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50
Optimal Clock Synchronization
 Journal of the ACM
, 2003
"... We present a simple, efficient, and unified solution to the problems of synchronizing, initializing, and integrating clocks for systems with different types of failures: crash, omission, and arbitrary failures with and without message authentication. This is the ft known solution that achieves optim ..."
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Cited by 153 (0 self)
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We present a simple, efficient, and unified solution to the problems of synchronizing, initializing, and integrating clocks for systems with different types of failures: crash, omission, and arbitrary failures with and without message authentication. This is the ft known solution that achieves optimal accuracy  the accuracy of synchronized clocks (with respect to real time) is as good as that specified for the underlying hardware clocks. The solution is also optimal with respect to the number of faulty processes that can be tolerated to achieve this accuracy.
A new faulttolerant algorithm for clock synchronization
 INFORMATION AND COMPUTATION
, 1988
"... ..."
Easy Impossibility Proofs for Distributed Consensus Problems
 DISTRIBUTED COMPUTING
, 1986
"... Easy proofs are given, of the impossibility of solving several consensus problems (Byzantine agreement, weak agreement, Byzantine firing squad, approximate agreement and clock synchronization) in certain communication graphs. It is shown that, in the presence of m faults, no solution to these proble ..."
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Cited by 96 (8 self)
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Easy proofs are given, of the impossibility of solving several consensus problems (Byzantine agreement, weak agreement, Byzantine firing squad, approximate agreement and clock synchronization) in certain communication graphs. It is shown that, in the presence of m faults, no solution to these problems exists for communication graphs with fewer than 3m+ 1 nodes or less than 2m+l connectivity. While some of these results had previously been proved, the new proofs are much simpler, provide considerably more insight, apply to more general models of computation, and (particularly in the case of clock synchronization) significantly strengthen the results.
Faulttolerance in collaborative sensor networks for target detection
 IEEE Transactions on Computers
, 2004
"... Abstract—Collaboration in sensor networks must be faulttolerant due to the harsh environmental conditions in which such networks can be deployed. This paper focuses on finding algorithms for collaborative target detection that are efficient in terms of communication cost, precision, accuracy, and n ..."
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Cited by 86 (4 self)
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Abstract—Collaboration in sensor networks must be faulttolerant due to the harsh environmental conditions in which such networks can be deployed. This paper focuses on finding algorithms for collaborative target detection that are efficient in terms of communication cost, precision, accuracy, and number of faulty sensors tolerable in the network. Two algorithms, namely, value fusion and decision fusion, are identified first. When comparing their performance and communication overhead, decision fusion is found to become superior to value fusion as the ratio of faulty sensors to fault free sensors increases. As robust data fusion requires agreement among nodes in the network, an analysis of fully distributed and hierarchical agreement is also presented. The impact of hierarchical agreement on communication cost and system failure probability is evaluated and a method for determining the number of tolerable faults is identified. Index Terms—Collaborative target detection, decision fusion, fault tolerance, sensor networks, value fusion. 1
Understanding Protocols for Byzantine Clock Synchronization
, 1987
"... All published faulttolerant clock synchronization protocols are shown to result from refining a single paradigm. This allows the differera clock synchronization protocols to be compared and permits presemation of a single correctness analysis that holds for all. The paradigm is based on a reliab ..."
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Cited by 80 (0 self)
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All published faulttolerant clock synchronization protocols are shown to result from refining a single paradigm. This allows the differera clock synchronization protocols to be compared and permits presemation of a single correctness analysis that holds for all. The paradigm is based on a reliable time source that periodically causes events; detection of such an event causes a processor to reset its clock. In a distributed system, the reliable time source can be approximated by combining the values of processor clocks using a generalization of a "faulttolerant average", called a convergence function. The performance of a clock synchronization protocol based on our paradigm can be quantified in terms of the two parameters that characterize the behavior of the convergence function used: accuracy and precision.
A New FaultTolerant Algorithm for Clock Synchronization
, 1984
"... We describe a new faulttolerant algorithm for solving a variant of Lamport's clock synchronization problem. The algorithm is designed for a system of distributed processes that communicate by sending messages. Each process has its own readonly physical clock whose drift rate from real time is ..."
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Cited by 79 (3 self)
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We describe a new faulttolerant algorithm for solving a variant of Lamport's clock synchronization problem. The algorithm is designed for a system of distributed processes that communicate by sending messages. Each process has its own readonly physical clock whose drift rate from real time is very small. By adding a value to its physical clock time, the process obtains its local time. The algorithm solves the problem of .maintaining closely synchronized local times, assuming that processes' local times are clo. sely synchronized initially. The algorithm is able to tolerate the failure of just under a third of the participating processes. It maintains synchronization to within a small constant, whose magnitude depends upon the rate of clock drift, the message delivery time, and the initial closeness of synchronization. We also give a characterization of how far the clocks drift from real time. Reintegration of a repaired process can be accomplished using a slight modification of the 'basic algorithm. A similar style algorithm can also be used to achieve synchronization initially.
