Information technology — Open systems interconnection — Part 1: The Directory: Overview of concepts, models and services

This document provides the directory capabilities required by many application layer standards and telecommunication services. Among the capabilities which it provides are those of "user-friendly naming", whereby objects can be referred to by names which are suitable for citing by human users (though not all objects need have user-friendly names); and "name-to-address mapping" which allows the binding between objects and their locations to be dynamic. The latter capability allows networks, for example, to be "self-configuring" in the sense that addition, removal and the changes of object location do not affect network operation. The Directory is not intended to be a general-purpose database system, although it may be built on such systems. It is assumed, for instance, that, as is typical with communication directories, there is a considerably higher frequency of "queries" than of updates. The rate of updates is expected to be governed by the dynamics of people and organizations, rather than, for example, the dynamics of networks. There is also no need for instantaneous global commitment of updates; transient conditions, where both old and new versions of the same information are available, are quite acceptable. It is a characteristic of the Directory that, except as a consequence of differing access rights or un-propagated updates, the results of directory queries will not be dependent on the identity or location of the inquirer. This characteristic renders the Directory unsuitable for some telecommunication applications, for example some types of routing. For cases where the results are dependent on the identity of the inquirer, access to directory information and updates of the Directory may be denied.

Technologies de l'information — Interconnexion de systèmes ouverts (OSI) — Partie 1: Titre manque

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Published
Publication Date
30-Nov-2020
Current Stage
6060 - International Standard published
Start Date
09-Nov-2020
Completion Date
01-Dec-2020
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INTERNATIONAL ISO/IEC
STANDARD 9594-1
Ninth edition
2020-11
Information technology — Open
systems interconnection —
Part 1:
The Directory: Overview of concepts,
models and services
Reference number
ISO/IEC 9594-1:2020(E)
ISO/IEC 2020
---------------------- Page: 1 ----------------------
ISO/IEC 9594-1:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO/IEC 2020

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO/IEC 2020 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/IEC 9594-1:2020(E)
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.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of document should be noted (see www.iso.org/directives).

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. Details

of any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www.iso.org/patents) or the IEC list of patent

declarations received (see http://patents.iec.ch).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see

www.iso.org/iso/foreword.html.

This document was prepared by ITU-T as ITU-T X.500 (10/2019) and drafted in accordance with

its editorial rules, in collaboration with Joint Technical Committee ISO/IEC JTC 1,

Information technology, Subcommittee SC 6, Telecommunications and information exchange between

systems.

This ninth edition cancels and replaces the eighth edition (ISO/IEC 9594-1:2017), which has been

technically revised.
A list of all parts in the ISO/IEC 9594 series can be found on the ISO website.

Any feedback or questions on this document should be directed to the user’s national standards body.

A complete listing of these bodies can be found at www.iso.org/members.html.
© ISO/IEC 2020 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO/IEC 9594-1:2020 (E)
CONTENTS
Page

1 Scope .............................................................................................................................................................. 1

2 Normative references...................................................................................................................................... 1

2.1 Identical Recommendations | International Standards ........................................................................ 1

3 Definitions ...................................................................................................................................................... 2

3.1 Communication model definitions ...................................................................................................... 2

3.2 Directory model definitions ................................................................................................................ 2

3.3 Distributed Operation definitions ........................................................................................................ 3

3.4 Replication definitions ........................................................................................................................ 3

3.5 Basic directory definitions .................................................................................................................. 3

4 Abbreviations ................................................................................................................................................. 3

5 Conventions .................................................................................................................................................... 4

6 Overview of the Directory .............................................................................................................................. 4

7 The Directory Information Base (DIB) .......................................................................................................... 5

8 The Directory service ..................................................................................................................................... 7

8.1 Introduction ......................................................................................................................................... 7

8.2 Service qualification ............................................................................................................................ 7

8.3 Directory interrogation ........................................................................................................................ 7

8.4 Directory modification ........................................................................................................................ 8

8.5 Other outcomes ................................................................................................................................... 8

9 The distributed Directory ............................................................................................................................... 9

9.1 Functional model ................................................................................................................................. 9

9.2 Organizational model .......................................................................................................................... 9

9.3 Operation of the model ........................................................................................................................ 9

10 Access control in the Directory ...................................................................................................................... 13

11 Service administration .................................................................................................................................... 14

12 Replication in the Directory ........................................................................................................................... 15

12.1 Introduction ......................................................................................................................................... 15

12.2 Forms of Directory replication ............................................................................................................ 15

12.3 Replication and consistency of Directory information ........................................................................ 16

12.4 Views of replication ............................................................................................................................ 16

12.5 Replication and Access Control .......................................................................................................... 17

13 Directory protocols ......................................................................................................................................... 17

Annex A – Applying the Directory ............................................................................................................................ 18

A.1 The Directory environment ................................................................................................................. 18

A.2 Directory service characteristics ......................................................................................................... 18

A.3 Patterns of use of the Directory ........................................................................................................... 18

Annex B – Amendments and corrigenda ................................................................................................................... 22

© ISO/IEC 2020 – All rights reserved
Rec. ITU-T X.500 (10/2019) v
---------------------- Page: 4 ----------------------
ISO/IEC 9594-1:2020 (E)
Introduction

This Recommendation | International Standard together with other Recommendations | International Standards, 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.

