ISO/IEC 9594-1:2020

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 2020
© ISO/IEC 2020
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ii © ISO/IEC 2020 – All rights reserved

Foreword
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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.
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© ISO/IEC 2020 – All rights reserved iii

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
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)
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
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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2 Rec. ITU-T X.500 (10/2019)
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
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)
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
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 – All rights reserved
6 Rec. ITU-T X.500 (10/2019)
8 The Directory service
NOTE – The definition of the abstract service of the Directory can be found in Rec. ITU-T X.511 | ISO/IEC 9594-3.
8.1 Introduction
This clause provides an overview of the service provided to users, as represented by their DUAs and/or LDAP clients, by
the Directory. All services are provided by the Directory in response to requests from DUAs and/or LDAP clients. There
are requests which allow interrogation of the Directory, as described in 8.3, and those for modification, as described in
8.4. In addition, requests for service can be qualified, as described in 8.2. The Directory always reports the outcome of
each request that is made of it. The form of the normal outcome is specific to the request, and is evident from the
description of the request. Most abnormal outcomes are common to several requests. The possibilities are described in 8.5.
The Directory ensures that changes to the DIB, whether the result of a Directory service request, or by some other (local)
means, result in a DIB which continues to obey the rules of the Directory schema.
A user and the Directory are bound together for a period of time at an access point to the Directory. At the time of binding,
the user and the Directory optionally verify each other's identity.
8.2 Service qualification
8.2.1 Service controls
A number of controls can be applied to the various service requests, primarily to allow the user to impose limits on the
use of resources that the Directory shall not surpass, but also to control the progress of the Directory operation. Controls
are provided on, among other things: the amount of time, the size of results, the scope of search, the interaction modes,
and the priority of the request.
8.2.2 Security parameters
Each request may be accompanied by information in support of security mechanisms for protecting the Directory
information. Such information may include the user's request for various kinds of protection; a digital signature of the
request, together with information to assist the correct party to verify the signature.
8.2.3 Filters
A number of requests, whose outcome involves information from or concerning a number of entries, may carry with them
one or more filters. A filter expresses one or more conditions that an entry or a compound entry shall satisfy in order to
be returned as part of the outcome. This allows the set of entries returned to be reduced to only those relevant.
8.3 Directory interrogation
8.3.1 Read
A read request is aimed at a particular entry, or a compound entry and causes the values of some or all of the attributes of
that entry to be returned. In the case of compound entries, family member information is contained in a package (of syntax
similar to that of an attribute) comprising the selected family information. Where only some attributes are to be returned,
the DUA supplies the list of attribute types of interest as part of the request. A DUA may also supply one or more contexts
for one or more attribute types of interest, in order to select only those values that apply in the specified contexts.
NOTE – LDAP clients do not support the Read operation.
8.3.2 Compare
A compare request is aimed at a particular attribute of a particular entry or a compound entry, and causes the Directory
to check whether a supplied value matches a value of that attribute. A DUA may also supply one or more contexts for the
attribute value of interest to constrain the comparison operation.
NOTE – For example, this can be used to carry out password checking, where the password, held in the Directory, might be
inaccessible for read, but accessible for compare.
8.3.3 List
A list request causes the Directory to return the list of immediate subordinates of a particular named entry in the DIT.
NOTE – LDAP clients do not support the List operation.
© ISO/IEC 2020 – All rights reserved
Rec. ITU-T X.500 (10/2019) 7
8.3.4 Search
A search request causes the Directory to return information from all of the entries or compound entries within one or more
portions of the DIT that satisfy some filters. The information returned from each entry consists of some or all of the
attributes of that entry as with read. Information returned from related entries may be combined according to some join
criteria.
It is possible to put restrictions on the types of searches that can be performed by use of search-rules. It is also possible,
as a facility of search-rules, to progressively relax or tighten up searches within a single Directory operation, if too few
or too many items of entry information would otherwise be returned.
8.3.5 Abandon
An abandon request, as applied to an outstanding interrogation request, informs the Directory that the originator of the
request is no longer interested in the request being carried out. The Directory may, for example, cease processing the
request, and may discard any results so far achieved.
8.4 Directory modification
8.4.1 Add entry
An add entry request causes a new leaf entry to be added to the DIT. Contexts may be included with the attribute values
for the new entry.
8.4.2 Remove entry
A remove entry request causes a leaf entry or, if required, the entries comprising a compound entry to be removed from
the DIT.
8.4.3 Modify entry
A modify entry request causes the Directory to execute a sequence of changes to a particular entry or family member.
Either all of the changes are made, or none of them, and the DIB is always left in a state consistent with the schema. The
changes allowed include the addition, removal, or replacement of attributes or attribute values. Contexts may be included
with attribute values that are added to the entry. This operation can only be used on a single family member, but cannot
manipulate a compound entry as a whole.
A Modify Entry operation can supply in the result, if required, the information contained in the entry or compound entry
after a successful modification has taken place.
8.4.4 Modify distinguished name
A modify distinguished name (DN) request is used to change the relative distinguished name of an entry (either an object
entry, an alias, or a family member) or to move an entry, if not a family member, to a new superior in the DIT. If an entry
has subordinates, then all subordinates are renamed or moved accordingly. In the case of family members, these can be
moved to have new superiors, provided that they remain within the same compound entry.
8.5 Other outcomes
8.5.1 Errors
Any service may fail, for example because of problems with the user supplied parameters, in which case an error is
reported. Information is returned with the error, where possible, to assist in correcting the problem. However, in general
only the first error encountered by the Directory is reported. Besides the above-mentioned example of problems with the
parameters supplied by the user (particularly invalid names for entries or invalid attribute types) errors may arise from
violations of security policy, schema rules, and service controls.
8.5.2 Referrals
A service may fail because the particular access point to which the DUA or LDAP client is bound is not the most suitable
for carrying out the request, e.g., because the information affected by the request is (logically) far away from the access
point. In this case the Directory may return a referral, which suggests an alternative access point at which the DUA or
LDAP client can make its request.
NOTE – The Directory and the DUA may each have a preference as to whether referrals are used, or whether the requests are
chained (see 9.3). The DUA can express its preference by means of service controls. The Directory makes the final decision as to
which approach is used.
© ISO/IEC 2020 – All rights reserved
8 Rec. ITU-T X.500 (10/2019)
9 The distributed Directory
NOTE – The models of the Directory are defined in Rec. ITU-T X.501 | ISO/IEC 9594-2, while the procedures for the operation
of the distributed Directory are specified in Rec. ITU-T X.518 | ISO/IEC 9594-4.
9.1 Functional model
The functional model of the Directory is shown in Figure 4.
A Directory System Agent (DSA) is an application process which is part of the Directory and whose role is to provide
access to the DIB to DUAs, LDAP clients and/or other DSAs. A DSA may use information stored in its local database or
interact with other DSAs or LDAP servers to carry out requests. Alternatively, the DSA may direct a requester to another
DSA which can help carry out the request. A DSA that is capable of issuing an LDAP request and understanding the
associated LDAP response is said to be an LDAP requester. A DSA that is capable of understanding an LDAP request
and responding to that LDAP request is said to be an LDAP responder. Local databases are entirely implementation
dependent.
An LDAP server is an application process which is part of the Directory, that responds to requests via the LDAP protocol,
and whose role is to provide access to the DIB to LDAP clients and/or LDAP requesters. An LDAP server may use
information stored in its local database or may direct a requester to another LDAP responder or LDAP server which can
help carry out the request. As with DSAs, local databases are entirely implementation dependent.

