ISO 7498:1984

International Standard 7498
INTERNATIONAL ORGANIZATION FOR STANDARDlZATION.MEXAYHAPOAHAR OPrAHM3AUMR il0 CTAHAAPTH3AUMM.ORGANlSATlON INTERNATIONALE DE NORMALISATION
Information processing systems - Open Systems
* Interconnection - Basic Reference Model
Systèmes de traitement de l'information - Interconnexion de systèmes ouverts - Modèle de Référence de base
First edition - 1984-10-15
-
UDC 681.3.01 Ref. No. 607498-1984 (E)
~; c
Descriptors : data processing, information interchange, open system interconnection, reference model
O
Price based on 40 pages
1-
Foreword
IS0 (the International Organization for Standardization) is a worldwide federation of
national standards bodies (IS0 member bodies). The work of preparing International
IS0 technical committees. Each member
Standards is normally carried out through
body interested in a subject for which a technical committee has been established has
the right to be represented on that committee. International organizations, govern-
mental and non-governmental, in liaison with ISO, also take part in the work.
Draft International Standards adopted by the technical committees are circulated to
the member bodies for approval before their acceptance as International Standards by
the IS0 Council. They are approved in accordance with IS0 procedures requiring at
least 75 % approval by the member bodies voting.
International Standard IS0 7498 was prepared by Technical Committee ISO/TC 97,
Information processing systems.
NOTE - Possible questions arising using International Standard IS0 7498 should be directed to
subcommittee ISO/TC 97/SC 21 - Information Processing Systems - Information Retrieval,
Transfer and Management for Open Systems interconnection - through its Secretariat (ANSI).
Subcommittee ISO/TC 97/SC 21 handles such questions and maintains a register of answers to
questions of general interest.
Information about the register and its availability may be obtained from :
Secretariat of iSO/TC 97/SC 21 or IS0 Central Secretariat
American National Standards institute 1, rue de Varembé
1430 Broadway Case postale 56
NEW YORK, N.Y. 10018 CH-1211 GENEVA 20
USA Switzerland
O International Organization for Standardization, 1984 0
Printed in Switzerland
Contents
Page
O Introduction , . . . . . . . . . . . . . <.<.<.
1 Scope and field of application . . . .
.... 2
,. <._. 2
2 Definitions . . . . . . . . . . . . . . . . . . . . . . . . .
.......... ....... . ..... <.........<...... 2
4 Introduction to Open Systems Interconnection (OsIl . . . . . . . . . . . . . . . . . 2
. . . . . . . . . . . . . . . . . . . . . . . . . <.<.
4.2 Open Systems Interconnection environment . . . . . . . . . . . . . . . . . . . . . . .
4.3 Modelling the OS1 environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5 Concepts of a layered architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . '"''.~'.".''.'
5.2 Principles of layering . 5
5.3 Communication between peer-entities . . . . . . . . . . 7
5.4 Identifiers . . . . . . . . . .
5.5 Properties of service-access-points . . . . . . . . . . . . . . . . . . . .
5.6 Data-units . . . . . , . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.7 Elements of layer operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.8 Routing .,. <<,. . <.<.<.<<<.<<. 16
5.9 Management Aspects of OS1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6 Introduction to the specific OS1 layers ._.,._ <<.
6.1 Specific layers. . . . . . . . . . . . . <.<.<.<<.<.
ciples used to determine the seven layers of the Reference
....... <...._.._. ..__.....__..._.. <<....... 20
6.3 Layer descriptions . . . . . . . . . . . .,.,. <.<. 20
7 Detailed description of the resulting OS1 architecture . . . . . . . . . . , . , . , , . . . .
7.1 Application Layer . <_.<._._._.<._. 20
7.2 Presentation Layer <<.<_.<<.<<<._._._._.<_. 22
iii
7.3 Session Layer . .
7.4 Transport Layer . . .
7.5 Network Layer . . 29
7.6 Data Link Layer. . . 33
7.7 Physical Layer. . . 34
Annexes
............ 38
Brief explanation of how the layers were chosen .
A
6 Alphabetical index to definitions . . 39
iv
INTERNATIONAL STANDARD IS0 7498-1984 (E)
Information processing systems - Open Systems
Interconnection - Basic Reference Model
e
While the scope of the general architectural principles required
O Introduction
for OS1 is very broad, this International Standard is primarily
concerned with systems comprising terminals, computers and
0.1 About this standard
associated devices and the means for transferring information
between such systems. Other aspects of OS1 requiring atten-
The purpose of this International Standard Reference Model of
tion are described briefly (see 4.2).
Open Systems Interconnection is to provide a common basis
for the coordination of standards development for the purpose
The justification for development of standards shall follow nor-
of systems interconnection, while allowing existing standards
mal administrative procedures even though such standards are
to be placed into perspective within the overall Reference
identified in the Reference Model.
