rfc892.txt

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Network Working Group                                   ISO
Request for Comments:  892                              December 1983


                     ISO Transport Protocol Specification

This document is distributed as an RFC for information only.
It does not specify a standard for the ARPA Internet.

Note:  This document appeared in:

ISO/TC97/SC16/WG6.  Information Processing Systems - Open Systems
    Interconnection - Transport Protocol Specification.  Computer
    Communication Review 12, 3-4 (July/October 1982), pp.  24-67.

and differs from it only in format.


Table of Contents

0.   Introduction
1.   Scope and Field of Application
2.   References

Section One - General

3.   Definitions
4.   Symbols and Abbreviations
5.   Overview

     5.1  Service provided by the transport layer
     5.2  Service assumed from the network layer
     5.3  Functions of the transport layer
     5.4  Model of the transport layer

Section Two - Transport Protocol Specification

6.   Protocol Mechanisms
     
     6.1  Assignment to network connection
     6.2  Transport protocol data unit (TPDU) transfer
     6.3  Data TPDU length and segmenting
     6.4  Concatenation and separation
     6.5  Connection establishment
     6.6  Connection refusal
     6.7  Release
     6.8  Implicit termination
     6.9  Spurious disconnect
     6.10 Data TPDU numbering
     6.11 Expedited data transfer
     6.12 Reassignment
     6.13 Reassignment after failure

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     6.14 Retention until acknowledgement of TPDUs
     6.15 Resynchronization
     6.16 Multiplexing and demultiplexing
     6.17 Explicit flow control
     6.18 Checksum
     6.19 Frozen references
     6.20 Retransmission on timeout
     6.21 Resequencing
     6.22 Inactivity control
     6.23 Treatment of protocol errors
     6.24 Splitting and recombining

7.   Protocol Classes
     7.0  Protocol description of class 0:  simple class
     7.1  Protocol description of class 1:  basic error recovery class
     7.2  Protocol description of class 2:  multiplexing class
     7.3  Protocol description of class 3:  error recovery and multiplexing
                                            class
     7.4  Protocol description of class 4:  error detection and recovery class

8.   Encoding

     8.1  Summary
     8.2  Structure
     8.3  Connection Request (CR)
     8.4  Connection Confirm (CC)
     8.5  Disconnect Request (DR)
     8.6  Disconnect Confirm (DC)
     8.7  Data (DT
     8.8  Expedited Data (ED)
     8.9  Data Acknowledgement (AK)
     8.10 Expedited Data Acknowledgement (EA)
     8.11 Reject (RJ)
     8.12 TPDU Error (ERR)

Section Three  -  Conformance

9.   Conformance

0.   Introduction

        The Transport Protocol Standard is one of a set of International
Standards produced to facilitate the interconection of computer
systems.  The set of standards covers the services and protocols
required to achieve such interconnection.

        The Transport Protocol Standard is positioned with respect to
other related standards by the layers defined in the Reference Model
for Open Systems Interconnection (ISO 7498).  It is most closely
related to, and lies within the field of application of the Transport

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Service Standard (DP aaaa).  It also uses and makes reference to the
Network Service Standard (DP bbbb), whose provisions it assumes in
order to accomplish the transport protocol's aims.  The
interrelationship of these standards is depicted in Figure 1.

-----------------------------------TRANSPORT SERVICE DEFINITION-----

 Transport                --Reference to aims---------------
 Protocol
 Specification            --Reference to assumptions--------

------------------------------------NETWORK SERVICE DEFINITION------

Figure 1  -  Relationship between the transport protocol and adjacent
             services

        The standard specifies a common encoding and a number of 
classes of transport protocol procedures to be used with different 
network qualities of service.

        It is intended that the Transport Protocol should be simple 
but general enough to cater for the total range of Network Service
qualities possible, without restricting future extensions.

        The protocol is structured to give rise to classes of protocol 
which are designed to minimize possible incompatibilities and 
implementation costs.

        The classes are selectable with respect to the Transport and 
Network Services in providing the required quality of service for the
interconnection of two session entities (note that each class provides
a different set of functions for enhancement of service qualities).

        This protocol standard is concerned with optimisation of network
tariffs and the following qualities of service:

     a)  different throughput rates;
     b)  different error rates;
     c)  integrity of data requirements;
     d)  reliability requirements.

        The aim of this standard is primarily to provide a definition 
for implementors.  Since the protocol is complex, the document contains
much material which is advisory or descriptive, but mandatory
requirements on implementations are clearly identified. 

        It should be noted that, as the number of valid protocol sequences 
is very large, it is not possible with current technology to verify that an
implementation will operate the protocol defined in this document
correctly under all circumstances.  It is possible by means of testing

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to establish confidence that an implementation correctly operates the
protocol in a representative sample of circumstances.  It is, however,
intended that this standard can be used in circumstances where two
implementations fail to communicate in order to determine whether one
or both have failed to operate the protocol correctly.

