The network layer provides node-to-node communication while the transport layer provides the service of application-to-application communication.

The application-to-application communication service is useful because there can be many different applications running in a node that want to communicate with one of the applications running in a different node.

Achieving transport layer service

Application-to-application communication is achieved via the concept of ports, where each node can have many different applications running and listening on different ports for data from the network.

Some of the protocols in the transport layer can also provide more services on top of this fundamental one. We will briefly cover the transmission control protocol (TCP) and the user datagram protocol (UDP), the two most commonly used and well-known protocols of the transport layer.

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