Figure 4 – Functional model of the Directory
9.2 Organizational model
A set of one or more DSAs and/or LDAP servers and zero or more DUAs and/or LDAP clients managed by a single
organization may form a Directory Management Domain (DMD). The organization concerned may or may not elect to
make use of the Directory Specifications to govern the communications among the functional components within
the DMD.
The other Directory Specifications specify certain aspects of the behaviour of DSAs. For this purpose, a group of DSAs
within one DMD may, at the option of the organization which manages the DMD, behave as a single DSA.
9.3 Operation of the model
The DUA or LDAP client interacts with the Directory by communicating with one or more DSAs and/or LDAP servers.
A DUA or LDAP client need not be bound to any particular DSA or LDAP server. It may interact directly with various
DSAs and/or LDAP servers to make requests. However, if a LDAP client binds to an LDAP server, it cannot have requests
served by other than the LDAP server to which it is bound. For some administrative reasons, it may not always be possible
to interact directly with the DSA or LDAP server which needs to carry out the request, e.g., to return some directory
information. It is also possible that the DUA or LDAP client can access the Directory through a single DSA. For this
purpose, DSAs will need to interact with each other and DSAs may need to interact with LDAP servers.
The DSA is concerned with carrying out the requests of DUAs and LDAP clients, and with obtaining the information
where it does not have the necessary information. It may take the responsibility to obtain the information by interacting
with other DSAs and/or LDAP servers on behalf of the DUA or LDAP client.
A number of cases of request handling have been identified, as illustrated in Figures 5 through 7, and described below.
In Figure 5a, DSA C receives a referral from DSA A and is responsible for either conveying the request to the DSA B
(named in the referral from DSA A), or conveying the referral back to the originating DUA.
© ISO/IEC 2020 – All rights reserved
Rec. ITU-T X.500 (10/2019) 9
Figure 5a – Referrals
NOTE 1 – If DSA C returns the referral to the DUA, the "request (to B)" will not occur. Similarly, if DSA C conveys the request
to DSA B, it will not return a referral to the DUA.
In Figure 5b, DSA C receives a referral from DSA A and is responsible for either conveying the request to the DSA B
(named in the referral from DSA A), or conveying the referral back to the originating LDAP client.