Model.
As standards emerge to meet the OS1 requirements, a small
The term Open Systems Interconnection (OSI) qualifies stan-
?umber of practical subsets should be defined by the standards
dards for the exchange of information among systems that are
developers from optional functions, to facilitate implementa-
"open" to one another for this purpose by virtue of their mutual
tion and compatibility.
use of the applicable standards.
The description of the Reference Model of OS1 given in this In-
The fact that a system is open does not imply any particular
ternational Standard is developed in stages :
systems implementation, technology or means of interconnec-
tion, but refers to the mutual recognition and support of the ap-
Clause 4 establishes the reasons for Open Systems Inter-
plicable standards.
connection, defines what is being connected, the scope of
the interconnection and, describes the modelling principles
It is also the purpose of this International Standard to identify
used in OSI;
areas for developing or improving standards, and to provide a
common reference for maintaining consistency of all related
Clause 5 describes the general nature of the architecture of
standards. It is not the intent of this International Standard
the Reference Model: namely that it is layered, what layer-
either to serve as an implementation specification, or to be a
ing means, and the principles used to describe lavers;
basis for appraising the conformance of actual implementa-
tions, or to provide a sufficient level of detail to define precisely
Clause 6 names, and introduces the specific layers of the ar-
the services and protocols of the interconnection architecture.
chitecture; and
Rather, this International Standard provides a conceptual and
functional framework which allows international teams of ex- Clause 7 provides the description of the specific layers.
perts to work productively and independently on the
An indication of how the layers were chosen is given in
development of standards for each layer of the Reference
Model of OSI. annex A to this International Standard.
The Reference Model has sufficient flexibility to accommodate The Reference Model serves as a framework for the definition
advances in technology and expansion in user demands. This of services and protocols which fit within the boundaries
flexibility is also intended to allow the phased transition from established by the Reference Model.
existing implementations to OS1 standards.
In those few cases where a feature is explicitly marked (op-
tional) in the Reference Model it should remain optional in the
NOTE - The Reference Model is expected to be subject to future ex-
corresponding service or protocol (even if at a given instant the
pansion. Some anticipated directions of expansion are indicated by
notes or footnotes in this International Standard. two cases of the option are not yet documented).
IS0 7498-1984 (E)
0.2 Related OS1 standards This notation is also used for other concepts in the model
which are related to these layers, for example (Ni-protocol,
Concurrently with the preparation of this International Stan- (N + 1 )-service.
dard, work is in progress within IS0 on the development of OS1
Clause 6 introduces names for individual layers. When referring
standards in the following areas :
to these layers by name, the (NI-, iN + 1)- and (N - 1)- prefixes
a) virtual terminal protocols;
are replaced by the names of the layers, for example transport-
protocol, session entity, and network-service.
b) file transfer, access and management protocols;
Ci job transfer and manipulation protocols;
4 Introduction to Open Systems
Interconnection (OsIl
di common application services and protocols;
NOTE - The general principles described in clauses 4 and 5 hold for all
e) presentation layer services and protocols;
layers of the Reference Model, unless layer specific statements to the
contrary are made in clauses 6 and 7.
f) Session Layer services and protocols;
g) Transport Layer services and protocols;
4.1 Definitions
h) Network Layer services and protocols;
4.1.1 real system : A set of one or more computers, the
m
associated software, peripherals, terminals, human operators,
j) Data Link Layer services and protocols;
physical processes, information transfer means, etc., that
k) Physical Layer services and protocols; and forms an autonomms whole capable of performing information
processing and/or information transfer.
mi OS1 management protocols.
4.1.2 real open system : A real system which complies with
The first five items in this list relate to the Application and
the requirements of OS1 standards in its communication with
Presentation Layers of the Reference Model.
other real systems.
In addition, liaison is maintained with CCITT in the develop-
4.1.3 open system : The representation within the
ment of OS1 standards.
Reference Model of those aspects of a real open system that
are Dertinent to OSI.
1 Scope and field of application
4.1.4 application-process : An element within a real open
This International Standard describes the Reference Model of
system which performs the information processing for a par-
Open Systems Interconnection. It establishes a framework for
ticular application.
coordinating the development of existing and future standards
for the interconnection of systems and is provided for reference
4.2 Open Systems Interconnection environment
by those standards.
In the concept of OSI, a real system is a set of one or more 0
This International Standard does not specify services and pro-
computers, associated software, peripherals, terminals, human
tocols for OSI. It is neither an implementation specification for
operators, physical processes, information transfer means,
systems, nor a basis for appraising the conformance of im-
plementations. etc., that forms an autonomous whole capable of performing
information processing and/or information transfer.
