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.
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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