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 introduces and models the concepts of the Directory and of the DIB and

overviews the services and capabilities which they provide. Other Recommendations | International Standards make use

of these models in defining the abstract service provided by the Directory, and in specifying the protocols through which

this service can be obtained or propagated.

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 ninth edition technically revises and enhances,

the eighth edition of this Recommendation | International Standard.

Annex A, which is an integral part of this Recommendation | International Standard, describes the types of use to which

the Directory can be applied.

Annex B, 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.

© ISO/IEC 2020 – All rights reserved
vi Rec. ITU-T X.500 (10/2019)
---------------------- Page: 5 ----------------------
ISO/IEC 9594-1:2020 (E)
INTERNATIONAL STANDARD
ITU-T RECOMMENDATION
Information technology – Open Systems Interconnection –
The Directory: Overview of concepts, models and services
1 Scope

The Directory provides the directory capabilities required by many application layer standards and telecommunication

services. Among the capabilities which it provides are those of "user-friendly naming", whereby objects can be referred

to by names which are suitable for citing by human users (though not all objects need have user-friendly names); and

"name-to-address mapping" which allows the binding between objects and their locations to be dynamic. The latter

capability allows networks, for example, to be "self-configuring" in the sense that addition, removal and the changes of

object location do not affect network operation.

The Directory is not intended to be a general-purpose database system, although it may be built on such systems. It is

assumed, for instance, that, as is typical with communication directories, there is a considerably higher frequency of

"queries" than of updates. The rate of updates is expected to be governed by the dynamics of people and organizations,

rather than, for example, the dynamics of networks. There is also no need for instantaneous global commitment of updates;

transient conditions, where both old and new versions of the same information are available, are quite acceptable.

It is a characteristic of the Directory that, except as a consequence of differing access rights or un-propagated updates,

the results of directory queries will not be dependent on the identity or location of the inquirer. This characteristic renders

the Directory unsuitable for some telecommunication applications, for example some types of routing. For cases where

the results are dependent on the identity of the inquirer, access to directory information and updates of the Directory may

be denied.
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.501 (2019) | ISO/IEC 9594-2:2020, Information technology – Open Systems

Interconnection – The Directory: Models.

– Recommendation ITU-T X.509 (2019) | ISO/IEC 9594-8:2020, Information technology – Open Systems

Interconnection – The Directory: Public-key and attribute certificate frameworks.

– Recommendation ITU-T X.511 (2019) | ISO/IEC 9594-3:2020, Information technology – Open Systems

Interconnection – The Directory: Abstract service definition.

– Recommendation ITU-T X.518 (2019) | ISO/IEC 9594-4:2020, Information technology – Open Systems

Interconnection – The Directory: Procedures for distributed operation.

– Recommendation ITU-T X.519 (2019) | ISO/IEC 9594-5:2020, Information technology – Open Systems

Interconnection – The Directory: Protocol specifications.

– Recommendation ITU-T X.520 (2019) | ISO/IEC 9594-6:2020, Information technology – Open Systems

Interconnection – The Directory: Selected attribute types.

– Recommendation ITU-T X.521 (2019) | ISO/IEC 9594-7:2020, Information technology – Open Systems

Interconnection – The Directory: Selected object classes.

– Recommendation ITU-T X.525 (2019) | ISO/IEC 9594-9:2020, Information technology – Open Systems

Interconnection – The Directory: Replication.
© ISO/IEC 2020 – All rights reserved
Rec. ITU-T X.500 (10/2019) 1
---------------------- Page: 6 ----------------------
ISO/IEC 9594-1:2020 (E)
3 Definitions

For the purposes of this Recommendation | International Standard, the following definitions apply.

