ISO/IEC 8073:1992

Information technology - Telecommunications and information exchange between systems - Open Systems Interconnection - Protocol for providing the connection-mode transport service

Technologies de l'information — Télécommunications et échange d'informations entre systèmes — Interconnexion de systèmes ouverts (OSI) — Protocole pour fourniture du service de transport en mode connexion

General Information

• Status: Withdrawn
• Publication Date: 16-Dec-1992
• Withdrawal Date: 16-Dec-1992
• ICS: 35.100.40 - Transport layer
• Technical Committee: ISO/IEC JTC 1/SC 6 - Telecommunications and information exchange between systems
• Drafting Committee: ISO/IEC JTC 1/SC 6 - Telecommunications and information exchange between systems
• Current Stage: 9599 - Withdrawal of International Standard
• Start Date: 04-Sep-1997
• Completion Date: 30-Oct-2025

Relations

• Revises: ISO/IEC 8073:1988/Cor 3:1990 - Information processing systems - Open Systems Interconnection - Connection oriented transport protocol specification - Technical Corrigendum 3
• Effective Date: 15-Apr-2008
• Revises: ISO/IEC 8073:1988 - Information processing systems - Open Systems Interconnection - Connection oriented transport protocol specification
• Effective Date: 15-Apr-2008
• Revised: ISO/IEC 8073:1997 - Information technology - Open Systems Interconnection - Protocol for providing the connection-mode transport service
• Effective Date: 15-Apr-2008

Standards Content (Sample)

I N TE R NAT I O NA L
ISO/IEC
S TA N DA R D
Third edition
1992-1 2-1
(r
Information technology -
Telecommunications and information
exchange between systems - Open
Systems Interconnection - Protocol for
providing the conneètion-mode transport
service
Technologies de l'information - Télécommunications et échange
d'informations entre systèmes - Interconnexion de systèmes ouverts
(OSII - Protocole pour fourniture du service de transport en mode
connexion
Reference number
ISOAEC 8073 : 1992 (E)
Contents Page
Foreword . iii
Introduction . iv
1 Scope . 1
2 Normative references . 1
Section one: General . 3
3 Definitions . 3
4 Symbols and abbreviations . 4
5 Overview of the Transport Protocol . 5
Section two: Transport protocol specification . 10
6 Elements of procedure . 10
7 Protocol classes . 30
8 Specification for class O: Simple class . 32
9 Specification for class 1: Basic error recovery class . 32
1 O Specification for class 2: Multiplexing class . 33
11 Specification for class 3: Error recovery and multiplexing class . 34
12 Specification for class 4: Error detection and recovery class . 36
13 Structure and encoding of TPDUs . 47
Section Three: Conformance . 64
14 Conformance . 64
c .
Annex A State tables . 66
Annex B Network connection management subprotocol . 84
Annex C PICS Proforma . 99
Annex D Checksum Algorithms . 126
Annex E State tables for operation of class 4 over connection-mode and
connectionless-mode network services . 127
O ISOllEC 1992
All rights reserved . No part of this publication may be reproduced or utilized in any form or by
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Printed in Switzerland
ii
O ISO/IEC ISOAEC 8073 : 1992 (E)
Foreword
IS0 (the International Organization for Standardization) and IEC (the International
Electrotechnical Commission) form the specialized system for worldwide
standardization. National bodies that are members of IS0 or IEC participate in the
development of International Standards through technical committees established by
the respective organization to deal with particular fields of mutual interest. Other
international organizations, governmental and non-governmental, in liaison with IS0
and IEC, also take part in the work.
In the field of information technology, IS0 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 least 75% of the national bodies
casting a vote.
International Standard ISO/IEC 8073 was prepared by Joint Technical Committee
ISOAEC JTC 1, information technology.
This third edition cancels and replaces the second edition (ISO/IEC 8073:1988) and
includes technical revisions that have been published as: IS0
8073:1986/Add.l:1988, ISOAEC 8073:1988/Add.2:1989, ISO/IEC 8073:1988/Am.3,
ISO/IEC 8073:1988/Tech.Cor.l:1990, ISO/IEC 8073:1988~ech.Cor.2:1990, ISOAEC
8073:1988/Tech.Cor.3:1990, ISO/IEC 8073:1988/Tech.Cor.4:1991, ISO/IEC
8073:1988/Tech.Cor.5:1991 and ISOAEC 8073:1988/Tech.Cor.6:1992. This edition
also includes ISO/IEC 8073:1988/Dam.4 and various technical revisions that have
been balloted together with the Draft International Standard of this edition.
