A transport-layer protocol provides process-to-process communication. A process is an application-layer entity (running program) that uses the services of the transport layer.

The network layer is responsible for communication at the computer level (host-to-host communication). A network layer protocol can deliver the message only to the destination computer.

A transport layer protocol is responsible for delivery of the message to the appropriate process. It is shown in figure.

The relationship between the communicating processes is the client-server relationship.