A new solution for the Byzantine agreement problem

Reliability is an important research topic in distributed computing systems consisting of a large number of processors. To achieve reliability, the fault-tolerance scheme of the distributed computing system must be revised. This kind of problem is known as a Byzantine agreement (BA) problem. It requires all fault-free processors to agree on a common value, even if some components are corrupt. Consequently, there have been significant studies of this agreement problem in distributed systems. However, the traditional BA protocols focus on running ⌊(n−1)/3⌋+1 rounds of message exchange continuously to make each fault-free processor reach an agreement. In other words, since having a large number of messages results in a large protocol overhead, those protocols are inefficient and unreasonable, especially for some network environments which have large number of processors. In this study, we propose a novel and efficient protocol to reduce the number of messages. Our protocol can collect, compare and replace the received values to find the reliable processors and replace the values sent by the unreliable processors. Subsequently, each processor can agree on a common value through three rounds of message exchange. Furthermore, the proposed protocol can use the minimum number of messages to tolerate the maximum number of faulty components in a distributed system.     

An optimal solution for byzantine agreement under a hierarchical cluster-oriented mobile ad hoc network

Mobile ad hoc NETworks (MANETs) are becoming more popular due to the advantage that they do not require any fixed infrastructure, and that communication among processors can be established quickly. For this reason, potential MANET applications include military uses, search and rescue and meetings or conferences. Therefore, the fault-tolerance and reliability of the MANET is an important issue, which needs to be considered. The problem of reaching agreement in the distributed system is one of the most important areas of research to design a fault-tolerant system. With an agreement, each correct processor can cope with the influence from other faulty components in the network to provide a reliable solution. In this research, a potential MANET with a dual failure mode is considered. The proposed protocol can use the minimum number of rounds of message exchange to reach a common agreement and can tolerate a maximum number of allowable faulty components to induce all correct processors to reach a common agreement within the MANET.     

Reaching Agreement among Virtual Subnets in Hybrid Failure Mode

Fault-tolerance is an important research topic in the study of distributed systems. To cope with the influence of faulty components, reaching a common agreement in the presence of faults before performing certain tasks is essential. However, the Byzantine Agreement (BA) problem is a fundamental problem in fault-tolerant distributed systems. In previous studies, protocols dealing with the BA problem focused on static networks; however, these do not perform well in dynamically changing mobile networks. The most well known mobile network is the Mobile Ad-hoc Network (MANET). To enhance fault-tolerance and MANET reliability, the BA problem in virtual subnets of MANET is revisited in this paper. The proposed protocol is called the Hybrid Agreement Protocol (HAP). It achieves agreement on a common value among all functional mobile processors in a minimal number of message exchange rounds, and can tolerate a maximal number of allowable faulty components in the virtual subnet of MANET.     

The incremental agreement

To achieve reliable distributed systems, the fault-tolerance must be studied. One of the most important problems of fault-tolerance issues lies in the Byzantine Agreement (BA) problem. The primary issue surrounding BA is that fault-free processors must obtain common agreement even in cases where faults persist. In this field, the fault diagnosis protocol has been proposed so that each fault-free processor detects/locates a common set of faulty processors. However, in this study, the incremental agreement is invoked to make each processor able to agreement upon executing the fault diagnosis protocol using minimal rounds of message exchange in the presence of dual failure characteristics of processors.     

Byzantine agreement in the presence of mixed faults on processorsand links

In early stage, the Byzantine agreement (BA) problem was studied with single faults on processors in either a fully connected network or a nonfully connected network. Subsequently, the single fault assumption was extended to mixed faults (also referred to as hybrid fault model) on processors. For the case of both processor and link failures, the problem has been examined in a fully connected network with a single faulty type, namely an arbitrary fault. To release the limitations of a fully connected network and a single faulty type, the problem is reconsidered in a general network. The processors and links in such a network can both be subjected to different types of fault simultaneously. The proposed protocol uses the minimum number of message exchanges and can tolerate the maximum number of allowable faulty components to make each fault-free processor reach an agreement     