Annex A, B and C form an integral part of this International Standard. Annexes D
and E are for information only.
iii
ISOAEC
ISOAEC 8073 : 1992 (E)
In trod uct ion
This International Standard is one of a set of international Standards produced to
facilitate the interconnection of information processing systems. This set of
International Standards covers the services and protocols required to achieve such
interconnection.
The Transport Protocol Standard is positioned with respect to other related
International Standards by the layers defined in the Reference Model for Open
Systems Interconnection (IS0 7498). It is most closely related to, and lies within the
field of application of the Transport Service Standard (IS0 8072). It also uses and
makes reference to the Network Service Standard (ISO/I EC 8348), whose provisions
it assumes in order to accomplish the transport protocol's aims. The interrelationship
of these International Standards is illustrated in Figure 1.
71- TRANSPORT SERVICE DEFINITION
Transport A
I-- -Referencetoaims - - - - -I
I Protocol
Specification
-Reference to assumptions - - 7
!
NETWORK SERVICE DEFINITION
Figure 1 - Relationship between the Transport Protocol and adjacent services
This International 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 (each class provides a different set of functions for enhancement of service
qualities).
This International Standard defines mechanisms that can be used to optimize
network tariffs and enhance the following qualities of service:
a) different throughput;
b) different error rates;
c) integrity of data requirements;
d) reliability requirements.
iv
O ISOAEC ISOAEC 8073 : 1992 (E)
It does not require an implementation to use all of these mechanisms, nor does it
define methods for measuring achieved quality of service or criteria for deciding
when to release transport connections following quality of service degradation.
The primary aim of this International Standard is to provide a set of rules for
communication expressed in terms of the procedures to be carried out by peer
entities at the time of communication. These rules for communication are intended
to provide a sound basis for development in order to serve a variety of purposes i.e.:
a) as a guide for implementors and designers;
b) for use in the testing and procurement of equipment;
c) as part of an agreement for the admittance of systems into the open systems
environment;
d) as a refinement of the understanding of OSI.
As it is expected that the initial users of this International Standard will be designers
and implementors of equipment this International Standard contains, in notes or in
annexes, guidance on the implementation of the procedures defined herein.
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 International Standard correctly under all circumstances. It is
possible by means of testing to establish confidence that an implementation correctly
operates the protocol in a representative sample of circumstances. It is, however,
intended that this International 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.
This International Standard contains a section on conformance of equipment
claiming to implement the procedures in this International Standard. To evaluate
conformance of a particular implementation, it is necessary to have a statement of
which capabilities and options have been implemented for a given OS1 protocol.
Such a statement is called a Protocol Implementation Conformance Statement
(PICS). A PICS proforma is provided in Annex C. Attention is drawn to the fact that
International Standard does not contain any tests to demonstrate this
this
conformance.
The variations and options available within this International Standard are essential
as they 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 International Standard with statements of the options
provided or required or with statements of the intended purpose of provision or use.
V
~
INTERNATIONAL STANDARD O ISO/IEC
Information technology - Telecommunications and
information exchange between systems - Open
Systems Interconnection - Protocol for providing the
connection-mode transport service
These procedures are applicable to instances of
1 Scope
communication between systems which support the
Transport Layer of the OS1 Reference Model and which
This International Standard specifies
wish to interconnect in an open systems environment.
a) five classes of procedures when operating over the
This International Standard specifies, in clause 14,
connection-mode network service:
conformance requirements for systems implementing these
1) class O: simple class; procedures and provides the PICS proforma in compliance
with the relevant requirements, and in accordance with the
2) class 1 : basic error recovery class;
relevant guidance, given in ISOAEC 9646-2. It does not
contain tests which can be used to demonstrate this
3) class 2: multiplexing class;
conformance.
4) class 3: error recovery and multiplexing class;
5) class 4: error detection and recovery class;
2 Normative references
for the connection-mode transfer of data and control
information from one transport entity to a peer transport
The following standards contain provisions which, through
entity;
reference in this text, constitute provisions of this
International Standard. At the time of publication, the
b) one class (class 4) of procedure when operating over editions indicated were valid. All standards are subject to
the connectionless-mode network service; revision, and parties to agreements based on this
International Standard are encouraged to investigate the
possibility of applying the most recent editions of the
c) the means of negotiating the class of procedures to
standards listed below. Members of IEC and IS0 maintain
be used by the transport entities;
registers of currently valid International Standards.
d) the structure and encoding of the transport protocol
IS0 7498:1984, Information processing systems - Open
data units used for the transfer of data and control
Systems Interconnection - Basic Reference Model.
information.
