`

`ISO/IEC 7498-1:1994(E)
`
`Contents
`
`2
`3
`4
`
`5
`
`6
`
`7
`
`Scope..............................................................................................................................................................
`Definitions ..................................................................................................................................................... .
`Notation ......................................................................................................................................................... .
`Introduction to Open Systems Interconnection (OSI) ................................................................................... .
`4.1 Definitions .......................................................................................................................................... .
`4.2 Open System Interconnection Environment ...................................................................................... .
`4.3 Modelling the OSI Environment ........................................................................................................ .
`Concepts of a layered architecture ................................................................................................................ .
`5.1
`Introduction ........................................................................................................................................ .
`Principles of layering ......................................................................................................................... .
`5.2
`5.3
`Communication between peer-entities ............................................................................................... .
`5.4
`Identifiers ........................................................................................................................................... .
`Properties of service-access-points .................................................................................................... .
`5.5
`5.6 Data-units ........................................................................................................................................... .
`5. 7
`The nature of the (N)-service ............................................................................................................. .
`Elements of layer operation ............................................................................................................... .
`5.8
`5.9 Routing ............................................................................................................................................... .
`5.10 Quality Of Service (QOS) .................................................................................................................. .
`Introduction to the specific OSI layers .......................................................................................................... .
`6.1
`Specific layers .................................................................................................................................... .
`6.2
`The principles used to determine the seven layers in the Reference Model ...... .................................
`6.3
`Layer descriptions...............................................................................................................................
`6.4
`Combinations of connection-mode and connectionless-mode............................................................
`6.5
`Configurations of OSI Open Systems.................................................................................................
`Detailed description of the resulting OSI architecture .................................................................................. .
`Application Layer .............................................................................................................................. .
`7 .1
`7 .2
`Presentation Layer ............................................................................................................................. .
`7 .3
`Session Layer......................................................................................................................................
`7.4
`Transport Layer...................................................................................................................................
`7 .5 Network Layer....................................................................................................................................
`7 .6 Data Link Layer..................................................................................................................................
`7.7
`Physical Layer.....................................................................................................................................
`
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`49
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`ISO/IEC 1994
`©
`All rights reserved. Unless otherwise specified, no part of this publication may be
`reproduced or utilized in any form or by any means, electronic or mechanical, including
`photocopying and microfilm, without permission in writing from the publisher.
`ISO/IEC Copyright Office • Case postale 56 • CH-1211 Geneve 20 • Switzerland
`Printed in Switzerland
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`©ISO/IEC
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`ISO/IEC 7498-1:1994(E)
`
`8
`
`Management aspects of OSI .......................................................................................................................... .
`8.1
`Definitions .......................................................................................................................................... .
`8.2
`Introduction ........................................................................................................................................ .
`Categories of management activities .................................................................................................. ·
`8.3
`8.4
`Principles for positioning management functions .............................................................................. .
`Compliance and Consistency with this reference model... ............................................................................ .
`9 .1
`Definitions .......................................................................................................................................... .
`9.2 Application of consistency and compliance requirements..................................................................
`Annex A - Brief explanation of how the layers were chosen ................................................................................
`
`9
`
`Annex B - Alphabetical index to definitions ···········································:·······························..............................
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`ISO/IEC 7498-1:1994(E)
`
`©ISO/IEC
`
`Foreword
`
`ISO (the International Organization for Standardization) and IEC (the Inter(cid:173)
`national Electrotechnical Commission) form the specialized system for worldwide
`standardization. National bodies that are members of ISO or IEC participate in the
`development of International Standards through technical committees established
`by the respective organization to deal with particular fields of technical activity.
`ISO and IEC technical committees collaborate in fields of mutual interest. Other
`international organizations, governmental and non-governmental, in liaison with
`ISO and IEC, also take part in the work.
`
`In the field of information technology, ISO and IEC have established a joint
`technical committee, ISO/IEC JTC 1. Draft International Standards adopted by the
`joint technical committee are circulated to national bodies for voting. Publication
`as an International Standard requires approval by at least 75 % of the national
`bodies casting a vote.
`
`International Standard ISO/IEC 7 498-1 was prepared by Joint Technical
`Committee ISO/IEC JTC 1, Information technology, in collaboration with ITU-T.
`The identical text is published as ITU-T Recommendation X.200.
`
`This second edition, along with parts 2, 3 and 4, cancels and replaces the first
`edition (ISO 7498:1984), which has been technically revised.
