1、BRITISH STANDARD BS ISO/IEC 8473-3:1995 Implementation of ISO/IEC 8473-3:1995 Information technology Protocolfor providingthe connectionless-mode network service: Provision of the underlying service by an X.25 subnetworkBSISO/IEC8473-3:1995 This British Standard, having been prepared under the direc
2、tionof the Information Systems Technology Assembly,was published underthe authority of the Standards Board and comesintoeffect on 15July1995 BSI 12-1999 The following BSI references relateto the work on this standard: Committee reference IST/6 Draft for comment 94/642336 DC ISBN 0 580 24515 2 Commit
3、tees responsible for this BritishStandard The preparation of this British Standard was entrusted to Technical Committee IST/6, Data communications, upon which the following bodies were represented: British Computer Society British Telecommunications plc CCTA (the Government Centre for Information Sy
4、stems) Digital Equipment Co Ltd. IBM United Kingdom Ltd. Institution of Electrical Engineers International Computers Limited Logica UK Ltd. Nine Tiles Computer Systems Ltd. Rank Xerox (UK) Ltd. Amendments issued since publication Amd. No. Date CommentsBSISO/IEC8473-3:1995 BSI 12-1999 i Contents Page
5、 Committees responsible Inside front cover National foreword ii Foreword iii Text of ISO/IEC 8473-3 1BSISO/IEC8473-3:1995 ii BSI 12-1999 National foreword This British Standard reproduces verbatim ISO/IEC 8473-3:1995 and implements it as the UK national standard. This British Standard is published u
6、nder the direction of the Information Systems Technology Assembly whose Technical Committee IST/6 has the responsibility to: aid enquirers to understand the text; present to the responsible international committee any enquiries on interpretation, or proposals for change, and keep UK interests inform
7、ed; monitor related international and European developments and promulgate them in the UK. NOTEInternational and European Standards, as well as overseas standards, are available from Customer Services, BSI, 389 Chiswick High Road, London W4 4AL. A British Standard does not purport to include all the
8、 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. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii,
9、theISO/IEC title page, pages ii to iv, pages 1 to 11 and abackcover. 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.ISO/IEC8473-3:1995(E) ii BSI 12-1999 Contents Page Foreword iii I
10、ntroduction 1 1 Scope 1 2 Normative references 1 2.1 Identical Recommendations|International Standards 1 2.2 Paired Recommendations|International Standards identical in technical content 1 2.3 Additional references 1 3 Definitions 2 3.1 Reference model definitions 2 3.2 Network layer architecture de
11、finitions 2 3.3 Network layer addressing definitions 2 3.4 X.25 definitions 2 4 Abbreviations 2 5 Subnetwork dependent convergence function 3 5.1 General model 3 5.2 Subnetwork user data 3 5.3 Subnetwork dependent convergence functions used with X.25 subnetworks 3 Annex A PICS proforma 7 A.1 Introdu
12、ction 7 A.2 Abbreviations and special symbols 7 A.3 Instructions for completing the PICS proforma 7 A.4 Identification 9 A.5 Subnetwork dependent convergence functions for use with X.25 subnetworks 10 Table 1 Subnetwork connection reference encoding 6ISO/IEC8473-3:1995(E) BSI 12-1999 iii Foreword IS
13、O (the International Organization for Standardization) and IEC (the International 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 committ
14、ees 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 wor
15、k. 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 le
16、ast75% of the national bodies casting a vote. International Standard ISO/IEC 8473-3 was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology, Subcommittee SC 6, Telecommunications and information exchange between systems, in collaboration with ITU-T. The identical text is publ
17、ished as ITU-T Recommendation X.622. ISO/IEC 8473 consists of the following parts, under the general title Information technology Protocol for providing the connectionless-mode network service: Part 1: Protocol specification; Part 2: Provision of the underlying service by an ISO/IEC 8802 subnetwork;
18、 Part 3: Provision of the underlying service by an X.25 subnetwork; Part 4: Provision of the underlying service by a subnetwork that provides the OSI data link service. Annex A forms an integral part of this part of ISO/IEC 8473.iv blankISO/IEC8473-3:1995(E) BSI 12-1999 1 Introduction This is one of
19、 a set of Recommendations and International Standards produced to facilitate the interconnection of open systems. The set covers the services and protocols required to achieve such interconnection. This Recommendation|International Standard is positioned with respect to other related Recommendations
