TMR processing without explicit clock synchronisation

Abstract

Replicated processing with majority voting is a well known method for achieving fault tolerance. Triple Modular Redundant (TMR) processing is the most commonly used version of that method. Replicated processing requires that the replicas reach agreement on the order in which messages are to be processed. Synchronous and deterministic ordering protocols published in the literature require that the replicas maintain an abstraction of clocks that are kept in known and bounded synchronism. We present a protocol for TMR systems that does not require this abstraction of synchronised clocks. We analyse the protocol performance and show that this protocol in practice can be at least as fast as any synchronised clock based ordering protocol. We also derive a faster protocol that has an improved performance in the absence of processor failures. We then build a TMR node and measure its performance to illustrate that the protocols developed here provide faster ordering and are easier to implement.