`
`ISO/IEC 7498 consists of the following parts, under the general title Information
`technology - Open Systems Interconnection - Basic Reference Model:
`
`Part 1: The Basic Model
`
`Part 2: Security Architecture
`
`Part 3: Naming and addressing
`
`Part 4: Management framework
`
`Annex B forms an integral part of this part of ISO/IEC 7498. Annex A is for
`information only.
`
`IV
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`ISO/IEC 7498-1:1994(E)
`
`Introduction
`
`This reference model provides a common basis for the coordination of standards development for the purpose of systems
`interconnection, while allowing existing standards to be placed into perspective within the overall reference model. It also
`identifies areas for developing and improving standards and provides a common reference for maintaining consistency among
`all related standards. The text was developed jointly with ITU-T and the main intent of this revision is to introduce the joint text,
`which incorporates inclusion of the concept of connectionless transmission, in addition to a number of technical and editorial
`refinements.
`
`v
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`INTERNATIONAL STANDARD
`
`CCITT RECOMMENDATION
`
`ISO/IEC 7498-1: 1994(E)
`
`INFORMATION TECHNOLOGY - OPEN SYSTEMS INTERCONNECTION
`BASIC REFERENCE MODEL: THE BASIC MODEL
`
`1
`
`Scope
`
`1.1
`The purpose of this Reference Model of Open Systems Interconnection is to provide a common basis for the
`coordination of standards development for the purpose of systems interconnection, while allowing existing standards to
`be placed into perspective within the overall Reference Model.
`
`1.2
`The term Open Systems Interconnection (OSI) qualifies standards for the exchange of information among
`systems that are "open" to one another for this purpose by virtue of their mutual use of the applicable standards.
`
`1.3
`The fact that a system is open does not imply any particular systems implementation, technology or means of
`interconnection, but refers to the mutual recognition and support of the applicable standards.
`
`1.4
`It is also the purpose of this Reference Model to identify areas for developing or improving standards, and to
`provide a common reference for maintaining consistency of all related standards. It is not the intent of this Reference
`Model either to serve as an implementation specification, or to be a basis for appraising the conformance of actual
`implementations, or to provide a sufficient level of detail to define precisely the services and protocols of the
`interconnection architecture. Rather, this Reference Model provides a conceptual and functional framework which
`allows international teams of experts to work productively and independently on the development of standards for each
`layer of the Reference Model for OSI.
`
`1.5
`The Reference Model has sufficient flexibility to accommodate advances in technology and expansion in user
`demands. This flexibility is also intended to allow the phased transition from existing implementations to OSI standards.
`
`1.6
`While the scope of the general architectural principles required for OSI is very broad, this Reference Model is
`primarily concerned with systems comprising terminals, computers, and associated devices and the means for
`transferring information between such systems. Other aspects of OSI requiring attention are described briefly (see 4.2).
`
`1.7
`
`The description of the Basic Reference Model of OSI is developed in stages:
`
`1.8
`Clause 4 establishes the reasons for Open Systems Interconnection, defines what is being connected, the scope
`of the interconnection, and describes the modelling principles used in OSI.
`
`1.9
`Clause 5 describes the general nature of the architecture of the Reference Model; namely that it is layered,
`what layering means, and the principles used to describe layers.
`
`1.10
`
`1.11
`
`1.12
`
`1.13
`
`1.14
`
`Clause 6 names, and introduces the specific layers of the architecture.
`
`Clause 7 provides the description of the specific layers.
`
`Clause 8 provides the description of Management Aspects of OSI.
`
`Clause 9 specifies compliance and consistency with the OSI Reference Model.
`
`An indication of how the layers were chosen is given in Annex A to this Basic Reference Model.
`
`1.15
`Additional aspects of this Reference Model beyond the basic aspects are described in several parts. The first
`part describes the Basic Reference Model. The second part describes the architecture for OSI Security. The third part
`describes OSI Naming and Addressing. The fourth describes OSI System Management.
`
`1.16
`The Basic Reference Model serves as a framework for the definition of services and protocols which fit within
`the boundaries established by the Reference Model.
`
`1.17
`In those few cases where a feature is explicitly marked (optional) in the Basic Reference Model it should
`remain optional in the corresponding service or protocol (even if at a given instant the two cases of the option are not yet
`documented).
