Fault-tolerant Consensus Protocol Research Articles

Liveness Attacks On HotStuff: The Vulnerability Of Timer Doubling Mechanism

Abstract Byzantine fault-tolerant (BFT) consensus protocols are essential in distributed computing. Most partially synchronous BFT protocols proceed in views and rely on a view synchronizer module to guarantee liveness by synchronizing honest replicas to the same view. HotStuff is a leading BFT consensus protocol known for achieving linear view change and optimistic responsiveness. To achieve these desirable properties, HotStuff relies on a candidate solution for the view synchronizer based on a recomposed timer doubling mechanism. However, a formal analysis of this mechanism is currently lacking. This paper delves into HotStuff with the recomposed timer doubling mechanism. To facilitate accurate analysis, we introduce a new specification for the view synchronizer, incorporating two paths for view switching as in HotStuff’s setting. Surprisingly, we observe that the adversary can disrupt the view synchronization and launch a liveness attack, stalling the confirmation process. Besides, the adversary can further recover or control the confirmation process at will. A repairment that retains the desirable feature of HotStuff is also presented. We simulate the liveness attack and the repairment, demonstrating their effectiveness. Specifically, the liveness attack can cause HotStuff’s throughput to drop and remain at 0. When equipped with our repairment, HotStuff can resist the attack and retain the throughput performance.

Proportional integral observer‐based robust fault‐tolerant consensus for nonlinear two‐time‐scale‐agent systems with heterogeneous sensor faults

AbstractThis article studies the fault‐tolerant consensus issue of a class of nonlinear two‐time‐scale‐agent systems with heterogeneous sensor faults and external disturbances. First, the decentralized proportional integral (PI) observer for each agent is constructed to estimate the states and sensor faults simultaneously. Then, a distributed adaptive protocol based on the decentralized PI observer is designed to guarantee the consensus of the underlying systems. Some sufficient existence conditions are given in terms of linear matrix inequalities (LMIs) for the decentralized PI observer and the fault tolerant consensus protocol. Note that performance index is employed to attenuate the influence of sensor faults and disturbances on state estimation and the consensus of the underlying systems. Finally, the effectiveness and superiority of the proposed method are demonstrated by an application example and a numerical example.

Analytical model for performability evaluation of Practical Byzantine Fault-Tolerant systems

Designing systems tolerant to faults is crucial to assure continuity of service for mission critical applications. However, their implementation may be costly and challenging. In this study, analytical models are presented for performance evaluation of systems equipped with Practical Byzantine Fault-Tolerant consensus protocols. Byzantine Fault Tolerance is particularly compelling, since it can provide a robust consensus mechanism to implement decentralized platforms, like Decentralized Ledger Technology and, notably, blockchains. The performability model is based on continuous-time Markov chains, in which the processes involved follow the exponential distribution. The numerical results presented report an inverse non-linear relation between number of nodes and performability. Performance decreases also as the ratio between break-down rate and repair rate increases.

Fully Distributed Fault-Tolerant Leaderless Consensus of Multi-Agent Systems Under Directed Communication Graph

This paper investigates a fault-tolerant leaderless consensus problem for general linear multi-agent systems (MASs) under a strongly connected directed communication graph. The influences of the actuator bias faults and loss of actuator effectiveness faults on MASs are considered in this paper. The designed fully distributed adaptive fault-tolerant consensus protocols (FTCPs) can make all agents' states achieve consensus, asymptotically. Using the features of the loss of actuator effectiveness faults and the strongly connected graph, novel positive definite Lyapunov functions are constructed skillfully to analyse the validity of the proposed protocols. Finally, simulation examples are presented to demonstrate the effectiveness of the proposed protocols.

Adaptive fixed-time consensus for stochastic multi-agent systems with uncertain actuator faults

In this paper, we study the adaptive fixed-time consensus control for stochastic multi-agent systems (SMASs) with uncertain actuator faults. Firstly, a fully distributed adaptive consensus protocol and an adaptive fault-tolerant consensus protocol are proposed, respectively, to ensure that the fixed-time consensus of SMASs with actuator faults can be reached. Secondly, an adaptive fault-tolerant containment consensus protocol is further proposed for the SMASs by leveraging the signum function, and this protocol can effectively solve the containment consensus in the unbalanced communication network. Finally, some simulation examples are given to verify the effectiveness of our consensus protocols.

