GB/T 9387.1-1998 Information technology Open Systems Interconnection Basic Reference Model Part 1: Basic Model
Some standard content:
GB/T9387.11998
This standard is equivalent to the international standard ISO/IEC7498-1:1994 "Basic Reference Model for Open Systems Interconnection of Information Technology: Basic Model".
The main technical differences between this standard and ISO/IEC7498-1 are as follows: a) The corresponding Chinese terms are also given in the original Appendix B\English Index"; b) In order to facilitate the retrieval of Chinese terms, a "Chinese Index" is added after the original appendix of this standard, namely Appendix C. c) In order to make the same level consistent with or without titles, this standard cancels the title of 7.1.4.3 and adds 5.10.2.1, 7.3.4.1, 7.3.4.2, 7.6.3.1, 7.3.6.2.7.7.3.1, 7.7.3.2, 7.7.4.1 and 7.7.4.2 Title. GB/T9387, under the general title of "Basic Reference Model for Open Systems Interconnection of Information Technology", currently includes the following 4 parts: Part 1 (i.e. GB/T9387.1): Basic Model; Part 2 (i.e. GB/T9387.2): Security Architecture Part 3 (i.e. GB/T9387.3): Naming and Addressing; Part 4 (i.e. GB/T9387.4): Management Framework. This standard replaces GB9387-88.
The main differences between this standard and GB9387-88 are: a) Addition of connectionless content to the relevant articles; b) Additions and deletions to the contents of some articles; c) Change of the original Article 5.9 to Chapter 8;
d) Addition of Chapter 9.
Appendices B and C of this standard are appendices to the standard. Appendix A of this standard It is a prompt appendix. This standard was proposed by the Ministry of Electronics Industry of the People's Republic of China. This standard is under the jurisdiction of the Standardization Institute of the Ministry of Electronics Industry. The drafting unit of this standard: Standardization Institute of the Ministry of Electronics Industry. The main drafters of this standard: Zhao Xiaofan, Zheng Hongren, Ma Rushan, Cao Dongqi. GB/T 9387. 1—1998
ISO/IEC Foreword
ISO (International Organization for Standardization) and IEC (International Electrotechnical Commission) are worldwide specialized standardization organizations. National member bodies (all of which are members of ISO or IEC) participate in the formulation of international standards for specific technical specifications through various technical committees established by international organizations. The technical committees of ISO and IEC cooperate in areas of common interest. Other official and non-official international organizations in contact with ISO and IEC may also participate in the formulation of international standards. For information technology, ISO and IEC have established a The draft international standards proposed by the joint technical committees are circulated to the national member bodies for voting. Publication of an international standard requires at least 75% of the national member bodies voting in favor.
International Standard ISO/IEC 7498-1 was prepared by the Joint Technical Committee ISO/IEC JTC1 "Information Technology" and ITU-T. The equivalent text is ITU-T Recommendation X.200. This is the second edition of this standard. It supersedes the first edition (ISO 7498: 1984) together with Parts 2.3 and 4. The second edition makes technical revisions to the first edition.
ISO/IEC 7498, under the general title "Basic Reference Model for Open Systems Interconnection of Information Technology", currently consists of the following four parts: Part 1: Basic Model;
- Part 2: Security Architecture;
- Part 3: Naming and Addressing;
Part 4: Management Framework.
Annex B forms an integral part of this standard. Appendix A is for reference only. 2
GB/T9387.1—1998
This reference model provides a common basis for the development of standards for coordinated system interconnection, while allowing existing standards to be reflected in the entire reference model. It also defines the scope for the development and completion of standards and provides a common reference for maintaining compatibility between all related standards. This standard was developed jointly with ITU-T, and the main purpose of this revision is to propose this joint text. In addition to careful revisions to certain technical content and editorial issues, this version also includes the concept of connectionless transmission. 1 Scope
National Standard of the People's Republic of China
Information technology-Open Systems Interconnection
Basic Reference Model--Part 1: The Basic ModelGB/T9387.1-1998
idt ISO/IEC 7498-1:1994
Replaces GB9387-88
1.1 The purpose of this Open Systems Interconnection Reference Model standard is to provide a common basis for the development of coordinated standards for systems interconnection, and at the same time, to enable existing standards to be reflected in the overall reference model. 1.2 The term Open Systems Interconnection (OSI) is limited to standards for the exchange of information between systems that are "open" to each other, and the systems achieve information exchange by jointly using appropriate standards. 1.3 The fact that a system is open does not imply a particular system implementation, nor does it imply a technology or method of interconnection, but rather that the systems recognize each other and support appropriate standards.
