IEC 61158-6-21:2019

IEC 61158-6-21 ®
Edition 2.0 2019-06
INTERNATIONAL
STANDARD
Industrial communication networks – Fieldbus specifications –
Part 6-21: Application layer protocol specification – Type 21 elements

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IEC 61158-6-21 ®
Edition 2.0 2019-06
INTERNATIONAL
STANDARD
Industrial communication networks – Fieldbus specifications –

Part 6-21: Application layer protocol specification – Type 21 elements

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 25.040.40; 35.100.70; 35.110 ISBN 978-2-8322-7013-4

– 2 – IEC 61158-6-21:2019 © IEC 2019
CONTENTS
FOREWORD . 6
INTRODUCTION . 8
1 Scope . 9
1.1 General . 9
1.2 Overview. 9
1.3 Specifications . 9
1.4 Conformance . 10
2 Normative references . 10
3 Terms, definitions, symbols, abbreviations and conventions . 10
3.1 Terms and definitions from other ISO/IEC standards . 11
3.1.1 ISO/IEC 7498-1 terms . 11
3.1.2 ISO/IEC 8822 terms . 11
3.1.3 ISO/IEC 8824-1 terms . 11
3.1.4 ISO/IEC 9545 terms . 11
3.2 Other terms and definitions . 11
3.3 Abbreviations and symbols . 17
3.4 Conventions . 18
3.4.1 General conventions . 18
3.4.2 Convention for the encoding of reserved bits and octets . 18
3.4.3 Conventions for the common coding of specific field octets . 18
3.4.4 Conventions for APDU abstract syntax definitions . 19
3.4.5 Conventions for APDU transfer syntax definitions . 19
3.4.6 Conventions for AE state machine definitions . 20
4 FAL syntax description . 21
4.1 General . 21
4.2 FAL-AR PDU abstract syntax . 21
4.2.1 Top level definition . 21
4.2.2 Confirmed send service . 21
4.2.3 Unconfirmed send service . 21
4.2.4 FalArHeader . 21
4.2.5 InvokeID . 21
4.2.6 ServiceType . 21
4.3 Abstract syntax of PDU body . 22
4.3.1 ConfirmedServiceRequest PDUs . 22
4.3.2 ConfirmedServiceResponse PDUs . 22
4.3.3 UnconfirmedServiceRequest PDUs . 22
4.3.4 Error information . 22
4.4 Protocol data units (PDUs) for application service elements (ASEs) . 23
4.4.1 PDUs for Application process ASE . 23
4.4.2 PDUs for Service data object ASE . 25
4.4.3 PDUs for Process data object ASE . 28
5 Transfer Syntax . 28
5.1 Overview of encoding. 28
5.2 APDU header encoding . 29
5.2.1 Encoding of FalArHeader field . 29
5.2.2 Encoding of InvokeID Field . 29

5.2.3 Encoding of Type field . 29
5.3 APDU body encoding . 30
5.3.1 General . 30
5.4 Encoding of Data types . 30
5.4.1 General description of data types and encoding rules . 30
5.4.2 Transfer syntax for bit sequences . 30
5.4.3 Encoding of a Boolean value . 31
5.4.4 Encoding of an unsigned integer value . 31
5.4.5 Encoding of a signed integer . 31
5.4.6 Encoding of a floating point value . 32
5.4.7 Encoding of an octet string value . 32
5.4.8 Encoding of a visible string value . 33
5.4.9 Encoding of a Unicode string value . 33
5.4.10 Encoding of a time of day value . 33
5.4.11 Encoding of a Time Difference value . 33
6 FAL protocol state machines . 34
7 AP context state machine . 36
8 FAL service protocol machine . 36
8.1 General . 36
8.2 Common parameters of the primitives . 36
8.3 AP ASE protocol machine . 36
8.3.1 Primitive definitions . 36
8.3.2 State machine . 38
8.4 Service data object ASE protocol machine (SDOM) . 40
8.4.1 Primitive definitions . 40
8.4.2 State machine . 41
8.5 Process data object ASE protocol machine (PDOM) . 44
8.5.1 Primitive definitions . 44
8.5.2 State machine . 44
9 AR protocol machine . 45
9.1 General . 45
9.2 Point-to-point user-triggered confirmed client/server AREP (PTC-AR) ARPM . 46
9.2.1 PTC-AR Primitive definitions . 46
9.2.2 DLL mapping of PTC-AREP class . 46
9.2.3 PTC-ARPM state machine . 47
9.3 Multipoint network-scheduled unconfirmed publisher/subscriber AREP
(MSU-AR) ARPM . 48
9.3.1 MSU-AR primitive definitions . 48
9.3.2 DLL mapping of MSU-AR class . 49
9.3.3 MSU-ARPM state machine. 49
9.4 Multipoint user-triggered unconfirmed publisher/subscriber AREP (MTU-AR)
ARPM . 51
9.4.1 MTU-AR primitive definitions . 51
9.4.2 DLL mapping of MTU-AR class . 51
9.4.3 MTU-ARPM state machine . 52
10 DLL mapping protocol machine . 53
10.1 Primitive definitions . 53
10.1.1 Primitives exchanged between DMPM and ARPM . 53
10.1.2 Parameters of ARPM/DMPM primitives . 53

