rfc905 RFC 905 ISO Transport Protocol specification ISO DP 8073
Network Working Group ISO
Request for Comments: 905 April 1984
ISO Transport Protocol Specification
ISO DP 8073
Status of this Memo:
This document is distributed as an RFC for information only. It
does not specify a standard for the ARPA-Internet.
Notes:
1) RFC 892 is an older version of the ISO Transport Protocol
Specification. Therefore this RFC should be assumed to
supercede RFC 892.
2) This document has been prepared by retyping the text of
ISO/TC97/SC16/N1576 and then applying proposed editorial
corrections contained in ISO/TC97/SC16/N1695. These two
documents, taken together, are undergoing voting within ISO
as a Draft International Standard (DIS).
3) Although this RFC has been reviewed after typing, and is
believed to be substantially correct, it is possible that
typographic errors not present in the ISO documents have been
overlooked.
Alex McKenzie
BBN
Table of Contents
1 SCOPE AND FIELD OF APPLICATION........................ 3
1.1 This International Standard specifies:.............. 3
1.2 The procedures are defined in terms of:............. 4
1.3 .................................................... 4
1.4 .................................................... 5
2 REFERENCES............................................ 5
3 DEFINITIONS........................................... 6
3.1 .................................................... 6
3.2 .................................................... 6
3.2.1 equipment:........................................ 7
3.2.2 transport service user:........................... 7
3.2.3 network service provider:......................... 7
3.2.4 local matter:..................................... 7
3.2.5 initiator:........................................ 7
3.2.6 responder:........................................ 8
3.2.7 sending transport entity:......................... 8
3.2.8 receiving transport entity:....................... 8
3.2.9 preferred class:.................................. 8
3.2.10 alternative class:............................... 8
3.2.11 proposed class:.................................. 9
3.2.12 selected class:.................................. 9
3.2.13 proposed parameter:.............................. 9
3.2.14 selected parameter:.............................. 9
3.2.15 error indication:................................ 9
3.2.16 invalid TPDU:................................... 10
3.2.17 protocol error:................................. 10
3.2.18 sequence number:................................ 10
3.2.19 transmit window:................................ 10
3.2.20 lower window edge:.............................. 11
3.2.21 upper window edge:.............................. 11
3.2.22 upper window edge allocated to the peer
entity:
.................................................... 11
3.2.23 closed window:.................................. 11
3.2.24 window information:............................. 11
3.2.25 frozen reference:............................... 12
3.2.26 unassigned reference:........................... 12
3.2.27 transparent (data):............................. 12
i
3.2.28 owner (of a network connection):................ 12
3.2.29 retained TPDU:.................................. 12
4 SYMBOLS AND ABBREVIATIONS............................ 13
4.1 Data units......................................... 13
4.2 Types of transport protocol data units............. 13
4.3 TPDU fields........................................ 13
4.4 Times and associated variables..................... 14
4.5 Miscellaneous...................................... 14
5 OVERVIEW OF THE TRANSPORT PROTOCOL................... 15
5.1 Service provided by the transport layer............ 15
5.2 Service assumed from the network layer............. 16
5.3 Functions of the Transport Layer................... 18
5.3.1 Overview of functions............................ 18
5.3.1.1 Functions used at all times.................... 19
5.3.1.2 Connection Establishment....................... 19
5.3.1.3 Data Transfer.................................. 20
5.3.1.4 Release........................................ 21
5.4 Classes and options................................ 21
5.4.1 General.......................................... 21
5.4.2 Negotiation...................................... 22
5.4.3 Choice of network connection..................... 22
5.4.4 Characteristics of Class 0....................... 23
5.4.5 Characteristics of Class 1....................... 23
5.4.6 Characteristics of Class 2....................... 24
5.4.6.1 General........................................ 24
5.4.6.2 Use of explicit flow control................... 24
5.4.6.3 Non-use of explicit flow control............... 24
5.4.7 Characteristics of Class 3....................... 24
5.4.8 Characteristics of Class 4....................... 25
5.5 Model of the transport layer....................... 25
6 ELEMENTS OF PROCEDURE................................ 27
6.1 Assignment to network connection................... 27
6.1.1 Purpose.......................................... 27
6.1.2 Network service primitives....................... 27
6.1.3 Procedure........................................ 28
