OSI Model: Transport Layer and its Function

The transport layer is responsible for delivery of entire message from source to destination. We know that the network layer ensures the delivery of individual packets, but its treats every packet individually and does not recognize the relationship between packets. The transport layer ensures that whole message transmits in an order way, ensures error control. Transport layer provides reliability through error control and flow control at source to destination level.

Examples of transport layer protocols are TCP/IP and UDP.

Functions of Transport Layer

Service Point Addressing: There are many programs running on a computer at the same time. Transport layer is responsible for process to process delivery of entire message. For process to process delivery of transmission, transport layer header includes service point address also known as port address. As network layer gets packets to the correct computer, transport layer gets message to the correct process on a computer.

Segmentation and Reassembling: Transport layer divides the message into segments and each segment contains a sequence number. The segment number enables transport layer to reassemble the message. When message is reassembled correctly upon arrival at the destination, this layer identifies and replaces packets which were lost in transmission.

Connection Control: The transport layer can be of two types, either connectionless or connection-oriented. In connectionless transport layer, each segment is considered as independent packets and delivers it to transport layer at destination machine. In connection-oriented transport layer, a connection is established with transport layer at destination machine before delivering packets. The connection is terminated when all data are transferred.

Flow Control: Flow control in this layer is performed process to process.

Error Control: Transport layer is also responsible error control. Error control is performed process to process. The sending transport layer ensures that message arrives at destination machine without any error.