From immediate agreement to eventual agreement: early stopping agreement protocol for dynamic networks with malicious faulty processors

With the rapid advancement of wireless networking technology, networks have evolved from static to dynamic. Reliability of dynamic networks has virtually become an important issue. Fortunately, a solution to the above issue can be derived from solutions to the Byzantine Agreement (BA) problem. BA problem can be solved by protocols that make processors reach an agreement through message exchange. Protocols used to solve the problem can be divided into Immediate Byzantine Agreement (IBA) protocols and Eventual Byzantine Agreement (EBA) protocols. In IBA protocols, the number of rounds of message exchange is determined by the total number of processors in the network. Even if no faulty processor is present in the network, IBA protocols still require a fixed number of rounds of message exchange, causing a waste of time. In contrast, EBA protocols dynamically adjust the number of rounds of message exchange according to the interference of faulty processors. In terms of efficiency, EBA protocols certainly outperform IBA protocols. Due to the fact that the existing EBA protocols have been designed for static networks, they cannot work on dynamic networks. In this paper, we revisit the EBA problem in dynamic networks to increase the reliability of dynamic networks. Simulations will be conducted to validate that the proposed protocol requires the minimum rounds of message exchange and can tolerate the maximum number of malicious faulty processors compared to other existing protocols.     

Byzantine Agreement in a generalized connected network

Traditionally, the Byzantine Agreement (BA) problem is studied either in a fully connected network or in a broadcast network. A generalized network model for BA is proposed in this paper. A fully-connected network or a broadcast network is a special case of the new network architecture. Under the new generalized network model, the BA problem is reexamined with the assumption of malicious faults on both processors and transmission medium (TM), as opposed to previous studies which consider malicious faults on processors only. The proposed algorithm uses the minimum number of message exchanges, and can tolerate the maximum number of allowable faulty components to make each healthy processor reach a common agreement for the cases of processor failures, TM failures, or processor/TM failures. The results can also be used to solve the interactive consistency problem and the consensus problem     

Grouping Byzantine Agreement

The reliability of the distributed system has always been an important topic of research. Byzantine Agreement (BA) protocol, which allows the fault-free processors to agree on a common value, is one of the most fundamental problems studied in a distributed system. In previous works, the problem was visited in a fully connected network or an unfully connected network with fallible processors. In this paper, the BA problem is reexamined in a group-oriented network, which has the feature of grouping, and the network topology does not have to be fully connected. We also enlarge the fault tolerant capability by allowing dormant faults and malicious faults (also called as the dual failure mode) to exist in a group-oriented network simultaneously. The proposed protocol is more efficient than the traditional BA protocols and can tolerate the maximum number of tolerable faulty processors.     

Visiting Byzantine Agreement underlying Ad Hoc environment

The Mobile Ad Hoc Network (MANET) has become more popular because the MANET is a self-organizing, self-configuring, and an instantly deployable multi-hop wireless network that responds to application needs without any fixed infrastructure. Moreover, the MANET is fault-tolerant and reliable. A mechanism is needed in the MANET that allows a set of nodes to agree on a common value. The distributed Byzantine Agreement (BA) problem is one of the most important issues in designing a fault-tolerant system. In many cases, reaching a common agreement among fault-free nodes in coping with the influence from faulty components is crucial in a fault-tolerant system. When a common agreement is achieved, all fault-free nodes in the system can produce stable results without any influence from the faulty components. In this study, the BA problem is visited in a MANET, in which the components are subject to a malicious fault. The proposed protocol can tolerate the maximum number of allowable faulty nodes using a minimum number of message exchange rounds. Each fault-free node can reach a common agreement value for the BA problem in a MANET. The text was submitted by the authors in English.     

