Transport layer

This is the fourth layer of the OSI model. The data 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 entire message reach 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 these segments to the network layer. At the receiver side, transport layer receives packet from the network layer. It detaches header from them and then reassembles the segments into original message and passes that message to the session layer.


  • Service-Point Addressing :- There may be number of program running at the same time on a computer. For this purpose, source-yo-destination delivery should also include process-to process delivery. Data must be transferred from a specific process on one computer to a specific process on another computer and to accomplish this, port address is requires. Thus, transport layer including service-point address in the header. Using these port addresses transport layer delivers the packets to the correct process.
  • Segmentation And Reassembly : A message is divided into segments with each segment having a sequence number. When the message arrive at the destination, with the help of these sequence number, the transport layer reassembles the message correctly. Sequence number 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 connection less service. In connection-oriented service, transport layer established a connection with the transport layer at the destination before transmission. In connection less service. 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 re transmitted to achieve error correction.