1.4 Another purpose of this reference model is to define the scope of the development or revision of standards and to provide a common reference for maintaining the compatibility of all related standards. This reference model is neither a specification of a specific implementation nor a basis for certifying the consistency of a specific implementation, nor does it provide detailed information that accurately defines the services and protocols of the interconnection architecture. This reference model only provides a conceptual and functional framework that allows experts to work independently and creatively in developing standards for each layer of the OSI reference model. 1.5 This reference model is flexible enough to adapt to the development of technology and the expansion of user requirements. This flexibility is also to enable existing implementations to gradually transition to conform to the OSI standard. 1.6 Although the scope of the general OSI architectural principles is very wide, this standard is mainly concerned with systems consisting of terminals, computers and related equipment, and the means of transmitting information between these systems. Other aspects of OSI that require attention are also briefly described (see 4.2).
1.7 The OSI Basic Reference Model is described in the following order in this standard: 1.8 Chapter 4 presents the rationale for open systems interconnection, defines the objects of connection and the scope of interconnection, and describes the modeling principles used in OSI.
1.9 Chapter 5 describes the general nature of the reference model architecture, namely that the model is layered, the meaning of the layers, and the principles used to describe the layers.
1.10 Chapter 6 names and describes the layers of the architecture. 1.11 Chapter 7 describes the layers.
1.12 Chapter 8 describes the management aspects of OSI. 1.13 Chapter 9 specifies conformity and compatibility with the OSI reference model. 1.14 Appendix A of this basic reference model gives guidelines for selecting each layer. 1.15 In addition to the basic aspects, other aspects of this reference model are described in several parts. Part 1 describes the basic reference model. Part 2 describes the OSI security architecture. Part 3 describes OSI naming and addressing. Part 4 describes OSI management. 1.16 The basic reference model is a framework for defining services and protocols within the scope of the reference model. 1.17_ In a few cases, options that are clearly marked in this basic reference model should still be options in the corresponding services or protocols (even if the two cases of the option have not been written when the State Administration of Quality and Technical Supervision approved it on April 10, 1998 and implemented it on October 1, 1998). GB/T 9387. 1--1998
1.18 This reference model does not specify OSI services and protocols. It is neither a system implementation specification nor a basis for identification of conformance. 1.19 For standards that meet (SI requirements, a few practical subsets may be defined from optional functions to facilitate implementation and compatibility. 2 Definitions
Term definitions are included at the beginning of each chapter and each clause. For ease of reference, Appendix B provides an English index of these terms in alphabetical order, and Appendix C provides a Chinese index of these terms in phonetic order. 3 Representation
3.1 The concept of layers is introduced in Chapter 5. (N), (N+1) and (N-1) are used to represent adjacent layers: (N) layer: a specific layer;
(N+1) layer: an adjacent higher layer;
(N-1) layer: an adjacent lower layer.
This notation also applies to other concepts related to these layers in the model, such as: (N) protocol, (N+1) service. 3.2 The names of the layers are introduced in Chapter 6. When referring to these layers by name, the prefixes (N), (N+1) and (N-1) are replaced by the name of the layer, such as: transport protocol, session entity and network service. 4 Open Systems Interconnection Introduction
Note: Except for special statements about layers in Chapters 6 and 7, the general principles described in Chapters 4 and 5 apply to all layers of the reference model. 4.1 Definitions
4.1.1 Real system
A collection of one or more computers, associated software, peripherals, terminals, operators, physical processes and information transfer means, forming an autonomous entity capable of performing information processing and/or information transfer. 4.1.2 Open system A real system that complies with the requirements of the OSI standard when communicating with other real systems. 4.1.3 Open system
The representation of the aspects of an open system that are relevant to OSI in the reference model. 4.1.4 Application process An element in an open system that performs information processing for a specific application. 4.1.5 Open system interconnection environment (OSIE) An abstract representation of the concepts, elements, functions, services, protocols, etc. defined by the OSI reference model, and the specific standards derived from them for communication between open systems.