Message-optimal protocols for Byzantine Agreement

It is often important for the correct processes in a distributed system to reach agreement, despite the presence of some faulty processes. Byzantine Agreement (BA) is a paradigm problem that attempts to isolate the key features of reaching agreement. We focus here on the number of messages required to reach BA, with particular emphasis on the number of messages required in thefailure-free runs, since these are the ones that occur most often in practice. The number of messages required is sensitive to the types of failures considered. In earlier work, Amduret al. (1992) established tight upper and lower bounds on the worst- and average-case number of messages required in failure-free runs for crash failures. We provide tight upper and lower bounds for all remaining types of failures that have been considered in the literature on the BA problem: receiving omission, sending omission, and general omission failures, as well as arbitrary failures with or without message authentication. We also establish a tradeoff between number of rounds and number of messages in the failure-free runs required to reach agreement in the case of crash, sending, and general omission failures.The work of V. Hadzilacos was supported, in part, by a grant from the Natural Sciences and Engineering Research Council of Canada.     

Reliable synchronization in distributed systems

In distributed computer systems, processors often need to be synchronized to maintain correctness and consistency. Unlike shared-memory parallel systems, the lack of shared memory and a clock considerably complicates the task of synchronization in distributed systems. The objective of this article is two-fold: (1) We present a new randomized agreement algorithm to synchronize cooperating processors in a distributed system. This algorithm achieves the desired agreement in expected five rounds of message exchanges, tolerating a maximum of one-fifth of the processors failures. The algorithm belongs to the class of broadcast-based synchronization problems. (2) We present a new self-stabilization algorithm for an acyclic directed-graph structured distributed systems. This new fault-tolerant algorithm survives all imaginable faults in distributed systems. The algorithm belongs to arbiter-based and broadcast-based synchronization problems.     

How to cope with faulty processors in a completely connected network of communicating processors

Computer systems consisting of a completely connected network of communicating processors should work reliable in spite of a number of malfunctioning processors that give conflicting information to different members of the system. It is necessary that all wellfunctioning processors agree on the message sent by any transmitting processor, regardless of the behaviour of the malfunctioning processors. Moreover, it is required that whenever the transmitter is wellfunctioning, the agreement reached should be equal to the message actually sent by the transmitter. In this paper it is shown by a mathematical proof that this is possible if and only if the number of malfunctioning processors is less than one-third of the total number of processors in the network. In cases where this requirement is fulfilled, we give a clear algorithm to reach agreement.     

A self-stabilizing ring orientation algorithm with a smaller numberof processor states

A distributed system is said to be self-stabilizing if it will eventually reach a legitimate system state regardless of its initial state. Because of this property, a self-stabilizing system is extremely robust against failures; it tolerates any finite number of transient failures. The ring orientation problem for a ring is the problem of all the processors agreeing on a common ring direction. This paper focuses on the problem of designing a deterministic self-stabilizing ring orientation system with a small number of processor states under the distributed daemon. Because of the impossibility of symmetry breaking, under the distributed daemon, no such systems exist when the number n of processors is even. Provided that n is odd, the best known upper bound on the number of states is 256 in the link-register model, and eight in the state-reading model. We improve the bound down to 6³=216 in the link-register model     

The optimal generalized Byzantine Agreement in Cluster-based Wireless Sensor Networks

A Wireless Sensor Network (WSN) is a wireless network consisting of spatially distributed autonomous devices using sensor nodes in a wide range of applications in various domains. In the future, WSNs are expected to be integrated into the “Internet of Things” (IoT), where sensor nodes join the Internet dynamically, and use them to collaborate and accomplish their tasks. Because of the communications of WSN will produce a broadcast storm, the Cluster-based Wireless Sensor Network (CWSN) was proposed to ameliorate the broadcast storm. However, the capability of the fault-tolerance and reliability of CWSNs must be carefully investigated and analyzed. To cope with the influence of faulty components, reaching a common agreement in the presence of faults before performing certain tasks is essential. Byzantine Agreement (BA) problem is a fundamental problem in fault-tolerant distributed systems. To enhance fault-tolerance and reliability of CWSN, the BA problem in CWSN is revisited in this paper. In this paper, a new BA protocol is proposed that adapts to the CWSN and derives its limit of allowable faulty components, while maintaining the minimum number of message exchanges.     