IS0 7498:1984/Add.l:1987, Information processing
The procedures are defined in terms of
systems - Open Systems Interconnection - Basic
Reference Model - Addendum I: Connectionless-mode
a) the interactions between peer transport entities
transmission.
through the exchange of transport protocol data units;
!SO 7498-3:1989, Information processing systerns - Open
b) the interactions between a transport entity and the
Systems Interconnection - Basic Reference Model - Part 3:
transport service user in the same system through the
Naming and addressing.
exchange of transport service primitives;
IS0 8072:1986, Information processing systems - Open
c) the interactions between a transport entity and the
Systems Interconnection - Transport service definition.
network service provider through the exchange of
network service primitives.
ISOAEC 8348:1992, Information processing systems -
Data communications - Network service definition.
These procedures are defined in the main text of this
International Standard supplemented by state tables in
annex A.
ISO/IEC 8073 : 1992 (E) O ISO/IEC
ISOAEC 9646-1 :1991, lnformation technology - Open ISO/IEC 11570:1992, Information technology -
Telecommunications and information exchange between
Systems lnterconnection - Conformance testing
systems - Open Systems lnterconnection - Transport
methodology and framework - Part 1: General concepts.
protocol identification mechanism.
ISO/IEC 9646-2:1991, lnformation technology - Open
CCITT X.224, Transport Protocol Specification for Open
Systems lnterconnection - Conformance testing
methodology and framework - Part 2: Abstract test suite Systems Interconnection for CCITT Applications Version
specification. 1988,
ISOAEC 8073 : 1992 (E)
O ISOAEC
Section one: General
3.2.8 receiving transport entity: A transport entity
3 Definitions
that receives a given TPDU.
NOTE - The definitions contained in this clause make use of
abbreviations defined in clause 4.
3.2.9 preferred class: The protocol class that the
initiator indicates in a CR TPDU as its first choice for use
over the transport connection.
3.1 This International Standard is based on the concepts
developed in IS0 7498 and IS0 7498/Add.l and ISOAEC
7498-3 and makes use of the following terms defined in it:
3.2.10 alternative class: A protocol class that the
initiator indicates in a CR TPDU as an alternative choice for
a) concatenation and separation; use over the transport connection.
b) segmenting and reassembling;
3.2.11 proposed class: A preferred class or an
alternative class.
c) multiplexing and demultiplexing;
3.2.12 selected class: The protocol class that the
d) splitting and recombining;
responder indicates in a CC TPDU that it has chosen for
use over the transport connection.
e) flow control;
3.2.13 proposed parameter: The value for a parameter
f) connectionless-mode transmission:
that the initiator indicates in a CR TPDU that it wishes to
use over the transport connection.
g) nil selector value.
3.2.14 selected parameter: The value for a parameter
3.2 For the purposes of this International Standard, the
that the responder indicates in a CC TPDU that it has
following definitions apply:
chosen for use over the transport connection.
3.2.1 equipment: Hardware or software or a
3.2.15 error indication: An N-RESET indication, or an
combination of both; it need not be physically distinct within
N-DISCONNECT indication with a reason code indicating
a computer system.
an error, that a transport entity receives from the NS-
provider.
3.2.2 transport service user: An abstract
representation of the totality of those entities within a single
3.2.16 invalid TPDU: A TPDU that does not comply
system that make use of the transport service.
with the requirements of this International Standard for
structure and encoding.
3.2.3 network service provider: An abstract machine
that models the totality of the entities providing the network
3.2.17 protocol error: A TPDU whose use does not
service, as viewed by a transport entity.
comply with the procedures for the class.
3.2.4 local matter: A decision made by a system
3.2.18 sequence number:
concerning its behavior in the Transport Layer that is not
subject to the requirements of this protocol.
a) the number in the TPDU-NR field of a DT TPDU that
indicates the order in which the DT TPDU was
transmitted by a transport entity;
3.2.5 initiator: A transport entity that initiates a CR
TPDU.
b) the number in the YR-TU-NR field of an AK or RJ
TPDU that indicates the sequence number of the next
3.2.6 responder: A transport entity with whom an
DT TPDU expected to be received by a transport entity.
initiator wishes to establish a transport connection.
NOTE - Initiator and responder are defined with respect to a single
3.2.19 transmit window: The set of consecutive
transport connection. A transport entity can be both an initiator and
sequence numbers which a transport entity has been
responder simultaneously. authorized by its peer entity to send at a given time on a
given transport connection.
3.2.7 sending transport entity: A transport entity that
sends a given TPDU.