6.2 Transport protocol data unit (TPDU) transfer....... 29
6.2.1 Purpose.......................................... 29
6.2.2 Network Service Primitives....................... 30
6.2.3 Procedure........................................ 30
6.3 Segmenting and reassembling........................ 30
6.3.1 Purpose.......................................... 30
6.3.2 TPDUs and parameter used......................... 31
6.3.3 Procedure........................................ 31
ii
6.4 Concatenation and separation....................... 31
6.4.1 Purpose.......................................... 31
6.4.2 Procedure........................................ 32
6.5 Connection establishment........................... 32
6.5.1 Purpose.......................................... 32
6.5.2 Network service primitives....................... 33
6.5.3 TPDUs and parameters used........................ 33
6.5.4 Procedure........................................ 34
6.6 Connection refusal................................. 40
6.6.1 Purpose.......................................... 40
6.6.2 TPDUs and parameters used........................ 40
6.6.3 Procedure........................................ 41
6.7 Normal release..................................... 41
6.7.1 Purpose.......................................... 41
6.7.2 Network service primitives....................... 42
6.7.3 TPDUs and parameters used........................ 42
6.7.4 Procedure for implicit variant................... 43
6.7.5 Procedure for explicit variant................... 43
6.8 Error Release...................................... 44
6.8.1 Purpose.......................................... 45
6.8.2 Network service primitives....................... 45
6.8.3 Procedure........................................ 45
6.9 Association of TPDUs with transport
connections
.................................................... 45
6.9.1 Purpose.......................................... 45
6.9.2 Network service primitives....................... 46
6.9.3 TPDUs and parameters uses........................ 46
6.9.4 Procedures....................................... 46
6.9.4.1 Identification of TPDUs........................ 46
6.9.4.2 Association of individual TPDUs................ 47
6.10 Data TPDU numbering............................... 49
6.10.1 Purpose......................................... 49
6.10.2 TPDUs and parameters used....................... 49
6.10.3 Procedure....................................... 50
6.11 Expedited data transfer........................... 50
6.11.1 Purpose......................................... 50
6.11.2 Network service primitives...................... 50
6.11.3 TPDUs and parameter used........................ 51
6.11.4 Procedures...................................... 51
6.12 Reassignment after failure........................ 52
6.12.1 Purpose......................................... 52
6.12.2 Network service primitives...................... 52
iii
6.12.3 Procedure....................................... 52
6.12.4 Timers.......................................... 54
6.13 Retention until acknowledgement of TPDUs.......... 56
6.13.1 Purpose......................................... 56
6.13.2 Network service primitives...................... 56
6.13.3 TPDUs and parameters used....................... 56
6.13.4 Procedures...................................... 57
6.14 Resynchronization................................. 60
6.14.1 Purpose......................................... 60
6.14.2 Network service primitives...................... 60
6.14.3 TPDUs and parameters used....................... 60
6.14.4 Procedure....................................... 61
6.14.4.1 Active resynchronization procedures........... 61
6.14.4.2 Passive resynchronization procedures.......... 62
6.14.4.3 Data Resynchronization Procedures............. 63
6.15 Multiplexing and demultiplexing................... 64
6.15.1 Purpose......................................... 64
6.15.2 TPDUs and parameters used....................... 64
6.15.3 Procedure....................................... 65
6.16 Explicit Flow Control............................. 65
6.16.1 Purpose......................................... 65
6.16.2 TPDUs and parameters used....................... 65
6.16.3 Procedure....................................... 66
6.17 Checksum.......................................... 66
6.17.1 Purpose......................................... 66
6.17.2 TPDUs and parameters used....................... 66
6.17.3 Procedure....................................... 67
6.18 Frozen references................................. 68
6.18.1 Purpose......................................... 68
6.18.2 Procedure....................................... 68
6.18.2.1 Procedure for classes 0 and 2................. 68
6.18.2.2 Procedure for classes 1 and 3................. 69
6.18.2.3 Procedure for classes 4....................... 70
6.19 Retransmission on time-out........................ 70
6.19.1 Purpose......................................... 70
6.19.2 TPDUs used...................................... 70
6.19.3 Procedure....................................... 70
6.20 Resequencing...................................... 70
