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Consensus and collision detectors in wireless ad hoc networks
 In PODC
, 2005
"... Abstract In this study, we consider the faulttolerant consensus problem in wireless ad hoc networks with crashprone nodes. Specifically, we develop lower bounds and matching upper bounds for this problem in singlehop wireless networks, where all nodes are located within broadcast range of each oth ..."
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Cited by 46 (21 self)
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Abstract In this study, we consider the faulttolerant consensus problem in wireless ad hoc networks with crashprone nodes. Specifically, we develop lower bounds and matching upper bounds for this problem in singlehop wireless networks, where all nodes are located within broadcast range of each other. In a novel break from existing work, we introduce a highly unpredictable communication model in which each node may lose an arbitrary subset of the messages sent by its neighbors during each round. We argue that this model better matches behavior observed in empirical studies of these networks. To cope with this communication unreliability we augment nodes with receiverside collision detectors and present a new classification of these detectors in terms of accuracy and completeness. This classification is motivated by practical realities and allows us to determine, roughly speaking, how much collision detection capability is enough to solve the consensus problem efficiently in this setting. We consider ten different combinations of completeness and accuracy properties in total, determining for each whether consensus is solvable, and, if it is, a lower bound on the number of rounds required. Furthermore, we distinguish anonymous and nonanonymous protocolswhere &quot;anonymous &quot; implies that devices do not have unique identifiersdetermining what effect (if any) this extra information has on the complexity of the problem. In all relevant cases, we provide matching upper bounds. Our contention is that the introduction of (possibly weak) receiverside collision detection is an important approach to reliably solving problems in unreliable networks. Our results, derived in a realistic network model, provide important feedback to ad hoc network practitioners regarding what hardware (and lowlayer software) collision detection capability is sufficient to facilitate the construction of reliable and faulttolerant agreement protocols for use in realworld deployments.
The Price of Anonymity: Optimal Consensus despite Asynchrony, Crash and Anonymity
, 2008
"... This paper addresses the consensus problem in asynchronous systems prone to process crashes, where additionally the processes are anonymous (they cannot be distinguished one from the other: they have no name and execute the same code). To circumvent the three computational adversaries (asynchrony, f ..."
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Cited by 14 (3 self)
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This paper addresses the consensus problem in asynchronous systems prone to process crashes, where additionally the processes are anonymous (they cannot be distinguished one from the other: they have no name and execute the same code). To circumvent the three computational adversaries (asynchrony, failures and anonymity) each process is provided with a failure detector of a class denoted ψ, that gives it an upper bound on the number of processes that are currently alive (in a nonanonymous system, the classes ψ and Pthe class of perfect failure detectors are equivalent). The paper first presents a simple ψbased consensus algorithm where the processes decide in 2t + 1 asynchronous rounds (where t is an upper bound on the number of faulty processes). It then shows one of its main results, namely, 2t + 1 is a lower bound for consensus in the anonymous systems equipped with ψ. The second contribution addresses earlydecision. The paper presents and proves correct an earlydeciding algorithm where the processes decide in min(2f + 2, 2t + 1) asynchronous rounds (where f is the actual number of process failures). This leads to think that anonymity doubles the cost (wrt synchronous systems) and it is conjectured that min(2f + 2, 2t + 1) is the corresponding lower bound. The paper finally considers the kset agreement problem in anonymous systems. It first shows that the
Anonymous and faulttolerant sharedmemory computing
, 2007
"... The vast majority of papers on distributed computing assume that processes are assigned unique identifiers before computation begins. But is this assumption necessary? What if processes do not have unique identifiers or do not wish to divulge them for reasons of privacy? We consider asynchronous sh ..."