O I Soil EC
ISO/IEC 8073 : 1992 (E)
4 Symbols and abbreviations
3.2.20 lower window edge: The lowest sequence
number in a transmit window.
4.1 Data units
3.2.21 upper window edge: The sequence number
which is one greater that the highest sequence number in
TPDU Transport-protocol-data-unit
the transmit window.
TSDU Transport-service-data-unit
NSDU Network-service-data-unit
3.2.22 upper window edge allocated to the peer
entity: The value that a transport entity communicates to
Types of Transport Protocol data units
4.2
its peer entity to be interpreted as its new upper window
edge.
CR TPDU Connection request TPDU
CC TPDU Connection confirm TPDU
closed window: A transmit window that contains DR TPDU Disconnect request TPDU
3.2.23
Disconnect confirm TPDU
no sequence number. DC TPDU
Data TPDU
DT TPDU
Expedited data TPDU
ED TPDU
3.2.24 window information: Information contained in a
Data acknowledge TPDU
AK TPDU
TPDU relating to the upper and the lower window edges.
Expedited acknowledge TPDU
EA TPDU
RJ TPDU Reject TPDU
Error TPDU
3.2.25 frozen reference: A reference that is not ER TPDU
available for assignment to a connection because of the
requirements of 6.18.
4.3 TPDU Fields
3.2.26 unassigned reference: A reference that is
Length indicator (field)
LI
neither currently in use for identifying a transport connection
CDT Credit (field)
nor which is in a frozen state.
TSAP-ID Transport-service-access-point ident if ier
(field)
DST-REF Destination reference (field)
3.2.27 transparent (data): TS-user data that is
SRC-REF Source reference (field)
transferred intact between transport entities and which is
EOT End of TSDU mark
unavailable for use by the transport entities.
TPDU-NR DT TPDU number (field)
ED-TPDU-NR ED TPDU number (field)
3.2.28 owner (of a network connection): The transport
YR-TU-NR Sequence number response (field)
entity that issued the N-CONNECT request leading to the
YR-EDTU-NR ED TPDU number response (field)
creation of that network connection. Only applicable when
ROA Request of acknowledgement mark
operating over the connection-mode network service.
4.4 Times and associated variables
3.2.29 retained TPDU: A TPDU that is subject to the
retransmission procedure or retention and
T1 Local retransmission time
acknowledgement procedure and is available for possible
N The maximum number of transmissions
retransmission.
Time bound on reference and sequence number
L
I Inactivity time
W Window time
3.3 This International Standard uses the following terms
Time to try reassignmenthesynchronization
defined in ISOAEC 8348: TTR
TWR Time to wait for reassignmenthesynchronization
TS 1 Supervisory timer 1
a) connection-mode network service
TS2 Supervisory timer 2
NSDU lifetime local-to-remote
MLR
b) connectionless-mode network service
NSDU lifetime remote-to-local
MRL
Expected maximum transit delay local-to-remote
ELR
This International Standard uses the following terms
3.4
Expected maximum transit delay remote-to-local
ERL
defined in ISOAEC 9646-1:
R Persistence time
Local acknowledgement time
AL
a) PICS proforma
Remote acknowledgement time
AR
Local inactivity time
IL
b) protocol implementation conformance statement
Remote inactivity time
IR
(PICS)
0 ISO/IEC ISOAEC 8073 : 1992 (E)
Service Assumed from the Network Layer
4.5 Miscellaneous 5.2
TS-user Transport-service user The protocol specified in this International Standard
TSAP Transport-se rvice-access-point assumes the use of the Network Service defined in ISO/IEC
NS-provider Network service provider 8348.
NSAP Network-service-access-point
QOS Quality of service
When operating over CONS, information is transferred to
CLNS Connectionless-mode network service
and from the NS-provider in the network service primitives
CONS Connection-mode network service
listed in Table 2a). When operating over CLNS, information
is transferred to and from the NS-provider in the network
service primitives listed in table 2b).
5 Overview of the Transport Protocol
NOTES
NOTE -This overview is not exhaustive and has been provided for
1 The parameters listed in Table 2a) are those in the current
guidance.
ISOAEC 8348).
connection-mode network sewice (see
5.1 Service provided by the Transport Layer
2 The parameters listed in table 2b) are those in the current
connectionless-mode network service (see ISOAEC 8348).
The protocol specified in this International Standard
3 The way the parameters are exchanged between the transport
supports the Transport Service defined in IS0 8072.
entity and the NS-provider is a local matter.