20、 and International Standards by the layers defined in ITU-T Rec. X.200|ISO/IEC 7498-1. In particular, it defines the way in which an X.25 subnetwork may be used within the Network layer to provide the abstract underlying service with respect to which the protocol defined by ITU-T Rec. X.233|ISO/IEC8
21、473-1is specified. In order to evaluate the conformance of a particular implementation of this protocol, it is necessary to have a statement of which of the protocols capabilities and options have been implemented. Such a statement is called a Protocol Implementation Conformance Statement (PICS), as
22、 defined in CCITT Rec. X.290|ISO/IEC 9646-1. A PICS proforma, from which a PICS may be prepared for a specific implementation, is included in this Recommendation|International Standard as normative Annex A. 1 Scope This Recommendation|International Standard specifies the way in which the underlying
23、service assumed by the protocol defined by ITU-T Rec. X.233|ISO/IEC 8473-1 is provided by a subnetwork that conforms to ITU-T Recommendation X.25 through the operation of a Subnetwork Dependent Convergence Function (SNDCF) as described in ISO/IEC 8648. This Recommendation|International Standard also
24、 provides the PICS proforma for this protocol, in compliance with the relevant requirements, and in accordance with the relevant guidance, given in CCITT Rec. X.290|ISO/IEC 9646-1. 2 Normative references The following Recommendations and International Standards contain provisions which, through refe
25、rence in this text, constitute provisions of this Recommendation|International Standard. At the time of publication, the editions indicated were valid. All Recommendations and Standards are subject to revision, and parties to agreements based on this Recommendation|International Standard are encoura
26、ged to investigate the possibility of applying the most recent editions of the Recommendations and Standards listed below. Members of IEC and ISO maintain registers of currently valid International Standards. The Telecommunication Standardization Bureau of the ITU maintains a list of currently valid
27、 ITU-T Recommendations. 2.1 Identical Recommendations|International Standards ITU-T Recommendation X.200 (1994)|ISO/IEC 7498-1:1994, Information technology Open Systems Interconnection Basic Reference Model. CCITT Recommendation X.213 (1992)|ISO/IEC 8348:1993, Information technology Network service
28、definition for Open Systems Interconnection. 2.2 Paired Recommendations|International Standards identical in technical content CCITT Recommendation X.290 (1992), OSI conformance testing methodology and framework for protocol Recommendations for CCITT applications General concepts. ISO/IEC 9646-1:199
29、1, Information technology Open Systems Interconnection Conformance testing methodology and framework Part 1: General concepts. 2.3 Additional references ITU-T Recommendation X.25 (1993), Interface between data terminal equipment (DTE) and data circuit-terminating equipment (DCE) for terminals operat
30、ing in the packet mode and connected to public data networks by dedicated circuit. CCITT Recommendation X.121 (1992), International numbering plan for public data networks. ISO/IEC 8208:1990, Information technology Data communications X.25 Packet Layer Protocol for Data Terminal Equipment. ISO/IEC 8
31、648:1988, Information processing systems Open Systems Interconnection Internal organization of the network layer.ISO/IEC8473-3:1995(E) 2 BSI 12-1999 3 Definitions 3.1 Reference model definitions This Recommendation|International Standard makes use of the following terms defined in ITU-T Rec. X.200|I
32、SO/IEC 7498-1: a) network entity; b) Network layer; c) service; d) service data unit; e) protocol control information. 3.2 Network layer architecture definitions This Recommendation|International Standard makes use of the following terms defined in ISO/IEC 8648: a) subnetwork; b) subnetwork dependen
33、t convergence protocol; c) subnetwork dependent convergence function; d) subnetwork access protocol. 3.3 Network layer addressing definitions This Recommendation|International Standard makes use of the following term defined in CCITT Rec. X.213|ISO/IEC 8348: subnetwork point of attachment. 3.4 X.25
34、definitions This Recommendation|International Standard makes use of the following terms defined in ITU-T Rec. X.25 and ISO/IEC 8208: a) data circuit-terminating equipment; b) data terminal equipment; c) logical channel; d) permanent virtual circuit; e) virtual circuit. 4 Abbreviations CLNP Connectio
