This chapter discusses two protocols that are used for the reliable exchange of data between two computing stations. In the ideal case, data would simply be exchanged by sending and receiving messages. Unfortunately, the possibility of communication errors cannot always be ignored; the messages must be transported via a physical medium, which may lose, duplicate, reorder, or garble messages transmitted through it. These errors must be detected and corrected by supplementary mechanisms, traditionally referred to as protocols, implemented in the computing stations.

The main function of these protocols is data transmission, i.e., the accepting of the information at one station and its delivery at the other station. Reliable data transmission includes the repeated sending of messages that are lost, rejecting or correcting messages that are garbled, and discarding duplicates of messages. To do so the protocol maintains state information, recording which data has already been sent, which data has been certified to be received, and so on. The necessity of using state information raises the issue of connection management, i.e., the initialization and discarding of state information. The initialization is called opening the connection and its discarding is called closing the connection. The difficulty of connection management is due to the possibility that a message remains in the communication channels when the connection is closed. Such a message could be received when no connection exists or during a later connection, and its receipt must not disturb the correct operation of the current connection.