Information is transferred to and from the TS-user in the
transport service primitives listed in Table 1.
Table 1 -Transport service primitives
1- Primitives Parameters
T-CONNECT request
Called address
indication Calling address
Expedited data option
Quality of service
TS-user data
T-CONNECT response Responding address
confirm Quality of service
Expedited data option
I TS-user-data .
T-DATA request 1 TS-user-data
indication
T-EXPEDITED DATA request I TS-user-data
indication
T-DISCONNECT request TS-user-data
T-DISCONNECT indication Disconnect reason
TS-user-data
ISOAEC 8073 : 1992 (E) O ISOAEC
Table 2a) - Connection-mode network service primitives
Primitives wy Parameters WIXIYIZ
N-CONNECT request X Called address X
Calling address X
Receipt confirmation selection Y
indication X
Expedited data selection Y
QOS parameter set X
NS-user-data Z
N-CONN ECT response X Responding address X
confirm X Receipt confirmation selection Y
Expedited data selection Y
QOS parameter set X
NS-user-data Z
N-user-data X
N-DATA request X
Confirmation request Y
indication Ix
N-DATA ACKNOWLEDGE request IY
indication IY
NS-user-data Y
N-EXPEDITED DATA request jY
indication jY
Reason W
N-RESET request Ix
Originator W
indication X
Reason W
-
N-RESET response X
-
confirm
Reason W
NS-user-data Z
Responding address Z
X Originator W
indication
Reason W
NS-user-data Z
Responding address Z
I
Kev:
w:- The usage of this parameter is a local matter, e.g. for diagnostic or to decide whether to attempt resynchronization.
x The Transport Protocol assumes that this facility is provided in all networks.
The Transport Protocol assumes that this facility is provided in some networks and a mechanism is provided to
Y:
optionally use the facility.
The Transport Protocol does not use this parameter.
Z
Table 2b) - Connectionless-mode network service primitives
Primitives wy Parameters wBvy/z
N-UN ITDATA request X Source address X
Destination address X
Quality of service X
NS-user-data X
X Source address X
indication
Destination address X
Quality of service X
NS-user-data X
Kev:
W: The usage of this parameter is a local matter, e.g. for diagnostic or to decide whether to attempt resynchronization.
X: The Transport Protocol assumes that this facility is provided in all networks.
Y: The Transport Protocol assumes that this facility is provided in some networks and a mechanism is provided to
optionally use the facility.
Z The Transport Protocol does not use this parameter.
0 ISOAEC ISOAEC 8073 : 1992 (E)
a) select the network service which best matches the
5.3 Functions of the Transport Layer
requirement of the TS-user taking into account charges
for various services (see 6.5);
5.3.1 Overview of functions
b) decide whether to multiplex multiple transport
The functions in the Transport Layer are those necessary to
connections onto a single network connection only when
bridge the gap between the services available from the
operating over CONS (see 6.5);
Network Layer and those to be offered to the TS-users.
c) establish the optimum TPDU size (see 6.5);
The functions in the Transport Layer are concerned with the
enhancement of quality of service, including aspects of cost
d) select the functions that will be operational upon
optimization.
entering the data transfer phase (see 6.5);
These functions are grouped below into those used at all
e) map transport addresses onto network addresses;
times during a transport connection and those concerned
with connection establishment, data transfer and release.
f) provide a means to distinguish between two different
transport connections (see 6.5);
NOTE - This Intemational Standard does not include the following
functions which are under consideration for inclusion in future
g) transport of TS-user data (see 6.5);
of this International Standard:
editions
a) encryption; h) exchange values of Inactivity timers (see 6.5).
b) accounting mechanisms;
5.3.1.3 Data transfer
c) status exchanges and monitoring of QOS;
The purpose of data transfer is to permit duplex
transmission of TSDUs between the two TS-users
d) blocking;
connected by the transport connection. This purpose is
achieved by means of two-way simultaneous
e) temporary release of network connections;
communication and by the following functions, some of
which are used or not used in accordance with the result of
9 alternative checksum algorithm.
