Degradable Agreement with Hybrid Faults ( An Algorithm and Reliability - Safety Analysis )

Achieving traditional Byzantine agreement in the presence of arbitrary faults requires that the total number of nodes be larger than three times the number of faulty nodes. Two approaches have been proposed in the literature to circumvent this requirement: (i) hybrid fault model approach 4, 7] considers three types of faults of varying degree of severity, and (ii) degradable agreement approach 10, 11] relaxes the deenition of agreement in the presence of excessive faults. This report combines the above two approaches and presents a degradable agreement algorithm for the hybrid fault model. The report deenes and evaluates reliability and safety measures for the proposed degradable agreement algorithm and concludes that degradable agreement can be used to trade reliability with safety. It is shown that the hybrid fault model results in better algorithms (with respect to reliability and/or safety) only when the probability of arbitrary faults is not very small.