Transport Layer

The main role of the transport layer is to provide the communication services directly to the application processes running on different hosts.

The transport layer provides logical communication between application processes running on different hosts. Although the application processes on different hosts are not physically connected, application processes use the logical communication provided by the transport layer to send messages to each other.

The transport layer protocols are implemented in the end systems but not in the network routers.

A computer network provides more than one protocol to the network applications. For example, TCP and UDP are two transport layer protocols that provide a different set of services to the network layer.

All transport layer protocols provide multiplexing/demultiplexing service. It also provides other services such as reliable data transfer,
bandwidth guarantees, and delay guarantees.

Each of the applications in the application layer has the ability to send a message by using TCP or UDP. The application communicates by using either of these two protocols. Both TCP and UDP will then communicate with the internet protocol in the internet layer. The applications can read and write to the transport layer. Therefore, we can say that communication is a two-way process.