Boosting the Efficiency of Byzantine-tolerant Reliable Communication

Reliable communication is a fundamental primitive in distributed systems prone to Byzantine (i.e. arbitrary, and possibly malicious) failures to guarantee integrity, delivery and authorship of messages exchanged between processes. Its practical adoption strongly depends on the system assumptions. One of the most general (and hence versatile) such hypothesis assumes a set of processes interconnected through an unknown communication network of reliable and authenticated links, and an upper bound on the number of Byzantine faulty processes that may be present in the system, known to all participants. To this date, implementing a reliable communication service in such an environment may be expensive, both in terms of message complexity and computational complexity, unless the topology of the network is known. The target of this work is to combine the Byzantine fault-tolerant topol-ogy reconstruction with a reliable communication primitive, aiming to boost the efficiency of the reliable communication service component after an initial (expensive) phase where the topology is partially reconstructed. We characterize the sets of assumptions that make our objective achievable, and we propose a solution that, after an initialization phase, guarantees reliable communication with optimal message complexity and optimal delivery complexity

Byzantine Fault Tolerant Symmetric-Persistent Circle Evacuation

We consider (n, f)-evacuation on a circle, an evacuation problem of a hidden exit on the perimeter of a unit radius circle for n> 1 robots, f of which are faulty. All the robots start at the center of the circle and move with maximum speed 1. Robots must first find the exit and then move there to evacuate in minimum time. The problem is considered complete when all the honest robots know the correct position of the exit and the last honest robot has evacuated through the exit. During the search, robots can communicate wirelessly. We focus on symmetric-persistent algorithms, that is, algorithms in which all robots move directly to the circumference, start searching the circle moving in the same direction (cw or ccw), and do not stop moving around the circle before receiving information about the exit. We study the case of (n, 1) and (n, 2) evacuation. We first prove a lower bound of 1+4πn+2sin(π2-πn) for one faulty robot, even a crash-faulty one. We also observe an almost matching upper bound obtained by means of an earlier search algorithm. We finally study the case with two Byzantine robots and we provide an algorithm that achieves evacuation in time at most 3+6πn, for n≥ 9, or at most 3+6πn+δ(n), for n< 9, where δ(n)≤2sin(3π2n)+2-4sin(3π2n)+4sin2(3π2n)-2. © 2021, Springer Nature Switzerland AG