3.1 Communication model definitions
The following terms are defined in Rec. ITU-T X.519 | ISO/IEC 9594-5:
a) application-entity;
b) application layer;
c) application process.
3.2 Directory model definitions
The following terms are defined in Rec. ITU-T X.501 | ISO/IEC 9594-2:
a) access control;
b) Administration Directory Management Domain;
c) alias;
d) ancestor;
e) attribute;
f) attribute type;
g) attribute value;
h) authentication;
i) compound entry;
j) context;
k) Directory Information Tree (DIT);
l) Directory Management Domain (DMD);
m) Directory System Agent (DSA);
n) Directory User Agent (DUA);
o) distinguished name;
p) entry;
q) family (of entries);
r) hierarchical group;
s) LDAP client;
t) LDAP requester;
u) LDAP responder;
v) LDAP server;
w) name;
x) object (of interest);
y) Private Directory Management Domain;
z) related entries;
aa) relative distinguished name;
bb) root;
cc) schema;
dd) security policy;
ee) subordinate object;
ff) superior entry;
gg) superior object;
hh) tree.
© ISO/IEC 2020 – All rights reserved
2 Rec. ITU-T X.500 (10/2019)
---------------------- Page: 7 ----------------------
ISO/IEC 9594-1:2020 (E)
3.3 Distributed Operation definitions
The following terms are defined in Rec. ITU-T X.518 | ISO/IEC 9594-4:
a) uni-chaining;
b) multi-chaining;
c) referral.
3.4 Replication definitions
The following terms are defined in Rec. ITU-T X.525 | ISO/IEC 9594-9:
a) caching;
b) cache copy;
c) entry copy;
d) master DSA;
e) replication;
f) shadow consumer;
g) shadow supplier;
h) shadowed information;
i) shadowing agreement.
3.5 Basic directory definitions
The following terms are defined in this Recommendation | International Standard:

3.5.1 the Directory: A collection of open systems cooperating to provide directory services.

3.5.2 directory information base (DIB): The set of information managed by the Directory.

3.5.3 (directory) user: The end user of the Directory, i.e., the entity or person which accesses the Directory.

4 Abbreviations

For the purposes of this Recommendation | International Standard, the following abbreviations apply:

ACI Access Control Information
DAP Directory Access Protocol
DIB Directory Information Base
DISP Directory Information Shadowing Protocol
DIT Directory Information Tree
DMD Directory Management Domain
DOP Directory Operational Binding Management Protocol
DSA Directory System Agent
DSP Directory System Protocol
DUA Directory User Agent
LDAP Lightweight Directory Access Protocol
OSI Open Systems Interconnection
RDN Relative Distinguished Name
5 Conventions

The term "Directory Specification" (as in "this Directory Specification") shall be taken to mean Rec. ITU-T X.500 |

ISO/IEC 9594-1. The term "Directory Specifications" shall be taken to mean the Rec. ITU-T X.500 | ISO/IEC 9594-1,

Rec. ITU-T X.501 | IO/IEC 9594-2, Rec. ITU-T X.511 | ISO/IEC 9594-3, Rec. ITU-T X.518 | ISO/IEC 9594-4,

© ISO/IEC 2020 – All rights reserved
Rec. ITU-T X.500 (10/2019) 3
---------------------- Page: 8 ----------------------
ISO/IEC 9594-1:2020 (E)

Rec. ITU-T X.519 | ISO/IEC 9594-5, Rec. ITU-T X.520 | ISO/IEC 9594-6, Rec. ITU-T X.521 | ISO/IEC 9594-7 and

Rec. ITU-T X.525 | ISO/IEC 9594-9.

If an International Standard or ITU-T Recommendation is referenced within normal text without an indication of the

edition, the edition shall be taken to be the latest one as specified in the normative references clause.

6 Overview of the Directory

The Directory is a collection of open systems which cooperate to hold a logical database of information about a set of

objects in the real world. The users of the Directory, including people and computer programs, can read or modify the

information, or parts of it, subject to having permission to do so. Each user is represented in accessing the Directory by a

Directory User Agent (DUA) or an LDAP client, each of which is considered to be an application-process. These concepts

are illustrated in Figure 1.

NOTE – The Directory Specifications refer to the Directory in the singular, and reflects the intention to create, through a single,

unified, name space, one logical directory composed of many systems and serving many applications. Whether or not these systems

choose to interwork will depend on the needs of the applications they support. Applications dealing with non-intersecting worlds

of objects may have no such need. The single name space facilitates later interworking should the needs change. For a variety of

reasons, such as security, connectivity, or business decisions, it is likely that some portions of the Directory may be unreachable

from other portions of the Directory using third edition operations. This results in differing views of the Directory. Such differing

views may contain related entries about a given real world object. Such related entries may or may not have the same distinguished

name. Using fourth or subsequent edition systems, it is possible to perform operations across multiple, differing views to provide

an integrated response to the user. Specifically:

– DMD administrators (see 9.2) may have a need to publish their own view (or views) of some specific real-world object; a

real-world object may thus be modelled by multiple independent entries in the directory. This may happen whether or not

they need to interwork. Interworking using DSP may also be unsupported.

