participate in a distributed operation, it is not aware of this cooperation.

Rec. ITU-T X.518 (10/2012) 7
SECTION 3 – DISTRIBUTED DIRECTORY MODELS
7 Distributed Directory system mo
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