Transport Layer (Layer 4)

The Transport Layer of the OSI Model’s responsibility is to provide end-to-end communication. There are following functions of Transport Layer

·        Addressing (port number)

·        Transport (Connection-Less and Connection-Oriented)

·        Error Correction

Addressing refer on Layer 4 as port number, suppose when you are using an internet and you are require to do two jobs. One is browsing using HTTP and two is download a file using FTP. Here the transport layer gives a port number to both HTTP and FTP (80 and 20-21) and the result is that the HTTP port 80’s data is not going to mix with the FTP 20-21’s data. Their Sessions at Layer 5 are going to handle separate and as well as at Layer 6 handle the port 80 and 20-21 differently according to the requirements and at Layer 7 port 80’s data compare up with the HTTP on the browser and port 20-21’s data compare up with the FTP.

Transport refer on Layer 4 as the connection-less and connection-oriented. The two terms are use here. One is connection-less which means when one device is sending a data to another, the sending device doesn't going to established a connection with the reviver (no handshaking), it simple send the data to the reviver and does not care weather it is going to reach at the destination or not. UDP, ICMP, DNS, T FTP and IP are some of the protocols of connection-less communication. The other term connection-oriented is basically the opposite of the connection-less, which means its is going to establish a connection before sending a data to the reviver (handshaking). When the data are send, it is going to be acknowledge. By that mean connection-oriented provide a reliable and guaranteed connection between the devices. TCP, Frame Relay, ATM and SCTP are some of the protocols of connection-oriented.

Error Correction is also in the function of the Layer 4. The connection-oriented protocols are mostly used this function, because it provide the data grantee. When the reviver revived the data the following methods it uses for the error detection parity bit, check sum and CRC. When errors are detected then its  going to acknowledge the sender to resend the data.