1、INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARD IZATI ON SECTOR OF ITU X.622 (07/94) DATA NETWORKS AND OPEN SYSTEM COM M UN IC ATION S OS1 NETWORKINGS AND SYSTEM ASPECTS - NETWORKING INFORMATION TECHNOLOGY - THE CONNECTIONLESS-MODE NETWORK PROTOCOL FOR PROVIDING SERVICE: PROVI
2、SION OF THE UNDERLYING SERVICE BY AN X.25 SUBNETWORK ITU-T Recommendation X.622 (Previously “CCITT Recommendation”) FOREWORD ITU (International Telecommunication Union) is the United Nations Specialized Agency in the field of telecommunications. The ITU Telecommunication Standardization Sector (IT-T
3、) is a permanent organ of the IT. Some i 79 member countries, 84 telecom operating entities, 145 scientific and industrial organizations and 38 international organizations participate in ITU-T which is the body which sets world telecommunications standards (Recommendations). The approval of Recommen
4、dations by the Members of ITU-T is covered by the procedure laid down in WTSC Resolution No. 1 (Helsinki. 1993). In addition, the World Telecommunication Standardization Conference (WTSC), which meets every four years, approves Recommendations submitted to it and establishes the study programme for
5、the following period. In some areas of information technology which fall within ITU-Ts purview, the necessary standards are prepared on a collaborative basis with IS0 and IEC. The text of -T Recommendation X.622 was approved on ist of July 1994. The identical text is also published as ISO/IEC Intern
6、ational Standard 8473-3. NOTE In this Recommendation, the expression “Administration” is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. O ITU 1994 AI1 rights reserved. No part of this publication may be reproduced or utilized in any form o
7、r by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU. ITU-T X-SERIES RECOMMENDATIONS DATA NETWORKS AND OPEN SYSTEM COMMUNICATIONS (February 1994) ORGANIZATION OF X-SERIES RECOMMENDATIONS Subject area PUBLIC DATA NETWORKS Recommend
8、ation series Services and facilities Interfaces X. 1-X. 19 X.20-X.49 Transmission. signalling and switching Network aspects X.50-X.89 X.90-X. 149 Maintenance Administrative arrangements x. 150-x. 179 X. 180-X. I99 OPEN SYSTEMS INTERCONNECTION Model and notation x.200-x.209 Service definitions Connec
9、tion-mode protocol srecifcations x.210-x.219 X. 220-X . 229 Connectionless-mode protocol specifications PICS Droformas I OS1 NETWORKING AND SYSTEM ASPECTS I I X.230-X.239 X.240-X.259 I Networking Protocol identification Security protocols I X.600-X.649 I X.260-X.269 X.270-X.279 Layer managed objects
10、 Conformance testing X.280-X.289 X.290-X.299 OS1 APPLICATIONS I I INTERWORKING BEIWEEN NETWORKS General Commitment, concurrency and recovery x.300-x.349 I I X.850-X.859 Mobile data transmission systems Management Transaction processing I X.860-X.879 I X.350-X.369 X.370-X.399 MESSAGE HANDLING SYSTEMS
11、 DIRECTORY x.400-x.499 x.500-x.599 Naming, addressing and registration Abstract Syntax Notation One (ASN.1) X.650-X.679 X.680-X.699 OS1 MANAGEMENT SECURITY x.700-x.799 X.800-X.849 Remote operations OPEN DISTRIBUTED PROCESSING X.880-X.899 x.900-x.999 Contents Introduction . 1 Scope 2 Normative refere
12、nces . 2 . i Identical Recommendations I International Standards 2.2 Paired Recommendations I International Standards identical in technical content . 2.3 Additional references 3 Definitions 3 .i Reference model definitions . 3.2 Network layer architecture definitions 3.3 Network layer addressing de
13、finitions . 3.4 Xi25 definitions . 4 Abbreviations . . 5 Subnetwork dependent convergence function 5.1 General model . 5.2 Subnetwork user data . 5.3 Subnetwork dependent convergence functions used with X.25 subnetworks A.1 Introduction . A.2 Abbreviations and special symbols . A.3 A.4 Identificatio
14、n A.5 Subnetwork dependent convergence functions for use with X.25 subnetworks . Annex A - PICS proforma Instructions for completing the PICS proforma ITU-T Rec . X.622 (1994 E) Page . 111 1 1 1 1 1 2 2 2 2 2 2 3 3 3 3 7 7 7 7 9 10 i Summary This Recommendation I International Standard specifies the
15、 mapping between an X.25 subnetwork and the Connectionless Layer Network Layer service assumed by the Connectionless Layer Network Layer Protocol as defined in Recornmendation X.233. II ITU-T RM. X.622 (1994 E) Introduction This is one of a set of Recommendations and International Standards produced
16、 to facilitate the interconnection of open systems. The set covers the services and protocols required to achieve such interconnection. This Recommendation I International Standard is positioned with respect to other related Recommendations and International Standards by the layers defined in ITU-T
17、Rec. X.200 I 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 I ISO/IEC 8473-1 is specified. In order to evaluate the conformanc
18、e 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 defined in CCITT Rec. X.290 I ISO/IEC 9646-1. A PICS
19、proforma, from which a PICS may be prepared for a specific implementation. is included in this Recommendation I Intemational Standard as normative Annex A. . ITU-T RW. X.622 (1994 E) 111 ISO/IEC 8473-3 : 1995 (E) INTERNATIONAL STANDARD ITU-T RECOMMENDATION INFORMATION TECHNOLOGY - PROTOCOL FOR PROVI
20、DING THE CONNECTIONLESS-MODE NETWORK SERVICE: PROVISION OF THE UNDERLYING SERVICE BY AN X.25 SUBNETWORK 1 Scope This Recommendation i International Standard specifies the way in which the underlying service assumed by the protocol defined by ITU-T Rec. X.233 I ISOAEC 8473-1 is provided by a subnetwo
21、rk that conforms to IT-T Recommendation X.25 through the operation of a Subnetwork Dependent Convergence Function (SNDCF) as described in ISO/IEC 8648. This Recommendation I International Standard also provides the PICS proforma for this protocol, in compliance with the relevant requirements, and in
22、 accordance with the relevant guidance, given in CCT Rec. X.290 I ISOAEC 9646-1. 2 Normative references The following Recommendations and International Standards contain provisions which, through reference in this text, constitute provisions of this Recommendation i International Standard. At the ti
