The transport layer is the layer in which direct logical end-to-end communication between two nodes is realized. Transparent data transport over the network connections can be controlled by flow control mechanisms.
The tasks of the transport layer
The task of the transport layer, performed by the transport protocol with its transport classes, is to provide transparent, seamless, and secure end-to-end data transmission to two communicating application processes, without regard to the media used in layers 1 to 3. The following elements or properties are characteristic: transparency, freedom from errors, network independence, end-to-end transport service, cost optimization, and transport addressing. Two types of transport services are distinguished: connection-oriented and connectionless. Connectionless transport services include, for example, datagram service.
The services of the transport layer
The following services are offered to the higher layers: The transport layer supports connection-oriented or connectionless communication. Normal and precedence data units are transferred in the transport layer.
If a network only allows the transport of small data units, layer 4 can split the large data packets arriving from layer 5, the communication control layer, into several small data units by means of segmentation, send them over the network and reassemble them to the original form in the transport layer of the target system, the reassembling.
If very small data units of layer 5 are to be transported through the network and if this transmission would be inefficient, the station can form a larger unit by concatenation, concatenation, which it transports through the network. The remote station separates these data units back into their original small units.
The classes of service of the transport layer
The transport layer supports the selection of quality of service parameters. For this purpose, 5 service classes, transport classes 0 to 4 with different performance characteristics, have been defined in the OSI transport protocols. The transport layer is able to select between several paths for a specific application relationship or to send data units to a destination via several paths for reasons of data throughput. In connection-oriented communication, the transport layer restores the correct sequence of received data units by assigning sequence numbers.
The DIN definition is: "The transport layer extends end system connections that lead from end system to end system to user connections that lead from user to user. A user is understood to be an association between a representation instance and a communication control instance".