Transport protocols run over the best-effort IP layer to provide a mechanism for applications to communicate with each other without directly interacting with the IP layer.
It’s important to note that the use of ports is crucial in the transport layer. Certainly, there can be multiple types of processes that may run on a single host at a particular time. In general, a single IP address is assigned to a host. A sender host sends the data to the receiver host with a port number. Furthermore, the receiver host can identify the port and will route the data to a specific process.
The services of Transport Layer
This layer is responsible for delivering the messages, not only from a host to another host but also from a specific process of a host to a specific process of another host.
End to end delivery
Reliability
Flow control
Error control
Multiplexing/Demultiplexing