– Notwithstanding the last sentence of the Note, it is also possible that particular DMDs may choose to publish information

about real-world objects within their own distinct directory name-spaces (i.e., in one of multiple DITs); in this case, it would

be possible to have a specific real-world object modelled by entries in the same or different DIT namespaces, with the same

or different distinguished names in each. Note that certain Directory facilities (e.g., the acquisition of certificates, and related

functions based on digital signatures) cannot be implemented when distinct objects are permitted to share distinguished

names.

– The objective of related entries is to provide a means whereby users can access such entries, bringing the resulting information

together, if possible. This would apply to the situation described by both of the preceding bullet points.

Figure 1 – Access to the Directory

The information held in the Directory is collectively known as the Directory Information Base (DIB). Clause 7 gives an

overview of its structure.

The Directory provides a well-defined set of access capabilities, known as the abstract service of the Directory, to its

users. This service, which is briefly described in clause 8, provides a simple modification and retrieval capability. This

can be built on with local DUA functions to provide the capabilities required by the end-users.

The Directory is distributed, both along functional and organizational lines. Clause 9 gives an overview of the

corresponding models of the Directory. These have been developed in order to provide a framework for the cooperation

of the various components to provide an integrated whole.

The Directory exists in an environment where various administrative authorities control access to their portion of the

information. Clause 10 gives an overview of access control.

When the Directory is distributed, it may be desirable to replicate information to improve performance and availability.

Clause 11 gives an overview of the Directory replication mechanism.
© ISO/IEC 2020 – All rights reserved
4 Rec. ITU-T X.500 (10/2019)
---------------------- Page: 9 ----------------------
ISO/IEC 9594-1:2020 (E)

The provision and consumption of the Directory services requires that the users (actually the DUAs and/or LDAP clients)

and the various functional components of the Directory should cooperate with one another. In many cases this will require

cooperation between application processes in different open systems, which in turn requires standardized application

protocols, briefly described in clause 11, to govern this cooperation.

The Directory has been designed so as to support multiple applications, drawn from a wide range of possibilities. The

nature of the applications supported governs which objects are listed in the Directory, which users access the information,

and which kinds of access they carry out. Applications may be very specific, such as the provision of distribution lists for

electronic mail, or generic, such as the 'inter-personal communications directory' application. The Directory provides the

opportunity to exploit commonness among the applications:

– A single object may be relevant to more than one application: Perhaps even the same piece of information

about the same object may be so relevant.

– To support this, a number of object classes and attribute types are defined, which are useful across a range

of applications. These definitions are contained in Rec. ITU-T X.520 | ISO/IEC 9594-6 and

Rec. ITU-T X.521 | ISO/IEC 9594-7.

– Certain patterns of use of the Directory are common across a range of applications: Annex A gives an

overview of this area.
7 The Directory Information Base (DIB)

NOTE 1 – The DIB, and its structure, are defined in Rec. ITU-T X.501 | ISO/IEC 9594-2.

The DIB is made up of information about objects. It is composed of (Directory) entries, each of which consists of a

collection of information on one object. An entry may be an aggregate of member entries each holding information about

a particular aspect of an object. Such an aggregate entry is called a compound entry. Each entry is made up of attributes,

each with a type and one or more values. The types of attribute which are present in a particular entry are dependent on

the class of object which the entry describes. Each value of an attribute may be tagged with one or more contexts that

specify information about a value that can be used to determine the applicability of the value.

The entries of the DIB are arranged in the form of a tree, the Directory Information Tree (DIT) where the vertices represent

the entries. Entries higher in the tree (nearer the root) will often represent objects such as countries or organizations, while

entries lower in the tree will represent people or application processes.

NOTE 2 – The services defined in the Directory Specifications operate only on a tree-structured DIT. The Directory Specifications

do not preclude the existence in the future of other structures (as the need arises).

Every entry has a distinguished name, which uniquely and unambiguously identifies the entry. These properties of the

distinguished name are derived from the tree structure of the information. The distinguished name of an entry is made up

of the distinguished name of its superior entry, together with specially nominated attribute values (the distinguished

values) from the entry.

Some of the entries at the leaves of the tree are alias entries, while other entries are object entries and compound entries.

Alias entries point to object entries, and provide the basis for alternative names for the corresponding objects.

A compound entry is an entry representing a single object and it is an aggregate of member entries each representing a

part of the information about the object.

The Directory enforces a set of rules to ensure that the DIB remains well-formed in the face of modifications over time.

