Null Messages, Information and Coordination

This paper investigates the transfer of information in fault-prone synchronous systems using null messages. The notion of an $f$-resilient message block is defined to capture the fundamental communication pattern for knowledge transfer. This pattern may involve null messages in addition to explicit messages, and hence, it provides a fault-tolerant extension of the classic notion of a message-chain. Based on the above, we provide tight necessary and sufficient characterizations of the generalized communication patterns, including actual messages and null messages, that can serve to solve the distributed tasks of (nice-run) Signalling and Ordered Response.