the selection performed in connection establishment. '
Functions used at all times
5.3.1.1
a) concatenation and separation (see 6.4): a function
used to collect several TPDUs into a single NSDU at the
The following functions, depending upon the selected class
sending transport entity and to separate the TPDUs at
and options, are used at all times during a transport
the receiving transport entity;
connection:
b) segmenting and reassembling (see 6.3): a function
a) transmission of TPDUs (6.2 and 6.9);
used to segment a single data TSDU into multiple
TPDUs at the sending transport entity and to
b) multiplexing and dernult iplexing '(see 6.15): A reassemble them into their original format at the
function used only when operating over CONS to share receiving transport entity;
a single network connection between two or more
transport connections;
c) splitting and recombining (see 6.23): a function
allowing, only when operating over CONS, the
c) error detections (see 6.10, 6.13 and 6.17): A simultaneous use of two or more network connections to
function used to detect the loss, corruption, duplication, support the same transport connection;
misordering, or misdelivery of TPDUs;
d) flow control (see 6.16): a function used to regulate
the flow of TPDUs between two transport entities on one
d) error recovery (see 6.12, 6.14, 6.18, 6.19, 6.20, 6.21,
and 6.22): A function used to recover from detected and transport connection;
signalled errors.
e) transport connection identification: a means to
uniquely identify a transport connection between the pair
5.3.1.2 Connection establishment
of transport entities supporting the connection during the
lifetime of the transport connection;
The purpose of connection establishment is to establish a
transport connection between two TS-users. The following
f) expedited data (see 6.1 1): a function used to bypass
functions of the transport layer during this phase match the
the flow control of normal data TPDU. Expedited data
TS-users' requested quality of service with the services
TPDU flow is controlled by separate flow control;
offered by the network layer:
ISOAEC 8073 : 1992 (E) O ISOAEC
A: Network connection with acceptable
g) TSDU delimiting (see 6.3): a function used to a) Type
determine the beginning and ending of a TSDU. residual error rate (for example, not signalled by
disconnect or reset) and acceptable rate of signalled
errors.
5.3.1 -4 Release
b) Type B: Network connections with acceptable
The purpose of release (see 6.7 and 6.8) is to provide
residual error rate (for example, not signalled by
disconnection of the transport connection, regardless of the
disconnect or reset) but unacceptable rate of signalled
current activity.
errors.
5.4 Classes and options when operating over CONS
c) Type C: Network connections with unacceptable
residual error rate.
5.4.1 General
It is assumed that each transport entity is aware of the
quality of service provided by particular network
The functions of the Transport Layer have been organized
connections.
into classes and options.
A class defines a set of functions. Options define those 5.4.4 Characteristics of class O
functions within a class which may or may not be used.
Class O provides the simplest type of transport connection
and is fully compatible with the CCITT Recommendation
This International Standard defines five classes of protocol:
T.70 for teletex terminals.
a) class O: simple class;
Class O has been designed to be used with type A network
connections.
b) class 1 : basic error recovery class;
c) class 2: multiplexing class; 5.4.5 Characteristics of class 1
d) class 3: error recovery and multiplexing class;
Class 1 provides a basic transport connection with minimal
overheads.
e) class 4: error detection and recovery class.
The main purpose of the class is to recover from network
NOTES
disconnect or reset.
1 Transport connections of classes 2,3 and 4 may be multiplexed
Selection of this class is usually based on reliability criteria.
together onto the same network connection.
1 has been designed to be used with type B network
Class
connections.
2 Classes O to 3 do not specify mechanisms to detect unsignalled
network transmission failures.
5.4.6 Characteristics of class 2
5.4.2 Negotiation
5.4.6.1 General
The use of classes and options is negotiated during
Class 2 provides a way to multiplex several transport
connection establishment. The choice made by the
connections onto a single network connection. This class
transport entities will depend upon
has been designed to be used with type A network
connections.
a) the TS-users' requirements expressed via T-
CONNECT service primitives;
5.4.6.2 Use of explicit flow control
b) the quality of the available network services;
The objective is to provide flow control to help avoid
c) the user required service versus cost ratio congestion at transport-connection-end-points and on the
network connection. Typical use is when traffic is heavy
acceptable to the TS-user.
and continuous, or when there is intensive multiplexing.
Use of flow control can optimize response times and
5.4.3 Choice of network connection
resource utilization.
The following list classifies network services in terms of
5.4.6.3 Non-use of explicit flow control
quality with respect to error behavior in relation to user
requirements; its main purpose is to provide a basis for the
decision regarding which class of transport protocol should The objective is to provide a basic transport connection with
be used in conjunction with given network connection: minimal overheads suitable when explicit disconnection of
the transport connection is desirable. The option would
ISOAEC 8073 : 1992 (E)
0 ISOAEC
to be detected include: TPDU loss, TPDU delivery out of
typically be used for unsophisticated terminals, and when no
sequence, TPDU duplication and TPDU corruption. These
multiplexing onto network connections is required.
errors may affect control TPDUs as well as data TPDUs.
Expedited data is never available.
