A transport-layer protocol provides for logical communication between application processes running on different hosts.
By logical communication, we mean
that from an application’s perspective, it is as if the hosts running the processes were
directly connected; in reality, the hosts may be on opposite sides of the planet, connected via numerous routers and a wide range of link types. Application processes
use the logical communication provided by the transport layer to send messages to
each other, free from the worry of the details of the physical infrastructure used to
carry these messages. Figure 3.1 illustrates the notion of logical communication.