        The variations and options available within this standard are
essential to enable a Transport Service to be provided for a wide
variety of applications over a variety of network qualities.  Thus, a
minimally conforming implementation will not be suitable for use in
all possible circumstances.  It is important therefore to qualify all
references to this standard with statements of the options provided or
required or with statements of the intended purpose of provision or
use.

1.      Scope and Field of Application

1.1     This International Standard Specifies:

        a)  five classes of procedures

                1) Class 0.  Simple class;
                2) Class 1.  Basic error recovery class;
                3) Class 2.  Multiplexing class;
                4) Class 3.  Error recovery class;
                5) Class 4.  Error detection and recovery class,

                for the transfer of data and control information from
                one transport entity to a peer transport entity;

        b)  the means of negotiating the class of procedures to be 
            used by the transport entities;

        c)  the encoding of the transport protocol data units used for
            the transfer of data and control information;

        d)  the functional requirements of equipment within a
            computer system claiming to implement these procedures.

1.2     The procedures are defined in terms of:

        a)  the interactions between peer transport entities through
            the exchange of transport protocol data units;

        b)  the interactions between a transport entity and the
            transport service user in the same system through the exchange of
            transport service primitives;

        c)  the interactions between a transport entity and the
            network service provider through the exchange of network

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            service primitives.

1.3     This International Standard is applicable to equipment which 
        supports the Transport Layer of the OSI Reference Model and which
        wishes to interconnect in an open systems environment.

2.      References

ISO 7498        Information processing systems - Open systems inter-
                connection - Basic Reference Model

DP aaaa         Information processing systems - Open systems inter-
                connection - Transport service definition (N1169).

DP bbbb         Information processing systems - Open systems inter-
                connection - Connection-oriented network service
                definition (N729)

Section One - General

3.      Definitions

3.1     Equipment:  Hardware or software or a combination of both; it
        need not be physically distinct within a computer system.

3.2     Transport service user:  An abstract representation of the
        totality of those entities within a single system that make 
        use of the transport service.

3.3     Network service provider:  An abstract machine which models 
        the totality of the entities providing the network service,
        as viewed by a transport entity.

        Explanatory Notes

        1.  Definitions 3.1 to 3.3 relate to terms used in clause 1.

        2.  This standard makes use of the terms, concepts, and 
            definition defined in ISO 7498.

4.      Symbols and Abbreviations

4.1     Data Units

        TPDU    Transport protocol data unit
        TSDU    Transport service data unit
        NSDU    Network service data unit

4.2     Types of transport protocol data units


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        CR TPDU         Connection request TPDU
        CC TPDU         Connection confirm TPDU
        DR TPDU         Disconnect request TPDU
        DC TPDU         Disconnect confirm TPDU
        DT TPDU         Data TPDU
        ED TPDU         Expedited data TPDU
        AK TPDU         Data acknowledge TPDU
        EA TPDU         Expedited acknowledge TPDU
        RJ TPDU         Rejected TPDU
        ERR TPDU        Error TPDU

4.3     TPDU fields

        LI              Length indicator (field)
        CDT             Credit (field)
        TSAP-ID         Transport service access point identifier
                        (field)
        DST-REF         Destination reference (field)
        SCE-REF         Source reference (field)
        EOT             End of TSDU mark
        TPDU-NR         DT TPDU number (field)
        ED-TPDU-NR      ED TPDU number (field)
        YR-TU-NR        Sequence number response (field)

4.4     Parameters

        T (R)           Receive sequence number
        T (S)           Send sequence number

4.5     Timer variables

        T1              Elapse time between retransmissions
        N               The maximum number of retransmissions
        L               Bound for the maximum time between the
                        decision to transmit a TPDU and the receipt of
                        any response relating to it
        T-WAIT          Maximum time for a reassignment to take place
                        before TC failure is assumed
        I               Inactivity timer - Maximum time allowed to 
                        elapse between receipt of TPDUs before TC 
                        failure is assumed
        W               Window timer - Maximum interval between trans-
                        mission of up to date window information

4.6     Other variables

        n               The number of bits in the sequence number
                        field
        p               The number of bits in the credit field of a 
                        CR, CC or AK TPDU

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4.7     Miscellaneous

        TS-user         Transport service user
        TSAP            Transport service access point
        NSAP            Network service access point
        TC              Transport connection
        NC              Network Connection

5.      Overview of the Transport Protocol

5.1     Service Provided by the Transport Layer

        The services provided by the protocol described in this
document are connection-oriented services.  They are defined in
document DP aaaa.  The Transport Service primitives provided are
summarized in Figure 1.

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