Reaching Approximate Byzantine Consensus in Partially-Connected Mobile Networks

We consider the problem of approximate consensus in mobile networks containing Byzantine nodes. We assume that each correct node can communicate only with its neighbors and has no knowledge of the global topology. As all nodes have moving ability, the topology is dynamic. The number of Byzantine nodes is bounded by f and known by all correct nodes. We first introduce an approximate Byzantine consensus protocol which is based on the linear iteration method. As nodes are allowed to collect information during several consecutive rounds, moving gives them the opportunity to gather more values. We propose a novel sufficient and necessary condition to guarantee the final convergence of the consensus protocol. The requirement expressed by our condition is not "universal": in each phase it affects only a single correct node. More precisely, at least one correct node among those that propose either the minimum or the maximum value which is present in the network, has to receive enough messages (quantity constraint) with either higher or lower values (quality constraint). Of course, nodes' motion should not prevent this requirement to be fulfilled. Our conclusion shows that the proposed condition can be satisfied if the total number of nodes is greater than 3f+1.Comment: No. RR-7985 (2012

Global Data Computation in a Dedicated Chordal Ring

Existing Global Data Computation (GDC) protocols for asynchronous systems are designed for fully connected networks. In this paper, we discuss GDC in a dedicated asynchronous chordal ring, a type of un-fully connected networks. The virtual links approach, which constructs t+1 (t<n) process-disjoint paths for each pair of processes without direct connection to tolerate failures (where t is the maximum number of processes that may crash and n is the total number of processes), can be applied to solve the GDC problem in the chordal but the virtual links approach incurs high message complexity. To reduce the high communication cost, we propose a non round-based GDC protocol for the asynchronous chordal ring with perfect failure detectors. The main advantage of our approach is that there is no notion of round, processes only send messages via direct connections and the implementation of failure detectors does not require process-disjoint paths. Analysis and comparison with the virtual links approach shows that our protocol reduces the message complexity significantly.Singapore-MIT Alliance (SMA

Using Failure Detection and Consensus in the General Omission Failure Model to Solve Security Problems

It has recently been shown that fair exchange, a security problem in distributed systems, can be reduced to a fault tolerance problem, namely a special form of distributed consensus. The reduction uses the concept of security modules which reduce the type and nature of adversarial behavior to two standard fault-assumptions: message omission and process crash. In this paper, we investigate the feasibility of solving consensus in asynchronous systems in which crash and message omission faults may occur. Due to the impossibility result of consensus in such systems, following the lines of unreliable failure detectors of Chandra and Toueg, we add to the system a distributed device that gives information about the failure of other processes. Then we give an algorithm using this device to solve the consensus problem. Finally, we show how to implement such a device in a asynchronous untrusted environment using security modules and some weak timing assumptions

The Failure Detector Abstraction

This paper surveys the failure detector concept through two dimensions. First we study failure detectors as building blocks to simplify the design of reliable distributed algorithms. More specifically, we illustrate how failure detectors can factor out timing assumptions to detect failures in distributed agreement algorithms. Second, we study failure detectors as computability benchmarks. That is, we survey the weakest failure detector question and illustrate how failure detectors can be used to classify problems. We also highlights some limitations of the failure detector abstraction along each of the dimensions

Contributions on agreement in dynamic distributed systems

139 p.This Ph.D. thesis studies the agreement problem in dynamic distributed systems by integrating both the classical fault-tolerance perspective and the more recent formalism based on evolving graphs. First, we developed a common framework that allows to analyze and compare models of dynamic distributed systems for eventual leader election. The framework extends a previous proposal by Baldoni et al. by including new dimensions and levels of dynamicity. Also, we extend the Time-Varying Graph (TVG) formalism by introducing the necessary timeliness assumptions and the minimal conditions to solve agreement problems. We provide a hierarchy of time-bounded, TVG-based, connectivity classes with increasingly stronger assumptions and specify an implementation of Terminating Reliable Broadcast for each class. Then we define an Omega failure detector, W, for the eventual leader election in dynamic distributed systems, together with a system model, , which is compatible with the timebounded TVG classes. We implement an algorithm that satisfy the properties of W in M. According to our common framework, M results to be weaker than the previous proposed dynamic distributed system models for eventual leader election. Additionally we use simulations to illustrate this fact and show that our leader election algorithm tolerates more general (i.e., dynamic) behaviors, and hence it is of application in a wider range of practical scenarios at the cost of a moderate overhead on stabilization times