– 4 – IEC 61158-6-21:2019 © IEC 2019
10.1.3 Primitives exchanged between DLL and DMPM . 53
10.1.4 Parameters of DMPM/DLL primitives . 54
10.2 DMPM state machine . 54
10.2.1 DMPM states . 54
10.2.2 DMPM state table . 54
10.2.3 Functions used by DMPM . 54
Bibliography . 55

Figure 1 – Common structure of specific fields . 19
Figure 2 – APDU overview . 29
Figure 3 – Type field . 30
Figure 4 – Encoding of Time of Day value . 33
Figure 5 – Encoding of Time Difference value . 34
Figure 6 – Primitives exchanged between protocol machines . 35
Figure 7 – State transition diagram of APAM . 38
Figure 8 – State transition diagram of SDOM . 41
Figure 9 – State transition diagram of PDOM . 44
Figure 10 – State transition diagram of PTC-ARPM . 47
Figure 11 – State transition diagram of MSU-ARPM . 50
Figure 12 – State transition diagram of MTU-ARPM . 52
Figure 13 – State transition diagram of DMPM . 54

Table 1 – Conventions used for AE state machine definitions . 20
Table 2 – Status code for the confirmed response primitive . 23
Table 3 – Encoding of FalArHeader field . 29
Table 4 – Transfer Syntax for bit sequences . 30
Table 5 – Transfer syntax for data type UNSIGNEDn . 31
Table 6 – Transfer syntax for data type INTEGERn . 32
Table 7 – Primitives exchanged between FAL-user and APAM . 37
Table 8 – Parameters used with primitives exchanged FAL-user and APAM . 38
Table 9 – APAM state table – Sender transitions . 38
Table 10 – APAM state table – Receiver transitions . 39
Table 11 – Functions used by the APAM . 39
Table 12 – Primitives exchanged between FAL-user and SDOM . 40
Table 13 – Parameters used with primitives exchanged FAL-user and SDOM . 41
Table 14 – SDOM state table – Sender transitions . 42
Table 15 – SDOM state table – Receiver transitions . 43
Table 16 – Functions used by the SDOM . 43
Table 17 – Primitives exchanged between FAL-user and PDOM . 44
Table 18 – Parameters used with primitives exchanged between FAL-user and PDOM . 44
Table 19 – PDOM state table – Sender transitions . 45
Table 20 – PDOM state table – Receiver transitions . 45
Table 21 – Functions used by the SDOM . 45
Table 22 – Primitives issued by user to PTC-ARPM . 46

Table 23 – Primitives issued by PTC-ARPM to user . 46
Table 24 – PTC-ARPM state table – sender transactions . 47
Table 25 – PTC-ARPM state table – receiver transactions . 48
Table 26 – Function BuildFAL-PDU . 48
Table 27 – Primitives issued by user to ARPM . 48
Table 28 – Primitives issued by ARPM to user . 48
Table 29 – MSU-ARPM state table – sender transactions . 50
Table 30 – MSU-ARPM state table – receiver transactions . 50
Table 31 – Function BuildFAL-PDU . 50
Table 32 – Primitives issued by user to ARPM . 51
Table 33 – Primitives issued by ARPM to user . 51
Table 34 – MTU-ARPM state table – sender transactions. 52
Table 35 – MTU-ARPM state table – receiver transactions . 52
Table 36 – Function BuildFAL-PDU . 53
Table 37 – Primitives issued by ARPM to DMPM . 53
Table 38 – Primitives issued by DMPM to ARPM . 53
Table 39 – Primitives issued by DMPM to DLL . 53
Table 40 – Primitives issued by DLL to DMPM . 53
Table 41 – DMPM state table – sender transactions. 54
Table 42 – DMPM state table – receiver transactions . 54