An application-process is an element within an open system
2 Definitions
which performs the information processing for a particular ap-
plication.
Definitions of terms are included at the beginning of individual
clauses and sub-clauses. An index of these terms is provided in
Application-processes can represent manual processes, com-
an annex B for easy reference.
puterized processes or physical processes.
3 Notation Some examples of application-processes that are applicable to
this open system definition are the following :
Layers are introduced in clause 5. An (Ni-, (N+ 1)- and
a) a person operating a banking terminal is a manual
(N - 1)- notation is used to identify and relate adjacent layers :
application-process;
(NI-layer : any specific layer;
b) a FORTRAN program executing in a computer centre
and accessing a remote database is a computerised
(N + 1)-layer : the next higher layer;
application-process; the remote database management
systems server is also an application-process; and
f N - 1)-layer : the next lower layer.
IS0 7498-1984 (E)
c) a process control program executing in a dedicated com- OS1 is concerned only with interconnection of systems. All
puter attached to some industrial equipment and linked into other aspects of systems which are not related to interconnec-
a plant control system is a physical application-process. tion are outside the scope of OSI.
OS1 is concerned with the exchange of information between OS1 is concerned not only with the transfer of information be-
open systems (and not the internal functioning of each in- tween systems, i.e. transmission, but also with their capability
dividual real open system). to interwork to achieve a common (distributed) task. In other
words, OS1 is concerned with the interconnection aspects of
cooperation') between systems, which is implied by the expres-
As shown in figure 1, the physical media for Open Systems In-
terconnection provides the means for the transfer of informa- sion "systems interconnection".
tion between open systems.
The objective of OS1 is to define a set of standards to enable
real open systems to cooperate. A system which complies with
the requirements of applicable OS1 standards in its cooperation
NOTE -- At this point, only telecommunications media have been con-
sidered The use of other interconnection media is for further study. with other systems is termed a real open system.
-------
Physical media
Figure 1 - Open systems connected by physical media
1 i Cooperation among open systems involves a broad range of activities of which the following have been identified :
a) interprocess communication, which concerns the exchange of information and the synchronization of activity between OS1 application-
processes;
b) data representation, which concerns all aspects of the creation and maintenance of data descriptions and data trarisformations for reformatting
data exchanged between open systems;
ci data storage, which concerns storage media, and file and database systems for managing and providing access to data stored on the media,
d) process and resource management, which concerns the means by which OS1 appliration-processes are declared, initiated and controlled, aiiri
the means by which they acquire OS1 resources;
e) integrity and security, which concern information processing constraints that have to be be preserved or assured during the operation of the
open systems; and
f) program support, which concerns the definition, compilation, linking, testing, storage, transfer, and access to the programs executed by OS1
application-processes.
Some of these activities may imply exchange of information between the interconnected open systems and their interconnection aspects may,
therefore, be of concern to OSI.
This International Standard covers the elements ot OS1 aspects of these activities which are essential for early development of OS1 standards.
1 IS0 7498-1984 (E)
The reader not familiar with the technique of abstract modelling
4.3 Modelling the OS1 environment
is cautioned that those concepts introduced in the description
The development of OS1 standards, i.e. standards for the inter- of open systems constitute an abstraction despite a similar ap-
pearance to concepts commonly found in real systems.
connection of real open systems, is assisted by the use of
abstract models. To specify the external behaviour of intercon- Therefore real open systems need not be implemented as
described by the Model.
nected real open systems, each real open system is replaced by
a functionally equivalent abstract model of a real open system
called an open system. Only the interconnection aspects of Throughout the remainder of this International Standard, only
these open systems would strictly need to be described. the aspects of real systems and application-processes which lie
However to accomplish this, it is necessary to describe both the within the OS1 environment are considered. Their interconnec-
internal and external behaviour of these open systems. Only the tion is illustrated throughout this International Standard as
external behaviour of open systems is retained as the standard depicted in figure 2.
of behaviour of real open systems. The description of the inter-
nal behaviour of open systems is provided in the Reference
Model only to support the definition of the interconnection
5 Concepts of a layered architecture
aspects. Any real system which behaves externally as an open
system can be considered to be a real open system.
5.1 Introduction
This abstract modelling is used in two steps.
Clause 5 sets forth the architectural concepts that are applied
First, basic elements of open systems and some key decisions in the development of the Reference Model of Open Systems
concerning their organization and functioning, are developed. Interconnection. Firstly, the concept of a layered architecture
This constitutes the Reference Model of Open Systems Inter-
(with layers, entities, service-access-points, protocols, connec-
connection described in this International Standard. tions, etc.) is described. Secondly, identifiers are introduced
for entities, service-access-points, and connections. Thirdly,
Then, the detailed and precise description of the functioning of service-access-points and data-units are described. Fourthly,
the open system is developed in the framework formed by the elements of layer operation are described including connec-
Reference Model. This constitutes the services and protocols tions, transmission of data, and error functions. Then, routing
for OS1 which are the subject of other International Standards. aspects are introduced and finally, management aspects are
discussed.