Figure 5b – Referrals
NOTE 2 – In addition to any other ITU-T X.500 protocols it may support, DSA C in Figure 5b shall also be an LDAP responder.
NOTE 3 – If DSA C returns the referral to the LDAP client, the "request (to B)" will not occur. Similarly, if DSA C conveys the
request to DSA B, it will not return a referral to the LDAP client.
NOTE 4 – If DSA C returns the referral to the LDAP client, the referral shall be in the form of an LDAP referral. If the referral
returned by DSA A is in the form of an LDAP referral, DSA C may return that referral directly to the LDAP client; otherwise,
DSA C shall either convey the request to DSA B or translate the referral into an LDAP referral. If DSA C returns the referral to
the LDAP client, the client will bind directly to DSA B, which shall also be an LDAP responder. It will also be necessary for DSA
B to be an LDAP responder if DSA A returns an LDAP referral and DSA C conveys the request directly to DSA B.
In Figure 5c, the DUA receives the referral from DSA C, and is responsible for reissuing the request directly to DSA A
(named in the referral from DSA C).

Figure 5c – Referrals
In Figure 5d, the LDAP client receives the referral from DSA C, and is responsible for reissuing the request directly
to DSA A (named in the referral from DSA C).
© ISO/IEC 2020 – All rights reserved
10 Rec. ITU-T X.500 (10/2019)
Figure 5d – Referrals
NOTE 5 – Both DSA A and DSA C in Figure 5d shall be LDAP responders. Alternatively, either of these two DSAs could be an
LDAP server.
NOTE 6 – The referral that is returned to the LDAP client shall be in the form of an LDAP referral.
Figures 6a through 6c show DSA uni-chaining, whereby the request can be passed through several DSAs before the
response is returned.
Figure 6a – Uni-chaining
Figure 6b – Uni-chaining
NOTE 7 – In addition to any other ITU-T X.500 protocols it may support, the DSA on the left in Figure 6b shall also be an LDAP
responder.
Figure 6c – Uni-chaining
© ISO/IEC 2020 – All rights reserved
Rec. ITU-T X.500 (10/2019) 11
NOTE 8 – In addition to any other ITU-T X.500 protocols it may support, the DSA in Figure 6c shall also be both an LDAP
responder and an LDAP requester.
Figures 7a through 7c show multi-chaining, where the DSA associated with the DUA or LDAP client carries out the
request by forwarding it to two or more other D
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