4.1.6 Local system environment (LSE) An abstract representation of the part of a real system that is not part of OSI. Note: LSE may contain functions that are not required for OSI communication. 4.1.7 Application-process-invocation application-process-invocation The specific use of some or all of the capabilities of a given application process in order to support a specific case of information processing. 4.1.8 Application-process-type application-process-type The description of a class of application processes using a set of information processing capabilities. 4.2 Open Systems Interconnection Environment
4.2.1 In the OSI concept, a real system is an autonomous entity that can perform information processing and/or information transmission. It is a collection of one or more computers, corresponding software, peripherals, terminals, operators, physical processes and information transmission means. 4.2.2 An application process is an element in an open real system that performs information processing for a specific application. 5
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4.2.3 An application process can be manifested as a human process, a computer process or a physical process. 4.2.4 The following are some examples of application processes that meet this open system definition: a) A person operating a bank terminal is a human application process; b) A FORTRAN program running in a computing center and accessing a remote database is a computer application process; a remote database management system server is also an application process; c) A process control program running on a dedicated computer connected to an industrial device and connected to a plant control system is a physical application process.
4.2.5 An application process represents a collection of resources (including processing resources) within an open real system that can be used to perform specific information processing activities. An application process can organize its interactions with other application processes in any way necessary to achieve specific information processing goals. This reference model does not place any restrictions on the form of these interactions or the relationships that may exist between them. 4.2.6 The activities of a given application process are represented by one or more application process calls. Cooperation between application processes occurs through the relationships established between application process calls. At a particular time, an application process represents one or more application process calls, or it is not called. An application process call is responsible for coordinating its interactions with other application process calls. This coordination is outside the scope of this reference model. 4.2.7 OSI is concerned with the exchange of information between open systems (not with the functions within each independent open real system). 4.2.8 As shown in Figure 1, the physical media used for open system interconnection provides the means for transmitting information between open systems. Open system A
Open system S
Physical media
Open system B
Open system C
Figure 1 Open systems connected by physical media
4.2.9 OSI is only concerned with the interconnection of systems. All other aspects not related to interconnection are outside the scope of OSI. 4.2.10 OSI is not only concerned with the transmission of information between systems, that is, transmission, but also with the ability of systems to cooperate with each other to complete common (distributed) tasks. In other words, OSI is concerned with the interconnection of systems for cooperation1, and uses the term "system interconnection" to express this. 4.2.11 () The purpose of OSI is to define a set of standards that enable open real systems to cooperate. A system that follows appropriate OSI standards when cooperating with other systems is called an open real system. 4.2.12 OSI standards are designed to specify a set of standards that enable autonomous systems to communicate. Any device that communicates in accordance with all applicable QS1 protocol standards is the real-world equivalent of the model's concept of an "open system" "terminal" class of device, that is, it requires human intervention for the majority of information processing, and when communicating with other open systems, it uses appropriate OSI standards to meet the conditions in the above clauses.
1) The range of activities involved in cooperation between open systems is very wide, and the following are some of the activities that have been recognized: a) Interprocess communication, which involves the exchange of information and synchronization of activities between () SI application processes; b) Data representation, which involves the creation and maintenance of data descriptions and all aspects of data conversion required to reformat data exchanged between open systems;
c) Data storage, which involves storage media and the file systems and database systems that manage and provide access to data stored on the media; d) Process and resource management, which involves the means of declaring, initiating and controlling OSI application processes, and the application processes obtaining OSI information. SI resources; e) integrity and security, which involves restrictions on information processing that must be maintained or observed during the operation of the open system; f) program support, which involves the definition, compilation, linking, testing, storage, transmission and access to programs executed by OSI application processes. Some of the above activities may imply the exchange of information between interconnected open systems, so the interconnection aspects of these activities may be related to (SI. This standard contains all elements of the OSI aspects of these activities, which are essential for the early development of OSI standards. 6
4.3 Modeling of the open system interconnection environment
GB/T 9387.1—1998
4.3.1 The development of the OSI standard (open real system interconnection standard) is aided by abstract models. In order to specify the external behavior of interconnected open real systems, each open real system is replaced by a functionally equivalent abstract model, called an open system. Only the interconnection aspects of these open systems need to be strictly described. However, in order to do this, it is necessary to describe the internal and external behaviors of these open systems. Only the external behavior of the open system serves as the standard for the behavior of the open real system. The description of the internal behavior of the open system in the basic reference model is only to support the definition of the interconnection aspects. Any real system that behaves as an open system externally can be considered an open real system. 4.3.2 Abstract modeling is divided into two steps.