It should be emphasized that the Reference Model does not, by
itself, specify the detailed and precise functioning of the open The concepts described in clause 5 are those required to
system and, therefore, it does not specify the external describe the Reference Model of Open Systems Interconnec-
behaviour of real open systems and does not imply the struc- tion. However, not all of the concepts described are employed
in each layer of the Reference Model.
~ ture of the implementation of a real open system.
Aspects of application-processes of concern to OSI,
I i.e., application-entities (see 7.1)
Open system S
Aspects of real open
systems of
concern to OS1
t \
Physical media for OS1
Figure 2 - Basic elements of OS1
IS0 7498-1984 E)
5.2.1.3 (NI-entity : An active element within an
Four elements are basic to the Reference Model (see figure 2) :
i NI-subsystem.
ai open systems;
5.2.1.4 peer-entities : Entities within the same layer.
the application-entities which exist within the OS1 en-
b)
5.2.1.5 sublayer : A subdivision of a layer,
vironment;
CI the connections (see 5.3) which join the application- 5.2.1.6 (NI-service : A capability of the (NI-layer and the
entities and permit them to exchange information (see layers beneath it, which is provided to (N + 1)-entities at the
note 1); and boundary between the (NI-layer and the (N + 1)-layer.
the physical media for OSI.
di
5.2.1.7 (NI-facility : A part of an (NI-service.
NOTES
5.2.1.8 (NI-function : A part of the activity of (Ni-entities.
1 This Basic Reference Model of Gpen Systems Interconnection is
based on the assumption that a connection is required for the transfer 5.2.1.9 (NI-service-access-point : The point at which
of data. An addendum to this International Standard is currently being (NI-services are provided by an (NI-entity to an (N + lbentity.
developed to extend the description to cover the connectionless forms
of data transmission which may be found in a wide variety of data com-
5.2.1.10 (NI-protocol : A set of rules and formats (semantic
munications techniques (for example local area networks, digital radio,
and syntatic) which determines the communication behaviour
etc.) and applications (for example remote sensing and banking).
of (NI-entities in the performance of (Ni-functions.
2 Security aspects which are also general architectural elements of
protocols are not discussed in this International Standard.
5.2.2 Description
The basic structuring technique in the Reference Model of
5.2 Principles of layering
Open Systems Interconnection is layering. According to this
technique, each open system is viewed as being logically com-
posed of ail ordered set of subsystems, represented for conve
5.2.1 Definitions
nience in the vertical sequence shown in figure 3. Adjacent
subsystems communicate through their common boundary.
5.2.1.1 (NI-subsystem : An element in a hierarchical division
Subsystems of the same rank (Ni collectively form the (NI-layer
of an open system which interacts directly only with elements
of the Reference Model of Open Systems Interconnection. An
in the next higher division or the next lower division of that
i NI-subsvstem consists of one or several (NI-entities. Entities
open system.
rxist in each layer. Entities in the same layer are termed peer-
entities. Note that the highest layer does not have an
(N + 1) layer above it and the lowest layer does not have an
5.2.1.2 (NI-layer : A subdivision of the OS1 architecture,
constituted by subsystems of the same rank (NI. (N -- 1)-layer below it.
Open Open Open Open
system A system B system C system S
Physical media for OS1
I
Figure 3 - Layering in co-operating open systems
IS0 7498-1984 (E)
layer which may be bypassed. The bypassing of all the sublayers of a
Not all peer (NI-entities need or even can communicate. There
layer is not allowed. A sublayer uses the entities and connections of its
may be conditions which prevent this communication (for
layer. The detailed definition or additional characteristics of a sublayer
example: they are not in interconnected open systems, or they
are for further study.
do not support the same protocol subsets).
Except for the highest layer, each (Ni-layer provides
NOTES
(N + 1)-entities in the (N + 1)-layer with (NI-services. The
1 The distinction between the type of some object and an instance of
highest layer is assumed to represent all possible uses of the
that object is a distinction of significance for OSI. A type is a descrip-
services which are provided by the lower layers.
tion of a class of objects. An instance of this type is any object that
conforms to this description. The instances of the same type constitute
NOTES
a class. A type, and any instançes of this type can be referred to by an
individual name. Each nameable instance and the type to which this in-
1 Not all open systems provide the initial source or final destination of
stance belongs should carry distinguishable names.
data. Such open systems need not contain the higher layers of the ar-
chitecture isee figures 6 and 13).