These rules, known as the Directory schema, prevent entries having the wrong types of attributes for its object class,

attribute values being of the wrong form for the attribute type, and even entries having subordinate entries of the wrong

class.
Figure 2 illustrates the above concepts of the DIT and its components.
© ISO/IEC 2020 – All rights reserved
Rec. ITU-T X.500 (10/2019) 5
---------------------- Page: 10 ----------------------
ISO/IEC 9594-1:2020 (E)
Figure 2 – Structure of the DIT and of entries

Figure 3 gives a hypothetical example of a DIT. The tree provides examples of some of the types of attributes used to

identify different objects. For example the name:
{C=GB, L=Winslow, O=Graphic Services, CN=Laser Printer}

identifies the application entity, "Laser Printer", which has in its distinguished name the geographical attribute of Locality.

The residential person, John Jones, whose name is {C=GB, L=Winslow, CN=John Jones}, has the same geographical

attribute in his distinguished name.

The growth and form of the DIT, the definition of the Directory schema, and the selection of distinguished names for

entries as they are added, is the responsibility of various authorities, whose hierarchical relationship is reflected in the

shape of the tree. The authorities shall ensure, for example, that all of the entries in their jurisdiction have unambiguous

distinguished names, by carefully managing the attribute types and values which appear in those names. Responsibility

is passed down the tree from superior to subordinate authorities, with control being exercised by means of the schema.

Figure 3 – A hypothetical Directory Information Tree

The hierarchical group function allows an alternative hierarchical relationship to be established among entries

independent of the hierarchical relationship reflected by the DIT structure. The Directory Search operation (see 8.3.4) can

return not only information from matched entries, but also other members of the hierarchical group to which the matched

entry might belong. The hierarchical group function also has the advantage that it allows hierarchical relationships to be

changed wi
...

INTERNATIONAL ISO/IEC
STANDARD 9594-1
Ninth edition
Information technology — Open
systems interconnection —
Part 1:
The Directory: Overview of concepts,
models and services
PROOF/ÉPREUVE
Reference number
ISO/IEC 9594-1:2020(E)
ISO/IEC 2020
---------------------- Page: 1 ----------------------
ISO/IEC 9594-1:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO/IEC 2020

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii PROOF/ÉPREUVE © ISO/IEC 2020 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/IEC 9594-1:2020(E)
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.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of document should be noted (see www.iso.org/directives).

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. Details

of any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www.iso.org/patents) or the IEC list of patent

declarations received (see http://patents.iec.ch).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see

www.iso.org/iso/foreword.html.

This document was prepared by ITU-T as ITU-T X.500 (10/2019) and drafted in accordance with its

editorial rules. It was adopted by Joint Technical Committee ISO/IEC JTC 1, Information technology,

Subcommittee SC 6, Telecommunications and information exchange between systems.
A list of all parts in the ISO/IEC 9594 series can be found on the ISO website.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www.iso.org/members.html.
© ISO/IEC 2020 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO/IEC 9594-1:2020 (E)
CONTENTS
Page

1 Scope .............................................................................................................................................................. 1

2 Normative references...................................................................................................................................... 1

2.1 Identical Recommendations | International Standards ........................................................................ 1

3 Definitions ...................................................................................................................................................... 2

3.1 Communication model definitions ...................................................................................................... 2

3.2 Directory model definitions ................................................................................................................ 2

3.3 Distributed Operation definitions ........................................................................................................ 3

3.4 Replication definitions ........................................................................................................................ 3

3.5 Basic directory definitions .................................................................................................................. 3

4 Abbreviations ................................................................................................................................................. 3

5 Conventions .................................................................................................................................................... 4

6 Overview of the Directory .............................................................................................................................. 4

7 The Directory Information Base (DIB) .......................................................................................................... 5

8 The Directory service ..................................................................................................................................... 7

8.1 Introduction ......................................................................................................................................... 7

8.2 Service qualification ............................................................................................................................ 7

8.3 Directory interrogation ........................................................................................................................ 7

8.4 Directory modification ........................................................................................................................ 8

8.5 Other outcomes ................................................................................................................................... 8

9 The distributed Directory ............................................................................................................................... 9

9.1 Functional model ................................................................................................................................. 9

9.2 Organizational model .......................................................................................................................... 9

9.3 Operation of the model ........................................................................................................................ 9

10 Access control in the Directory ...................................................................................................................... 13

11 Service administration .................................................................................................................................... 14

12 Replication in the Directory ........................................................................................................................... 15

12.1 Introduction ......................................................................................................................................... 15

12.2 Forms of Directory replication ............................................................................................................ 15

12.3 Replication and consistency of Directory information ........................................................................ 16

12.4 Views of replication ............................................................................................................................ 16

12.5 Replication and Access Control .......................................................................................................... 17

13 Directory protocols ......................................................................................................................................... 17

Annex A – Applying the Directory ............................................................................................................................ 18

A.1 The Directory environment ................................................................................................................. 18

A.2 Directory service characteristics ......................................................................................................... 18

A.3 Patterns of use of the Directory ........................................................................................................... 18

Annex B – Amendments and corrigenda ................................................................................................................... 22

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ISO/IEC 9594-1:2020 (E)
Introduction

This Recommendation | International Standard together with other Recommendations | International Standards, 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.