4.3.3 First, study the basic elements of the open system and make 4.3.5 It should be emphasized that the Basic Reference Model does not itself specify the detailed and precise functionality of open systems, and therefore does not specify the external behavior of open real systems, nor does it imply the implementation structure of open real systems. 4.3.6 Readers who are not familiar with abstract modeling techniques should note that although the abstract model is superficially similar to concepts that are common in real systems, only those concepts introduced in the description of open systems constitute the abstract model. Therefore, it is not necessary to implement open real systems as described in this model.
4.3.7 The remainder of this standard considers only the various aspects of real systems and application processes within the OSI environment, the interconnections of which are shown in Figure 2 of this standard.
4.3.8 Due to the different degrees of application of OSIE concepts through the use of (SI standards, several subsets of OSIE may be generated. These subsets correspond to partially disjoint sets of open real systems, and physical OSI communication cannot be carried out between these open real systems. 5 The concept of layered architecture
5.1 Introduction
5.1.1 This chapter begins to introduce the architectural concepts applicable in the development of the open systems interconnection reference model. First, the concept of layered architecture (including layers, entities, service access points, protocols, connections, etc.) is described. Second, identifiers used for entities, service access points, and connections are introduced. Third, service access points and data units are described. Fourth, the elements of layer operations are described, including connections, data transmission and error functions. Routing is then introduced. Management is discussed last. 5.1.2 This chapter describes the concepts required by the OSI reference model, but the concepts described here are not necessarily used in every layer of the reference model.
5.1.3 The following four elements are basic to the reference model (see Figure 2): Aspects of the application process related to OSI, that is, application entities (see 7.1) Open system A
Open entities related to OSI
Physical media used by OSI
Open system B
Open system C
Figure 2 Basic elements of OSI
Open system S
a) Open system;
GB/T 9387.1--1998
b) Application entities that exist within the OSI environment (see 7.1); c) Connections that connect application entities and enable them to exchange information (see 5.3); d) Physical media used by OSI.
Note: Security aspects are also architectural elements of the protocol and are discussed in GB/T 9387.2. 5.2 Layering principle
5.2.1 Definition
5.2.1.1 (N)-subsystem (N)-subsystem An element in the hierarchical division of an open system that can only interact directly with elements in the upper or lower division of the open system.
5.2.1.2 (N)-layer
(N)-layer
A subdivision of the OSI architecture consisting of subsystems in the same (N) row. 5.2.1.3 Peer-(N)-entities Entities in the same (N) layer.
5.2.1.4 Sublayer
Subdivision within a layer.
5.2.1.5 (N)service (N)-service
A capability of the (N) layer and the layers below it, provided to the (N+1) entity at the boundary between the (N) layer and the (N+1) layer.
5.2.1.6 (N)facility (N)-facility
Part of the (N) service.
5.2.1.7 (N)function (N)-function
Part of the (N) entity's activity.