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Cited by 6 (2 self)
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The vast majority of papers on distributed computing assume that processes are assigned unique identifiers before computation begins. But is this assumption necessary? What if processes do not have unique identifiers or do not wish to divulge them for reasons of privacy? We consider asynchronous sharedmemory systems that are anonymous. The shared memory contains only the most common type of shared objects, read/write registers. We investigate, for the first time, what can be implemented deterministically in this model when processes can fail. We give anonymous algorithms for some fundamental problems: timestamping, snapshots and consensus. Our solutions to the first two are waitfree and the third is obstructionfree. We also show that a shared object has an obstructionfree implementation if and only if it satisfies a simple property called idempotence. To prove the sufficiency of this condition, we give a universal construction that implements any idempotent object.
Naming and counting in anonymous unknown dynamic networks
 In 15th International Symposium on Stabilization, Safety, and Security of Distributed Systems (SSS
, 2013
"... Abstract. In this work, we study the fundamental naming and counting problems (and some variations) in networks that are anonymous, unknown, and possibly dynamic. In counting, nodes must determine the size of the network n and in naming they must end up with unique identities. By anonymous we mean ..."
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Cited by 4 (4 self)
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Abstract. In this work, we study the fundamental naming and counting problems (and some variations) in networks that are anonymous, unknown, and possibly dynamic. In counting, nodes must determine the size of the network n and in naming they must end up with unique identities. By anonymous we mean that all nodes begin from identical states apart possibly from a unique leader node and by unknown that nodes have no a priori knowledge of the network (apart from some minimal knowledge when necessary) including ignorance of n. Network dynamicity is modeled by the 1interval connectivity model [KLO10], in which communication is synchronous and a (worstcase) adversary chooses the edges of every round subject to the condition that each instance is connected. We first focus on static networks with broadcast where we prove that, without a leader, counting is impossible to solve and that naming is impossible to solve even with a leader and even if nodes know n. These impossibilities carry over to dynamic networks as well. We also show that a unique leader suffices in order to solve counting in linear time. Then we focus on dynamic networks with broadcast. We conjecture that dynamicity renders nontrivial computation impossible. In view of this, we let the nodes know an upper bound on the maximum degree that will ever appear and show that in this case the nodes can obtain an upper bound on n. Finally, we replace broadcast with onetoeach, in which a node may send a different message to each of its neighbors. Interestingly, this natural variation is proved to be computationally equivalent to a fullknowledge model, in which unique names exist and the size of the network is known. 1
Counting on Anonymous Dynamic Networks through Energy Transfer
"... In this paper we address the problem of counting the size of the network on anonymous dynamic networks with broadcast using a leader. In an anonymous network a node has no knowledge on the network including that nodes have no unique identifiers. The dynamicity of the network is modeled using 1inter ..."
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In this paper we address the problem of counting the size of the network on anonymous dynamic networks with broadcast using a leader. In an anonymous network a node has no knowledge on the network including that nodes have no unique identifiers. The dynamicity of the network is modeled using 1interval connectivity. In order to tackle the difficulties that arise from the dynamic environment and the absence of unique IDs, we introduce a new approach based on the concept of energy transfer. Each process owns a fixed energy charge, and tries to discharge itself exchanging, at each round, at most half of its charge with neighbors. The leader acts as a sink collecting energy. This simple algorithm enforces, at each round, an invariant on the sum of energy among networks’ node: energy is not created or destroyed. This invariant allows the leader to eventually converge to a precise count of the network size. Moreover, using an additional assumption on the network knowledge, such as an upper bound on the number of nodes present in the network or upper bound on the maximum node’s degree/access to a local counting oracle, the leader can count the size of the network and detect the termination in a finite time. To point out the relevance of the energy notion, as a final result we show that if the network’s size is known, it is possible, without a leader, to count
P.: Computing in dynamic networks
 Chapter 6 in Computational Network Theory: Theoretical Foundations and Applications, 1st edn
, 2015
"... 1.1 MotivationState of the art Distributed computing systems are more and more becoming dynamic. The static and relatively stable models of computation can no longer represent the plethora of recently established and rapidly emerging information and communication technologies. In recent years, we h ..."
