1、BRITISH STANDARD BS 7221:1989 ISO 8648:1988 Incorporating Technical Corrigendum No. 1 Open Systems Interconnection: Specification for internal organization of the Network Layer UDC 681.324:681.016.006.3/.8BS7221:1989 This British Standard, having been prepared under the directionof the Information S
2、ystems Technology Standards Policy Committee, was publishedunder the authority ofthe Board of BSI and comes intoeffect on 31 December 1989 BSI11-1999 The following BSI references relate to the work on this standard: Committee reference IST/6 Draftfor comment DD 135 ISBN 0 580 17734 3 Committees resp
3、onsible for this British Standard The preparation of this British Standard was entrusted by the Information Systems Technology Standards Policy Committee (IST/-) to Technical Committee IST/6, upon which the following bodies were represented: Association for Payment Clearing Services British Computer
4、 Society British Telecommunications plc Business Equipment and Information Technology Association Department of Trade and Industry (Information Technology Division) Department of Trade and Industry (National Physical Laboratory) Electricity Supply Industry in England and Wales Electronic Engineering
5、 Association HM Treasury (Central Computer and Telecommunications Agency) Information Technology Users Standards Association Inter-Universities Computing Committee Joint Network Team LAMSAC National Computing Centre Ltd. National Health Services OFTEL (Offices of Telecommunications) Post Office Tele
6、communication Engineering and Manufacturing Association Telecommunications Managers Association User Standards Forum for Information Technology (Institute of Data Processing Management) Amendments issued since publication Amd. No. Date of issue Comments 7168 Corr.1 July 1992 Indicated by a sideline
7、in the marginBS7221:1989 BSI 11-1999 i Contents Page Committees responsible Inside front cover National foreword ii 0 Introduction 1 1 Scope and field of application 1 2 References 2 3 Definitions 2 4 Abbreviations 3 5 Network Layer concepts and terminology 3 6 Organization of the Network Layer 6 7
8、Application of the Network Layer Internal organization 12 8 Interconnection scenarios 14 Figure 1 “Direct” communication between end systems (without an intermediate system) 6 Figure 2 End systems communicating through an intermediate system 6 Figure 3 Interconnection of real subnetworks that includ
9、e integral interworking unit functionality 7 Figure 4 Interconnection of real subnetworks by a separate interworking unit 8 Figure 5 Relationship between subnetwork service and data transmission service 9 Figure 6 Network Layer protocol rules 11 Figure 7 Provision of CO-NS or CL-NS over a single dat
10、a-link 16 Figure 8 Provision of CO-NS or CL-NS over a single subnetwork 17 Figure 9 Interconnection of subnetworks utilizing a single NL protocol to provide the CO-NS or CL-NS over two CO- or CL-subnetworks 18 Figure 10 Provision of the CO-NS over two CO-subnetworks using hop-by-hop harmonization 19
11、 Figure 11 Provision of the CO-NS over two CO-subnetworks using aninternetworking protocol approach 22 Figure 12 Provision of the CL-NS over two CL-subnetworks using hop-by-hop harmonization 23 Figure 13 Provision of the CL-NS over two CL-subnetworks using an internetworking protocol approach 24 Fig
12、ure 14 Provision of the CL-NS over two CO-subnetworks using hop-by-hop harmonization 25 Figure 15 Provision of the CL-NS over two CO-subnetworks using an internetworking protocol approach 26 Figure 16 Provision of the CO-NS over two CL-subnetworks using hop-by-hop harmonization 27 Figure 17 Provisio
13、n of the CO-NS over two CL-subnetworks using an internetworking protocol approach 28 Figure 18 Provision of the CL-NS over a CL-subnetwork and a CO-subnetwork using hop-by-hop harmonization 29 Figure 19 Provision of the CL-NS over a CL-subnetwork and a CO-subnetwork using an internetworking protocol
14、 approach 30 Figure 20 Provision of the CO-NS over a CL-subnetwork and a CO-subnetwork using hop-by-hop harmonization 31BS7221:1989 ii BSI 11-1999 Page Figure 21 Provision of the CO-NS over a CL-subnetwork and a CO-subnetwork using an internetworking protocol approach 32 Table 1 Correspondence betwe
15、en real world objects and abstract elements 5 Table 2 Summary of interconnection scenarios 14 Publications referred to Inside back coverBS7221:1989 BSI 11-1999 iii National foreword This British Standard, prepared under the direction of the Information Systems Technology Standards Policy Committee,