For example, given that a programmer has written a computer pro-
gram, that programmer has generated a type of something where in-
2 Classes of service may be defined within the (Ni-services. The
stances of that are created every time that particular program is invok-
precise definition of the term ”classes of service” is for further study.
ed into execution by a computer. Thus, a FORTRAN compiler is a type
and each occasion where a copy of that program is invoked in a data
processing machine one displays an instance.
Each service provided by an (NI-layer may be tailored by the
selection of one or more (NI-facilities which determine the at-
Consider now an (Ni-entity in the OS1 context. It too, has two aspects,
tributes of that service. When a single (Ni-entity cannot by
a type and a collection of instances. The type of an (Ni-entity is defined
it is able to perform. An in- itself fully support a service requested by an (N + 1 )-entity it
by the specific set of (NI-layer functions
stance of that type of (Ni-entity is a specific invocation of whatever it is calls upon the co-operation of other (Ni-entities to help com-
within the relevant open system that provides the (Ni-layer functions
plete the service request. In order to co-operate, (NI-entities in
called for by its type for a particular occasion of communication. It
any layer, other than those in the lowest layer, communicate by
follows from these observations that (Ni-entity types refer only to the
means of the set of services provided by the (N - 1 )-layer (see
properties of an association between peer (Ni-entities, while an
figure 4). The entities in the lowest layer are assumed to com-
(Ni-entity instance refers to the specific, dynamic occasions of actual
municate directly via the physical media which connect them.
information exchange.
It is important to note that actual communication occurs only between
The services of an (NI-layer are provided to the (N + 1 )-layer,
(Ni-entity instances at all layers. It is only at connection establishment
using the (NI-functions performed within the (N)-layer and as
time (or its logical equivalent during a recovery process) that (Ni-entity
necessary the services available from the (N - 1 )-layer.
types are explicitly relevant. Actual connections are always made to
specific (NI-entity instances, although a request for connection may
An (NI-entity may provide services to one or more
well be made for arbitrary (NI-entity instances of a specified type.
(N+ 1)-entities and use the services of one or more
Nothing in this International Standard, however, precludes the request
(N - 1)-entities. An (NI-service-access-point is the point at
for a connection with a specific (named) instance of a peer (Ni-entity.
If an (NI-entity instance is aware of the name of its peer (Ni-entity in- which a pair of entities in adjacent layers use or provide services
stance, it should be able to request another connection to that isee figure 7).
(Ni-entity instance.
Co-operation between (NI-entities is governed by one or more
2 It may be necessary to further divide a layer into small substructures
(NI-protocols. The entities and protocols within a layer are il-
called sublayers and to extend the technique of layering to cover other
lustrated in figure 5.
dimensions of OSI. A sublayer is defined as a grouping of functions in a
(N + 1)-layer
/
i Ni-layer
Figure 4 - (N + 1)-entities in the (N,+ 1)-layer communicate through the (Nl-layer
IS0 7498-1984 (E)
(Ni-entities
(Ni-protocol
5 - (NI-protocols between (NI-entities
Figure
5.3.1.11 (NI-simplex-transmission : ( NI-data-transmission
5.3 Communication between peer-entities
in one pre-assigned direction. 1)
e 5.3.1 Definitions
5.3.1 .I2 (NI-data-communication : An (NI-function which
transfers (NI-protocol-data-units (see 5.6.1.3) according to an
5.3.1 .I (NI-connection : An association established by the (NI-protocol, over one or more (N - l)-connedions.l)
(NI-layer between two or more (N + 1 bentities for the transfer
of data.
5.3.1.13 (NI-two-way-simultaneous communication :
(NI-data-communication in both directions at the same time.
5.3.1.2 (NI-connection-endpoint : A terminator at one end
5.3.1.14 (NI-two-way-alternate communication :
of an (NI-connection within an (NI-service-access-point.
(NI-data-communication in both directions, one direction at a
time.
5.3.1.3 multi-endpoint-connection : A connection with
5.3.1 .I5 (NI-one-way-communication : (Ni-datacommuni-
more than two connection-endpoints.
cation in one pre-assigned direction.
5.3.1.4 correspondent (NI-entities : (N)-entities with an
5.3.2 Description
(N - 1 )-connection between them.
For information to be exchanged between two or more
(N + 1 bentities, an association shall be established between
5.3.1.5 (NI-relay : An (NI-function by means of which an
them in the (NI-layer using an (NI-protocol.
(NI-entity forwards data received from one correspondent
(NI-entity to another correspondent (NI-entity.
NOTE - Classes of protocols may be defined within the (N)-protocols.
The precise definition of the term "classes of protocols" is for further
study.
