ITU-T X 222-1995 Use of X 25 LAPB Compatible Data Link Procedures to Provide the OSI Connection-Mode Data Link Service - Data Networks and Open Systems Communications - Open Systempeci.pdf

1、 ITU-T RECMN*X.222 75 = 4862573 0603237 T57 INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU x.222 (04/95) DATA NETWORKS AND OPEN SYSTEMS COMM UNICATIONS OPEN SYSTEMS INTERCONNECTION-CONNECTION- MODE PROTOCOL SPECIFICATIONS USE OF X.25 LAPB COMPATIBLE DATA

2、LINK PROCEDURES TO PROVIDE THE OS1 CONNECTION-MODE DATA LINK SERVICE ITU-T Recommendation X.222 (Previously “CCITT Recommendation”) ITU-T RECMN*X*222 95 4862593 Ob03220 779 FOREWORD The ITU-T (Telecommunication Standardization Sector) is a permanent organ of the International Telecommunication Union

3、 (ITU). The ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommen- dations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Conference (WTSC), which meets every four years, establishes

4、 the topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics. The approval of Recommendations by the Members of the ITU-T is covered by the procedure laid down in WTSC Resolution No. 1 (Helsinki, March 1-12, 1993). ITU-T Recommendation X.222 was prepa

5、red by ITU-T Study Group 7 (1993-1996) and was approved under the WTSC Resolution No. 1 procedure on the 10th of April 1995. NOTE In this Recommendation, the expression “Administration” is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. O I

6、TU 1995 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU. ITU-T RECMN*X-222 95 I 4Bh259L Ob03221 bo5 Services and Facilities Interface

7、s IT-T X-SERIES RECOMMENDATIONS DATA NETWORKS AND OPEN SYSTEM COMMUNICATIONS (February 1994) ORGANIZATION OF X-SERIES RECOMMENDATIONS X. 1 -X. 19 X.20-X.49 I Subject area I Recommendation Series I Administrative Arrangements OPEN SYSTEMS INTERCONNECTION IPUBLIC DATA NETWORKS -1 X. 180-X. 199 1 Conne

8、ctionless-mode Protocol Specifications PICS Proformas X.230-X.239 X.240-X.259 I Transmission, Signalling and Switching I X.50-X.89 I Security Protocols Layer Managed Objects I Network Asuects X.270-X.279 X.280-X.289 I X.90-X.149 1 Conformance Testing INTERWORKTNG BETWEEN NETWORKS I Maintenance I X.1

9、50-X.179 I X.290-X.299 Mobile Data Transmission Systems Management I Model and Notation I X.200-X.209 I X.350-X.369 X.370-X.399 I Service Definitions MESSAGE HANDLING SYSTEMS DIRECTORY I X.210-X.219 I X.400-X.499 X.500-X.599 I Connection-mode Protocol Specifications I X.220-X.229 I Networking Naming

10、, Addressing and Registration X.600-X. 649 X .650-X. 679 I Protocol Identification I X.260-X.269 I Abstract Syntax Notation One (ASN.l) OS1 MANAGEMENT X.680-X.699 X.700-X.799 SECURITY OS1 APPLICATIONS I General I X.300-X.349 I X. 800-X. 849 Commitment, Concurrency and Recovery Transaction Processine

11、 X.850-X.859 X.860-X.879 I OS1 NETWORKING AND SYSTEM ASPECTS I I Remote Operations OPEN DISTRIBUTED PROCESSING X. 880-X. 899 x.900-x.999 - ITU-T RECMN*Xm222 95 m 48b2CL Ob03222 541 m CONTENTS 1 Introduction 2 References 2.1 Identical Recommendations I International Standards 2.2 Paired Recommendatio

12、ns I International Standards equivalent in technical content 2.3 Additional references 3 Definitions 4 Abbreviations 5 . General principles of the protocol mappings . 5.1 Data Link architecture 5.2 Modelling of service primitives 5.3 Relationships between service features and protocol functions Prot

13、ocol mapping for X.25 LAPB single link procedure . 6.1 General protocol functions . 6.2 DL connection establishment 6.3 DL-connection release 6.4 Data transfer 6.5 DL connection reset 6 Recommendation X.222 (04/95) Page 1 1 1 1 1 2 2 3 3 4 5 5 5 6 7 7 7 1 ITU-T RECMN*X.222 95 4862593 Ob03223 488 SUM

14、MARY This Recommendation defines a method for providing the OS1 connection-mode data link service through the use of the X.25 LAFB-compatible data link procedures as described in Recommendations X.25 and X.75. This Recommendation provides the mapping between the abstract primitives and parameters of