Votes-as-a-Proof (VaaP): Permissioned Blockchain Consensus Protocol Made Simple

With the development of Blockchain technology, permissioned Blockchains are getting more and more attention from researchers because applications based on permissioned Blockchains are more practical and easier to be carried out. This paper aims to design a dedicated consensus protocol for permissioned Blockchains. The existing consensus protocols applied to permissioned Blockchains are either derived from public Blockchains such as Proof of Work (PoW) or Proof of Stake (PoS), with full decentralization, resulting in low transaction processing efficiency; or derived from traditional Byzantine fault-tolerant (BFT) consensus protocols such as Practical BFT (PBFT) or HoneyBadgerBFT, with high communication complexity of the consensus process, resulting in low scalability. Therefore, we propose a dedicated consensus protocol for permissioned Blockchains called Votes-as-a-Proof (VaaP) with high transaction processing efficiency while ensuring high scalability. Every node in VaaP runs a simple consensus process based on voting in parallel. Faulty nodes will only deprive themselves of using consensus service. We present the comparison of VaaP and Sphinx, one of the state-of-the-art consensus protocols, analytically and experimentally (up to 500 nodes). The results indicate that VaaP outperforms Sphinx in throughput, latency and scalability.

A Consensus Algorithm Based on Risk Assessment Model for Permissioned Blockchain

Blockchain is characterized by privacy, traceability, and security features as a novel framework of distributed ledger technologies. Blockchain technology enables stakeholders to conduct trusted data sharing and exchange without a trusted centralized institution. These features make blockchain applications attractive to enhance trustworthiness in very different contexts. Due to unique design concepts and outstanding performance, blockchain has become a popular research topic in industry and academia in recent years. Every participant is anonymous in a permissionless blockchain represented by cryptocurrency applications such as Bitcoin. In this situation, some special incentive mechanisms are applied to the permissionless blockchain, such as “mined” native cryptocurrency to solve the trust issues of the permissionless blockchain. In many use cases, permissionless blockchain has bottlenecks in transaction throughput performance, which restricts further application in the real world. A permissioned blockchain can reach a consensus among a group of entities that do not establish an entire trust relationship. Unlike permissionless blockchains, the participants must be identified in permissioned blockchains. By relying on the traditional crash fault-tolerant consensus protocols, permissioned blockchains can achieve high transaction throughput and low latency without sacrificing security. However, how to balance the security and consensus efficiency is still the issue that needs to be solved urgently in permissioned blockchains. As the core module of blockchain technology, the consensus algorithm plays a vital role in the performance of the blockchain system. Thus, this paper proposes a new consensus algorithm for permissioned blockchain, the Risk Assessment-based Consensus (RAC) protocol, combined with the decentralized design concept and the risk-node assessment mechanism to address the unbalance issues of performance in speed, scalability, and security.

Recent progress on BFT in the era of blockchains.

This perspective highlights some recent progress on the research of Byzantine fault tolerant (BFT) consensus protocol in the era of blockchains, including both partially synchronous BFT and asynchronous BFT protocols, their fundamental building blocks, and their variants.

A consensus protocol for unmanned aerial vehicle networks in the presence of Byzantine faults

This paper discusses the fault-tolerant consensus problem in Unmanned Aerial Vehicle Networks (UAVNets). In recent years, the applications of UAVNets have become more and more popular. Related applications include aerial photography, geological and topographic surveys, disaster monitoring, military applications, and so on. Therefore, it is very important to build a highly reliable and fault-tolerant UAVNets. However, the network architecture of UAVNets is very different from previous network architectures. Because UAVs fly at a high speed, the topology also varies quickly. Hence, how to collect sufficient messages to reach a consensus on the network for UAVs is a challenge. In this paper, the characteristics of the distributed UAVNets will be explored first. Then, based on the characteristics of the UAVNets, a new fault-tolerant consensus protocol called the UAV Consensus Protocol (UCP) is proposed. The proposed UCP consists of two phases: the message exchanging phase and the consensus making phase. The proposed UCP can solve the consensus problem with ⌞(nO-1)/3⌟+1 rounds of message exchange in the presence of ⌞(nO-1-aO)/3⌟ Byzantine faulty UAVs and aO away UAVs, where nO is the number of AUVs in the UAVNet. Moreover, the correctness of the proposed UCP is also proved in this paper.