非完全互连同构系统上的静态任务调度

在分布式内存多处理机DMM(distributed memory multiprocessor)系统中,不同处理机上运行的任务之间的通信开销仍然很大,有时甚至抵消了多处理机并行所带来的好处.为了使并行程序在DMM系统上能得以高效的执行,必须采用合理的调度技术将任务分配给处理机.文章首先分别给出了任务调度系统中的任务模型、处理机模型以及调度问题的形式化描述,然后在此基础上研究了任务调度中3个最重要的问题,即(1) 如何顺序选择参与调度的任务,(2) 如何选择路由,(3) 如何分配任务给处理机.其中,路由选择     

Byzantine Agreement under dual failure mobile network

Networks are trending towards wireless systems that provide support for mobile computing. The Byzantine Agreement (BA) protocols used in static networks do not perform well in a dynamically changing mobile environment. Mobile commerce and related applications are necessary for wireless networks. There are numerous properties in a wireless network that play important roles. For example, the processors in a wireless network have highly mobile capabilities. Processors can immigrate into or move away from the network at any time. Although mobile technology has brought greater convenience, it is comparatively more dangerous. Wireless systems are susceptible to security flaws such as attacks by hackers. The number of allowable faulty components within the system is also decreased. To increase the number of allowable faulty components and ensure network security, a simple, secure and efficient protocol, BAM, is proposed to handle the BA problem. The fault symptoms include malicious and dormant faults. Furthermore, the proposed protocol uses the minimum number of message exchange rounds to make all healthy processors agree on a common value and can tolerate the maximum number of allowable faulty components. The proposed method will also ensure message security and increase the system's fault tolerant capability.     

An efficient and non-interactive hierarchical key agreement protocol

The non-interactive identity-based key agreement schemes are believed to be applicable to mobile ad-hoc networks (MANETs) that have a hierarchical structure such as hierarchical military MANETs. It was observed by Gennaro et al. (2008) that there is still an open problem on the security of the existing schemes, i.e., how to achieve the desirable security against corrupted nodes in the higher levels of a hierarchy? In this paper, we propose a novel and very efficient non-interactive hierarchical identity-based key agreement scheme that solves the open problem and outperforms all existing schemes in terms of computational efficiency and data storage.     

On exploiting task duplication in parallel program scheduling

One of the main obstacles in obtaining high performance from message-passing multicomputer systems is the inevitable communication overhead which is incurred when tasks executing on different processors exchange data. Given a task graph, duplication-based scheduling can mitigate this overhead by allocating some of the tasks redundantly on more than one processor. In this paper, we focus on the problem of using duplication in static scheduling of task graphs on parallel and distributed systems. We discuss five previously proposed algorithms and examine their merits and demerits. We describe some of the essential principles for exploiting duplication in a more useful manner and, based on these principles, propose an algorithm which outperforms the previous algorithms. The proposed algorithm generates optimal solutions for a number of task graphs. The algorithm assumes an unbounded number of processors. For scheduling on a bounded number of processors, we propose a second algorithm which controls the degree of duplication according to the number of available processors. The proposed algorithms are analytically and experimentally evaluated and are also compared with the previous algorithms     

Broadcasting multiple messages in the multiport model

We consider the problem of broadcasting multiple messages from one processor to many processors in the k-port model for message-passing systems. In such systems, processors communicate in rounds, where in every round, each processor can send k messages to k processors and receive k messages from k processors In this paper, we first present a simple and practical algorithm based on variations of k complete k-ary trees. We then present an optimal algorithm up to an additive term of one for this problem for any number of processors, any number of messages, and any value for k     

A note on consensus on dual failure modes

F.J. Meyer and D.K. Pradhan (1991) proposed the MS (for “mixed-sum”) algorithm to solve the Byzantine Agreement (BA) problem with dual failure modes: arbitrary faults (Byzantine faults) and dormant faults (essentially omission faults and timing faults). Our study indicates that this algorithm uses an inappropriate method to eliminate the effects of dormant faults and that the bound on the number of allowable faulty processors is overestimated. This paper corrects the algorithm and gives a new bound for the allowable faulty processors