`
`ITU-T Rec. X.200 (1994 E)
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`ISO/IEC 7498-1 : 1994(E)
`
`This Reference Model does not specify services and protocols for OSI. It is neither an implementation
`1.18
`specification for systems, nor a basis for appraising the conformance of implementations.
`
`For standards which meet the OSI requirements, a small number of practical subsets are defined from optional
`1.19
`functions, to facilitate implementation and compatibility.
`
`2
`
`Definitions
`
`Definitions of terms are included at the beginning of individual clauses and sub-clauses. An index of these terms is
`provided in Annex B for easy reference.
`
`3
`
`Notation
`
`3.1
`layers:
`
`Layers are introduced in clause 5. An (N)-, (N+ 1)- and (N-1)- notation is used to identify and relate adjacent
`
`(N)-layer: any specific layer;
`
`(N+l)-layer: the next higher layer;
`
`(N-1)-layer: the next lower layer.
`
`This notation is also used for other concepts in the model which are related to these layers, for example (N)-protocol,
`(N+ 1 )-service.
`
`Clause 6 introduces names for individual layers. When referring to these layers by name, the (N)-, (N+l)(cid:173)
`3.2
`and (N-1)- prefixes are replaced by the names of the layers, for example transport-protocol, session-entity, and
`network-service.
`
`4
`
`Introduction to Open Systems Interconnection (OSI)
`NOTE - The general principles described in clauses 4 and 5 hold for all layers of the Reference Model, unless layer
`specific statements to the contrary are made in clauses 6 and 7.
`
`4.1
`
`Definitions
`
`real system: A set of one or more computers, the associated software, peripherals, terminals, human
`4.1.1
`operators, physical processes, information transfer means, etc., that forms an autonomous whole capable of performing
`information processing and/or information transfer.
`
`real open system: A real system which complies with the requirements of OSI standards in its
`4.1.2
`communication with other real systems.
`
`open system: The representation within the Reference Model of those aspects of a real open system that are
`4.1.3
`pertinent to OSI.
`
`application process: An element within a real open system which performs the information processing for a
`4.1.4
`particular application.
`
`Open System Interconnection Environment (OSIE): An abstract representation of the set of concepts,
`4.1.5
`elements, functions, services, protocols, etc., as defined by the OSI Reference Model and the derived specific standards
`which, when applied, enable communications among open systems.
`
`Local System Environment (LSE): An abstract representation of that part of the real system that is not
`4.1.6
`pertinent to OSI.
`NOTE - The LSE may include functions necessary for non-OSI communication.
`
`application-process-invocation: A specific utilization of part or all of the capabilities of a given application
`4.1.7
`process in support of a specific occasion of information processing.
`
`application-process-type: A description of a class of application processes in terms of a set of information
`4.1.8
`processing capabilities.
`
`2
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`ISO/IEC 7498-1: 1994(E)
`
`4.2.10 OSI is concerned not only with the transfer of information between systems, i.e. transmission, but also with
`their capability to interwork to achieve a common (distributed) task. In other words, OSI is concerned with the
`interconnection aspects of cooperation1> between systems, which is implied by the expression "systems interconnection."
`
`4.2.11
`The objective of OSI is to define a set of standards to enable real open systems to cooperate. A system which
`complies with the requirements of applicable OSI standards in its cooperation with other systems is termed a real open
`system.
`
`4.2.12 The design intent of the OSI standards is to specify a set of standards that make it possible for autonomous
`systems to communicate. Any equipment which communicates in conformance with all applicable OSI protocol
`standards is a real world equivalent of the model concept "open system". Equipment that is in the "terminal" category,
`that is, one that requires human intervention for the dominant parts of information processing, may satisfy the conditions
`of the previous sentences when the appropriate OSI standards are employed in communication with other open systems.
`
`4.3
`
`Modelling the OSI Environment
`
`4.3.1
`The development of OSI standards, i.e. standards for the interconnection of real open systems, is assisted by
`the use of abstract models. To specify the external behavior of interconnected 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 these open systems would strictly need to be described. However to accomplish this, it is
`necessary to describe both the internal and external behavior of these open systems. Only the external behavior of open
`systems is retained for the definition of standards for real open systems. The description of the internal behavior of open
`systems is provided in the Basic Reference Model only to support the definition of the interconnection aspects. Any real
`system which behaves externally as an open system can be considered to be a real open system.
`
`4.3.2
`
`This abstract modelling is used in two steps.