NOTE -The transport entity is incapable of distinguishing between
5.4.7 Characteristics of class 3
failure of the network service and failure of the peer entity, except
optionally, by some local means, in the case of the failure of the
Class 3 provides the characteristics of class 2 plus the
local interface to the network service (e.g., in the failure of the local
ability to recover from network disconnect or reset.
transceiver on a local area network).
Selection of this class is usually based upon reliability
3 has been designed to be used with type B
criteria. Class
There is no indication given to the transport entity about the
network connections.
ability of the network entity to fulfill the service requirements
given in the N-UNITDATA primitive. However, it can be a
5.4.8 Characteristics of class 4 local matter to make transport entities aware of the
availability and characteristics (QOS) of connectionless-
mode network services, as the corresponding NSAP
Class 4 provides the characteristics of class 3, plus the
associations, exist logically by the nature of the
capability to detect and recover from errors which occur as
connectionless-mode network service and may be
a result of the low grade of service available from the NS-
recognized by network entities.
provider. The kind of errors to be detected include: TPDU
loss, TPDU delivery out of sequence, TPDU duplication and
TPDU corruption. These errors may affect control TPDUs
Model of the transport layer
5.6
as well as data TPDUs.
A transport entity communicates with its TS-users through
This class also provides for increased throughput capability
one or more TSAPs by means of the service primitives as
and additional resilience against network failure.
defined by the transport service definition (see IS0 8072).
Service primitives will cause or be the result of transport
Class 4 has been designed to be used with type C network
protocol data unit exchanges between the peer transport
connections.
entities supporting a transport connection. These protocol
exchanges are effected using the services of the Network
Layer as defined by the network service definition (see
5.5 Characteristics of class 4 transport protocol
ISOAEC 8348) through one or more NSAPs.
when operating over CLNS
Transport connection endpoints are identified in end
In operation over a connectionless-mode network service
systems by an internal, implementation dependent,
the class 4 transport procotol provides flow control between
mechanism so that the TS-user and the transport entity can
to
communicating peer transport entities, the capability
refer to each transport connection.
detect and recover from errors which occur as a result of a
low grade of service available from the NS-provider, and
resilience from failure of the peer entity. The kinds of error
TSAP
TSAP 1
Transport entity Transport entity
NSAP NSAP .
NOTE - For the purposes of illustration, figure 2 shows only one TSAP and one NSAP for each transport entity. In certain
instances, more than one TSAP andlor more than one NSAP may be associated with a particular transport entity.
Figure 2 - Model of the transport layer
ISOAEC 8073 : 1992 (E) O I SO/I EC
Section two: Transport protocol specification
During the resynchronization (see 6.1 4) and reassignment
6 Elements of procedure
after failure (see 6.12) procedures, the initiator may reassign
a transport connection to another network connection
This clause contains elements of procedure which are used
joining the same NSAPs, provided that it is the owner of the
in the specification of protocol classes in clauses 7 to 12.
network connection and that the transport connection is
These elements are not meaningful on their own.
assigned to only one network connection at any given time.
The procedures define the transfer of TPDUs whose
During the splitting procedure (see 6.23), a transport entity
structure and coding is specified in clause 13. Transport
may assign a transport connection to any additional network
entities shall accept and respond to any TPDU received in a
connection joining the same NSAPs, provided that it is the
valid NSDU and may issue TPDUs initiating specific
owner of the network connection and that either the network
elements of procedure specified in this clause.
connection does not have another transport connection
assigned to it; or multiplexing is possible on the network
NOTE -Where network service primitives, TPDUs and parameters
connection,
used are not significant for a particular element of procedure,
they have not been included in the specification.
The transport entity that did not initiate the assignment
becomes aware of the assignment when it receives:
6.1 Use of the network service
a) a CR TPDU during the connection establishment
6.1.1 Assignment to network connection when procedure (see 6.5); or
operating over CONS
b) an RJ TPDU or a retransmitted CR or DR TPDU
This procedure is used only when operating over the
during the resynchronization (see 6.1 4) and
connection-mode network service.
reassignment after failure (see 6.1 2) procedures: or
c) any TPDU when splitting (see 6.23) is used.
6.1.1.1 Purpose
NOTES
The procedure is used in all classes to assign transport
connections to network connections.
1 When a new network connection is created, the quality of
service requested is a local matter, although it will normally be
6.1.1.2 Network service primitives
related to the requirements of transport connection(s) expected to
be assigned to it.
The procedure uses the following network service
2 An existing network connection may also not be suitable if, for
primitives:
example, the quality of service requested for the transport
connection cannot be attained by using or enhancing the network
a) N-CONNECT;
connection.
b) N-DISCONNECT.
