Together with the network layer,the transport layer is the heart of the protocol（  ）The network layer provides end-to-end（  ） delivery using datagrams or virtual circuits.The transport layer builds on the network layer to provide data transport from a process on a（  ）machine to a process on a destination machine with a desired level of reliability that is independent of the physical （  ） currently in use.It provides the abstractions that applications nedd to use the network..Without the（  ） layer,the whole concept of layered protocols would make little sense.