1、INTERNATIONAL STANDARD lSO/IEC 10028 First edition 1993-l l-01 Information technology - Telecommunications and information exchange between systems - Definition of the relaying functions of a Network layer intermediate system Technologies de /information - TI nrcehzary element of the Network relayin
2、g functions, but it does not specify how those routing functions are to be realized. The definition of Network relaying functions applies borh to a subnetwork supporting all elements of the Network Service (as defined in IS0 8648). and to a Network relay system interconnecting two subnetworks accord
3、ing IO any of the three approaches defined in IS0 8648. In the case of the hop-by-hop harmonization approach to interconnection. the definition of Network relaying functions applies to the result of harmonizing the subnetwork service(s) to the level of the Network Service. The harmonization function
4、s, which require protocol mechanisms to be specified, are outside the scope of this International Sr;md;lrd. This International Standard is intended for use in guiding the design and application of real interworking units and real subnetworks (eg, local area networks and private packet switched netw
5、orks) which are to support the OS1 NS. It is also intended for use in the development of standards for the Network layer, to ensure that the requirements deriving from the need for Network layer relaying are taken into account. There are no requirements for conformance to this International Standard
6、. NOTE - Conformance rcquircmcnts relating IO Ncrwork Iaycr relaying arc to bc found in the spccificiltions for mappings between the Network Internal Layer Service and Network protocols. 2 Normative references The following standards contain provisions which, through reference in this text, constitu
7、te provisions of this International Standard. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this International Standard are encouraged to investigate the possibility of applying the most recent editions of the
8、 standards listed below. Members of IEC and IS0 maintain registers of currently valid Internation;d Standards. IS0 7498: 1984, Informotiou processing sstettts - Opcw S.vstetrt.s Ittc,cottcctio, - Busic Rcfcrence Model NOTE - See also CCITT Recommendation X.200 (1993). 1 ISOAEC 10028 : 1993 (E) 0 ISO
9、AEC CCITT Recommendation X.213 (1992) 1 ISO/IEC 8348:1993, I f n ormation technology - Open Systems Interconnection - Network service definition. ISO/TR 8509: 1987, Information processing systems-Open Systems Interconnection -Service conventions. IS0 8648: 1987, Information processing systems - Open
10、 Systems Interconnection-Internal organization of the Network layer. 3 Definitions 3.1 Reference model definitions This International Standard makes use of the following terms defined in IS0 7498: a) b) cl 4 4 f) g) h) j) k) Network layer Network service Network service access point Network service
11、access point address Network entity Network relay correspondent Network entities Network connection Network connection endpoint Network function 3.2 Network layer architecture definitions This International Standard makes use of the following terms defined in IS0 8648: 4 real subnetwork b) subnetwor
12、k cl interworking unit d) Network relay system e) intermediate system fl end system 3.3 Service conventions definitions This International Standard makes use of the following terms defined in ISO/TR XSOY: 4 Network service-user b) Network service-provider cl primitive 4 request (primitive) d indicat
13、ion (primitive) t) response (primitive) g) confirm (primitive) 3.4 Network relaying definitions For the purposes of this International Standard the following definitions also apply: 3.4.1 domain: A subset of those open systems containing Network entities, in which there is a common understanding of
14、values for cer- tain domain-related parameters of NILS primitives. 3.4.2 domain-related parameter: A parameter of NILS primitives that does not have a set of values defined by this International Stan- dard - whether directly, or indirectly by reference to ISO/IEC 8348 - to be applicable throughout t
15、he Network layer. 3.4.3 hop: The communication path supporting the transfer of information by NILS primitives, or by NILS primitives and Network service primitives, for an instance of communication between two Network entities that are adjacent in the modelling chosen for the Network layer interconn
16、ection. 0 ISOAEC ISOllEC 10028 : 1993 (E) 4 Abbreviations and names of NILS elementary sequences 4.1 General abbreviations co EES ENSDU ISB N- NC NHEF NILS NISAP NS NSAP NSDU OS1 QOS connection-mode extended end system expedited Network service data unit internal service bearer Network- Network conn
17、ection Network hop endpoint Network internal layer service Network internal service access point Network service Network service access point Network service data unit open systems interconnection quality of service 4.2 Names of NILS elementary sequences of primitives for N-connections The following
18、 is a summary of the names defined in clause 8 for the elementary sequences of primitives that are used in defining the behaviour of a Network connection according to the Network internal layer service. Each item includes a brief description of the nature of the sequences function, and a reference t
19、o the figure defining the sequence. ABORT NC request aborted (figure 4) DT Normal data transfer (figure 7) EC Expedited data confirmation (figure 7) ED Expedited data transfer (figure 7) ESTAB NC confirmed (figure 6) FR Generated flow control, of received data (figure 7) Fr External flow control, of
20、 transmitted data (figure 7) RC Receipt confirmation (figure 7) RELC NC release. generated collision (figure 9) RELF NC release, forwarded (figure 9) RELG NC release, generated (figure 9) RELX NC release, external collision (figure 9) REQ NC request completed (figure 4) RESC Confirmation of forwarde