6.20.1 Purpose......................................... 71
6.20.2 TPDUs and parameters used....................... 71
6.20.3 Procedure....................................... 71
6.21 Inactivity control................................ 71
6.21.1 Purpose......................................... 71
iv
6.21.2 Procedure....................................... 72
6.22 Treatment of protocol errors...................... 72
6.22.1 Purpose......................................... 72
6.22.2 TPDUs and parameters used....................... 72
6.22.3 Procedure....................................... 72
6.23 Splitting and recombining......................... 74
6.23.1 Purpose......................................... 74
6.23.2 Procedure....................................... 74
7 Protocol Classes..................................... 76
8 SPECIFICATION FOR CLASS 0. SIMPLE CLASS.............. 79
8.1 Functions of class 0............................... 79
8.2 Procedures for class 0............................. 79
8.2.1 Procedures applicable at all times............... 79
8.2.2 Connection establishment......................... 79
8.2.3 Data transfer.................................... 80
8.2.4 Release.......................................... 80
9 SPECIFICATION FOR CLASS 1: BASIC ERROR
RECOVERY CLASS
.................................................... 81
9.1 Functions of Class 1............................... 81
9.2 Procedures for Class 1............................. 81
9.2.1 Procedures applicable at all times............... 81
9.2.2 Connection establishment......................... 82
9.2.3 Data Transfer.................................... 82
9.2.3.1 General........................................ 82
9.2.3.2 Expedited Data................................. 83
9.2.4 Release.......................................... 84
10 SPECIFICATION FOR CLASS 2 - MULTIPLEXING
CLASS
.................................................... 85
10.1 Functions of class 2.............................. 85
10.2 Procedures for class 2............................ 85
10.2.1 Procedures applicable at all times.............. 85
10.2.2 Connection establishment........................ 86
10.2.3 Data transfer when non use of explicit
flow control
.................................................... 86
10.2.4 Data transfer when use of explicit flow
control
.................................................... 86
10.2.4.1 General....................................... 86
10.2.4.2 Flow control.................................. 87
10.2.4.3 Expedited data................................ 88
v
10.2.5 Release......................................... 89
11 SPECIFICATION FOR CLASS 3: ERROR RECOVERY AND
MULTIPLEXING CLASS
.................................................... 90
11.1 Functions of Class 3.............................. 90
11.2 Procedures for Class 3............................ 90
11.2.1 Procedures applicable at all times.............. 90
11.2.2 Connection Establishment........................ 91
11.2.3 Data Transfer................................... 91
11.2.3.1 General....................................... 91
11.2.3.2 Use of RJ TPDU................................ 92
11.2.3.3 Flow Control.................................. 93
11.2.3.4 Expedited data................................ 93
11.2.4 Release......................................... 94
12 SPECIFICATION FOR CLASS 4: ERROR DETECTION
AND RECOVERY CLASS
.................................................... 95
12.1 Functions of Class 4.............................. 95
12.2 Procedures for Class 4............................ 95
12.2.1 Procedures available at all times............... 95
12.2.1.1 Timers used at all times...................... 95
12.2.1.1.1 NSDU lifetime (MLR, MRL).................... 98
12.2.1.1.2 Expected maximum transit delay (ELR,
ERL)
.................................................... 98
12.2.1.1.3 Acknowledge Time (AR, AL)................... 99
12.2.1.1.4 Local retransmission time (T1).............. 99
12.2.1.1.5 Persistence Time (R)........................ 99
12.2.1.1.6 Bound on References and Sequence
Numbers (L)
................................................... 100
12.2.1.2 General Procedures........................... 100
12.2.2 Procedures for Connection Establishment........ 102
12.2.2.1 Timers used in Connection Establishment...... 102
12.2.2.2 General Procedures........................... 103
12.2.3 Procedures for Data Transfer................... 104
12.2.3.1 Timers used in Data Transfer................. 104
12.2.3.2 General Procedures for data transfer......... 104
12.2.3.3 Inactivity Control........................... 105
12.2.3.4 Expedited Data............................... 105
12.2.3.5 Resequencing................................. 106
12.2.3.6 Explicit Flow Control........................ 107
12.2.3.7 Sequencing of received AK TPDUs.............. 108
vi
12.2.3.8 Procedure for transmission of AK TPDUs....... 109
12.2.3.8.1 Retransmission of AK TPDUs for window
synchronization
................................................... 109
12.2.3.8.2 Sequence control for transmission of
AK TPDUs
................................................... 109
12.2.3.8.3 Retransmission of AK TPDUs after CDT