`
`4.3.3
`First, basic elements of open systems and some key decisions concerning their organization and functioning,
`are developed. This constitutes the Basic Reference Model of Open Systems Interconnection described in this
`Recommendation I Part of this International ·standard.
`
`4.3.4
`Then, the detailed and precise description of the functioning of the open system is developed in the framework
`formed by the Basic Reference Model. This constitutes the services and protocols for OSI which are the subject of other
`Recommendations and/or International Standards.
`
`4.3.S
`It should be emphasized that the Basic Reference Model does not, by itself, specify the detailed and precise
`functioning of the open system and, therefore, it does not specify the external behavior of real open systems and does not
`imply the structure of the implementation of a real open system.
`
`4.3.6
`The reader not familiar with the technique of abstract modelling is cautioned that those concepts introduced in
`the description of open systems constitute an abstraction despite a similiar appearance to concepts commonly found in
`real systems. Therefore, real open systems need not be implemented as described by the Model.
`
`1) 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 OSI application processes;
`data representation, which concerns all aspects of the creation and maintenance of data descriptions and data
`transformations for reformatting data exchanged between open systems;
`data storage, which concerns storage media, and file and database systems for managing and providing access to data
`stored on the media;
`process and resource management, which concerns the means by which OSI application processes are declared,
`initiated and controlled, and the means by which they acquire OSI resources;
`
`b)
`
`c)
`
`d)
`
`e)
`
`f)
`
`integrity and security, which concern information processing constraints that have to be preserved or assured during
`the operation of the open systems; and
`program support, which concerns the definition, compilation, linking, testing, storage, transfer, and access to the
`programs executed by OSI 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 Basic Reference Model covers the elements of OSI aspects of these activities which are essential for early development of OSI
`standards.
`
`4
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`ITU-T Rec. X.200 (1994 E)
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`ISO/IEC 7498-1: 1994(E)
`
`5.1.3
`
`Four elements are basic to the Reference Model (see Figure 2):
`
`a)
`
`open systems;
`
`b)
`
`the application-entities which exist within the OSI Environment (see 7.1);
`
`c)
`
`the associations (see 5.3) which join the application-entities and permit them to exchange information;
`and
`
`d)
`
`the physical media for OSI.
`
`NOTE - Security aspects which are also general architectural elements of protocols are discussed in CCITI Rec. X.800 I
`ISO 7498-2.
`
`5.2
`
`Principles of layering
`
`5.2.1
`
`Definitions
`
`(N)-subsystem: An element in a hierarchical division of an open system which interacts directly only with
`5.2.1.1
`elements in the next higher division or the next lower division of that open system.
`
`5.2.1.2
`
`(N)-layer: A subdivision of the OSI architecture, constituted by subsystems of the same rank (N).
`
`5.2.1.3 peer-(N)-entities: Entities within the same (N)-layer.
`
`5.2.1.4
`
`sublayer: A subdivision of a layer.
`
`(N)-service: A capability of the (N)-layer and the layers beneath it, which is provided to (N+ 1 )-entities at the
`5.2.1.5
`boundary between the (N)-layer and the (N+l)-layer.
`
`5.2.1.6
`
`(N)-facility: A part of an (N)-service.
`
`5.2.1. 7
`
`(N)-function: A part of the activity of (N)-entities.
`
`(N)-service-access-point, (N)-SAP: The point at which (N)-services are provided by an (N)-entity to an
`5.2.1.8
`(N+l)-entity.
`
`(N)-protocol: A set of rules and formats (semantic and syntactic) which determines the communication
`5.2.1.9
`behavior of (N)-entities in the performance of (N)-functions.
`
`5.2.1.10 (N)-entity-type: A description of a class of (N)-entities in terms of a set of capabilities defined for the
`(N)-layer.
`
`5.2.1.11 (N)-entity: An active element within an (N)-subsystem embodying a set of capabilities defined for the
`(N)-layer that corresponds to a specific (N)-entity-type (without any extra capabilities being used).
`
`5.2.1.12 (N)-entity-invocation: A specific utilization of part or all of the capabilities of a given (N)-entity (without
`any extra capabilities being used).
`
`5.2.2
`
`Description
`
`5.2.2.1 The basic structuring technique in the Reference Model of Open Systems Interconnection is layering.
