Fault-Tolerant Simulation of Message-Passing Algorithms by Mobile Agents

The recently established computational equivalence between the traditional message-passing model and the mobile-agents model is based on the existence of a mobile-agents algorithm that simulates the execution of message-passing algorithms. Like most existing protocols for mobile agents, this simulation protocol works correctly only if the agents are fault-free. We consider the problem of performing the simulation of message-passing algorithms when the simulating agents may crash unexpectedly. We show how to simulate any distributed algorithm for the message-passing model in a mobile-agents system with k agents, tolerating up to f ≤ k−1 crashes during the simulation. Two fault-tolerant simulation algorithms are presented, one for non-anonymous settings (i.e., where either the networks nodes or the agents or both have distinct identities), and one for anonymous systems (where both the network nodes and the agents are anonymous). In both cases, the simulation overhead is polynomial. An interesting feature of the algorithm for the anonymous setting is that it allows for a self-balancing fault-tolerant simulation: Even though the agents may crash at any time, the algorithm ensures that the simulation proceeds flawlessly irrespective of the agent crashes and the system always stabilizes to a state where the workload is equally distributed among the remaining agents. Finally, unlike the existing fault-free simulation algorithm, both our protocols are able to detect termination even if the simulated algorithm has no explicit termination detection.