set to zero
................................................... 110
12.2.3.8.4 Retransmission procedures following
reduction of the
................................................... 111
12.2.3.9 Use of Flow Control Confirmation
parameter
................................................... 112
12.2.4 Procedures for Release......................... 113
12.2.4.1 Timers used for Release...................... 113
12.2.4.2 General Procedures for Release............... 113
13 STRUCTURE AND ENCODING OF TPDUs.................... 114
13.1 Validity......................................... 114
13.2 Structure........................................ 116
13.2.1 Length indicator field......................... 117
13.2.2 Fixed part..................................... 117
13.2.2.1 General...................................... 117
13.2.2.2 TPDU code.................................... 117
13.2.3 Variable part.................................. 118
13.2.3.1 Checksum Parameter (Class 4 only)............ 120
13.2.4 Data Field..................................... 120
13.3 Connection Request (CR) TPDU..................... 120
13.3.1 Structure...................................... 120
13.3.2 LI............................................. 121
13.3.3 Fixed Part (Octets 2 to 7)..................... 121
13.3.4 Variable Part (Octets 8 to p).................. 122
13.3.5 User Data (Octets p+1 to the end).............. 127
13.4 Connection Confirm (CC) TPDU..................... 128
13.4.1 Structure...................................... 128
13.4.2 LI............................................. 128
13.4.3 Fixed Part (Octets 2 to 7)..................... 128
13.4.4 Variable Part (Octet 8 to p)................... 129
13.4.5 User Data (Octets p+1 to the end).............. 129
13.5 Disonnect Request (DR) TPDU...................... 129
13.5.1 Structure...................................... 129
vii
13.5.2 LI............................................. 129
13.5.3 Fixed Part (Octets 2 to 7...................... 130
13.5.4 Variable Part (Octets 8 to p).................. 131
13.5.5 User Data (Octets p+1 to the end).............. 131
13.6 Disconnect Confirm (DC) TPDU..................... 132
13.6.1 Structure...................................... 132
13.6.2 LI............................................. 132
13.6.3 Fixed Part (Octets 2 to 6)..................... 132
13.6.4 Variable Part.................................. 133
13.7 Data (DT) TPDU................................... 133
13.7.1 Structure...................................... 133
13.7.2 LI............................................. 134
13.7.3 Fixed Part..................................... 134
13.7.4 Variable Part.................................. 135
13.7.5 User Data Field................................ 135
13.8 Expedited Data (ED) TPDU......................... 135
13.8.1 Structure...................................... 135
13.8.2 LI............................................. 136
13.8.3 Fixed Part..................................... 136
13.8.4 Variable Part.................................. 137
13.8.5 User Data Field................................ 137
13.9 Data Acknowledgement (AK) TPDU................... 137
13.9.1 Structure...................................... 137
13.9.2 LI............................................. 138
13.9.3 Fixed Part..................................... 138
13.9.4 Variable Part.................................. 139
13.10 Expedited Data Acknowledgement (EA) TPDU........ 140
13.10.1 Structure..................................... 140
13.10.2 LI............................................ 141
13.10.3 Fixed Part.................................... 141
13.10.4 Variable Part................................. 141
13.11 Reject (RJ) TPDU................................ 141
13.11.1 Structure..................................... 142
13.11.2 LI............................................ 142
13.11.3 Fixed Part.................................... 142
13.11.4 Variable Part................................. 143
13.12 TPDU Error (ER) TPDU............................ 143
13.12.1 Structure..................................... 143
13.12.2 LI............................................ 143
13.12.3 Fixed Part.................................... 144
13.12.4 Variable Part................................. 144
14 CONFORMANCE........................................ 145
14.1 ................................................. 145
viii
14.2 ................................................. 145
14.3 ................................................. 145
14.4 ................................................. 145
14.5 ................................................. 146
14.6 Claims of Conformance Shall State................ 146
ix
INTRODUCTION
The Transport Protocol Standard is one of a set of International
Standards produced to facilitate the interconnection 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 Service Standard (DP 8072). It also uses and makes
reference to the Network Service Standard (DP 8348), whose
provisions it assumes in order to accomplish the transport
protocol's aims. The interelationship of these standards is
depicted in figure 1.