5.3.1.6 (NI-data-source : An (NI-entity that sends
(N - 1 )-service-data-units (see 5.6.1.7) on an
This association is called an (NI-connection. (NI-connections
(N - 1)-connection.1)
are provided by the (NI-layer between two or more (NI-service-
access-points. The terminator of an (NI-connection at an
5.3.1.7 (NI-data-sink : An (NI-entity that receives
(NI-service-access-point is called an ( NI-connection-endpoint.
(N - II-service-data-units on an (N - l)-connection.l)
A connection with more than two connection-endpoints is
termed a multi-endpoint-connection. (NI-entities with a con-
nection between them are termed correspondent (Ni-entities.
5.3.1.8 (NI-data-transmission : An (NI-facility which con-
veys (NI-service-data-units from one (N + 1)-entity to one or
(N + 1 )-entities can communicate only by using the services of
more (N + 1 bentities. 1)
the (NI-layer. There are instances where services provided by
the (NI-layer do not permit direct access between all of the
(N + 1 bentities which have to communicate. If this is the case,
5.3.1.9 (NI-duplex-transmission : (NI-data transmission in
+ II-entity can
communication can still occur if some other (N
both directions at the same time.1)
act as a relay between them (see figure 6).
The fact that communication is relayed by a chain of
5.3.1.10 (NI-half-duplex-transmission : (NI-data transmis-
(N+1)-entities is known neither by the (NI-layer nor by the
sion in either direction, one direction at a time; the choice of
(N + 2)-layer.
direction is controlled by an (N + 1)-entity. 1)
These definitions are not for use in this International Standard but are for use in future OS1 standards.
1)
IS0 7498-1984 (E)
relay entity
/
<.I
.......
........
.......
........
.......
........
.......
........
.......
........
.......
........
.......
........
.......
........
........
........ ......
....... ......
....... ......
........
....... ............
........ ......
........ ......
.......
........ ............
....... ......
....... ......
.......
I
', /i/
- //
(Ni-layer '*-/ /
Figure 6 - Communication through a relay
5.4.1 .I1 (NI-connection-endpoint-suffix : A part of an
5.4 Identifiers
(NI-connection-endpoint-identifier which is unique within the
scope of an (Ni-service-access-point.
5.4.1 Definitions
5.4.1.12 multi-connection-endpoint-identifier : An iden-
5.4.1.1 title : A permanent identifier for an entity.
tifier which specifies the connection-endpoint of a multi-
endpoint-connection which should accept the data that is
being transferred.
5.4.1.2 title-domain : A subset of the title space of the OS1
environment.
5.4.1 .I3 (NI-service-connection-identifier : An identifier
which uniquely specifies an (NI-connection within the environ-
5.4.1.3 title-domain-name : An identifier which uniquely
ment of the correspondent (N + 1 )-entities.
identifies a title-domain within the OS1 environment.
5.4.1.14 (NI-protocol-connection-identifier : An identifier
NOTE - Title-domains of primary importance are the layers. In this
specific case, the title-domain-name identifies the (Ni-layer.
which uniquely specifies an individual (NI-connection within
the environment of the multiplexed (N - 1 )-connection.
5.4.1.4 local-title : A title which is unique within a title-
5.4.1.15 (NI-suffix : A part of an (NI-address which is unique
domain.
within the (NI-service-access-point.
5.4.1.5 global-title : A title which is unique within the OS1
environment and comprises two parts, a title-domain-name and 5.4.2 Description
a local-title.
An (NI-service-access-point-address, or (NI-address for short,
identifies a particular (N)-service-access-point to which an
5.4.1.6 (NI-address; (NI-service-access-point-address :
(N + 1)-entity is attached (see figure 7). When the (N + 1 )-entity
An identifier which tells where an (NI-service-access-point may
is detached from the (NI-service-access-point, the (Ni-address
be found.
no longer provides access to the (N+1)-entity. If the
(NI-service-access-point is reattached to a different
5.4.1.7 (NI-directory : An (NI-function by which the global
(N + 1 )-entity, then the (Nbaddress identifies the new
title of an (NI-entity is translated into the (N - 1)-address of an
(N + 1 )-entity and not the old one.
(N - 1 )-service-access-point to which the (Ni-entity is attached.
The use of an (NI-address to identify an (N + 1)-entity is the
most efficient mechanism if the permanence of attachment be-
5.4.1.8 (NI-address-mapping : An (Ni-function which pro-
tween the (N + l )-entity and the (NI-service-access-point can
vides the mapping between the (NI-addresses and the
be assured. If there is a requirement to identify an (N + 1 )-entity
(N - 1 )-addresses associated with an (NI-entity.
regardless of its current location, then the global-title assures
correct identification.
5.4.1.9 routing : A function within a layer which translates
the title of an entity or the service-access-point-address to
An (NI-directory is an (NI-function which translates global-
which the entity is attached into a path by which the entity can
titles of peer (Nbentities into the (N - 1 baddresses through
be reached.
which they cooperate.
