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This study presents a consensus region approach to designing distributed consensus protocols for multi-agent systems with general continuous-time linear node dynamics. The consensus region approach has a favourable decoupling feature, which decouples the design of the feedback gain matrices of the consensus protocols from the communication graph. Multi-step algorithms are presented to construct the fixed-gain consensus protocols, which requires the smallest non-zero eigenvalue of the Laplacian matrix. To remove this limitation, distributed adaptive protocols with time-varying coupling weights are designed for the cases with undirected and directed graphs, which can be implemented in a fully distributed fashion. The robustness issue of the adaptive protocols in the presence of external disturbances is also discussed. For the case where there exists a leader of bounded unknown control input, distributed discontinuous and continuous controllers are designed to solve the distributed tracking problem.

Consensus protocols are a fundamental part of any blockchain; although several protocols have been in operation for several years, they still have drawbacks. For instance, some may be susceptible to a 51% attack, also known as a majority attack, which may suppose a high risk to the trustworthiness of the blockchains. Although this attack is theoretically possible, executing it in practice is often regarded as arduous because of the premise that, with sufficiently active members, it is not ’straightforward’ to have much computing power. Since it represents a possible vulnerability, the community has made efforts to solve this and other blockchain problems, which has resulted in the birth of alternative consensus protocols, e.g., the proof of accuracy protocol. This paper presents a detailed proposal of a proof-of-accuracy protocol. It aims to democratize the miners’ participation within a blockchain, control the miners’ computing power, and mitigate the majority attacks.

Two distributed consensus problems, including leadless consensus and tracking consensus, for multiple quadrotors with bounded disturbances under a directed topology are solved in this study. First, the model of a single quadrotor is divided into two subsystems. Some coupling terms between the two subsystems are regarded as external perturbations. For the case of leadless consensus, a distributed sliding-mode controller for the first subsystem and a state-feedback controller for the second subsystem are proposed to guarantee that the consensus is achieved with bounded errors that are only associated with disturbances. For the case of tracking consensus, by using the proposed sliding-mode consensus protocols combining with two sliding-mode observers, it is shown that all quadrotors can track the leader with bounded errors which can be manipulated suitably. Simulations show the effectiveness of the proposed control protocols.

The role of the blockchain technology in decentralizing modern information systems is discussed. A general model of functioning a blockchain system is analyzed and the idea of constructing a new type of consensus protocols (“proof-of-accuracy” protocols) is proposed that, according to the authors opinion, combines advantages of “proof-of-work” and “proof-of-stake” protocols. Ways of implementing a “proof-of-accuracy” protocol are investigated.

Blockchain networks are based on cryptographic notions that include asymmetric-key encryption, hash functions and consensus protocols. Despite their popularity, mainstream protocols, such as Proof of Work or Proof of Stake still have drawbacks. Efforts to enhance these protocols led to the birth of alternative consensus protocols, catering to specific areas, such as medicine or transportation. These protocols remain relatively unknown despite having unique merits worth investigating. Although past reviews have been published on popular blockchain consensus protocols, they do not include most of these lesser-known protocols. Highlighting these alternative consensus protocols contributes toward the advancement of the state of the art, as they have design features that may be useful to academics, blockchain practitioners and researchers. In this paper, we bridge this gap by providing an overview of alternative consensus protocols proposed within the past 3 years. We evaluate their overall performance based on metrics such as throughput, scalability, security, energy consumption, and finality. In our review, we examine the trade-offs that these consensus protocols have made in their attempts to optimize scalability and performance. To the best of our knowledge, this is the first paper that focuses on these alternative protocols, highlighting their unique features that can be used to develop future consensus protocols.