-------------------------TRANSPORT SERVICE DEFINITION------------
Transport | --- Reference to aims --------------
Protocol |
Specification | --- Reference to assumptions -------
-------------------------NETWORK SERVICE DEFINITION--------------
Relationaship between Transport Protocol and adjacent services
Figure 1 .
The 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.
1
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 defines mechanisms that can be used to
optimize network tariffs and enhance the following qualities of
service:
a) different throughput rates;
b) different error rates;
c) integrity of data requirements;
d) reliability requirements.
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:
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.
It is expected that the initial users of the International
Standard will be designers and implementors of equipment and the
International Standard contains, in notes or in annexes, guidance
on the implementation of the procedures defined in the standard.
2
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. Attention is drawn to the fact that the
standard does not contain any tests to demonstrate this
conformance.
The variations and options available within this International
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
International 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 and multiplexing class;
5) Class 4. Error detection and recovery class,
3
for the connection oriented 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 structure and encoding of the transport protocol data
units used for the transfer of data and control
information;
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
service primitives.
These procedures are defined in the main text of the standard
supplemented by state tables in annex A.
1.3
These procedures are applicable to instances of communication
between systems which support the Transport Layer of the OSI
Reference Model and which wish to interconnect in an open systems
environment.
4
1.4
This International Standard also specifies conformance
requirements for systems implementing these procedures. It does
not contain tests which can be used to demonstrate this
conformance.
2 REFERENCES
ISO 7498 Information processing systems - Open systems
interconnection - Basic Reference Model
DP 8072 Information processing systems - Open systems
interconnection - Transport service definition
DP 8348 Information processing systems - Open systems
interconnection - Connection-oriented network service
definition.
5
SECTION ONE. GENERAL
3 DEFINITIONS
NOTE - The definitions contained in this clause make use of
abbreviations defined in clause 4.
3.1
This International Standard is based on the concepts developed in
the Reference Model for Open Systems Interconnection (DIS 7498)
and makes use of the following terms defined in that standard:
a) concatenation and separation;
b) segmenting and reassembling;
c) multiplexing and demultiplexing;
d) splitting and recombining;
e) flow control.
3.2
For the purpose of this International Standard, the following
definitions apply:
6
3.2.1 equipment:
Hardware or software or a combination of both; it need not be
physically distinct within a computer system.
3.2.2 transport service user:
An abstract representation of the totality of those entities
within a single system that make use of the transport service.
3.2.3 network service provider:
An abstract machine that models the totality of the entities
providing the network service, as viewed by a transport entity.
3.2.4 local matter:
A decision made by a system concerning its behavior in the
Transport Layer that is not subject to the requirements of this
protocol.
3.2.5 initiator:
A transport entity that initiates a CR TPDU.
7
3.2.6 responder:
A transport entity with whom an initiator wishes to establish a
transport connection.
NOTE - Initiator and responder are defined with respect to a
single transport connection. A transport entity can be both an
initiator and responder simultaneously.
3.2.7 sending transport entity:
A transport entity that sends a given TPDU.
3.2.8 receiving transport entity:
A transport entity that receives a given TPDU.
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.
3.2.10 alternative class:
A protocol class that the initiator indicates in a CR TPDU as an
alternative choice for use over the transport connection.
8
3.2.11 proposed class:
A preferred class or an alternative class.
3.2.12 selected class:
The protocol class that the responder indicates in a CC TPDU that
it has chosen for use over the transport connection.
3.2.13 proposed parameter:
The value for a parameter that the initiator indicates in a CR
TPDU that it wishes to use over the transport connection.
3.2.14 selected parameter:
The value for a parameter that the responder indicates in a CC
TPDU that it has chosen for use over the transport connection.
3.2.15 error indication:
An N-RESET indication, or an N-DISCONNECT indication with a
reason code indicating an error, that a transport entity receives
from the NS-provider.
