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 (请作答此空) layer,the whole concept of layered protocols would make little sense.

答案：

解析：