Consider a communication network in which processors want to transmit many short messages to each other. The processors are not necessarily connected by a communication channel. Usually this service is provided for by protocols in the transport layer. A protocol can incorporate such a message in a packet and send the packet to the destination processor. As discussed in chapter 1, in the transport layer it is again necessary that communication errors are considered, even though we can assume that the communication over channels is handled correctly by the lower layers. -Thus we have to assume that the communication network can lose packets, copy packets (due to necessary retransmissions), delay packets arbitrarily long, and deliver packets in a different order than the order in which they were sent.

We consider the design of some protocols that handle the communication of messages correctly, in the sense that there is no loss or duplication of messages (cf. Belsnes [Bel76]). To specify this more precisely, suppose processor i wants to transmit a message m to processor j. The message m is said to be lost if i thinks that j received m while this is not the case, and m is said to be duplicated if j receives two or more copies of m from i and thinks that they are different messages.

If a processor i has a message or a sequence of messages to send to j, it sets up a temporary connection with j, which is closed as soon as i knows that j received the message(s) (or that j is not in a position to receive them).