Reliable Communication Over Unreliable Channels

Abstract

Layered communication protocols frequently implement a FIFO message facility on top of an unreliable non-FIFO service such as that provided by a packet-switching network. This paper investigates the possibility of implementing a reliable message layer on top of an underlying layer that can lose packets and deliver them out of order, with the additional restriction that the implementation uses only a fixed finite number of different packets. A new formalism is presented to specify communication layers and their properties, the notion of their implementation by I/O automata, and the properties of such implementations. An I/O automaton that implements a reliable layer over an unreliable layer is presented. In this implementation, the number of packets needed to deliver each succeeding message increases permanently as additional packet-loss and reordering faults occur. A proof is given that no protocol can avoid such performance degradation.