On Composability of Reliable Unicast and Broadcast

In the recent past composability has emerged as a key requirement for various distributed protocols. It is not enough for a protocol to be robust when it runs in isolation or in a “stand-alone” setting but it should be robust even in an environment where several copies of the same protocol or other protocol(s) are running simultaneously. In this work, we investigate the composability for protocols that tolerate a bounded adversary modeled as a probabilistic polynomial time Turing machine. We examine composability of protocols for two fundamental problems in distributed computing reliable unicast and reliable broadcast. We show that any composable protocol – for reliable unicast tolerating an adversary, that corrupts up to any t nodes, requires 2t+ 1 connectivity and for reliable broadcast tolerating an adversary, that corrupts up to any t nodes, requires n > 3t and 2t+ 1 connectivity.