5.4.1.10 (NI-connection-endpoint-identifier : An identifier Interpretation of the correspondence between the
(Nl-addresses served by an (Ni-entity and the (N - 1 )-addresses
of an (Ni-connection-endpoint which can be used to identify
used for accessing (N-1)-services is performed by an
the corresponding (NI-connection at an (NI-service-access-
(NI-address-mapping function.
point.
IS0 7498-1984 (E)
Two particular kinds of (Nbaddress-mapping functions may b) an (NI-suffix which makes the (NI-service-access-point
exist within a layer : uniquely identifiable within the scope of the (N - 1 )-address.
a) hierarchical (NI-address-mapping; and Within a given layer, a hierarchical structure of addresses
simplifies (NI-address-mapping functions because of the per-
b) (NI-address-mapping by tables. manent nature of the mapping it presupposes. It is not imposed
by the model in all layers in order to allow more flexibility in
If an (NI-address is always mapped into only one (NI-address-mappings and to cover the case where an
(N - 1 baddress then hierarchical construction of addresses can (NI-entity attached to more than one (N - 1 )-service-access-
be used (see figure 8). The (NI-address-mapping function need point supports only one (N)-service-access-point.
only recognize the hierarchical structure of an (NI-address and
extract the (N - 1 )-address it contains.
If the previous condition is not true, i.e. either an (NI-address
can be mapped into several (N- 1)-addresses, or an
In this case, an (NI-address consists of two parts :
(NI-address is not permanently mapped into the same
(N - 1 )-address, then hierarchical construction of an address is
a) an (N - II-address of the (NI-entity which is supporting
not possible and the (NI-address-mapping function may use
the current (NI-service-access-point of the (N + 1 )-entity;
tables to translate (NI-addresses into (N - 1 )-addresses.
(NI-connection-
endpoint-identifier
/
/-
(N I-service-
(NI-address /- /
W
NOTE - Dashed arrows refer to identifiers.
Figure 7 - Entities, service-access-points, and identifiers
Ax Ay Ar Bi Bj Bk Bl
(N baddresses
Figure 8 - Hierarchical (NI-address-mapping
@SO 7498-1984 (E)
An (NI-service-access-point may be detached from an
The structure of an (NI-address is known by the (NI-entity
(Ni-entity and reattached to the same or another (NI-entity.
which is attached to the identified (NI-service-access-point.
+ 1)-entity does not know this structure.
However, the (N
An ( NI-service-access-point is located by means of its
t an (N + 1 )-entity has two or more (NI-service-access-points (NI-address. An (NI-address is used by an (N + 1)-entity to re-
quest an (NI-connection.
with either the same (NI-entity or different (NI-entities, the
(NI-entities have no knowledge of this fact. Each (NI-service-
access-point is considered to identify a different (N + 1)-entity
5.6 Data-units
from the perspective of the (NI-entities.
5.6.1 Definitions
A routing function translates the (NI-address of an
(N + 1 I-entity into a path or route by which the (N + 1 I-entity
5.6.1.1 (NI-protocol-control-information : Information ex-
may be reached.
changed between (NI-entities, using an (N - 1 )-connection, to
An (N + lbentity may establish an (NI-connection with another co-ordinate their joint operation.
(N + 1)-entity by using an (NI-service. When an (N + 1 I-entity
establishes an (NI-connection with another (N + 1)-entity, each
5.6.1.2 (NI-user-data : The data transferred between
(N + 1 I-entity is given an (NI-connection-endpoint-identifier by
(NI-entities on behalf of the (N + 1 )-entities for whom the
its supporting (NI-entity. The (N + 1 )-entity can then
(NI-entities are providing services.
distinguish the new connection from all other (N)-connections
accessible at the (NI-service-access-point it is using. This
5.6.1.3 (NI-protocol-data-unit : A unit of data specified in
(NI-connection-endpoint-identifier shall be unique within the
an (NI-protocol and consisting of (NI-protocol-information and
scope of the (N + 1)-entity which will use the (Nbconnection.
possibly (NI-user-data.
The (N)-connection-endpoint-identifier consists of two parts :
5.6.1.4 (NI-interface-control-information : Information
a) the (NI-address of the (NI-service-access-point which
transferred between an (N + II-entity and an (NI-entity to co-
will be used in conjunction with the (NI-connection; and
ordinate their joint operation.
b) an (NI-connection-endpoint-suffix which is unique
within the scope of the (NI-service-access-point.
