Transport layer of the OSI model - Definition

Transport layer

This is the fourth layer of the OSI model. The data unit at this layer is known as segment. The network layer deals with source-to-destination delivery of individual packets and does not recognize any relationship between them. On the other hand, the transport layer ensures that the entries message reaches in order and handles error control and flow control at the source-to destination level.

Transport layer receives data from the session layer .it divides the data into segments with each segment having its own header and passes this segment to the network layer. At the receiver side, transport layer receives packets from the network layer. It detaches headers from them and then reassemblies the segments into original message and passes that massage to the session later

Responsibilities of transport layer are:

☼Service-point addressing -there may be number of programs running at the same time on a computer. For this purpose, source-to-destination delivery should also include process-to-process delivery. Data must be transferred from a specific process (running program) on one computer to a specific process on another computer and to accomplish this, port address is required. Thus, transport layer includes service-point address (also referred to as port address) in the header. Using these port addresses transport layer delivers the packets to the correct process.

☼Segmentation and reassembly – a massage is divided into segments with each segment having a sequence number. When the message arrives at the destination, with the help of these sequence numbers, the transport layer reassembles the message correctly. Sequence numbers also enable transport layer to determine and replace the segments that were lost in the transmission.

☼Connection control –transport layer provides either connection-oriented or connectionless service. In connection-oriented service, transport layer establishes a connection with the transport layer at the destination before transmitting segments and terminates the connection when all the data is transferred. in connectionless service, transport layer treats each segment as individual packet and transmits it to the transport layer treats each segment as individual packet and transmits it to the transport layer at destination.

☼Error control – transport layer also performs error checking. Error control takes place end to end. Transport layer at the sending machine ensures that the entire message is delivered at the receiving machine without an error. Segments are retransmitted to achieve error correction.