`According to this technique, each open system is viewed as logically composed of an ordered set of (N)-subsystems,
`represented for convenience in the vertical sequence shown in Figure 3. Adjacent (N)-subsystems communicate through
`their common boundary. (N)-subsystems of the same rank (N) collectively form the (N)-layer of the Reference Model
`of Open Systems Interconnection. There is one and only one (N)-subsystem in an open system for layer N. An
`(N)-subsystem consists of one or several (N)-entities. Entities exist in each (N)-layer. Entities in the same (N)-layer are
`termed peer-(N)-entities. Note that the highest layer does not have an (N+l)-layer above it and the lowest layer does not
`have an (N-1)-layer below it.
`
`5.2.2.2 Not all peer-(N)-entities need or even can communicate. There may be conditions which prevent this
`communication (for example: they are not in interconnected open systems, or they do not support the same protocol
`subsets). Communication among peer-(N)-entities which reside in the same (N)-subsystem is provided by the LSE and
`therefore is out of the scope of OSI.
`
`6
`
`ITU-T Rec. X.200 (1994 E)
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`ISO/IEC 7498-1: 1994(E)
`
`5.3
`
`Communication between peer-entities
`
`5.3.1
`
`Definitions
`
`5.3.1.1
`
`(N)-association: A cooperative relationship among (N)-entity-invocations.
`
`(N)-connection: An association requested by an (N+ 1 )-entity for the transfer of data betweeen two or more
`5.3.1.2
`(N+ 1)-entities. The association is established by the (N)-layer and provides explicit identification of a set of (N)-data(cid:173)
`transmissions and agreement concerning the (N)-data-transmission services to be provided for the set.
`
`5.3.1.3 · (N)-connection-endpoint: A terminator at one end of an (N)-connection within an (N)-service-access-point.
`
`5.3.1.4 multi-endpoint-connection: A connection with more than two connection-endpoints.
`
`5.3.1.5
`
`correspondent (N)-entities: (N)-entities with an (N-1)-connection between them.
`
`(N)-relay: An (N)-function by means of which an (N)-entity forwards data received from one peer-(N)-entity
`5.3.1.6
`to another peer-(N)-entity.
`
`5.3.1.7
`
`(N)-data-source: An (N)-entity that sends (N-1)-service-data-units (see 5.6.1.7) on an (N-1)-connection.2)
`
`5.3.1.8
`
`(N)-data-sink: An (N)-entity that receives (N-1 )-service-data-units on an (N-1 )-connnection2)
`
`(N)-data-transmission: An (N)-facility which conveys (N)-service-data-units from one (N+l)-entity to one
`5.3.1.9
`or more (N+ 1 )-entities.
`
`5.3.1.10 (N)-duplex-transmission: (N)-data-transmission in both directions at the same time.2)
`
`5.3.1.11 (N)-half-duplex-transmission: (N)-data-transmission in either direction, one direction at a time; the choice of
`direction is controlled by an (N+l)-entity.2)
`
`5.3.1.12 (N)-simplex-transmission: (N)-data-transmission in one pre-assigned direction.2)
`
`5.3.1.13 (N)-data-communication: An (N)-function which transfers (N)-protocol-data-units (see 5.6.1.3) according to
`an (N)-protocol, over one or more (N-1)-connections.2)
`
`5.3.1.14 (N)-two-way-simultaneous-communication: (N)-data-communication in both directions at the same time.
`
`5.3.1.15 (N)-two-way-alternate-communication: (N)-data-communication in both directions, one direction at a time.
`
`5.3.1.16 (N)-one-way-communication: (N)-data-communication in one pre-assigned direction.
`
`5.3.1.17 (N)-connection-mode transmission: (N)-data-transmission in the context of an (N)-connection.
`
`5.3.1.18 (N)-connectionless-mode transmission: (N)-data-transmission not in the context of an (N)-connection and
`not required to maintain any logical relationship between (N)-service-data-units.
`
`5.3.2
`
`Description
`
`5.3.2.1 For information to be exchanged between two or more (N+l)-entities, an association is established between
`them in the (N)-layer using an (N)-protocol.
`NOTE - Classes of protocols may be defined within the (N)-protocols.
`
`5.3.2.2 The rules and formats of an (N)-protocol are instantiated in an (N)-subsystem by an (N)-entity. An (N)-entity
`may support one or more (N)-protocols. (N)-entities may support (N)-protocols which are connection-mode or
`connectionless-mode or both. (N)-entities when supporting connection-mode maintain the binding of (N)-connections to
`the appropriate (N+ 1 )-entities at the appropriate (N)-SAPs. (N)-entities when supporting connectionless-mode maintain
`a binding with the appropriate (N)-SAPs for delivering the connectionless data to the (N+ 1 )-entities.