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 introduces and models the concepts of the Directory and of the DIB and

overviews the services and capabilities which they provide. Other Recommendations | International Standards make use

of these models in defining the abstract service provided by the Directory, and in specifying the protocols through which

this service can be obtained or propagated.

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 ninth edition technically revises and enhances,

the eighth edition of this Recommendation | International Standard.

Annex A, which is an integral part of this Recommendation | International Standard, describes the types of use to which

the Directory can be applied.

Annex B, 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.

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ISO/IEC 9594-1:2020 (E)
INTERNATIONAL STANDARD
ITU-T RECOMMENDATION
Information technology – Open Systems Interconnection –
The Directory: Overview of concepts, models and services
1 Scope

The Directory provides the directory capabilities required by many application layer standards and telecommunication

services. Among the capabilities which it provides are those of "user-friendly naming", whereby objects can be referred

to by names which are suitable for citing by human users (though not all objects need have user-friendly names); and

"name-to-address mapping" which allows the binding between objects and their locations to be dynamic. The latter

capability allows networks, for example, to be "self-configuring" in the sense that addition, removal and the changes of

object location do not affect network operation.

The Directory is not intended to be a general-purpose database system, although it may be built on such systems. It is

assumed, for instance, that, as is typical with communication directories, there is a considerably higher frequency of

"queries" than of updates. The rate of updates is expected to be governed by the dynamics of people and organizations,

rather than, for example, the dynamics of networks. There is also no need for instantaneous global commitment of updates;

transient conditions, where both old and new versions of the same information are available, are quite acceptable.

It is a characteristic of the Directory that, except as a consequence of differing access rights or un-propagated updates,

the results of directory queries will not be dependent on the identity or location of the inquirer. This characteristic renders

the Directory unsuitable for some telecommunication applications, for example some types of routing. For cases where

the results are dependent on the identity of the inquirer, access to directory information and updates of the Directory may

be denied.
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.501 (2019) | ISO/IEC 9594-2:2020, Information technology – Open Systems

Interconnection – The Directory: Models.

– Recommendation ITU-T X.509 (2019) | ISO/IEC 9594-8:2020, Information technology – Open Systems

Interconnection – The Directory: Public-key and attribute certificate frameworks.

– Recommendation ITU-T X.511 (2019) | ISO/IEC 9594-3:2020, Information technology – Open Systems

Interconnection – The Directory: Abstract service definition.

– Recommendation ITU-T X.518 (2019) | ISO/IEC 9594-4:2020, Information technology – Open Systems

Interconnection – The Directory: Procedures for distributed operation.

– Recommendation ITU-T X.519 (2019) | ISO/IEC 9594-5:2020, Information technology – Open Systems

Interconnection – The Directory: Protocol specifications.

– Recommendation ITU-T X.520 (2019) | ISO/IEC 9594-6:2020, Information technology – Open Systems

Interconnection – The Directory: Selected attribute types.

– Recommendation ITU-T X.521 (2019) | ISO/IEC 9594-7:2020, Information technology – Open Systems

Interconnection – The Directory: Selected object classes.

– Recommendation ITU-T X.525 (2019) | ISO/IEC 9594-9:2020, Information technology – Open Systems

Interconnection – The Directory: Replication.
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3 Definitions

For the purposes of this Recommendation | International Standard, the following definitions apply.

3.1 Communication model definitions
The following terms are defined in Rec. ITU-T X.519 | ISO/IEC 9594-5:
a) application-entity;
b) application layer;
c) application process.
3.2 Directory model definitions
The following terms are defined in Rec. ITU-T X.501 | ISO/IEC 9594-2:
a) access control;
b) Administration Directory Management Domain;
c) alias;
d) ancestor;
e) attribute;
f) attribute type;
g) attribute value;
h) authentication;
i) compound entry;
j) context;
k) Directory Information Tree (DIT);
l) Directory Management Domain (DMD);
m) Directory System Agent (DSA);
n) Directory User Agent (DUA);
o) distinguished name;
p) entry;
q) family (of entries);
r) hierarchical group;
s) LDAP client;
t) LDAP requester;
u) LDAP responder;
v) LDAP server;
w) name;
x) object (of interest);
y) Private Directory Management Domain;
z) related entries;
aa) relative distinguished name;
bb) root;
cc) schema;
dd) security policy;
ee) subordinate object;
ff) superior entry;
gg) superior object;
hh) tree.
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3.3 Distributed Operation definitions
The following terms are defined in Rec. ITU-T X.518 | ISO/IEC 9594-4:
a) uni-chaining;
b) multi-chaining;
c) referral.
3.4 Replication definitions
The following terms are defined in Rec. ITU-T X.525 | ISO/IEC 9594-9:
a) caching;
b) cache copy;
c) entry copy;
d) master DSA;
e) replication;
f) shadow consumer;
g) shadow supplier;
h) shadowed information;
i) shadowing agreement.
3.5 Basic directory definitions
The following terms are defined in this Recommendation | International Standard:

3.5.1 the Directory: A collection of open systems cooperating to provide directory services.