9
3.2.16 invalid TPDU:
A TPDU that does not comply with the requirements of this
International Standard for structure and encoding.
3.2.17 protocol error:
A TPDU whose use does not comply with the procedures for the
class.
3.2.18 sequence number:
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.
b) The number in the YR-TU-NR field of an AK or RJ TPDU that
indicates the sequence number of the next DT TPDU expected
to be received by a transport entity.
3.2.19 transmit window:
The set of consecutive sequence numbers which a transport entity
has been authorized by its peer entity to send at a given time on
a given transport connection.
10
3.2.20 lower window edge:
The lowest sequence number in a transmit window.
3.2.21 upper window edge:
The sequence number which is one greater than the highest
sequence number in the transmit window.
3.2.22 upper window edge allocated to the peer entity:
The value that a transport entity communicates to its peer entity
to be interpreted as its new upper window edge.
3.2.23 closed window:
A transmit window that contains no sequence number.
3.2.24 window information:
Information contained in a TPDU relating to the upper and the
lower window edges.
11
3.2.25 frozen reference:
A reference that is not available for assignment to a connection
because of the requirements of 6.18.
3.2.26 unassigned reference:
A reference that is neither currently in use for identifying a
transport connection or which is in a frozen state.
3.2.27 transparent (data):
TS-user data that is transferred intact between transport
entities and which is unavailable for use by the transport
entities.
3.2.28 owner (of a network connection):
The transport entity that issued the N-CONNECT request leading to
the creation of that network connection.
3.2.29 retained TPDU:
A TPDU that is subject to the retransmission procedure or
retention until acknowledgement procedure and is available for
possible retransmission.
12
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
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 Reject TPDU
ER 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)
SRC-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)
YR-EDTU-NR ED TPDU number response (field)
13
4.4 Times and associated variables
T1 Elapsed time between retransmissions
N The maximum number of transmissions
L Bound on reference
I Inactivity time
W Window time
TTR Time to try reassignment/resynchronization
TWR Time to wait for
reassignment/resynchronization
TS1 Supervisory timer 1
TS2 Supervisory time 2
MLR NSDU lifetime local-to-remote
MRL NSDU lifetime remote-to-local
ELR Expected maximum transit delay
local-to-remote
ERL Expected maximum transit delay
remote-to-local
R Persistence time
AL Local acknowledgement time
AR Remote acknowledgement time
4.5 Miscellaneous
TS-user Transport service user
TSAP Transport service access point
NS-provider Network service provider
NSAP Network service access point
QOS Quality of service
14
5 OVERVIEW OF THE TRANSPORT PROTOCOL
NOTE - This overview is not exhaustive and has been provided for
guidance to the reader of this International Standard.
5.1 Service provided by the transport layer
The protocol specified in this International Standard supports
the transport service defined in DP 8072.
Information is transferred to and from the TS-user in the
transport service primitives listed in table 1.
15
+-------------------------------------------------------------+
| Primitive | Parameter |
|--------------------------------|----------------------------|
|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, |
| | TS User-Data. |
|--------------------------------|----------------------------|
|T-DATA request | TS User-Data. |
| indication | |
|--------------------------------|----------------------------|
|T-EXPEDITED DATA request | TS User-Data. |
| indication | |
|--------------------------------|----------------------------|
|T-DISCONNECT request | TS User-Data. |
|--------------------------------|----------------------------|
|T-DISCONNECT indication | Disconnect reason, |
| | TS User-Data. |
+--------------------------------|----------------------------+
Table 1. Transport service primitives
5.2 Service assumed from the network layer
The protocol specified in this International Standard assumes the
use of the network service defined in DP 8348.
Information is transferred to and from the NS-provider in the
network service primitives listed in table 2.
