ISO/IEC 8824-2:2021

INTERNATIONAL ISO/IEC
STANDARD 8824-2
Sixth edition
2021-06
Information technology — Abstract
Syntax Notation One (ASN.1) —
Part 2:
Information object specification
Technologies de l'information — Notation de syntaxe abstraite
numéro un (ASN.1) —
Partie 2: Spécification des objets informationnels
Reference number
©
ISO/IEC 2021
© ISO/IEC 2021
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ii © ISO/IEC 2021 – All rights reserved

Foreword
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This document was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology,
Subcommittee SC 6, Telecommunications and information exchange between systems, in collaboration
with ITU-T. The identical text is published as ITU-T X.681 (02/2021).
This sixth edition cancels and replaces the fifth edition (ISO/IEC 8824-2:2015), which has been
technically revised. It also incorporates ISO/IEC 8824-2:2015/Cor 1:2018.
A list of all parts in the ISO/IEC 8824 series can be found on the ISO and IEC websites.
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© ISO/IEC 2021 – All rights reserved iii

CONTENTS
Page
Introduction . iv
1 Scope . 1
2 Normative references . 1
2.1 Identical Recommendations | International Standards . 1
3 Definitions . 1
3.1 Specification of basic notation . 1
3.2 Constraint specification . 1
3.3 Parameterization of ASN.1 specification . 1
3.4 Additional definitions . 2
4 Abbreviations . 3
5 Convention . 3
6 Notation . 3
6.1 Assignments . 3
6.2 Types . 3
6.3 Values . 4
6.4 Elements . 4
7 ASN.1 lexical items . 4
7.1 Information object class references . 4
7.2 Information object references . 4
7.3 Information object set references . 4
7.4 Type field references . 4
7.5 Value field references . 4
7.6 Value set field references . 5
7.7 Object field references . 5
7.8 Object set field references . 5
7.9 Word . 5
7.10 Additional keywords . 5
8 Referencing definitions . 5
9 Information object class definition and assignment . 6
10 Syntax List . 9
11 Information object definition and assignment . 12
12 Information object set definition and assignment . 14
13 Associated tables . 15
14 Notation for the object class field type . 16
15 Information from objects . 18
Annex A – The TYPE-IDENTIFIER information object class . 21
Annex B – Abstract syntax definitions . 22
Annex C – The instance-of type . 23
Annex D – Examples . 25
D.1 Example usage of simplified OPERATION class . 25
D.2 Example usage of "ObjectClassFieldType" . 26
D.3 Illustrate usage of objects and object sets. 27
Annex E – Tutorial annex on the ASN.1 model of object set extension . 28
Annex F – Summary of the notation . 29

Rec. ITU-T X.681 (02/2021) iii
© ISO/IEC 2021 – All rights reserved

Introduction
An application designer frequently needs to design a protocol which will work with any of a number of instances of some
class of information objects, where instances of the class may be defined by a variety of other bodies, and may be added
to over time. Examples of such information object classes are the "operations" of Remote Operations Service (ROS) and
the "attributes" of the OSI Directory.
This Recommendation | International Standard provides notation which allows information object classes as well as
individual information objects and information object sets thereof to be defined and given reference names.
An information object class is characterized by the kinds of fields possessed by its instances. A field may contain:
– an arbitrary type (a type field); or
– a single value of a specified type (a fixed-type value field); or
– a single value of a type specified in a (named) type field (a variable-type value field);
– a non-empty set of values of a specified type (a fixed-type value set field); or
– a non-empty set of values of a type specified in a (named) type field (a variable-type value set field); or
– a single information object from a specified information object class (an object field);
– an information object set from a specified information object class (an object set field).
A fixed-type value field of an information object class may be selected to provide unique identification of information
objects in that class. This is called the identifier field for that class. Values of the identifier field, if supplied, are required
to be unique within any information object set that is defined for that class. They may, but need not, serve to
unambiguously identify information objects of that class within some broader scope, particularly by the use of object
identifier as the type of the identifier field.
An information object class is defined by specifying:
– the names of the fields;
– for each field, the form of that field (type, fixed-type value, variable-type value, fixed-type value set,
variable-type value set, object, or object set);
– optionality and default settings of fields;
– which field, if any, is the identifier field.
An individual information object in the class is defined by providing the necessary information for each field.
The notation defined herein permits an ASN.1 type to be specified by reference to a field of some information object
class – the object class field type. In Rec. ITU-T X.682 | ISO/IEC 8824-3, notation is provided to enable this type to be
restricted by reference to some specific information object set.
It can be useful to consider the definition of an information object class as defining the form of an underlying conceptual
table (the associated table) with one column for each field, and with a completed row defining an information object. The
form of the table (determined by the information object class specification) determines the sort of information to be
collected and used to complete some protocol specification. The underlying conceptual table provides the link between
those specifying information objects of that class and the protocol which needs that information to complete its
specification. Typically, the actual information object set used to complete a particular protocol specification will be a
parameter of that protocol (see Rec. ITU-T X.683 | ISO/IEC 8824-4).
The "InformationFromObjects" notation referencing a specific object or object set (probably a parameter) can be used to
extract information from cells of conceptual tables.
This Recommendation | International Standard:
– Specifies a notation for defining an information object class, and for identifying it with a reference name
(see clause 9).
– Specifies a notation by which the definer of an information object class can provide a define
...