Transport layer classes
- Class 0 - Simple class
- Class 1 - Basic error recovery class
- Class 2 - Multiplexing class
- Class 3 - Error Recovery and multiplexing class
- Class 4 - Error detection and recovery class
TRANSPORT LAYER STANDARDS
slide tran 4 - Connection request - Variable part
slide tran 6 - Functions within the transport layer
slide tran 7 - Services Provided to the session layer
Example Teletex call
1.0 - Introduction
slide tran 0
slide tran 1
Application --> | Mail Box | File transfer | Distributed Database | virtual terminal | |||
Presentation | |||||||
Session | |||||||
TRANSPORT | |||||||
Network --> | local | circuit | Packet | ||||
Link --> | area | switching | switching | ||||
Physical --> |
The transport layer hides the beculiarities of the network being used from
the layers above.
In the case of a packet switched network the following would be hidden,
- Limitations of packet size (by segmenting and concaternation - see below)
- limitations of throughput (by multiplexing or splitting)
- Loss of data caused by network resets and clears
slide tran 5 -
Recovery from disconnect & reset | muliplex several transport onto one network connection | error recovery | |
Class 0 | no | no | no |
Class 1 | yes | no | no |
Class 2 | no | yes | no |
Class 3 | yes | yes | no |
Class 4 | yes | yes | yes |
Transport layer classes
Class 0 - Simple class
Class 1 - Basic error recovery class
Class 2 - Multiplexing class
Class 3 - Error recovery and multiplexing class
Class 4 - Error detection and recovery class
-----------------------------------------------------------------------------------------------------------
Class 0 - Simple class
- Minimum facility
- Fully compatible with S.70 Teletex terminals
- For use over type A network connections
- Only functions for establishment, data transfer with segmenting and error reporting are available.
- NO multiplexing, disconnection(closed user network connection cleared), flow control, error recovery, expedited data transfer, user dateline during connection.
- Only address and TPDU size parameters are allowed.
Class 1 - Basic error recovery class
- Basic transport connection with minimal overheads
- 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.
Class 2 - Multiplexing class
- 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.
Class 3 - Error Recovery and multiplexing class
- Class 3 provides the characteristics of class 2 plus the ability to recover from network disconnect or reset.
- Class 3 has been designed to be used with type B network connections
Class 4 - Error detection and recovery class
- 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
- Class 4 has been designed to be used with type C network connections.
slide tran2 - jargon
TRANSPORT LAYER STANDARD
ISO 7498 Functions of transport later.
ISO 8072 Transport service definition.
ISO 8073 Transport protocol specification.
S.70 Teletex-network independent basic transport service.
slide tran3 - transport data units
Transport Layer
---------------
Byte: | 1 | 2 | 3 4 | 5 6 | 7 | 8 9 | 10 |
Connection Request | LI | CR-CDT | DST-REF | SRC-REF | Class | Variable | User |
E x | 0 0 | Options | Part | Data | |||
Connection confirm | LI | CC-CDT | DST-REF | SRC-REF | Class | Variable | User |
D x | Options | Data | |||||
Disconnect Request | LI | DR | DST-REF | SRC-REF | Reason | Variable | User |
80 | Data | ||||||
Disconnect Confirm | LI | DC | DST-REF | SRC-REF | Variable Part | ||
C0 | |||||||
Data(Class 0+1) | LI | DT | TPDU-NR | Variable part | |||
FO | + EOT | ||||||
Data(Class 2,3+4) | DST-REF | TPUD-NR | User | ||||
Data | |||||||
Extended Format -> | TPDU-NR + EOT | ||||||
Expidited(Class | LI | ED | DST-REF | TPUD-NR | Variable Part | User | |
Data 1,2,3,4) | 10 | Data | |||||
Extended Format -> | ED-TPDU-NR + EOT | ||||||
Data | LI | AK-CDT | DST-REF | YR-TU-NR | Variable Part | ||
Acknowledgement | 6 x | ||||||
YR-TU-NR | CDT | ||||||
Extended Format -> | |||||||
Expediated Data | LI | EA | DST-REF | YR-TU-NR | Variable part | ||
Acknowledgement | 20 | ||||||
Extended Format -> | YR-TU-NR | ||||||
Reject | LI | RJ-CDT | DST-REF | YR-TU-NR | |||
5 x | |||||||
YR-TU-NR | CDT | ||||||
Error | LI | ER | DST-REF | REJECT | Variable Part | ||
70 | Cause |
Where:
TPDU = Transport Protocol data unit
LI = Length indicator (1 byte) = Header length (fixed + variable part)
CDT = Credit Allocation (0 in Classes 0 + 1)
TSAP-ID = Transport service access point identifier
DST-REF = Destination reference - reference identifying the transport connection at the remote transport entity
SRC-REF = Source reference - Reference selected by the transport entity initiating to identify the requested transport connection
EOT = End of TSDU mark
TPDU-NR = Data TPDU number
ED-TPDU-NR = Expidited TPDU number
YR-TU-NR = Sequence number response (indicating the next expected TPDU)
YR-EDTU-NR = Expidited TPDU number response
blank
slide tran 4 - Connection request - Variable part
Parameter Code | Parameter Length | Parameter Value | |
D1 | not defined | Transport service access point identifier - called | |
C2 | not defined | - calling | |
C0 | 01 | 0D | TPDU size - 8192 octets |
0C | 4096 | ||
0B | 2048 | ||
0A | 1024 | ||
09 | 512 | ||
08 | 256 | ||
07 | 128 | ||
C4 | 01 | 01 | version number |
C5 | user defined | security parameters | |
checksum - always present in class 4 | |||
C6 | 01 | Additional option selection | |
bit 4 | use of network expidited in class 1 | ||
bit 3 | use of receipt conformation in class 1 | ||
bit 2 | 16 bit checksum defined in class 4 | ||
bit 1 | use of transport expidited data transfer service | ||
C7 | n | Alternative protocol class(es) | |
85 | 2 n | Acknowledge time | |
89 | 12or24 | Throughput | |
86 | 3 | Residual error rate | |
87 | 2 | Priority | |
88 | 8 | Transit delay | |
8B | 2 | Reassignment time |
blank
slide tran 6 - Functions within the transport layer
- mapping transport-address onto network-address;
- multiplexing (end-to-end) transport-connections onto network-connections;
- establishment and release of transport-connections;
- end to end sequence control on individual connections;
- end-to-end error detection and any necessary monitoring of the quality of service;
- end-to-end error recovery;
- end-to-end segmenting, blocking and concatenation;
- end-to-end flow control on individual connections;
- supervisory functions;
- expidited transport-service-data-unit transfer.