3 A network connection with no transport connection(s) assigned
to it, may be available after initial establishment, or because all of
the transport connections previously assigned to it have been
6.1.1.3 Procedure
released. It is recommended that only the owner of such a network
connection should release it. Furthemore, it is recommended that
Each transport connection shall be assigned to a network
it not be released immediately after the transmission of the final
connection. The initiator may assign the transport
TPDU of a transport connection; either a DR TPDU in response to
connection to an existing network connection of which it is
CR TPDU or a DC TPDU in response to DR TPDU. An
the owner or to a new network connection (see note 1)
appropriate delay will allow the TPDU concerned to reach the other
which it creates for this purpose.
transport entity allowing the freeing of any resources associated
with the transport connection concerned.
The initiator shall not assign or reassign the transport
connection to an existing network connection if the protocol
4 After the failure of a network connection, transport connections
class(es) proposed for the class in use for the transport
which were previously multiplexed together may be assigned to
connection are incompatible with the current usage of the
different network connections, and vice versa.
network connection with respect to multiplexing (see note
2).
ISOAEC 8073 : 1992 (E)
O ISOAEC
TPDUs as NS-user data parameters of N-EXPEDITED
DATA primitives.
6.1.2 Transmission over CLNS
In all other cases, transport entities shall transmit and
This procedure is used only when operating over the
receive TPDUs as NS-user data parameters of N-DATA
connectionless-mode network service.
primitives.
6.1.2.1 Purpose When a TPDU is put into an NS-user data parameter, the
significance of the bits within an octet and the order of
The procedure is used to transmit TPDUs over the octets within a TPDU shall be defined in 13.2.
connectionless-mode network service.
NOTE - TPDUs may be concatenated (see 6.4).
6.1.2.2 Network service primitives
6.3 Segmenting and reassembling
The procedure makes use of the following network service
primitive:
6.3.1 Purpose
N-UNITDATA.
The segmenting and reassembling procedure is used in all
classes to map TSDUs onto TPDUs.
6.1.2.3 Procedure
6.3.2 TPDU and Parameter Used
Each TPDU shall be transmitted in a single invocation of
the connectionless-mode network service, over a pre-
The procedure makes use of the following TPDU and
existing association between a pair of NSAPs. The
parameter:
association is considered by transport entities as
permanently established and available.
DT TPDU
- EndofTSDU.
6.2 Transport protocol data unit (TPDU) transfer
6.3.3 Procedure
6.2.1 Purpose
A transport entity shall map a TSDU onto an ordered
The TPDU transfer procedure is used in all classes to
sequence of one or more DT TPDUs. This sequence shall
convey transport protocol data units in user data fields of
not be interrupted by other DT TPDUs on the same
network service primitives.
transport connection.
6.2.2 Network service primitives
All DT TPDUs except the last DT TPDU in a sequence
greater than one shall have a length of data greater than
The procedure uses the following network service primitives zero.
when operating over CONS:
NOTES
a) N-DATA
1 The EOT parameter of a DT TPDU indicates whether or not
there are subsequent DT TPDUs in the sequence.
b) N-EXPEDITED DATA.
2 There is no requirement that the DT TPDUs shall be of the
The procedure uses the following network service primitive
maximum length selected during connection establishment.
when operating over CLNS:
6.4 Concatenation and separation
N-UN ITDATA.
6.4.1 Purpose
6.2.3 Procedure
The procedure for concatenation and separation is used in
The transport protocol data units (TPDUs) defined for the
classes 1, 2, 3 and 4 to convey multiple TPDUs in one
protocol are listed in 4.2.
NSDU.
When operating over CLNS, the transport entities shall
transmit and receive all TPDUç as NS-user data parameters 6.4.2 Procedure
of N-UNITDATA primitives.
A transport entity may concatenate TPDUs from the same
When operating over CONS and when the network or different transport connections, while maintaining the
expedited variant has been selected for class 1, the order of TPDUs for a given transport connection compatible
transport entities shall transmit and receive ED and EA with the protocol operation.
Q ISO/IEC
ISOllEC 8073 : 1992 (E)
- CLASS and OPTIONS (i.e. preferred class, use
A valid set of concatenated TPDUs may contain
of extended format, non-use of explicit flow control in
class 2);
a) any number of TPDUs from the following list: AK,
- calling TSAP-ID:
EA, RJ, ER, DC TPDUs, provided that these TPDUs
come from different transport connections:
- called TSAP-ID:
- TPDU size (proposed);
b) no more than one
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