Abbadi, ‘‘The Tree Quorum Protocol: An Efficient Approach for Managing Replicated Data
 Proc. 16th VLDB Conf
, 1990
"... In this paper, we present a lowcost faulttolerant protocol for managing replicated data. We impose a logical tree structure on the set of copies of an object and develop a protocol that uses the information available in the logical structure to reduce the communication requirements for read and wr ..."
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Cited by 71 (2 self)
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In this paper, we present a lowcost faulttolerant protocol for managing replicated data. We impose a logical tree structure on the set of copies of an object and develop a protocol that uses the information available in the logical structure to reduce the communication requirements for read and write operations. The tree quorum protocol is a generalization of the static voting protocol with two degrees of freedom for choosing quorums. In general, this results in significantly lower communication costs for comparable data availability. The protocol exhibits the property of graceful degradation, i.e., communication costs for executing operations are minimal in a failurefree environment but may increase as failures occur. This approach in designing distributed systems is desirable since it provides faulttolerance without imposing unnecessary costs on the failurefree mode of operations.
WaitFree Data Structures in the Asynchronous PRAM Model
 In Proceedings of the 2nd Annual Symposium on Parallel Algorithms and Architectures
, 2000
"... In the asynchronous PRAM model, processes communicate by atomically reading and writing shared memory locations. This paper investigates the extent to which asynchronous PRAM permits longlived, highly concurrent data structures. An implementation of a concurrent object is waitfree if every operati ..."
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Cited by 62 (13 self)
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In the asynchronous PRAM model, processes communicate by atomically reading and writing shared memory locations. This paper investigates the extent to which asynchronous PRAM permits longlived, highly concurrent data structures. An implementation of a concurrent object is waitfree if every operation will complete in a finite number of steps, and it is kbounded waitfree, for some k > 0, if every operation will complete within k steps. In the first part of this paper, we show that there are objects with waitfree implementations but no kbounded waitfree implementations for any k, and that there is an infinite hierarchy of objects with implementations that are kbounded waitfree but not Kbounded waitfree for some K > k. In the second part of the paper, we give an algebraic characterization of a large class of objects that do have waitfree implementations in asynchronous PRAM, as well as a general algorithm for implementing them. Our tools include simple iterative algorithms for waitfree approximate agreement and atomic snapshot.
SelfStabilizing Clock Synchronization in the presence of Byzantine faults
 Journal of the ACM
, 1995
"... We initiate a study of bounded clock synchronization under a more severe fault model than that proposed by Lamport and MelliarSmith [LM85]. Realistic aspects of the problem of synchronizing clocks in the presence of faults are considered. One aspect is that clock synchronization is an ongoing tas ..."
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Cited by 58 (11 self)
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We initiate a study of bounded clock synchronization under a more severe fault model than that proposed by Lamport and MelliarSmith [LM85]. Realistic aspects of the problem of synchronizing clocks in the presence of faults are considered. One aspect is that clock synchronization is an ongoing task, thus the assumption that in any period of the execution at least two thirds of the processors are nonfaulty is too optimistic. To cope with this reality we suggest selfstabilizing protocols that stabilize in any (long enough) period in which less than a third of the processors are faulty. Another aspect is that the clock value is bounded. A single transient fault may cause the clock to reach the upper bound. Therefore we suggest a bounded clock that wraps around when appropriate. We present two randomized selfstabilizing protocols for synchronizing bounded clocks in the presence of Byzantine processor failures. The rst protocol assumes that processors have a common pulse, while the second protocol does not. A new type of distributed counter based on the Chinese remainder theorem is used as part of the rst protocol. 1
Are WaitFree Algorithms Fast?
, 1991
"... The time complexity of waitfree algorithms in "normal" executions, where no failures occur and processes operate at approximately the same speed, is considered. A lower bound of log n on the time complexity of any waitfree algorithm that achieves approximate agreement among n processes i ..."
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Cited by 39 (11 self)
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The time complexity of waitfree algorithms in "normal" executions, where no failures occur and processes operate at approximately the same speed, is considered. A lower bound of log n on the time complexity of any waitfree algorithm that achieves approximate agreement among n processes is proved. In contrast, there exists a nonwaitfree algorithm that solves this problem in constant time. This implies an (log n) time separation between the waitfree and nonwaitfree computation models. On the positive side, we present an O(log n) time waitfree approximate agreement algorithm; the complexity of this algorithm is within a small constant of the lower bound.