In Proceedings of the 11th Pacific Rim International Symposium on Dependable Computing (PRDC), Changsha, China, pages 305-309, December 2005.
Nowadays, one of the major concerns about the services provided
over the Internet is related to their availability. Replication is
a well known way to increase the availability of a service.
However, replication has some associated costs, namely it is
necessary to guarantee a correct coordination among the replicas.
Moreover, being the Internet such an unpredictable and insecure
environment, coordination correctness should be tolerant to
Byzantine faults and immune to timing failures. Several past works
address agreement and replication techniques that tolerate
Byzantine faults under the asynchronous model, but they all make
the assumption that the number of faulty replicas is bounded and
known. Assuming a maximum number of f faulty replicas under the
asynchronous model is dangerous - there is no way of guaranteeing
that no more than f faults will occur during the execution of
the system. In this paper, we describe a resilient f
fault/intrusion-tolerant state machine replication system, which
guarantees that no more than f faults ever occur. The system is
asynchronous in its most part and it resorts to a synchronous
oracle to periodically remove the effects of faults/attacks from