23、me of publication, the editions indicated were valid. All Recommendations and Standards are subject to revision, and parties to agreements based on this Recommendation i International Standard are encouraged to investigate the possibility of applying the most recent editions of the Recommendations a
24、nd Standards listed below. Members of IEC and IS0 maintain registers of currently valid International Standards. The Telecommunication Standardization Bureau of the ITU maintains a list of currently valid ITU-T Recommendations. 2.1 Identical Recommendations I International Standards - ITU-T Recommen
25、dation X.200 (1994) I ISO/IEC 7498- 1 : 1994, Information technology - Open Systems Interconnection - Basic Reference Model. CCITT Recommendation X.213 (1992) I ISOAEC 8348: 1993, Information technology - Network service definition for Open Systems Interconnection. - 2.2 Paired Recommendations I Int
26、ernational Standards identical in technical content - CCITT Recommendation X.290 (1992). OS1 conformance testing methodology and framework for protocol Recommendations for CCITT applications - General concepts. ISOAEC 9646- 1 : 199 1. Information technology - Open Systems Interconnection - Conforman
27、ce testing methodology and framework - Part I: 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 operating in the packet mode and connected to public data tierwor
28、ks by dedicated circuit. CCIT Recommendation X. 12 1 ( 1992). International numbering plan for public dafa networks. ISOiEC 8208: 1990. Information technology - Data communications - X.25 Packet Layer Protocol for Data Terminai Equipment. ISO/IEC 8648: 1988, Information processing systems - Open Sys
29、tems Interconnection - Internal organization of the network layer. - - - ITU-T Rec. X.622 (1994 E) 1 3 Definitions 3.1 Reference model definitions This Recommendation I International Standard makes use of the following terms defined in ITU-T Rec. X.200 I ISO/IEC 7498- 1 a) network entity; b) Network
30、 layer; c) service; d) service data unit; e) protocol control information. 3.2 Network layer architecture definitions This Recommendation I International Standard makes use of the following terms defined in ISOEC 8648: a) subnetwork; b) subnetwork dependent convergence protocol; c) subnetwork depend
31、ent convergence function; d) subnetwork access protocol. 3.3 Network layer addressing definitions This Recommendation I International Standard makes use of the following term defined in CCITT Rec. X.213 I ISOAEC 8348: - subnetwork point of attachment. 3.4 X.25 definitions This Recommendation I Inter
32、national Standard makes use of the following terms defined in IT-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 Connectionless-mode network protocol DCE Data c
33、ircuit-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 protocol SMCP Subnetwork independent converg
34、ence protocol SNAcP Subnetwork access protocol SNPA Subnetwork point of attachment 2 ITU-T RW. X.622 (1994 E) ISO/IEC 8473i3 : 1995 (E) SNCR Subnetwork connection reference SNSDU Subnetwork service data unit 5 Subnetwork dependent convergence function 5.1 General model The general model for providin
35、g the underlying 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 su
36、bnetwork dependent 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 assum
37、ed by the CLNP 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-
38、data. If a connection 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 conne
39、ction-mode subnetwork 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 po
40、ints of attachment. The underlying service assumed by the CLNP is required to support a service data unit size of at least 512 octets 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 segmenta
41、tion is not required, 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 I ISO/IEC 8473- 1) shall be processed according to this Recommendation I International St
42、andard. NOTE - Data with 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 The connection-mode service offered by subnetworks that use the X.25 Packet Level Protocol defined in IS
43、OAJZC 8208 or IT-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 explicit subnetwork dependent convergence proto
44、col 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 the CCITT Rec. X. 12 1 DTE addresses used by th
45、e 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 Request packet, in the case that the subnetwork do
46、es not include this parameter but permits its inclusion by DES. NOTE - Some subnetworks which use the X.25 PLP employ addressing schemes other than CCIIT Rec. X.121. The use of addressing schemes other than CCIIT Rec. X.121 (e.g. CCIIT Recommendations E.163 and E.164) is not precluded. The SN-User-d
47、ata parameter carries user data up to a maximum size specified by the subnetwork authority. The underlying service assumed by ITU-T Rec. X.233 i ISO/IEC 8473-1 requires that a subnetwork be capable of supporting a minimum service data unit size of 5 12 octets. NOTE - The M-bit may be used in cases w
48、here an X.25 subnetwork cannot directly support a minimum packet size of 512 octets as well as in situations where a service data unit size greater than the minimum is required, e.g. where the non-segmenting protocol subset is used. Subnetwork dependent convergence functions used with X.25 subnetwor
49、ks ITU-T Rec. X.622 (1994 E) 3 5.3.1 Call setup considerations The mechanism and timing for opening a virtual circuit prior to the transmission of SN-User-data are a local matter. The opening of a virtual circuit may be initiated by: the arrival of an SNSDU to be transmitted over an X.25 subnetwork at a time when no suitable virtual circuit is available; the local queue of requests waiting for an existing virtual circuit reaching a threshold size at which an additional virtual circuit shall be made available (if possible) to maintain the requested QoS; or the exp
