20.2.4 Point-to-point Communication
Network technologies were developed to connect components (in this case individual computer systems) separated by larger distances, i.e., distances measured in meters instead of centimeters. Communicating over these larger distances led to different design tradeoffs. In early networks, information was sent as a sequence of bits over the shared communication medium. The bits were organized into packets, each containing the address of the destination. Packets also included a checksum used to detect errors in transmission and the protocol supported the ability to request the retransmission of corrupted packets. The software controlling the network is divided into a "stack" of modules, each implementing a different communication abstraction. The lowest-level physical layer is responsible for transmitting and receiving an individual packet of bits. Bit errors are detected and corrected, and packets with uncorrectable errors are discarded. There are different physical...