5.2.1.8 (N)service-access-point (N)-service-access-point, the point at which the (N)-SAP (N) entity provides the (N) service to the (N+1) entity. 5.2.1.9 (N)protocol (N)-protocol
A set of rules and formats (semantics and syntax) that determine the communication behavior of (N) entities when performing (N) functions. 5.2.1.10 (N)entity type (N)-entity-type A description of a class (N) entity with a set of capabilities defined for the (N) layer. 5.2.1.11 (N)entity (N)-entity
An active element within the (N) subsystem that is a specialization of a set of capabilities defined for the (N) layer corresponding to a specific (N) entity type (excluding any additional capabilities being used). 5.2.1.12 (N)entity invocation (N)-entity-invocation A specific use of some or all of the capabilities of a given (N) entity (excluding any additional capabilities being used). 5.2.2 Description
5.2.2.1 The basic construction technique in the Open Systems Interconnection Reference Model is layering. According to this technology, each open system is regarded as logically composed of an ordered set of subsystems. For convenience, the vertical sequence shown in Figure 3 is used for representation. Adjacent (N) subsystems communicate through their common boundaries. The (N) subsystems in the same (N) row together constitute the (N) layer of the open system interconnection reference model. For the (N) layer, there is only one (N) subsystem in an open system. An (N) subsystem consists of one or more (N) entities. There are entities in each layer. Entities in the same (N) layer are called peer (N) entities. It should be noted that there is no (N+1) layer above the highest layer and no (N-1) layer below the lowest layer. 8
Highest layer
(N+1) layer
(N) layer
(N-1) layer
Lowest layer
Open system A
GB/T9387.1—1998
Open system B
Open system C
Physical media used by OSI
Figure 3 Layers in a cooperating open system
Open system S
5.2.2.2 Some peer (N) entities need not or cannot communicate because there may be circumstances that prevent such communication (for example, the peer (N) entities are not in the interconnected open system or do not support the same protocol subset). Communications between peer (N) entities residing in the same (N) subsystem are provided by LSE and are therefore outside the scope of (SI). Note: For (SI, it is meaningful to distinguish between an object type and its instances. A type is a description of a class of objects, and an instance of the type is any object that meets its description. Instances of the same type constitute a class. A type and any of its instances can be referenced by their respective names. Each nameable instance and the type to which it belongs should have a distinguishable name. For example, a programmer writes a computer program, the programmer generates a type of certain instances, and each time the computer calls to execute the program, an instance of the type is created. For example. Therefore, a FORTRAN compiler is a type. Each call to its copy in a data processor represents an instance.
This general concept of instantiation also applies within OSI. Now consider the (N) entity in the OSI context. It also has two aspects: a type and a set of instances. The type of an (N) entity is defined by a specific set of (N) layer functions that the (N) entity can perform. In the relevant open system, it is the instance of the (N) entity type that provides the (N) layer functions, which is any special call to the (N) entity type. These (N) layer functions are required by the (N) entity type for a specific communication. The above conclusion is based on the following facts. Implementation: (N)Entity Types merely indicate the nature of the connection between peer (N)Entities, whereas (N)Entity Invocations represent a "sub-specific, dynamic, actual exchange of information. It is important that actual communication at all layers occurs only between (N)Entity Invocations at all layers. In connection mode (see 5.3.3), the (N)Entity Types are only explicitly associated when a connection is established (or, logically equivalently, during the recovery process). Although connection requests are often made to any (N)Entity of a particular type, actual connections are always established only to specific (N)Entity Invocations. However, this standard does not exclude requests to establish a connection with a specific (named) peer (N)Entity instance. If - If each (N) entity call knows the name of its peer (N) entity call, the first (N) entity call can request to establish another connection with its peer (N) instance call. 2 It may be necessary to further divide a layer into substructures called sublayers and extend the layering technique to other aspects of OSI. A sublayer is defined as a set of functions in a layer that can be bypassed, but bypassing all sublayers in a layer is not allowed. A sublayer uses the entities and communication services of its layer. The detailed definition of sublayers or additional features is for further study. 5.2.2.3 Except for the highest layer, each (N) layer provides (N) services to (N+1) entities in the (N+1) layer at the (N)-SAP. In 5.5.2.2.4 Each service provided by the (N) layer can be tailored by selecting one or more (N) facilities that can determine the attributes of the service. When a single (N) entity cannot fully support the service requested by the (N+1) entity on its own, it requires the cooperation of other (N) entities to help complete the service request. In order to cooperate, (N) entities in any layer except the lowest layer communicate through the services provided by the (N-1) layer (see Figure 4). The entities in the lowest layer can be considered to communicate directly through the physical media connecting them. 5.2.2.5 Use the (N) functions performed in the (N) layer and, when necessary, use services from the (N-1) layer to provide the (N+1) layer with various services of the (N) layer.
GB/T 9387.1—1998
Note: Since a certain facility is already available at the (N-1) service boundary, it does not exclude that no protocol action is required in the (N) layer to support the given (N) facility. However, it is not allowed to completely abandon all (N) protocols. (N+1) layer
(N) layer
(N+1) entity
Figure 4 (N+1) entities in the (N+1) layer communicate through the (N) layer 5.2.2.6 (N) entities can provide services to one or more (N+1) entities and can use the services of one or more (N-1) entities. The (N) service access point is the point at which a pair of entities in adjacent layers use or provide services (see Figure 7). 5.2.2.7 Cooperation between (N) entities is managed by one or more protocols. The entities and protocols within a layer are shown in Figure 5. (N) Entity
(N) Layer
(N) Protocol
Figure 5 (N) Protocol between (N) Entities
5.3 Communication between Peer Entities
5.3.1 Definition
5.3.1.1 (N) Association (N)-association A cooperative relationship between (N) entities.