5.6.1.5 (NI-interface-data : Information transferred from an
(N + 1 )-entity to an (NI-entity for transmission to a correspon-
A multi-endpoint-connection requires multi-connection-
dent (N + 1 )-entity over an (NI-connection, or conversely, infor-
endpoint-identifiers. Each such identifier is used to specify
mation transferred from an (NI-entity to an (N + 1 I-entity after
which connection-endpoint should accept the data which is
being received over an (NI-connection from a correspondent
being transferred. A multi-connection-endpoint-identifier shall
(N + 1 )-entity.
be unique within the scope of the connection within which it is
used.
5.6.1.6 (NI-interface-data-unit : The unit of information
The (NI-layer may provide to the (N + II-entities an (Ni-service-
transferred across the (Ni-service-access-point between an
connection-identifier which uniquely specifies the
(N+ II-entity and an (NI-entity in a single interaction. Each
(NI-connection within the environment of the correspondent
(NI-interface-data-unit contains (NI-interface-control-
(N + 1 bentities.
information and may also contain the whole or part of an
(NI-service-data-unit.
5.5 Properties of service-access-points
5.6.1.7 (NI-service-data-unit : An amount of (Nbinterface-
An (N + II-entity requests (NI-services via an (N)-service-
data whose identity is preserved from one end of an
access-point which permits the (N + II-entity to interact with
(Nbconnection to the other.
an (NI-entity.
5.6.1.8 expedited ( NI-service-data-unit, (NI-expedited-
Both the (Ni- and (N + 1)-entities attached to an (Nbservice-
data-unit : A small (NI-service-data-unit whose transfer is ex-
access-point are in the same system.
pedited. The (NI-layer ensures that an expedited-data-unit will
not be delivered after any subsequent service-data-unit or ex-
+ 1)-entity may concurrently be attached to one or more
An (N
pedited unit sent on that connection.
(NI-service-access-points attached to the same or different
(NI-entities.
5.6.2 Description
An (NI-entity may concurrently be attached to one or more
(N + 1 )-entities through (NI-service-access-points. Information is transferred in various types of data-units be-
tween peer-entities and between entities attached to a specific
An (NI-service-access-point is attached to only one (Ni-entity service-access-point. The data-units are defined in 5.6.1 and
and to only one (N + 1)-entity at a time. the relationships among them are illustrated in figures9 and IO.
An (NI-service-access-point may be detached from an Except for the relationships defined in figures 9 and IO, there is
no overall architectural limit to the size of data-units. There may
(N+l)-entity and reattached to the same or another
(N + 1 )-entity. be other size limitations at specific layers.
IS0 7498-1984 (E)
The size of (N)-interface-data-units is not necessarily the same Data may be held within a connection until a complete service-
at each end of the connection. data-unit is put into the connection.
Control Data Combined
I I I
(N)-protocol.
(NI - (N) (N)-protocol- (Ni-user-
peer- control- I data data-
entities information units
(N+ 1) -(N) (NI-interface- (Nbinterface- (Nbinterface-
adjacent control- data data-
layers information unit
Figure 9 - Relationships among data-units
(Nklayer
(N - I)-PCI (N - I)-SDU
51 a
(N - 1 )-layer
I
(N - 1)-PDU
PCI = protocol-control-information
= protocol-data-unit
PDU
SDU = service-data-unit
NOTES
1 This figure assumes that neither segmenting nor blocking of (N)-service-data-units is performed (see 5.7.6.5).
2 This figure does not imply any positional relationship between protocol-control-information and user-data in protocol-data-units.
3 An (Ni-protocol-data-unit may be mapped one-to-one into an (N - 1)-service-data-unit, but other relationships are possible (see figure 11).
Figure 10 - An illustration of mapping between data-units in adjacent layers
5.7 Elements of layer operation 5.7.1.11 blocking : A function performed by an (NI-entity to
map multiple (NI-service-data-units into one (Ni-protocol-data-
unit.
5.7.1 Definitions
5.7.1.12 deblocking : A function performed by an (Ni-entity
5.7.1.1 (NI-protocol-identifier : An identifier used between
to identify multiple (NI-service-data-units which are contained
correspondent (NI-entities to select a specific (Ni-protocol to
in one (N)-protocol-data-unit. It is the reverse function of
be used on a particular (N - 1 )-connection.
blocking.
5.7.1.2 centralized multi-endpoint-connection : A multi-
5.7.1.13 concatenation : A function performed by an
endpoint-connection where data sent by the entity associated
(Ni-entity to map multiple (NI-protocol-data-units into one
with the central connection-endpoint is received by all other en-
tities, while data sent by one of the other entities is only re- (N - 1)-service-data-unit.
ceived by the central entity.
5.7.1.14 separation : A function performed by an (Ni-entity
5.7.1.3 decentralized multi-endpoint-connection : A
to identify multiple (N)-protocol-data-units which are contained
multi-endpoint-connection such that data sent by an entity
in one (
...