– 6 – IEC 61158-6-21:2019 © IEC 2019
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INDUSTRIAL COMMUNICATION NETWORKS –
FIELDBUS SPECIFICATIONS –
Part 6-21: Application layer protocol specification –
Type 21 elements
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
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agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
Attention is drawn to the fact that the use of the associated protocol type is restricted by its
intellectual-property-right holders. In all cases, the commitment to limited release of
intellectual-property-rights made by the holders of those rights permits a layer protocol type to
be used with other layer protocols of the same type, or in other type combinations explicitly
authorized by its intellectual-property-right holders.
NOTE Combinations of protocol types are specified in IEC 61784-1 and IEC 61784-2.
International Standard IEC 61158-6-21 has been prepared by subcommittee 65C: Industrial
networks, of IEC technical committee 65: Industrial process measurement, control and
automation.
This second edition cancels and replaces the first edition published in 2010. This edition
constitutes a technical revision.

This edition includes the following significant technical changes with respect to the previous
edition:
• added WriteAndRead service;
• miscellaneous editorial corrections.
The text of this International standard is based on the following documents:
FDIS Report on voting
65C/948/FDIS 65C/956/RVD
Full information on the voting for the approval of this International standard can be found in
the report on voting indicated in the above table.
This publication has been drafted in accordance with ISO/IEC Directives, Part 2.
A list of all parts of the IEC 61158 series, published under the general title Industrial
communication networks – Fieldbus specifications, can be found on the IEC web site.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be:
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

– 8 – IEC 61158-6-21:2019 © IEC 2019
INTRODUCTION
This document is one of a series produced to facilitate the interconnection of automation
system components. It is related to other standards in the set as defined by the “three-layer”
fieldbus reference model described in IEC 61158–1.
The application protocol provides the application service by making use of the services
available from the data-link or other immediately lower layer. The primary aim of this
document is to provide a set of rules for communication expressed in terms of the procedures
to be carried out by peer application entities (AEs) at the time of communication. These rules
for communication are intended to provide a sound basis for development in order to serve a
variety of purposes:
• as a guide for implementers and designers;
• for use in the testing and procurement of equipment;
• as part of an agreement for the admission of systems into the open systems environment;
• as a refinement to the understanding of time-critical communications within OSI.
This document is concerned, in particular, with the communication and interworking of
sensors, effectors and other automation devices. By using this document together with other
standards positioned within the OSI or fieldbus reference models, otherwise incompatible
systems may work together in any combination.

INDUSTRIAL COMMUNICATION NETWORKS –
FIELDBUS SPECIFICATIONS –
Part 6-21: Application layer protocol specification –
Type 21 elements
1 Scope
1.1 General
This part of IEC 61158 is one of a series produced to facilitate the interconnection of
automation system components. It is related to other standards in the set as defined by the
three-layer fieldbus reference model described in IEC 61158-1.
This International Standard contains material specific to the Type 21 communication protocol.
1.2 Overview
The Fieldbus Application Layer (FAL) provides user programs with a means to access the
fieldbus communication environment. In this respect, the FAL can be viewed as a window
between corresponding application programs.
This document provides common elements for basic time-critical and non-time-critical
messaging communications between application programs in an automation environment, as
well as material specific to Type 21. The term “time-critical” is used to represent the presence
of a time-window, within which one or more specified actions must be completed with some
defined level of certainty. Failure to complete specified actions within the required time risks
the failure of the applications requesting the actions, with attendant risk to equipment, plant,
and possibly human life.
This document defines interactions between remote applications. It also defines the externally
visible behavior provided by the Type 21 application layer in terms of:
a) the formal abstract syntax defining the application layer protocol data units (APDUs)
conveyed between communicating application entities;
b) the transfer syntax defining encoding rules that are applied to the APDUs;
c) the application context state machine defining the application service behavior visible
between communicating application entities;
d) the application relationship state machines defining the communication behavior visible
between communicating application entities.
The purpose of this document is to:
a) describe the wire-representation of the service primitives defined in IEC 61158-5-21;
b) describe the externally visible behavior associated with their transfer.
This document defines the protocol of the Type 21 application layer in conformance with the
OSI Basic Reference Model (ISO/IEC 7498) and the OSI application layer structure
(ISO/IEC 9545).
1.3 Specifications
The principal objective of this document is to specify the syntax and behavior of the
application layer protocol that conveys the Type 21 application layer services.