35、nless-mode network protocol DCE Data circuit-terminating equipment DTE Data terminal equipment PDU Protocol data unit PVC Permanent virtual circuit QoS Quality of service SDU Service data unit SN Subnetwork SNDCF Subnetwork dependent convergence function SNDCP Subnetwork dependent convergence protoc
36、ol SNICP Subnetwork independent convergence protocol SNAcP Subnetwork access protocol SNPA Subnetwork point of attachment SNCR Subnetwork connection referenceISO/IEC8473-3:1995(E) BSI 12-1999 3 5 Subnetwork dependent convergence function 5.1 General model The general model for providing the underlyi
37、ng service assumed by the protocol in conjunction with a real subnetwork that uses a connectionless subnetwork access protocol is as follows. The generation of an SN-UNITDATA Request by the CLNP results in the generation of a corresponding subnetwork-specific UNITDATA request by the subnetwork depen
38、dent convergence function. The receipt of a subnetwork-specific UNITDATA indication associated with delivery of a connectionless data unit to its destination causes the SNDCF to generate an SN-UNITDATA Indication to the CLNP. The general model for providing the underlying service assumed by the CLNP
39、 in conjunction with a real subnetwork that uses a connection-mode subnetwork access protocol is as follows. The generation of an SN-UNITDATA Request by the CLNP causes a connection (logical channel, logical link, or the equivalent) to be made available for the transmission of SN-User-data. If a con
40、nection cannot be made available, the SN-UNITDATA Request is discarded. The receipt of subnetwork-specific PDUs containing SN-User-data causes the SNDCF to generate an SN-UNITDATA Indication to the CLNP. Where a real subnetwork is designed to use either a connectionless-mode or a connection-mode sub
41、network access protocol, the provision of the underlying service assumed by the CLNP is achieved by using the connectionless-mode alternative. 5.2 Subnetwork user data The SN-Userdata is an ordered multiple of octets, and is transferred transparently between the specified subnetwork points of attach
42、ment. The underlying service assumed by the CLNP is required to support a service data unit size of at least512octets. If the minimum service data unit sizes supported by all of the subnetworks involved in the transmission of a particular PDU are known to be large enough that segmentation is not req
43、uired, then either the full protocol or the non-segmenting protocol subset may be used. Data received from a subnetwork with protocol identification specifying this protocol (see ITU-T Rec. X.233|ISO/IEC 8473-1) shall be processed according to this Recommendation|International Standard. NOTEData wit
44、h other protocol identification should be ignored, since it may have been sent by an implementation supporting additional protocols intended for use with this protocol. 5.3 Subnetwork dependent convergence functions used with X.25 subnetworks The connection-mode service offered by subnetworks that u
45、se the X.25 Packet Level Protocol defined in ISO/IEC 8208 or ITU-T Rec. X.25 is manipulated by the subnetwork dependent convergence function so that a virtual circuit is made available for the transmission of SN-User-data following the generation of an SN-UNITDATA Request by the CLNP. In general, no
46、 explicit subnetwork dependent convergence protocol control information is exchanged between peer network entities during the data phase of operation in order to provide this mapping of service. The SN-Destination-Address and SN-Source-Address parameters in the SN-UNITDATA request and indication are
47、 the CCITT Rec. X.121 DTE addresses used by the X.25 subnetwork. If the X.25 subnetwork does not provide calling DTE information, a null SN-Source-Address parameter is supplied in the SN-UNITDATA Indication. The SNDCF shall include its own DTE address in the “calling DTE” field of the X.25 Call Requ
48、est packet, in the case that the subnetwork does not include this parameter but permits its inclusion by DTEs. NOTESome subnetworks which use the X.25 PLP employ addressing schemes other than CCITT Rec. X.121. The use of addressing schemes other than CCITT Rec. X.121 (e.g. CCITT Recommendations E.16
49、3 and E.164) is not precluded. The SN-User-data parameter carries user data up to a maximum size specified by the subnetwork authority. The underlying service assumed by ITU-T Rec. X.233|ISO/IEC8473-1requires that a subnetwork be capable of supporting a minimum service data unit size of512octets. NOTEThe M-bit may be used in cases where an X.25 subnetwork cannot directly support a minimum packet size of512octets as well as in situations where a service data unit size greater than the minimum is required, e.g. where the