5.3.1.2 (N) Connection (N)-connection An association requested by an (N+1) entity for the purpose of transferring data between two or more (N+1) entities. This association is established by the (N) layer and explicitly identifies a set of (N) data transfers and an agreement regarding the (N) data transfer services to be provided for them. 5.3.1.3 (N) Connection Endpoint (N)-connection-endpoint An endpoint at one end of an (N) connection within an (N) service access point. 5.3.1.4 Multi-endpoint-connection multi-endpoint-connection A connection with more than two connection endpoints. 5.3.1.5 correspondent (N)-entities correspondent (N)-entities (N)-entities between which there is an (N~-1) connection. 5.3.1.6 (N)-relay
(N)-function of an (N)-entity to forward data received from one corresponding (N)-entity to another corresponding (N)-entity. 5.3.1.7 (N)-data-source (N)-entity 2 that sends (N-1) service data units (see 5.6.1.7) on an (N-1) connection. 5.3.1.8 (N)-data-sink (N)-entity 2 that receives (N-1) service data units on an (N-1) connection. 5.3.1.9 (N)-data-transmission (N)-facility that delivers (N) service data units from an (N+1)-entity to one or more (N+1)-entities. 5.3.1.10 (N) duplex transmission (N)-duplex-transmission (N) data transmission in two directions simultaneously. 5.3.1.11 (N) half-duplex transmission (N)-half-duplex-transmission2) These definitions are not used in this standard, but are used by other OSI standards. 10
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(N) data transmission that can be carried out in both directions, but only in one direction at a time. The direction selection is controlled by the (N+1) entity. 5.3.1.12 (N) simplex transmission (N)-simplex-transmission (N) data transmission in one or more pre-specified directions2) 5.3.1.13 (N) data communication (N)-data-communication (N) protocol data units are transmitted on one or more (N-1) connections according to the (N) protocol (see the (N) function of 5.6.1.3)2). 5.3.1.14 (N) Two-way simultaneous communication (N)-two-way-simultaneous-communication (N) data communication carried out in two directions simultaneously. 5.3.1.15 (N) Two-way alternating communication (N)-two-way-alternate-comunication (N) data communication that can be carried out in both directions, but only in one direction at a time. 5.3.1.16 (N) One-way communication (N)-one-way-communication (N) data communication carried out in one pre-specified direction. 5.3.1.17 (N) Connection-mode transmission (N)-connection-mode-transmission (N) data transmission in the context of (N) connection. 5.3.1.18 (N) Connectionless-mode transmission (N)-connectionless-mode transmission (N) data transmission that is not in the context of (N) connection and does not require the maintenance of logical relationships between (N) service data units. 5.3.2 Description
5.3.2.1 In order to exchange information between two or more (N+1) entities, a connection is established between these (N+1) entities using the (N) protocol in the (N) layer.
Note: Multiple protocol classes can be defined within the (N) protocol. 5.3.2.2 The (N) entity instantiates the rules and format of the (N) protocol within the (N) subsystem. --An (N) entity can support one or more (N) protocols. The (N) entity can support both connection-mode and connectionless (N) protocols, or only one of the two. When the connection mode is supported, the (N) entity maintains a binding with the appropriate (N+1) entity at the appropriate (N)-SAP. When the connectionless mode is supported, the (N) entity maintains a binding with the appropriate (N)-SAP to deliver connectionless data to the (N+1) entity. 5.3.2.3 (N+1) entities can only communicate by using the services of the (N) layer. Sometimes, all (N+1) entities that need to communicate are not allowed to directly access the services provided by the (N) layer. In this case, if some other (N+1) entities can act as relays between them, communication is still possible (see Figure 6). Relay entity
(N+1) layer
(N) layer
Figure 6 Communication through relay
5.3.2.4 Neither the (N) layer nor the (N+2) layer is aware of the relay communication being performed by the (N+1) entity. 5.3.3 Communication mode
5.3.3.1 Introduction
—(N+1) Entities bzxZ.net
5.3.3.1.1 The (N) layer may provide both connection-mode and connectionless-mode services to the (N+1) layer, or only one of the two, using services provided by the (N-1) layer. Any instance of transmission between (N+1) entities must use the same (N) service mode. 5.3.3.1.2 Both the (N) connection-mode service and the (N) connectionless-mode service are characterized by the facilities they provide to the (N+1) entity and the quality of service seen by the (N+1) entity. For both (N) connection-mode service and (N) connectionless-mode service, functions that may be provided by the (N) layer are used to enhance the facilities provided by the (N-1) layer to the (N) layer and the quality of service seen by the (N+1) entity. Conversions are made between the two service modes if necessary.