– 10 – IEC 61158-6-21:2019 © IEC 2019
A secondary objective is to provide migration paths from previously existing industrial
communications protocols.
1.4 Conformance
This document does not restrict individual implementations or products, nor does it constrain
the implementations of application layer entities in industrial automation systems.
Conformance is achieved through implementation of this application layer protocol
specification.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
NOTE All parts of the IEC 61158 series, as well as IEC 61784-1 and IEC 61784-2 are maintained simultaneously.
Cross-references to these documents within the text therefore refer to the editions as dated in this list of normative
references.
IEC 61158-3-21:2019, Industrial communication networks – Fieldbus specifications –
Part 3-21: Data-link layer service definition – Type 21 elements
IEC 61158-4-21:2019, Industrial communication networks – Fieldbus specifications –
Part 4-21: Data-link layer protocol specification – Type 21 elements
IEC 61158-5-21:2019, Industrial communication networks – Fieldbus specifications –
Part 5-21: Application layer service definition – Type 21 elements
ISO/IEC 7498-1, Information technology – Open Systems Interconnection – Basic Reference
Model: The Basic Model
ISO/IEC/IEEE 8802-3, Information technology – Telecommunications and information
exchange between systems – Local and metropolitan area networks – Specific requirements –
Part 3: Standard for Ethernet
ISO/IEC 8822, Information technology – Open Systems Interconnection – Presentation
service definition
ISO/IEC 8824-1, Information technology – Abstract Syntax Notation One (ASN.1):
Specification of basic notation
ISO/IEC 9545, Information technology – Open Systems Interconnection – Application layer
structure
ISO/IEC 10731, Information technology – Open Systems Interconnection – Basic Reference
Model – Conventions for the definition of OSI services
ISO/IEC 9899, Information technology – Programming Languages – C
IEEE 754-2008, IEEE Standard for Binary Floating-Point Arithmetic
3 Terms, definitions, symbols, abbreviations and conventions
For the purposes of this document, the following terms, definitions, symbols, abbreviations
and conventions apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1 Terms and definitions from other ISO/IEC standards
3.1.1 ISO/IEC 7498-1 terms
For the purposes of this document, the following terms as defined in ISO/IEC 7498-1 apply:
a) application entity
b) application process
c) application protocol data unit
d) application service element
e) application entity invocation
f) application process invocation
g) application transaction
h) real open system
i) transfer syntax
3.1.2 ISO/IEC 8822 terms
For the purposes of this document, the following terms as defined in ISO/IEC 8822 apply:
a) abstract syntax
b) presentation context
3.1.3 ISO/IEC 8824-1 terms
For the purposes of this document, the following terms as defined in ISO/IEC 8824-1 apply:
a) object identifier
b) type
3.1.4 ISO/IEC 9545 terms
For the purposes of this document, the following terms as defined in ISO/IEC 9545 apply:
a) application-association
b) application-context
c) application context name
d) application-entity-invocation
e) application-entity-type
f) application-process-invocation
g) application-process-type
h) application-service-element
i) application control service element
3.2 Other terms and definitions
3.2.1
application
function or data structure for which data are consumed or produced

– 12 – IEC 61158-6-21:2019 © IEC 2019
3.2.2
application objects
multiple object classes that manage and provide a runtime exchange of messages across the
network and within the network device
3.2.3
application process
part of a distributed application on a network, which is located on one device and addressed
unambiguously
3.2.4
application process object
component of an application process that is identifiable and accessible through an FAL
application relationship
Note 1 to entry: Application process object definitions are composed of a set of values for the attributes of their
class (see the definition for “application process object class”). Application process object definitions may be
accessed remotely using the services of the FAL Object Management ASE. FAL Object Management services can
be used to load or update object definitions, to read object definitions, and to create and delete application objects
and their corresponding definitions dynamically.
3.2.5
application process object class
class of application process objects defined in terms of the set of their network-accessible
attributes and services
3.2.6
application relationship
cooperative association between two or more application-entity-invocations for the purpose of
exchange of information and coordination of their joint operation
Note 1 to entry: This relationship is activated either by the exchange of application-protocol-data-units or as a
result of preconfiguration activities.
3.2.7
application relationship application service element
application-service-element that provides the exclusive means for establishing and
terminating all application relationships
3.2.8
application relationship endpoint
context and behavior of an application relationship as seen and maintained by one of the
application processes involved in the application relationship
Note 1 to entry: Each application process involved in the application relationship maintains its own application
relationship endpoint.
3.2.9
attribute
description of an externally visible characteristic or feature of an object
Note 1 to entry: The attributes of an object contain information about variable portions of an object. Typically,
they provide status information or govern the operation of an object. Attributes may also affect the behavior of an
object. Attributes are divided into class attributes and instance attributes.
3.2.10
behavior
indication of how an object responds to particular events
3.2.11
channel
single physical or logical link of an input or output application object of a server to the process

