Uniform Agreement Despite Process Omission Failures

A process fails by omission if it “forgets” to send or receive messages. Considering omission failures is crucial for distributed systems, as such failures model both crash failures and incorrect behavior of process input/output buffers (such as buffer overflow). So, designing protocols that cope not only with crash failures but also with omission failures is a real challenge as soon as one is interested in obtaining real-time dependable distributed systems. While the consensus problem has received a lot of attention in the crash failure model and in the Byzantine failure model, it has received less attention in the omission failure model. This paper presents a simple uniform consensus protocol for synchronous systems made up of n processes where up to t can commit crash or omission failures. This protocol requires t + 1 communication steps. Interestingly, as this bound is tight for crash failures and those are included in omission failures, this shows that t + 1 is a tight lower bound for protocols solving uniform consensus in synchronous systems prone to process omission failures. The protocol assumes t < n=2 that is a necessary requirement on the maximum number of faulty processes that can be tolerated by any uniform consensus protocol in presence of omission failures. The proposed protocol is then extended, at no additional cost, to solve the Interactive Consistency problem.