blank
slide tran 7 - Services Provided to the session layer
- transpoonnection establishment;
- data transfer;
- transport-connection release.
Establishment phase
- Obtain a network-connection which best matches the requirements of the session-entity taking into account cost and quality of services.
- Decide whether multiplexing or splitting is needed to optimise the use of network connections.
- Establish the optimum transport- protocol-data-unit size
- Select the functions that will be operational upon entering the data phase
- Map transport-addresses onto network-addresses
- provide identification of different transport-connections between the same pair of transport-service- connections between the same pair of transport-service-access-points. (connection identification function)
- Transfer of data.
Data transfer phase
- sequencing
- blocking
- concatination - map multiple transport-protocol-data-units onto one network-service-data-unit.
- segmenting - map one transport-service-data-unit onto multiple network-protocol-data-units.
- multiplexing or spitting.
- flow control
- error detection
- error recovery
- expediatid data transfer
- transport-service-data-unit delimiting
- transport-connection id
Release Phase
- Notification of reason for release
- Identification of the transport-connection released
- Transfer of data
Primitave | Parameters |
T - CONNECT request | To transport address from transport address |
T - CONNECT indication | options, quality of service, TS user data. |
T - CONNECT reponse | responding address, options, |
T - CONNECT confirm | quality of service, TS user data. |
T - DISCONNECT request | TS - user data |
T - DISCONNECT indication | document reason, TS - user data |
T - DATA request | TS - user data |
T - DATA indication | TS - user data |
T - EXPEDITED DATA | TS - user data |
T - EXPEDITED DATA | TS - user data |
indication |
Example Teletex call
from network:
xx xx lg lc 0B CE 23 42 12 34 56 78 90 23 42 12 34 56 78 00 02 00 11 C0
from DTE:
call accept
from network:
xx xx yy yy yy 09 E0 00 00 31 31 00 C0 01 07 0D 88
from DTE:
xx xx yy yy yy 08 D0 54 43 41 42 00
from network:
xx xx yy yy yy 02 F0 80 0D 40 01 2E 0A 18 31 32 33 34 35 36 3D 49 44 41
20 20 20 20 20 20 20 20 20 20 20 20 20 20 0B 0E 38 32 2D 30 33 2D 30 30
2D 30 35 2D 31 32 0C 02 30 31 02 06 0D 01 01 0E 01 FE 08 01 01 41 03 4A
01 01 59 34
Call from terminal to network
from host:
xx xx 14 00 0B 0E 99 99 88 88 00 02
from network:
xx xx 14 00 OF 0E 00
from terminal:
xx xx 14 00 yy 06 E0 00 00 00 01 00
from network:
xx xx 14 00 yy 09 D0 00 01 31 31 00 C0 01 07
from terminal:
xx xx 14 00 yy 02 F0 80 0E 34 01 2A 09 18 32 33 34 32 2D 31 32 33 34 35
36 30 32 2D 31 32 33 3D 41 42 43 44 0B 0E 38 34 2D 30 33 2D 30 35 2D 31
31 3A 31 39 08 01 01 41 03 4A 01 01
Example Teletex call
--------------------
from network:
xx xx lg lc 0B CE 23 42 12 34 56 78 90 23 42 12 34 56 78 00 02 00 11 C0
from DTE:
call accept
from network:
xx xx yy yy yy 09 E0 00 00 31 31 00 C0 01 07 0D 88
from DTE:
---------
xx xx yy y yy 08 D0 54 43 41 42 00
from network:
-------------
xx xx
yy yy yy
02 F0 80 0D 40 01 2E 0A 18 31 32 33 34 35 36 3D 49 44 41
(123456=IDA)
20 20 20 20 20 20 20 20 20 20 20 20 20 20 0B 0E 38 32 2D 30 33 2D 30 30
(82-03-00)
2D 30 35 2D 31 32 0C 02 30 31 02 06 0D 01 01 0E 01 FE 08 01 01 41 03 4A
(-0512 01)
01 01 59 34
Call from terminal to network
-----------------------------
from host:
----------
xx xx 14 00 0B 0E 99 99 88 88 00 02
from network:
-------------
xx xx 14 00 OF 0E 00
from terminal:
--------------
xx xx 14 00 yy 06 E0 00 00 00 01 00
from network:
-------------
xx xx 14 00 yy 09 D0 00 01 31 31 00 C0 01 07
from terminal:
--------------
xx xx 14 00 yy 02 F0 80 0E 34 01 2A 09 18 32 33 34 32 2D 31 32 33 34 35
36 30 32 2D 31 32 33 3D 41 42 43 44 0B 0E 38 34 2D 30 33 2D 30 35 2D 31
31 3A 31 39 08 01 01 41 03 4A 01 01