With the increasing growth rate of smart home devices and their interconnectivity via the Internet of Things (IoT), security threats to the communication network have become a concern. This paper proposes a learning engine for a smart home communication network that utilizes blockchain-based secure communication and a cloud-based data evaluation layer to segregate and rank data on the basis of three broad categories of Transactions (T), namely Smart T, Mod T, and Avoid T. The learning engine utilizes a neural network for the training and classification of the categories that helps the blockchain layer with improvisation in the decision-making process. The contributions of this paper include the application of a secure blockchain layer for user authentication and the generation of a ledger for the communication network; the utilization of the cloud-based data evaluation layer; the enhancement of an SI-based algorithm for training; and the utilization of a neural engine for the precise training and classification of categories. The proposed algorithm outperformed the Fused Real-Time Sequential Deep Extreme Learning Machine (RTS-DELM) system, the data fusion technique, and artificial intelligence Internet of Things technology in providing electronic information engineering and analyzing optimization schemes in terms of the computation complexity, false authentication rate, and qualitative parameters with a lower average computation complexity; in addition, it ensures a secure, efficient smart home communication network to enhance the lifestyle of human beings.

In the digital economy era, the development of a distributed robust economy system has become increasingly important. The blockchain technology can be used to build such a system, but current mainstream consensus protocols are vulnerable to attack, making blockchain systems unsustainable. In this paper, we propose a new Robust Proof of Stake (RPoS) consensus protocol, which uses the amount of coins to select miners and limits the maximum value of the coin age to effectively avoid coin age accumulation attack and Nothing-at-Stake (N@S) attack. Under a comparison framework, we show that the RPoS equals or outperforms Proof of Work (PoW) protocol and Proof of Stake (PoS) protocol in three dimensions: energy consumption, robustness, and transaction processing speed. To compare the three consensus protocols in terms of trade efficiency, we built an agent-based model and find that RPoS protocol has greater or similar trade request-satisfied ratio than PoW and PoS. Hence, we suggest that RPoS is very suitable for building a robust digital economy distributed system.

As the core of a blockchain system, the consensus mechanism not only helps to maintain the consistency of node data, but also gets involved in the issuance of tokens and prevention of attacks. Since the first blockchain system was born, it has been continuously improved with the development of blockchain technology and evolved into multiple new branches. Starting with the basic introduction of consensus and the classic Byzantine Generals Problem in distributed computing area, this survey utilizes a thorough classification to explain current consensus protocols in the blockchain system, presents the characteristics of mainstream protocols (PoW, PoS, DPoS, PBFT, etc.) and analyzes the strengths and weaknesses of them. Then we evaluate the performance qualitatively and quantitatively. In the end, we highlight several research directions for developing more practical consensus protocols for the future.

Stable-protocol (SP) output consensus analysis and design problems for high-order linear time-invariant swarm systems with time-varying delays are investigated. First, a dynamic output feedback consensus protocol is proposed on the basis of the observable decomposition, and a necessary and sufficient condition for SP output consensus is given, which transforms the SP output consensus problem into asymptotical stability problems of multiple lower-dimensional subsystems. Then, in terms of linear matrix inequalities (LMIs), a sufficient condition for SP output consensualisation is presented, which can guarantee the scalability of swarm systems since it includes only five LMI constrains independent of the number of agents. Furthermore, an explicit expression of the output consensus function, which is independent of the time-varying delay, is shown and the impacts of initial states of agents and consensus protocols on the output consensus function are determined, respectively. Finally, a numerical example is given to demonstrate theoretical results.

Consensus algorithms are applied in the context of distributed computer systems to improve their fault tolerance. The explosive development of distributed ledger technology following the proposal of ‘Bitcoin’ led to a sharp increase in research activity in this area. Specifically, public and permissionless networks require robust leader selection strategies resistant to Sybil attacks in which malicious attackers present bogus identities to induce byzantine faults. Our goal is to analyse the entire breadth of works in this area systematically, thereby uncovering trends and research directions regarding Sybil attack resistance in today’s blockchain systems to benefit the designs of the future. Through a systematic literature review, we condense an immense set of research records (N = 21,799) to a relevant subset (N = 483). We categorise these mechanisms by their Sybil attack resistance characteristics, leader selection methodology, and incentive scheme. Mechanisms with strong Sybil attack resistance commonly adopt the principles underlying ‘Proof-of-Work’ or ‘Proof-of-Stake’ while mechanisms with limited resistance often use reputation systems or physical world linking. We find that only a few fundamental paradigms exist that can resist Sybil attacks in a permissionless setting but discover numerous innovative mechanisms that can deliver weaker protection in system scenarios with smaller attack surfaces.