16
+---------------------------------------------------------------+
| Primitives |X/Y| Parameters |X/Y/Z|
|----------------------------|---|------------------------|-----|
|N-CONNECT request | X | Called Address, | X |
| indication | X | Calling Address, | X |
| response | X | NS User-Data, | Z |
| confirm | X | QOS parameter set, | X |
| | | Responding address, | Z |
| | | Receipt confirmation | Y |
| | | selection. | |
|----------------------------|---|------------------------|-----|
|N-DATA request | X | NS User-Data, | X |
| indication | X | Confirmation request | Y |
|----------------------------|---|------------------------|-----|
|N-DATA ACKNOWLEDGE | | | |
| request | Y | | |
| indication | Y | | |
|----------------------------|---|------------------------|-----|
|N-EXPEDITED DATA | | | |
| request | Y | NS User-Data. | Y |
| indication | Y | | |
|----------------------------|---|------------------------|-----|
|N-RESET request | X | Originator, | Z |
| indication | X | Reason. | Z |
| response | X | | |
| confirm | X | | |
|----------------------------|---|------------------------|-----|
|N-DISCONNECT request | X | NS User-Data. | Z |
| indication | X | Originator, | Z |
| | | Reason. | Z |
+---------------------------------------------------------------+
Table 2. Network service primitives
17
Key:
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.
NOTES:
1 - The parameters listed in this table are those in the
current network service (first DP 8348).
2 - The way the parameters are exchanged between the transport
entity and the NS-provider is a local matter.
5.3 Functions of the Transport Layer
5.3.1 Overview of functions
The functions in the Transport Layer are those necessary to
bridge the gap between the services available from the Network
Layer and those to be offered to the TS-users.
The functions in the Transport Layer are concerned with the
enhancement of quality of service, including aspects of cost
optimization.
These functions are grouped below into those used at all times
during a transport connection and those concerned with connection
establishment, data transfer and release.
NOTE - This International Standard does not include the following
functions which are under consideration for inclusion in future
editions of this standard:
a) encryption;
18
b) accounting mechanisms;
c) status exchanges and monitoring of QOS;
d) blocking;
e) temporary release of network connections;
f) alternative checksum algorithm.
5.3.1.1 Functions used at all times
The following functions, depending upon the selected class and
options, are used at all times during a transport connection:
a) transmission of TPDUs (see 6.2 and 6.9);
b) multiplexing and demultiplexing (see 6.15), a function
used to share a single network connection between two or
more transport connections;
c) error detection (see 6.10, 6.13 and 6.17), a function used
to detect the loss, corruption, duplication, misordering
or misdelivery of TPDUs;
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
signalled errors.
5.3.1.2 Connection Establishment
The purpose of connection establishment is to establish a
transport connection between two TS-users. The following
functions of the transport layer during this phase must match the
TS-users' requested quality of service with the services offered
by the network layer:
19
a) select network service which best matches the requirement
of the TS-user taking into account charges for various
services (see 6.5);
b) decide whether to multiplex multiple transport connections
onto a single network connection (see 6.5);
c) establish the optimum TPDU size (see 6.5);
d) select the functions that will be operational upon
entering the data transfer phase (see 6.5);
e) map transport addresses onto network addresses;
f) provide a means to distinguish between two different
transport connections (see 6.5);
g) transport of TS-user data (see 6.5).
5.3.1.3 Data Transfer
The purpose of data transfer is to permit duplex transmission of
TSDUs between the two TS-users connected by the transport
connection. This purpose is achieved by means of two-way
simultaneous communication and by the following functions, some
of which are used or not used in accordance with the result of
the selection performed in connection establishment:
a) concatenation and separation (see 6.4), a function used to
collect several TPDUs into a single NSDU at the sending
transport entity and to separate the TPDUs at the
receiving transport entity;
b) segmenting and reassembling (see 6.3), a function used to
segment a single data TSDU into multiple TPDUs at the
sending transport entity and to reassemble them into their
original format at the receiving transport entity;
20
c) splitting and recombining (see 6.23), a function allowing
the simultaneous use of two or more network connections to
support the same transport connection;
d) flow control (see 6.16), a function used to regulate the
flow of TPDUs between two transport entities on one
transport connection;
e) transport connection identification, a means to uniquely
identify a transport connection between the pair of
transport entities supporting the connection during the
lifetime of the transport connection;
f) expedited data (see 6.11), a function used to bypass the
flow control of normal data TPDU. Expedited data TPDU
flow is controlled by separate flow control;
g) TSDU delimiting (see 6.3), a function used to determine
the beginning and ending of a TSDU.