16、is identical with ISO 8648:1988, including Technical Corrigendum 1, “Information processing systems Open Systems Interconnection Internal organization of the Network Layer” published by the International Organization for Standardization (ISO). It supersedes DD 135:1985 which is withdrawn. A related
17、Draft for Development to ISO 8802-3:1989 is DD 98-1:1984 “CSMA/CD Local area networks”. The Technical Committee has reviewed the provisions of CCITT Recommendations X.21 and X.25, to which reference is made in the text, and has decided that they are acceptable for use in connection with this standar
18、d. It is envisaged that British Standards identical with ISO 8802-4, ISO 8802-5 and ISO 8802-7 will be published in due course. The footnote to clause 2, referring to ISO 8802-2 and ISO 8802-3, is no longer applicable. ISO/DP 8802-2 (IEEE 802.2 Draft E March 1984) was implemented as an identical Dra
19、ft for Development, DD 99-1:1985. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Cross-r
20、eferences International Standard Corresponding British Standard ISO 7498:1984 BS 6568 Reference model of open systems interconnection Part 1:1988 Basic reference model (incorporating connectionless-mode transmission) (Identical) ISO 8208:1987 BS 7219:1989 Specification for X.25 packet level protocol
21、 for data terminal equipment for information processing systems (Identical) ISO 8348:1987 BS 7220:1989 Information processing systems Data communications Network service definition (Identical) ISO/TR 8509:1987 DD 108:1988 Open Systems Interconnection service conventions (Identical) Summary of pages
22、This document comprises a front cover, an inside front cover, pagesi toiv, pages1to32, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.iv blank
23、BS7221:1989 BSI 11-1999 1 0 Introduction This International Standard defines an architectural organization of the Network Layer of the OSI Reference Model. It is concerned with the functional organization of Network Layer entities in open systems, and with the ways in which this organization can be
24、mapped to “real world” components (e.g. “real” networks, switches, transmission media, etc.). This International Standard relates those “real world” objects which must be dealt with to a set of abstract elements. There may be a variety of mappings between an abstract element and the physical equipme
25、nt used to realize it. The description of such mappings requires a clear distinction between architectural terms and real world terms. The architectural organization defined in this International Standard identifies and categorizes the way in which functions can be performed within the Network Layer
26、 by Network Layer protocols. In so doing, it provides a uniform framework for describing how different Network Layer protocols, operating either individually or cooperatively, can be used to provide the OSI Network Service. By focusing attention on the common functional elements of Network Layer pro
27、tocols, this framework is intended to a) simplify the use of Network Layer protocols to provide the Network Service in different situations; b) limit the uncoordinated proliferation of Network Layer protocols with overlapping functions; and c) clarify the requirements for, and guide the development
28、of, future Network Layer protocol standards. This detailed internal structure is necessary in the case of the Network Layer for two reasons: 1) The Network Layer provides its users with a uniform Network Service, regardless of potentially wide variations in underlying “real-world” network services,
29、technologies, and administrative organization. It is important to understand how the underlying components are organized and interact within the Network Layer, and how they can be efficiently and effectively used. 2) ISO 7498 specifies that the Network Layer performs routeing and relaying functions
30、and may contain entities residing in intermediate systems as well as in end systems. It is necessary to describe events that take place within the Network Layer in both types of entities; i.e. in end systems (within which the Network Service is provided to NS users) and in intermediate systems (with
31、in which network-entities providing these relay and routeing functions do not provide the Network Service to NS users). 1 Scope and field of application This International Standard provides an architectural model of the OSI Network Layer as a framework for OSI Network Layer standardization, allowing
32、 incorporation of existing networks within the OSI architecture. This framework encourages the design of real subnetworks that fully support the OSI Network Service whilst accommodating the participation, in the OSI environment, of other subnetworks that do not fully support the OSI Network Service.
