Phase Clocks for Transient Fault Repair

Phase clocks are synchronization tools that implement a form of logical time in distributed systems. For systems tolerating transient faults by self-repair of damaged data, phase clocks can enable reasoning about the progress of distributed repair procedures. This paper presents a phase clock algorithm suited to the model of transient memory faults in asynchronous systems with read/write registers. The algorithm is self-stabilizing and guarantees accuracy of phase clocks within O(k) time following an initial state that is k-faulty. Composition theorems show how the algorithm can be used for the timing of distributed procedures that repair system outputs.Comment: 22 pages, LaTe

Self-stabilizing wormhole routing in hypercubes

Wormhole routing is an efficient technique used to communicate message packets between processors when they are not completely connected. To the best of our knowledge, this is the first attempt at designing a self-stabilizing wormhole routing algorithm for hypercubes. Our first algorithm handles all types of faults except for node/link failures. This algorithm achieves optimality in terms of routing path length by following only the preferred dimensions. In an n-dimensional hypercube, those dimensions in which source and destination address bits differ are called preferred dimensions. Our second algorithm handles topological changes. We propose an efficient scheme of rerouting flits in case of node/link failures. Similar to the first algorithm, this algorithm also tries to follow preferred dimensions if they are nonfaulty at the time of transmitting the flits. However, due to topological faults it is necessary to take non-preferred dimensions resulting in suboptimality of path selection. Formal proof of correctness for both solutions is given. (Abstract shortened by UMI.)

Stabilizing Maximal Independent Set in Unidirectional Networks is Hard

A distributed algorithm is self-stabilizing if after faults and attacks hit the system and place it in some arbitrary global state, the system recovers from this catastrophic situation without external intervention in finite time. In this paper, we consider the problem of constructing self-stabilizingly a \emph{maximal independent set} in uniform unidirectional networks of arbitrary shape. On the negative side, we present evidence that in uniform networks, \emph{deterministic} self-stabilization of this problem is \emph{impossible}. Also, the \emph{silence} property (\emph{i.e.} having communication fixed from some point in every execution) is impossible to guarantee, either for deterministic or for probabilistic variants of protocols. On the positive side, we present a deterministic protocol for networks with arbitrary unidirectional networks with unique identifiers that exhibits polynomial space and time complexity in asynchronous scheduling. We complement the study with probabilistic protocols for the uniform case: the first probabilistic protocol requires infinite memory but copes with asynchronous scheduling, while the second probabilistic protocol has polynomial space complexity but can only handle synchronous scheduling. Both probabilistic solutions have expected polynomial time complexity

Optimization of a Self-Stabilizing Role Assignment Algorithm for Actuator/Sensor Networks

La tesi si pone come obiettivo quello di proporre un algoritmo ottimizzato per l'assegnamento di ruoli nelle reti di sensori e attuatori. L'algoritmo è costituito da uno stack ti protocolli self-stabilizing che basa direttamente sull'interfaccia radio. Partendo dal basso lo Spanning Tree struttura la rete ad albero eleggendo dunque un root node. Basandosi su questa struttura il secondo livello dello stack implementa un paradigma di comunicazione gerarchico di tipo publish/subscribe, tramite cui ogni nodo notifica verso il root node le proprie funzionalità. In cima allo stack il Role Assignment analizza i ruoli richiesti dalle applicazioni per poter girare e, dinamicamente, provvede ad assegnarli ai nodi in base alle loro funzionalità. La tesi espone dunque l'implementazione dell'algoritmo per Network Simulator 2 (NS2) e i grafici ottenuti, riferenti agli indici di maggiore interesse

Best-effort Group Service in Dynamic Networks

We propose a group membership service for dynamic ad hoc networks. It maintains as long as possible the existing groups and ensures that each group diameter is always smaller than a constant, fixed according to the application using the groups. The proposed protocol is self-stabilizing and works in dynamic distributed systems. Moreover, it ensures a kind of continuity in the service offer to the application while the system is converging, except if too strong topology changes happen. Such a best effort behavior allows applications to rely on the groups while the stabilization has not been reached, which is very useful in dynamic ad hoc networks

Extending Byzantine Fault Tolerance to Replicated Clients

Byzantine agreement protocols for replicated deterministic state machines guarantee that externally requested operations continue to execute correctly even if a bounded number of replicas fail in arbitrary ways. The state machines are passive, with clients responsible for any active ongoing application behavior. However, the clients are unreplicated and outside the fault-tolerance boundary. Consequently, agreement protocols for replicated state machines do not guarantee continued correct execution of long-running client applications. Building on the Castro and Liskov Byzantine Fault Tolerance protocol for unreplicated clients (CLBFT), we present a practical algorithm for Byzantine fault-tolerant execution of long-running distributed applications in which replicated deterministic clients invoke operations on replicated deterministic servers. The algorithm scales well to large replica groups, with roughly double the latency and message count when compared to CLBFT, which supports only unreplicated clients. The algorithm supports both synchronous and asynchronous clients, provides fault isolation between client and server groups with respect to both correctness and performance, and uses a novel architecture that accommodates externally requested software upgrades for long-running evolvable client applications