15、 the connection-mode data link service and the real elements of the protocol. ii Recommendation X.222 (04/95) - ITU-T RECMN*X.222 95 48b2591 0603224 314 W Recommendation X.222 USE OF X.25 LAPB-COMPATIBLE DATA LINK PROCEDURES TO PROVIDE THE OS1 CONNECTION-MODE DATA LINK SERVICE1) (Geneva, 1995) 1 Int

16、roduction This Recommendation defines a method for providing the OS1 Connection-mode Data Link Service (CO-DLS) through the use of the X.25 LAPB-compatible DTE data link procedures as described in Recommendations X.25 and X.75 (abbreviated to X.25/LAPB, for the remainder of this Recommendation). Thi

17、s Recommendation specifies the detailed mappings between the CO-DLS and X.25LAPB-compatible DTE single link procedures as described in Recommendation X.25. 2 References The following Recommendations and other references contain provisions which, through reference in this text, constitute provisions

18、of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations and other references are subject to revision: all users of this Recomendation are therefore encouraged to investigate the possibility of applying the most recent edition of the Recommendations

19、and other references listed below. A list of the currently valid ITU-T Recommendations is regularly published. 2.1 Identical Recommendations I International Standards - ITU-T Recommendation X.200 (1994) I ISO/IEC 7498-1: 1994, Information technology - Open Systems Interconnection - Basic Reference M

20、odel: The Basic Model. ITU-T Recommendation X.210 (1993) I ISO/IEC 10731:1994, Information technology - Open Systems Interconnection - Conventions for the definition of OSI services. - 2.2 Paired Recommendations I International Standards equivalent in technical content - CCITT Recommendation X.212 (

21、1988), Data Link Service definition for Open Systems Interconnection for CCIT applications. ISO/IEC 8886: 1992, Information technology - Telecommunications and information exchange between systems - Data link service definition for Open Systems Interconnection. - 2.3 Additional references - ITU-T Re

22、commendation X.25 (1993), Interface between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for terminals operating in packet mode and connected to public data networks by dedicated circuit. ITU-T Recommendation X.75 (1993), Packet switched signalling system between public

23、 networks providing data transmission services. - 1 This text is a technically aligned subset of ISOiEC 11575, Information Technology - Telecommunications and Information Exchange between Systems - Protocol Mappings for the OS1 Data Link Service. Recommendation X.222 (04/95) 1 - ITU-T RECMN*X.222 95

24、 4862591 Ob03225 250 3 Definitions This Recommendation uses the following terms defined in ITU-T Rec. X.200 I ISOEC 7498-1: - DL-address; - DL-connection; - DL-entity; - DL-layer; - DL-protocol-data-unit; - DL-service access point; - DL-service access point address; - DL-service-data-unit; - DL-subs

25、ystem. This Recommendation uses the following terms defined in ITU-T Rec. X.210 I ISO/IEC TR 10731: - DLS provider; - DLSuser; - primitive; - request (primitive); - indication (primitive); - response (primitive); - confirm (primitive); For the purposes of this Recommendation, the following definitio

26、ns apply: - frame; - instance of DL-communication; - aDL-PDU; - a DL connection. 4 Abbreviations For the purposes of this Recommendation, the following abbreviations apply: CO Connection-mode DISC Disconnect DCE Data Circuit-terminating Equipment DL Data Link DLC Data Link Connection DLS Data Link S

27、ervice DLSAP DLSDU DM Disconnected Mode DTE Data Terminal Equipment FRMR Frame Reject HDLC High-Level Data Link Control I Information Data Link Service Access Point Data Link Service Data Unit 2 Recommendation X.222 (04/95) ITU-T RECMN*X.Z22 95 4862591 Ob03226 197 W LAPB os1 PDU RR SABM SABME SLP UA

28、 Link Access Protocol Balanced Open Systems Interconnection Protocol Data Unit Receiver Ready Set Asynchronous Balanced Mode Set Asynchronous Balanced Mode Extended Single Link Procedure Unnumbered Acknowledgment 5 General principles of the protocol mappings 5.1 Data Link architecture The OS1 Data L

29、ink service defines the properties of individual instances of DL-communication between pairs of DLS users. The definition is abstractly expressed in terms of primitives and parameters exchanged, at Data Link Service Access Points (DLSAPs), between each DLS user and a single DLS provider. This is ill

30、ustrated in Figure 1. FIGURE 1Bc.222 Model of Data Link service provision Operation of the DLS provider is modelled in terms of the exchange of DL-PDUs, in accordance with DL-protocols, between DL-entities (see Figure 2). Each DLSAP is attached to a unique DL-entity; a given DL-entity can have one o

31、r more DLSAPs attached to it, depending upon system configuration and the nature of the underlying DL-protocols. When real equipment is considered, a data link consists of two Data Link Entities communicating according to X.25 LAPB-compatible protocol together with the interconnecting media supporti