21、d reset (figure 8) RESF Forwarded reset (figure 8) RESG Generated local reset (figure 8) RESX External local reset (figure 8) RETRY NC establishment retry (figure 5) RJl NC request rejected (figure 4) SD NC status discarded (figure 5) SF NC status forwarded (figure 5) SG NC status generated (figure
22、5) 5 The model of relaying by an intermediate system 5.1 Elements of the relaying model This International Standard uses IS0 8648s structuring for intermediate systems as the basis for defining the operation of such systems in supporting the OS1 connection-mode Network service. Figure 1 illustrates,
23、 in a form adapted from IS0 8648. the general model of an intermediate system and the information flow through it. That flow represents. for each instance of Network layer communication handled by the intermediate system, the use made of the Network service by the NS-users in the end systems connect
24、ed to the two sides of the intermediate system: the end systems may be connected either directly or through other intermediate systems. The sides of the intermediate system are labelled A and B for convenience in referring to them. ISO/lEC 8348 defines the Network service in terms of primitives that
25、 occur in end systems, across the upper boundary of the Network layer. The first task of this International Standard is to provide corresponding definitions to allow discussion and specification of the information flow that represents those primitives in intermediate systems, and entirely within the
26、 Network layer. This is done by defining the Network internal layer service (NILS), including the correspondence between elements of the NILS and those of the Network service. The NILS is defined in terms of primitives with associated parameters, of the four types defined (but for OS1 layer services
27、) in IS0 TR 8509. The NILS is an abstraction of the interface in an intermediate system between the protocol support for the Network service and the N- relaying and N-routing functions. It is defined at a level corresponding to IS0 8648s subnetwork independent convergence protocol 3 ISOAEC 10028 : 1
28、993 (E) 0 ISOAEC (SNICP) role. Like the OS1 Network service, therefore, it is independent of the details of particular protocols and subnetworks. The definition of the NILS, however, involves less abstraction from the level of detail that is typical of protocols than does the Network service: this r
29、eflects the role of the NILS as an expression of general Network layer internal mechanisms, as opposed to just the Network layer functionality that needs to be made visible to an NS user. In figure I, the items IA and IB represent instances of the NILS. Clauses 7 to 9 contain the definition of the N
30、ILS. Item 2 in figure 1 represents the N-relaying function, which relates the elements of the two instances of the NILS. The N-relaying function is defined in clauses 8 and 9. N-protocols . b Ba, Bb - Data Link layer _- Physical layer Figure 1 - Intermediate-system model Item 3 represents the N-rout
31、ing function needed to perform the relaying. The N-relaying functions interface to this is defined in clause 9. Items 4A and 4B in figure 1 represent the mappings of the NILS to the protocol combinations used on the two sides of the intermediate system. These mappings for OSI standard Network protoc
32、ols are specified in other standards. NOTES 1 The protocol combinations on the two sides of an intermediate system may be different. If thcrc are K such combinations, the relaying model implies that only the K protocol mappings for the NILS need to bc spccificd in o or they may be comhincd “horizont
33、:llly”. as for inslancc when tin N-routing-protocol exchange is invoked in suppon of a particular attempt at NC establishment. 3 The OSI abstract modclling tcchniquc means that the NILS primilivcs al a given side of an incrmcdiatc system arc to bc considcrcd as a direct abstraction from the NPDUs tr
34、ansmitted and received hy the system. Thus ilcms IA and 48 in figure I truly rcprcscnl mappings between those NPDUS and NILS primitives: they do not represent. for example. Network protocol itllplclllnl;llionb. wikh hc implied possibility that thcsc can intcrvcnc. helow the NILS interface, in the fl
35、ow of Network-service-level information reprcscnicd hy 111. NILS. To the cxtcnl char such interventions are ohscrvahle outside the intcrmcdiate system. the modcl ascribes them all - apan perhaps front those rcsullinp front proltxol violations - IO the rcluying function shown as item 2. 5.2 Interconn
36、ection of intermediate systems This subclause considers how the model of a single intermediate system, introduced in 5.1, is used where multiple Network entities are interconnected in supporting an instance of Network layer communication. The concern throughout is with Network layer operation consid
37、ered at the level of the NILS or the OSI Network service. In particular, this subclause provides the framework for defining the correspondence between the OSI Network service, at end systems, and the operation of the NILS at intermediate systems involved in supporting the Network service. The simple
38、st interconnection scenario involving relaying is that shown in figure 2A, where two end systems communicate via a single intermediate system (representing, for example, a real subnetwork). The concern with the Network layer only. and with the NS and NILS level of abstraction within that layer, is e
39、xpressed in a diagrammatic representation that is simplified and abstracted from that used in figures I and ?A. Figure 2C is the representation corresponding to figure 2A, when the relaying function of the intermediate system is isolated for separate consideration. NOTE I - Figure 28 is provided pur
40、ely IO clarify the derivalion of figure 2C from figure ZA, hy illuslrating an intermediate step. Figures 3A and 3B show a more complex interconnection scenario and its representation. The new feature here is the communication between two intermediate systems, considered as Network relay systems. via a third intermediate system considered as a subnetwork. 4