3.2.12
class
set of objects, all of which represent the same type of system component
Note 1 to entry: A class is a generalization of an object, a template for defining variables and methods. All objects
in a class are identical in form and behavior, but usually contain different data in their attributes.
3.2.13
class attributes
attribute shared by all objects within the same class
3.2.14
class code
unique identifier assigned to each object class
3.2.15
class-specific service
service defined by a particular object class to perform a required function that is not
performed by a common service
Note 1 to entry: A class-specific object is unique to the object class that defines it.
3.2.16
client
a) object that uses the services of another (server) object to perform a task
b) initiator of a message to which a server reacts
3.2.17
consume
act of receiving data from a producer
3.2.18
consumer
node or sink that receives data from a producer
3.2.19
consuming application
application that consumes data
3.2.20
conveyance path
unidirectional flow of APDUs across an application relationship
3.2.21
cyclic
repetitive in a regular manner
3.2.22
data consistency
means for coherent transmission and access of the input- or output-data object between and
within client and server
3.2.23
device
physical hardware connected to the link
Note 1 to entry: A device may contain more than one node.

– 14 – IEC 61158-6-21:2019 © IEC 2019
3.2.24
device profile
collection of device-dependent information and functionality providing consistency between
similar devices of the same device type
3.2.25
diagnostic information
all data available at the server for maintenance purposes
3.2.26
end node
producing or consuming node
3.2.27
endpoint
one of the communicating entities involved in a connection
3.2.28
error
discrepancy between a computed, observed, or measured value or condition and the specified
or theoretically correct value or condition
3.2.29
error class
general grouping for related error definitions and corresponding error codes
3.2.30
error code
identification of a specific type of error within an error class
3.2.31
event
instance of a change of conditions
3.2.32
FIFO variable
variable object class composed of a set of homogeneously typed elements, where the first
written element is the first element that can be read
Note 1 to entry: In a fieldbus system, only one complete element can be transferred as a result of one service
invocation.
3.2.33
frame
simplified synonym for data link protocol data unit (DLPDU)
3.2.34
group
general term for a collection of objects
3.2.35
group
when describing an address, an address that identifies more than one entity
3.2.36
invocation
act of using a service or other resource of an application process

Note 1 to entry: Each invocation represents a separate thread of control that may be described by its context.
Once the service completes, or use of the resource is released, the invocation ceases to exist. For service
invocations, a service that has been initiated but not yet completed is referred to as an outstanding service
invocation. For service invocations, an Invoke ID may be used to identify the service invocation unambiguously and
differentiate it from other outstanding service invocations.
3.2.37
index
address of an object within an application process
3.2.38
instance
actual physical occurrence of an object within a class that identifies one of many objects in
the same object class
EXAMPLE California is an instance of the object class US-state.
Note 1 to entry: The terms object, instance, and object instance are used to refer to a specific instance.
3.2.39
instance attributes
attribute that is unique to an object instance and not shared by the object class
3.2.40
instantiated
object that has been created in a device
3.2.41
logical device
specific FAL class that abstracts a software component or a firmware component as an
autonomous self-contained facility of an automation device
3.2.42
manufacturer ID
identification of each product manufacturer by a unique number
3.2.43
management information
network-accessible information that supports management of the operation of the fieldbus
system, including the application layer
Note 1 to entry: Managing includes functions, such as controlling, monitoring, and diagnosis.
3.2.44
network
set of nodes connected by some type of communication medium, including any intervening
repeaters, bridges, routers, and lower-layer gateways
3.2.45
object
abstract representation of a particular component within a device, usually a collection of
related data in the form of variables, and methods (procedures) for operating on that data that
have clearly defined interface and behavior
3.2.46
object dictionary
collection of definitions, communication-specific attributes and parameters, and application-
dependent data
– 16 – IEC 61158-6-21:2019 © IEC 2019
3.2.47
object-specific service
service unique to the object class that defines it
3.2.48
physical device
automation or other network device
3.2.49
point-to-point connection
connection that exists between exactly two application objects
3.2.50
pre-established AR endpoint
AR endpoint placed in an established state during configuration of the AEs that control its
endpoints
3.2.51
process data
object(s) that are already pre-processed and transferred cyclically for the purpose of
information or further processing
3.2.52
produce
act of sending data to be received by a consumer
3.2.53
producer
node that is responsible for sending data
3.2.54
property
general term for descriptive information about an object
3.2.55
provider
source of a data connection
3.2.56
publisher
role of an AR endpoint that transmits APDUs onto the fieldbus for consumption by one or
more subscribers
Note 1 to entry: A publisher may not be aware of the identity or number of subscribers.
3.2.57
publishing manager
role of an AR endpoint in which it issues on
...