32、ng information exchange among the Data Link Entities. Figure 3 depicts the configuration applicable in this Recommendation. The Data Link service model deals primarily with the properties of individual instances of DL-communication, each occumng between a pair of DL-entities. DL-protocols have to de

33、al with multiple instances of communication between a given pair of DL-entities; representing the protocol facilities that support this forms a part of the specification of the mapping between the protocol and the DLS. Aspects to be considered include the number of DLSAPs supported by a given DL-ent

34、ity and the number of DL-connections that can be active simultaneously at a DLSAP. Recommendation X.222 (04/95) 3 r I I I DLSpfovider , I I FIGURE 2X.222 Structure of DLS provider Point-tepoint data link, two poor Data Link Entities only T07203894/d03 FIGURE 3K.222 Type data link configuration 5.2 M

35、odelling of service primitives Primitives are abstractions of the behaviour of real systems engaging in data communication: in specifying the mapping between these abstract primitives and the activity of real implementations of DL-protocol entities, this allows freedom in modelling the timing of whe

36、n primitives occur, so as to simplify the mapping specification. NOTE 1 - Recommendation X.212 explicitly allows this freedom; it defines the constraints on the sequence in which primitives can occur, but states that other constraints affect the ability of a DLS user or DLS provider to issue a primi

37、tive at any particular time. For primitives issued by the DLS user - those of types request and response - this Recommendation uses a rendezvous model: that is, a primitive can only occur if both the DLS user and the local DLS provider are prepared for it to occur. This provides two valuable simplif

38、ications: a) occurrence of DLS-user issued primitives can always be related to the externally observable transmission of corresponding frames - the ability to transmit the frames is considered to be an essential part of the DLS provider being prepared for the primitive to occur; and b) there is no n

39、eed to complicate the mapping by, for example, introducing any queueing of primitives that have been issued by the DLS user but have not yet resulted in any protocol activity. 4 Recommendation X.222 (04/95) ITU-T RECMN*X.222 95 48b259L Ob03228 TbT W Conversely, for primitives issued by the DLS provi

40、der - those of types indication and confirm - it is convenient to simplify the model by considering primitives to occur as soon as the DLS provider is ready. NOTE 2 -An implementation of a DL-protocol is free to use an interface that queues requests, e.g., for data transmission; however, the issuing

41、 of corresponding DLS primitives is modelled as occumng after the requests are removed from such a queue, not when they are entered into the queue. NOTE 3 - Any queueing mechanisms in real systems are matters of implementation detail; as in the case described in Note 2, the boundary between DLS prov

42、ider and DLS user is modelled as being at the DLS providers end of the queue. NOTE 4 - This model does not impose a requirement to support queues of unbounded size; interface flow control by the DLS user will in general affect the behaviour of the DL-protocol entity and prevent excessive demands. 5.

43、3 Relationships between service features and protocol functions All of the mappings covered by this Recommendation use natural relationships between functions of the various protocols and corresponding abstractions as Data Link service features. The primary function in each mapping is that of transf

44、erring units of user data. For each mapping, the correspondence is between the DLSDU of a DL-DATA primitive and the basic delimited unit of data transfer in the protocol: that is, the contents of the Information field of a single frame conveying user data. NOTE 1 - It is possible that future Data Li

45、nk layer protocols could provide intrinsic support for segmentation and reassembly of user data across sequences of frames; the absence of this feature from the X.25 LAPB does not preclude the possibility of single DLSDUs mapping to multiple frames of such future DL-protocols. The other functions of

46、 DL-protocols are defined to complement the primary data-transfer function, and the correspondences in the DLS mappings are similarly direct. For connection-mode operation, protocol functions for setting up, disconnecting, and resetting the connections used for data transfer are mapped to DL-connect

47、ion establishment, DL-connection release and DL connection reset. NOTE 2 - Recommendation X.212 defines a somewhat idealized connection-mode service, which does not fully represent all the peer-to-peer interactions that can occur when real DL-protocols such as X.25LAPB are used. The differences affe

48、ct only link Set-up, disconnection and reset, and not any successfully established period of data transfer. They occur typically when DL-PDUs responding to link Set-up, disconnection or reset are lost, and are more likely to occur if, at the same time, one of the DL-entities undergoes two or more ch

49、anges in its readiness to participate in data transfer. In such circumstances, one DL-entity may observe, for example, a single successful DLC-establishment, whereas the other observes a rejected incoming DLC establishment attempt followed by a successful incoming DLC establishment; or one DL-entity may observe a single DLC reset where the other observes two DLC resets, with no data received between the two. These do not represent malfunctions of the protocols, since they never affect the integrity of any successful transfers of use