SAZyzz: Scaling AZyzzyva to Meet Blockchain Requirements

We present SAZyzz, a leader-based Byzantine Fault Tolerant consensus protocol for partially synchronous networks. SAZyzz exhibits a better performance/scalability compared to the state-of-the-art leader-based BFT consensus protocols. It is built on top of AZyzzyva and has adopted a tree-based communication model which enables it to enhance the scalability of AZyzzyva. Additionally, SAZyzz reduces the communication complexity to <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</i> (log <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> ) in two paths of the protocol. However, the tree-based topology has been argued that has a shortcoming when used in designing BFT consensus protocols. This refers to the strong assumption that all the internal nodes of the tree are honest, which leads to a trade-off between tolerating Byzantine faults and better performance and scalability. This paper shows that, with the current technological infrastructures available for industrial systems, such as Trusted Execution Environment (TEE) and Public Key Infrastructure (PKI), this assumption is realistic. SAZyzz comprises of fast-path and backup-path, each of which has two modes: <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">simple mode</i> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">scalable mode</i> . To demonstrate the efficiency and feasibility of SAZyzz's adoption for blockchain systems, we designed and implemented the ZyConChain blockchain system based on SAZyzz. The evaluation results show that SAZyzz can significantly improve the performance/scalability of blockchain systems.

Consensus Sliding-Mode Fault-Tolerant Control for Second-Order Multi-Agent Systems

This study investigates the consensus problem of second-order nonlinear multi-agent systems (MASs) with actuator faults via a sliding mode control approach. The consensus error dynamic is given based on the relative states of the neighbors. Then, a sliding mode surface based on consensus errors is proposed, and the asymptotic stability of the sliding mode is proved using the Lyapunov theory. Furthermore, a sliding-mode fault-tolerant consensus protocol is proposed to compensate for actuator faults. According to the sliding mode control theory, the proposed sliding-mode fault-tolerant controller ensures that the consensus of the MASs can be reached in a finite time. Finally, a simulation example of a second-order multi-robot system is presented to demonstrate the effectiveness of the proposed controller.

Reaching Consensus with Byzantine Faulty Controllers in Software-Defined Networks

The combination of the Internet of Things (IoT) and Cloud computing are both popular applications that are based on the Internet. However, the traditional networking structure can no longer support the transmission of the huge number of packets required by cloud computing and IoT. Therefore, a new-generation architecture, software-defined networking (SDN), came into being. The new-generation SDN can control routing through software, making flexible and convenient management a key feature of SDN. However, attacks and threats are prevalent in today’s networking environment. When an SDN controller encounters a hacker attack or virus infection, it will not function properly. Hence, we need to design a fault-tolerant mechanism for the above environment. In this paper, a fault-tolerant consensus protocol is proposed to improve the fault tolerance of SDNs with multiple controllers.

Concordia: A Streamlined Consensus Protocol for Blockchain Networks

In this paper, we present a novel Byzantine fault-tolerant consensus protocol for sharded blockchain networks that does not rely on expensive leader-driven communication. The proposed protocol selects a single block proposer at a time and uses threshold signatures as a voting mechanism to confirm the validity of the proposed block. By using a gossip-like communication scheme, each node can collect and recover the group signature within <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathrm {O(log}N)$ </tex-math></inline-formula> steps. With only one block proposer per consensus round, there is no possibility of conflicting blocks and resultant forks. Therefore, our consensus protocol requires only one round of one-way communication to achieve finality for each block. Our protocol guarantees safety and liveness while tolerating up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f$ </tex-math></inline-formula> faulty participants among <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2f+1$ </tex-math></inline-formula> nodes. Our performance study shows that the proposed protocol enables hundreds of nodes to participate in the agreement process, and can finalize large blocks in approximately 10 seconds.

A Blockchain-enabled decentralized settlement model for IoT data exchange services

IoT data exchange services are emerging to connect various and distributed IoT data sources, which facilitate data owners exchange their IoT data flexibly. Traditional IoT data exchange services rely on a centralized third-party to negotiate the settlement of trades between data consumers and providers. Such settlement model suffers from issues including single point of failure, extra settlement fee, un-transparency of settlement details, to name a few. New settlement model is imperative to overcome such limitations. Blockchain is an innovative technology that is competent in governing the decentralized network, which poses great opportunity to implement fair settlement in decentralized manner. In this paper, we propose a Blockchain-enabled settlement model for decentralized IoT data exchange services. Firstly, Bitcoin-based time commitment scheme is adopted to build fair and autonomous settlement model. Furthermore, to enhance the accountability of all transactions, an optimized practical Byzantine fault tolerant consensus protocol named ReBFT, is proposed to enable all members involved in the IoT data exchange application achieve identical shared ledger recording all transactions. Finally, experiments are conducted to verify the feasibility of our proposal.