5.3.1.4 Release
The purpose of release (see 6.7 and 6.8) is to provide
disconnection of the transport connection, regardless of the
current activity.
5.4 Classes and options
5.4.1 General
The functions of the Transport Layer have been organized into
classes and options.
A class defines a set of functions. Options define those
functions within a class which may or may not be used.
This International Standard defines five classes of protocol:
21
a) Class 0: Simple Class;
b) Class 1: Basic Error recovery Class;
c) Class 2: Multiplexing Class;
d) Class 3: Error Recovery and Multiplexing Class;
e) Class 4: Error Detection and Recovery Class.
NOTE - Transport connections of classes 2, 3 and 4 may be
multiplexed together onto the same network connection.
5.4.2 Negotiation
The use of classes and options is negotiated during connection
establishment. The choice made by the transport entities will
depend upon:
a) the TS-users' requirements expressed via T-CONNECT service
primitives;
b) the quality of the available network services;
c) the user required service versus cost ratio acceptable to
the TS-user.
5.4.3 Choice of network connection
The following list classifies network services in terms of
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 be
used in conjunction with given network connection:
22
a) Type A. Network connection with acceptable residual error
rate (for example not signalled by disconnect or reset)
and acceptable rate of signalled errors.
b) Type B. Network connections with acceptable residual
error rate (for example not signalled by disconnect or
reset) but unacceptable rate of signalled errors.
c) Type C. Network connections with unacceptable residual
error rate.
It is assumed that each transport entity is aware of the quality
of service provided by particular network connections.
5.4.4 Characteristics of Class 0
Class 0 provides the simplest type of transport connection and is
fully compatible with the CCITT recommendation S.70 for teletex
terminals.
Class 0 has been designed to be used with type A network
connections.
5.4.5 Characteristics of Class 1
Class 1 provides a basic transport connection with minimal
overheads.
The main purpose of the class is to recover from network
disconnect or reset.
Selection of this class is usually based on reliability criteria.
Class 1 has been designed to be used with type B network
connections.
23
5.4.6 Characteristics of Class 2
5.4.6.1 General
Class 2 provides a way to multiplex several transport connections
onto a single network connection. This class has been designed
to be used with type A network connections.
5.4.6.2 Use of explicit flow control
The objective is to provide flow control to help avoid congestion
at transport-connection-end-points and on the network connection.
Typical use is when traffic is heavy and continuous, or when
there is intensive multiplexing. Use of flow control can
optimize response times and resource utilization.
5.4.6.3 Non-use of explicit flow control
The objective is to provide a basic transport connection with
minimal overheads suitable when explicit disconnection of the
transport connection is desirable. The option would typically be
used for unsophisticated terminals, and when no multiplexing onto
network connections is required. Expedited data is never
available.
5.4.7 Characteristics of Class 3
Class 3 provides the characteristics of Class 2 plus the ability
to recover from network disconnect or reset. Selection of this
class is usually based upon reliability criteria. Class 3 has
been designed to be used with type B network connections.
24
5.4.8 Characteristics of Class 4
Class 4 provides the characteristics of Class 3, plus the
capability to detect and recover from errors which occur as a
result of the low grade of service available from the NS-
provider. The kinds of errors to be detected include: TPDU
loss, TPDU delivery out of sequence, TPDU duplication and TPDU
corruption. These errors may affect control TPDUs as well as
data TPDUs.
This class also provides for increased throughput capability and
additional resilience against network failure. Class 4 has been
designed to be used with type C network connections.
5.5 Model of the transport layer
A transport entity communicates with its TS-users through one or
more TSAPs by means of the service primitives as defined by the
transport service definition DP 8072. Service primitives will
cause or be the result of transport protocol data unit exchanges
between the peer transport entities supporting a transport
connection. These protocol exchanges are effected using the
services of the Network Layer as defined by the Network Service
Definition DP 8348 through one or more NSAPs.
Transport connection endpoints are identified in end systems by
an internal, implementation dependent, mechanism so that the TS-
user and the transport entity can refer to each transport
connection.
25
+------+ +------+
----------| TSAP |------------------------| TSAP |----------
+------+ +------+
| |
+---------------+ +---------------+
| Transport | | Transport |
| entity | | entity |
&n 