3.5.2 directory information base (DIB): The set of information managed by the Directory.

3.5.3 (directory) user: The end user of the Directory, i.e., the entity or person which accesses the Directory.

4 Abbreviations

For the purposes of this Recommendation | International Standard, the following abbreviations apply:

ACI Access Control Information
DAP Directory Access Protocol
DIB Directory Information Base
DISP Directory Information Shadowing Protocol
DIT Directory Information Tree
DMD Directory Management Domain
DOP Directory Operational Binding Management Protocol
DSA Directory System Agent
DSP Directory System Protocol
DUA Directory User Agent
LDAP Lightweight Directory Access Protocol
OSI Open Systems Interconnection
RDN Relative Distinguished Name
5 Conventions

The term "Directory Specification" (as in "this Directory Specification") shall be taken to mean Rec. ITU-T X.500 |

ISO/IEC 9594-1. The term "Directory Specifications" shall be taken to mean the Rec. ITU-T X.500 | ISO/IEC 9594-1,

Rec. ITU-T X.501 | IO/IEC 9594-2, Rec. ITU-T X.511 | ISO/IEC 9594-3, Rec. ITU-T X.518 | ISO/IEC 9594-4,

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Rec. ITU-T X.519 | ISO/IEC 9594-5, Rec. ITU-T X.520 | ISO/IEC 9594-6, Rec. ITU-T X.521 | ISO/IEC 9594-7 and

Rec. ITU-T X.525 | ISO/IEC 9594-9.

If an International Standard or ITU-T Recommendation is referenced within normal text without an indication of the

edition, the edition shall be taken to be the latest one as specified in the normative references clause.

6 Overview of the Directory

The Directory is a collection of open systems which cooperate to hold a logical database of information about a set of

objects in the real world. The users of the Directory, including people and computer programs, can read or modify the

information, or parts of it, subject to having permission to do so. Each user is represented in accessing the Directory by a

Directory User Agent (DUA) or an LDAP client, each of which is considered to be an application-process. These concepts

are illustrated in Figure 1.

NOTE – The Directory Specifications refer to the Directory in the singular, and reflects the intention to create, through a single,

unified, name space, one logical directory composed of many systems and serving many applications. Whether or not these systems

choose to interwork will depend on the needs of the applications they support. Applications dealing with non-intersecting worlds

of objects may have no such need. The single name space facilitates later interworking should the needs change. For a variety of

reasons, such as security, connectivity, or business decisions, it is likely that some portions of the Directory may be unreachable

from other portions of the Directory using third edition operations. This results in differing views of the Directory. Such differing

views may contain related entries about a given real world object. Such related entries may or may not have the same distinguished

name. Using fourth or subsequent edition systems, it is possible to perform operations across multiple, differing views to provide

an integrated response to the user. Specifically:

– DMD administrators (see 9.2) may have a need to publish their own view (or views) of some specific real-world object; a

real-world object may thus be modelled by multiple independent entries in the directory. This may happen whether or not

they need to interwork. Interworking using DSP may also be unsupported.

– Notwithstanding the last sentence of the Note, it is also possible that particular DMDs may choose to publish information

about real-world objects within their own distinct directory name-spaces (i.e., in one of multiple DITs); in this case, it would

be possible to have a specific real-world object modelled by entries in the same or different DIT namespaces, with the same

or different distinguished names in each. Note that certain Directory facilities (e.g., the acquisition of certificates, and related

functions based on digital signatures) cannot be implemented when distinct objects are permitted to share distinguished

names.

– The objective of related entries is to provide a means whereby users can access such entries, bringing the resulting information

together, if possible. This would apply to the situation described by both of the preceding bullet points.

Figure 1 – Access to the Directory

The information held in the Directory is collectively known as the Directory Information Base (DIB). Clause 7 gives an

overview of its structure.

The Directory provides a well-defined set of access capabilities, known as the abstract service of the Directory, to its

users. This service, which is briefly described in clause 8, provides a simple modification and retrieval capability. This

can be built on with local DUA functions to provide the capabilities required by the end-users.

The Directory is distributed, both along functional and organizational lines. Clause 9 gives an overview of the

corresponding models of the Directory. These have been developed in order to provide a framework for the cooperation

of the various components to provide an integrated whole.