33、 This International Standard is intended for use both in the design and application of Network Layer protocols, operating between network-entities either in end systems providing the OSI Network Service, or in intermediate systems providing the routeing and relay functions. It shall be used to a) pr
34、ovide a common set of concepts and terminology for use in Network Layer standards (such standards shall reference this International Standard); b) analyse Network Layer functionality and classify Network Layer protocols; c) specify how “real networks” should be used in supporting or providing the OS
35、I Network Service, in particular, in circumstances where multiple “real networks” are to be interconnected and used. The organization defined herein does not address Network Layer management; in particular it is not concerned with the relationships among Network Layer entities which may be required
36、for Network Layer management purposes. In addition, this International Standard does not specify the operational requirements for relay functions in intermediate systems, nor address how specific combinations of permitted functions might be practically and efficiently utilized.BS7221:1989 2 BSI 11-1
37、999 2 References ISO 7498, Information processing systems Open Systems Interconnection Basic Reference Model. NOTESee also CCITT Recommendation X.200 Reference Model of Open Systems Interconnection for CCITT Applications. ISO 7498/Add.1, Information processing systems Open Systems Interconnection Ba
38、sic Reference Model Addendum 1: Connectionless-mode transmission. ISO 8208, Information processing systems Data communications X.25 Packet Level Protocol for Data Terminal Equipment. ISO 8348, Information processing systems Data communications Network service definition. NOTESee also CCITT Recommend
39、ation X.213 Network Service Definition for Open Systems Interconnection (OSI) for CCITT Applications. ISO 8348/Add. 1, Information processing systems Data communications Network service definition Addendum 1: Connectionless-mode transmission. ISO/TR 8509, Information processing systems Open Systems
40、Interconnection Service conventions. NOTESee also CCITT Recommendation X.210 Open Systems Interconnection (OSI) Layer Service Definition Conventions. ISO 8802-2, Information processing systems Data Communications Local Area Networks Logical Link Control 1) . ISO 8802-3, Carrier Sense Multiple Access
41、 with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications 1) . ISO 8802-4, Token-Passing Bus Access Method and Physical Layer Specifications 1) . ISO 8802-5, Token Ring Access Method and Physical Layer Specifications 1) . ISO 8802-7, Slotted Ring Access Method and Physical
42、Layer Specifications 1) . CCITT Recommendation X.21, General purpose interface between data terminal equipment and data circuit-terminating equipment for synchronous operation on public data networks. CCITT Recommendation X.25, Interface between data terminal equipment and data circuit-terminating e
43、quipment for terminals operating in the packet mode on public data networks. 3 Definitions 3.1 Reference model definitions This International Standard makes use of the following terms defined in ISO 7498: a) OSI Network Layer b) OSI Network Service c) OSI network-service-access-point d) OSI network-
44、service-access-point-address (network-address) e) OSI network-entity f) network-relay g) routeing h) service j) protocol k) protocol-control-information 3.2 Service conventions definitions This International Standard makes use of the following terms defined in ISO/TR 8509: l) service user m) service
45、 provider 3.3 Network Layer architecture definitions For the purpose of this International Standard, the following definitions apply: n) real subnetwork: A collection of equipment and physical media which forms an autonomous whole and which can be used to interconnect real systems for purposes of co
46、mmunication. p) subnetwork: An abstraction of a real subnetwork. q) interworking unit: One or more items of equipment, or a part of an item of equipment, whose operation provides a network-relay function (that is, a real system which receives data from one correspondent network-entity and forwards i
47、t to another correspondent network-entity). NOTESuch equipment may be integrated into a real subnetwork which is being interconnected. r) network-relay system: An abstraction of the equipment forming an interworking unit. s) intermediate system: An abstraction of a real system providing a network-re
48、lay function. t) data transmission service: The set of capabilities available from a real subnetwork which may be utilized by users of the real subnetwork for the purpose of sending and receiving data. 1) At present at the stage of draft; publication anticipated in due courseBS7221:1989 BSI 11-1999
49、3 NOTEThis service is defined between the points where each user is connected to and accesses the real subnetwork; e.g. a DTE/DCE interface, and should not be confused with the notion of a service at a layer boundary as defined in ISO 7498. 4 Abbreviations For the purpose of this International Standard, the following abbreviations apply: 5 Network Layer concepts and terminology This clause gives precise meaning to a number of terms used in the OSI Network Layer standards which have a range of int
