In this section, we discuss transport-layer multiplexing and demultiplexing, that is,
extending the host-to-host delivery service provided by the network layer to a
process-to-process delivery service for applications running on the hosts. In order to
keep the discussion concrete, we’ll discuss this basic transport-layer service in the
context of the Internet. We emphasize, however, that a multiplexing/demultiplexing
service is needed for all computer networks.