ResilientDB

Recent developments in blockchain technology have inspired innovative new designs in resilient distributed and database systems. At their core, these blockchain applications typically use Byzantine fault-tolerant consensus protocols to maintain a common state across all replicas, even if some replicas are faulty or malicious. Unfortunately, existing consensus protocols are not designed to deal with geo-scale deployments in which many replicas spread across a geographically large area participate in consensus. To address this, we present the Geo-Scale Byzantine Fault-Tolerant consensus protocol (GeoBFT). GeoBFT is designed for excellent scalability by using a topological-aware grouping of replicas in local clusters, by introducing parallelization of consensus at the local level, and by minimizing communication between clusters. To validate our vision of high-performance geo-scale resilient distributed systems, we implement GeoBFT in our efficient ResilientDB permissioned blockchain fabric. We show that GeoBFT is not only sound and provides great scalability, but also outperforms state-of-the-art consensus protocols by a factor of six in geo-scale deployments.

A Decentralized Context Broker Using Byzantine Fault Tolerant Consensus

A context broker is a reliable message-relaying service used to connect devices by integrating all device protocols and communication methods, and reliably transporting messages while isolating data from other application service layers and networking complexities. A highly scalable decentralized context broker stack is composed of three layers—starting with a peer-to-peer network connecting a byzantine fault-tolerant (i.e., blockchain-based) consensus protocol—and it manages the communication using a web-socket streaming protocol as interface to other applications. This paper presents such a concept for a decentralized context broker stack for intercommunication between heterogeneous materials handling systems, and deploys the stack as proof-of-concept using ROS-based robots in a logistics scenario.

Adaptive Fault-Tolerant Consensus Protocols for Multiagent Systems With Directed Graphs.

This paper investigates the problem of adaptive fault-tolerant tracking control for the multiagent systems (MASs) under the time-varying actuator faults and bounded unknown control input of the leader. On the basis of the local state information of neighboring agents, an adaptive fault-tolerant control protocol, which consists of the adaptive estimation of faults, is constructed to compensate for the loss of actuator effectiveness in the leader-follower consensus of MASs. Moreover, the modification term in the adaptive estimation can avoid high-frequency oscillations. It is shown that the tracking errors converge to a neighborhood around the origin in the presence of actuator faults, and the performance of the tracking problem is improved. Furthermore, the protocol is distributed in the sense that the coupling gains are independent. Finally, two examples are given to show the effectiveness of the proposed control protocol.

Distributed Fault-Tolerant Consensus Protocol for Fuzzy Multi-Agent Systems

A distributed actuator fault-tolerant strategy for fuzzy multi-agent systems is investigated in this paper. The proposed strategy utilizes an adaptive methodology in designing the consensus control. Compared with well-known consensus analysis, this paper devotes to the fault effect in analyzing multi-agent consensus problem. We conducted a Lyapunov stability analysis to unfold the advantages of proposed method in the distributed fault-tolerant control. Simulation example is provided to demonstrate the effectiveness of the theoretical results.

EventML: Specification, verification, and implementation of crash-tolerant state machine replication systems

Abstract Distributed programs are known to be extremely difficult to implement, test, verify, and maintain. This is due in part to the large number of possible unforeseen interactions among components, and to the difficulty of precisely specifying what the programs should accomplish in a formal language that is intuitively clear to the programmers. We discuss here a methodology that has proven itself in building a state of the art implementation of Multi-Paxos and other distributed protocols used in a deployed database system. This article focuses on the logical foundations as well as the basic ideas of formal EventML programming, illustrated by implementing a fault-tolerant consensus protocol and showing how we prove its safety properties with the Nuprl proof assistant.

Adaptive finite-time fault-tolerant consensus protocols for multiple mechanical systems

This paper investigates the problem of finite-time fault-tolerant consensus protocols for a class of uncertain multiple mechanical systems. On the basis of the recursive design method and finite-time control theory, distributed consensus protocols and adaptive laws are developed by using locally available information. Fuzzy logic systems are employed to approximate the unknown functions. It is proved that if the designed parameters and functions in the protocols and adaptive laws are suitably chosen, the position errors and the velocity errors between any two mechanical systems will converge to a small neighborhood of zero in finite time. Finally, an example is given to demonstrate the effectiveness of the proposed method.