2) These definitions are not used by this standard but are used by other OSI standards. 11
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5.3.3.1.3 Since connection-mode transmission and connectionless-mode transmission are complementary concepts, and especially since connectionless-mode transmission is most easily defined in terms of its relationship with the concept of connection, it is best to introduce the two in parallel for the sake of understanding. 5.3.3.1.4 In order for (N+1) entities to be able to communicate using a connection-mode service or a connectionless-mode service, a prearranged association shall exist between the two (N+1) entities, each of which has knowledge of the other, at least to initiate the use of such a service. This association is established by means not described in detail in this basic reference model and contains the following four basic knowledge: a) the addresses of the peer (N) entities involved; b) a protocol agreed upon by the peer (N) entities for at least initial communication; c) the availability of the peer (N) entities for communication; d) the quality of service available from the (N) service. NOTE: Knowledge of the prearranged association may be obtained in a variety of ways, some examples of which are: a) information obtained manually when exchanging contracts with service providers; b) information obtained by network management in a directory or by querying a database; c) information that may be known from previous instances of communication; d) information that may be provided dynamically through operational management collaboration. The full knowledge constituting the prearranged association may be obtained using a combination of the above methods. 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) entity to the (N-1) entity of the adjacent lower layer. The adjacent lower layer provides the (N) entity in a given (N) layer with the ability to establish and release a connection and transfer data on the connection in the form of connection mode services. The peer (N) entity uses the connection mode service through the following three distinct phases:
a) connection establishment,
b) data transfer,
c) connection release.
5.3.3.2.2 In addition to the distinct lifetimes indicated by the three phases, a connection has the following characteristics: a) it involves the establishment and maintenance of a two or more party agreement to transfer data and to use (N-1) service providers between the involved peer (N) entities;
b) it allows negotiation between all parties involved of parameters and options that will be used to control the transfer of data; c) it provides connection identification and serves to avoid overhead in address resolution and transmission when data is transferred; d) it provides a context by which successively transmitted data units between peer entities can be logically associated and sequence maintained and flow control provided for the transfer.
5.3.3.2.3 The characteristics of connection-mode transmission are particularly suitable for applications that require relatively long lifetimes and the existence of a stream of interactions between entities in a stable configuration. Examples of such applications include long-term connections between terminals directly using remote computers, file transfers, and remote job entry stations. In these cases, the entities involved first discuss their requirements and agree on the terms of their interaction, reserve resources that may be required, transfer a series of related data units to achieve the goals of each party, explicitly terminate the interaction, and release previously reserved resources. The nature of connection-mode transmission is also relevant to other applications to a large extent. 5.3.3.2.4 Connection-mode transmission is achieved by using (N) connections. (N) connections are provided by the (N) layer between two or more (N) service access points. The end point of an (N) connection at an (N) service access point is called an (N) connection endpoint. The (N) layer provides an (N) connection between two or more (N) service access points when the calling (N+1) entity makes a request to support an (N+1) entity connected to the (N) service access point involved in the (N) connection. An (N) connection with more than two endpoints is called a multi-endpoint connection. The (N) entities between which a connection exists are called connected (N) entities.
Note: Data transmission using the (N) connection-mode service includes the establishment of an (N) connection before the data is transmitted. In addition to the associations described in 5.3.2, an association between the (N+1) entity and the (N) connection mode service is established dynamically. This association involves elements that are not part of the pre-arranged association described in 5.3.3.1.4, in particular the following: a) The willingness of one or more peer (N) entities to conduct a specific communication and the willingness of the lower-level services supporting the communication: 12
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