`
`5.3.2.3
`(N+l)-entities can communicate only by using the services of the (N)-layer. There are instances where services
`provided by the (N)-layer do not permit direct access between all of the (N+ 1 )-entities which have to communicate. If
`this is the case communication can still occur if some other (N+ 1 )-entities can act as relays between them (see Figure 6).
`
`2) These definitions are not for use in this Basic Reference Model, but are for use in other OSI standards.
`
`ITU-T Rec. X.200 (1994 E)
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`5.3.3.2 Connection mode
`
`5.3.3.2.1 A connection is an association established for the transfer of data between two or more peer-(N)-entities. This
`association binds the peer-(N)-entities together with the (N-1)-entities in the next lower layer. The ability to establish
`and release a connection and to transfer data over it is provided to the (N)-entities in a given (N)-layer by the next lower
`layer as a connection-mode service. The use of a connection-mode service by peer-(N)-entities proceeds through three
`distinct phases:
`
`ISO/IEC 7498-1 : 1994(E)
`
`a)
`
`connection establishment;
`
`b) data transfer; and
`
`c)
`
`connection release.
`
`5.3.3.2.2 In addition to the clearly distinguishable lifetime exhibited by these phases,a connection has the following
`fundamental characteristics:
`
`a)
`
`b)
`
`c)
`
`d)
`
`it involves establishing and maintaining a two or more party agreement concerning the transmission of
`data among the peer-(N)-entities concerned, and using the provider of the (N-1 )-service;
`
`it allows the negotiation between all the parties concerned of the parameters and options that will govern
`the transmission of data;
`
`it provides connection identification by means of which the overheads involved in address resolution and
`transmission can be avoided on data transfers;
`
`it provides a context within which successive units of data transmitted between the peer-entities are
`logically related, and makes it possible to maintain sequence and provide flow control for those
`transmissions.
`
`5.3.3.2.3 The characteristics of connection-mode transmission are particularly attractive in applications which call for
`relatively long-lived, stream-oriented interactions between entities in stable configurations. Examples are provided by
`direct terminal use of a remote computer, file transfer, and long-term attachment of remote job entry stations. In these
`cases, the entities involved initially discuss their requirements and agree to the terms of their interaction, reserving
`whatever resources they may need, transfer a series of related units of data to accomplish their mutual objective, and
`explicitly end their interaction, releasing the previously reserved resources. The properties of connection-mode
`transmission are also relevant in a wide range of other applications.
`
`5.3.3.2.4 Connection-mode transmission is accomplished through the use of (N)-connections. (N)-connections are
`provided by the (N)-layer between two or more (N)-service-access-points. The terminator of an (N)-connection at an
`(N)-service-access-point is called an (N)-connection-endpoint. An (N)-connection is provided by the (N)-layer between
`two or more (N)-service-access-points at the request of a calling (N+ 1 )-entity in support of the (N+ 1 )-entities attached to
`the (N)-service-access-points involved in the (N)-connection. An (N)-connection with more than two endpoints is
`termed a multi-endpoint-connection. (N)-entities with a connection between them are termed correspondent (N)-entities.
`NOTE - Data transfer using an (N)-connection-mode service involves the establishment of an (N)-connection prior to the
`data transfer. This dynamically sets up an association between the (N+l)-entities and the (N)-connection-mode service in addition to
`the association identified in 5.3.2. This association involves elements which are not part of the pre-arranged association described in
`5.3.3.1.4, in particular:
`
`a)
`
`knowledge of the willingness of the peer-(N)-entity or entities to undertake a specific communication, and of the
`willingness of the underlying service to support it and;
`
`b)
`
`the ability for the peer-(N)-entities to negotiate and renegotiate the characteristics of the communication.
`
`5.3.3.3 Connectionless mode
`
`5.3.3.3.1 Connectionless-mode transmission is the transmission of a single unit of data from a source service-access(cid:173)
`point to one or more destination service-access-points without establishing a connection. A connectionless-mode service
`allows an entity to initiate such a transmission by the performance of a single service access.
`
`5.3.3.3.2 In contrast to a connection, an instance of the use of a connectionless-mode service does not hav