(第2空)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.