Distributed computer systems and networks have brought about big changes in the way modern society functions in its commercial, educational and research activities. Critical infrastructures such as banking and commerce, telecommunication, transportation, power distribution and medicine, are all increasingly dependent on networked systems. It is estimated that technology glitches in the U.S. cost at least $100 billion a year in lost productivity. This has led to a surge of interest in techniques for making network and software systems highly secure and reliable.

Increasing complexity in modern systems necessitates much greater reliability even to maintain the same apparent performance. For example, contemporary versions of Microsoft's Windows operating system have a mean time between failures (MTBF) of around 1 day. It is expected that by Moore's law, personal computer chip speeds will increase within ten years to over 10 GHz, in comparison with about 500 MHz at present. This means that...