The Directory exists in an environment where various administrative authorities control access to their portion of the

information. Clause 10 gives an overview of access control.

When the Directory is distributed, it may be desirable to replicate information to improve performance and availability.

Clause 11 gives an overview of the Directory replication mechanism.
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The provision and consumption of the Directory services requires that the users (actually the DUAs and/or LDAP clients)

and the various functional components of the Directory should cooperate with one another. In many cases this will require

cooperation between application processes in different open systems, which in turn requires standardized application

protocols, briefly described in clause 11, to govern this cooperation.

The Directory has been designed so as to support multiple applications, drawn from a wide range of possibilities. The

nature of the applications supported governs which objects are listed in the Directory, which users access the information,

and which kinds of access they carry out. Applications may be very specific, such as the provision of distribution lists for

electronic mail, or generic, such as the 'inter-personal communications directory' application. The Directory provides the

opportunity to exploit commonness among the applications:

– A single object may be relevant to more than one application: Perhaps even the same piece of information

about the same object may be so relevant.

– To support this, a number of object classes and attribute types are defined, which are useful across a range

of applications. These definitions are contained in Rec. ITU-T X.520 | ISO/IEC 9594-6 and

Rec. ITU-T X.521 | ISO/IEC 9594-7.

– Certain patterns of use of the Directory are common across a range of applications: Annex A gives an

overview of this area.
7 The Directory Information Base (DIB)

NOTE 1 – The DIB, and its structure, are defined in Rec. ITU-T X.501 | ISO/IEC 9594-2.

The DIB is made up of information about objects. It is composed of (Directory) entries, each of which consists of a

collection of information on one object. An entry may be an aggregate of member entries each holding information about

a particular aspect of an object. Such an aggregate entry is called a compound entry. Each entry is made up of attributes,

each with a type and one or more values. The types of attribute which are present in a particular entry are dependent on

the class of object which the entry describes. Each value of an attribute may be tagged with one or more contexts that

specify information about a value that can be used to determine the applicability of the value.

The entries of the DIB are arranged in the form of a tree, the Directory Information Tree (DIT) where the vertices represent

the entries. Entries higher in the tree (nearer the root) will often represent objects such as countries or organizations, while

entries lower in the tree will represent people or application processes.

NOTE 2 – The services defined in the Directory Specifications operate only on a tree-structured DIT. The Directory Specifications

do not preclude the existence in the future of other structures (as the need arises).

Every entry has a distinguished name, which uniquely and unambiguously identifies the entry. These properties of the

distinguished name are derived from the tree structure of the information. The distinguished name of an entry is made up

of the distinguished name of its superior entry, together with specially nominated attribute values (the distinguished

values) from the entry.

Some of the entries at the leaves of the tree are alias entries, while other entries are object entries and compound entries.

Alias entries point to object entries, and provide the basis for alternative names for the corresponding objects.

A compound entry is an entry representing a single object and it is an aggregate of member entries each representing a

part of the information about the object.

The Directory enforces a set of rules to ensure that the DIB remains well-formed in the face of modifications over time.

These rules, known as the Directory schema, prevent entries having the wrong types of attributes for its object class,

attribute values being of the wrong form for the attribute type, and even entries having subordinate entries of the wrong

class.
Figure 2 illustrates the above concepts of the DIT and its components.
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Figure 2 – Structure of the DIT and of entries

Figure 3 gives a hypothetical example of a DIT. The tree provides examples of some of the types of attributes used to

identify different objects. For example the name:
{C=GB, L=Winslow, O=Graphic Services, CN=Laser Printer}

identifies the application entity, "Laser Printer", which has in its distinguished name the geographical attribute of Locality.

The residential person, John Jones, whose name is {C=GB, L=Winslow, CN=John Jones}, has the same geographical

attribute in his distinguished name.

The growth and form of the DIT, the definition of the Directory schema, and the selection of distinguished names for

entries as they are added, is the responsibility of various authorities, whose hierarchical relationship is reflected in the

shape of the tree. The authorities shall ensure, for example, that all of the entries in their jurisdiction have unambiguous

distinguished names, by carefully managing the attribute types and values which appear in those names. Responsibility

is passed down the tree from superior to subordinate authorities, with control being exercised by means of the schema.

Figure 3 – A hypothetical Directory Information Tree

The hierarchical group function allows an alternative hierarchical relationship to be established among entries

independent of the hierarchical relationship reflected by the DIT structure. The Directory Search operation (see 8.3.4) can

return not only information from matched entries, but also other members of the hierarchical group to which the matched

entry might belong. The hierarchical group function also has the advantage that it allows hierarchical relationships to be

changed without changing the DIT structure and thereby the distinguished names of the entries.

© ISO/IEC 2020
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