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1.1 MotivationState of the art Distributed computing systems are more and more becoming dynamic. The static and relatively stable models of computation can no longer represent the plethora of recently established and rapidly emerging information and communication technologies. In recent years, we have seen a
1.4 The Consensus Problem In Wireless Ad Hoc Networks...................
, 2006
"... In this study, we consider the faulttolerant consensus problem in wireless ad hoc networks with crashprone nodes. Specifically, we develop lower bounds and matching upper bounds for this problem in singlehop wireless networks, where all nodes are located within broadcast range of each other. In a ..."
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In this study, we consider the faulttolerant consensus problem in wireless ad hoc networks with crashprone nodes. Specifically, we develop lower bounds and matching upper bounds for this problem in singlehop wireless networks, where all nodes are located within broadcast range of each other. In a novel break from existing work, we introduce a highly unpredictable communication model in which each node may lose an arbitrary subset of the messages sent by its neighbors during each round. We argue that this model better matches behavior observed in empirical studies of these networks. To cope with this communication unreliability we augment nodes with receiverside collision detectors and present a new classification of these detectors in terms of accuracy and completeness. This classification is motivated by practical realities and allows us to determine, roughly speaking, how much collision detection capability is enough to solve the consensus problem efficiently in this setting. We consider ten different combinations of completeness and accuracy properties in total, determining for each whether consensus is solvable, and, if it is, a lower bound on the number of rounds required. Furthermore, we distinguish anonymous and nonanonymous protocols—where “anonymous ” implies that devices do not have unique identifiers—determining what effect (if any) this extra information has on
DOI 10.1007/s0044601301903 Byzantine agreement with homonyms
"... Abstract So far, the distributed computing community has either assumed that all the processes of a distributed system have distinct identifiers or, more rarely, that the processes are anonymous and have no identifiers. These are two extremes of the same general model: namely, n processes use diffe ..."
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Abstract So far, the distributed computing community has either assumed that all the processes of a distributed system have distinct identifiers or, more rarely, that the processes are anonymous and have no identifiers. These are two extremes of the same general model: namely, n processes use different identifiers, where 1 ≤ ≤ n. In this paper, we ask how many identifiers are actually needed to reach agreement in a distributed system with t Byzantine processes. We show that having 3t + 1 identifiers is necessary and sufficient for agreement in the synchronous case but, more surprisingly, the number of identifiers must be greater than n+3t2 in the partially synchronous case. This demonstrates two differences from the classical model (which has = n): there are situations where relaxing synchrony to partial synchrony renders agreement impossible; and, in the partially synchronous case, increasing the number of correct processes can actually make it harder to reach agreement. The impossibility proofs use the fact that a Byzantine process can send multiple messages to the same recipient in a round. We show that removing this ability makes agreement easier: then, t + 1 identifiers are sufficient for agreement, even in the partially synchronous model, assuming processes can count the number of messages with the same identifier they receive in a round. C. DelporteGallet (B) · H. Fauconnier · H. TranThe
To Transmit Now or Not To Transmit Now
"... Abstract—This paper studies how a reliable communication service can be built, with high probability synchrony, over a wireless channel. We investigate environments where energy consumption is crucial, e.g., sensor networks. We consider a Partially Observable Markov Decision Process (POMDP) setting ..."
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Abstract—This paper studies how a reliable communication service can be built, with high probability synchrony, over a wireless channel. We investigate environments where energy consumption is crucial, e.g., sensor networks. We consider a Partially Observable Markov Decision Process (POMDP) setting in which the channel state: (i) changes according to a classic Markovian model and (ii) can be only partially observed, through feedback relative to previous transmissions. We perform a thorough analytic study under four Ack/Nack feedback mechanisms, which to our knowledge represent all Ack/Nack feedback variations. Despite the general intractability of POMDPs, we prove that our communication service, under reliable feedback, can be inexpensively implemented. We precisely obtain a closed form solution specifying when to transmit over the channel, which allows to derive an energyoptimal dependable implementation. We show that an easy implementable structure for our communication service can also be obtained under lossy feedback, depending on the feedback mechanism. I.