Blockchain is an emerging decentralized and distributed technology. Along with the beneficial features of decentralization, transparency, and security the consensus algorithms of blockchains form key building blocks for this technology. Consensus protocol/algorithm helps to provide a decentralized decision making process. An efficient consensus algorithm is inclusive that engages all the participants to make their decision based on the conflicts of the blockchain networks. These consensus decisions lead to better quality outcomes of the blockchains and help to obtain the finality. Rigorous research is in process to upgrade or optimize the existing consensus protocols. The optimized or enhanced consensus protocols objectify to be suitable for Internet-of-Thing (IoT) as the current versions of the protocols are not suitable for the resource-constrained environments due their complexity, hard configurations, mining techniques, high resource consumption, and explicit security loophole. In this paper, we present a survey of consensus protocols with a purpose to identify and discuss the existence of various consensus protocols available in literature. We emphasize on the genesis of the consensus protocols, particularly for Proof-of-X, byzantine fault tolerance, Paxos, and RAFT; we also include Directed Acyclic Graph (DAG) orientation of some contemporary algorithms. We discuss the variants of these genesis protocols. Our survey analyzes the advantages, disadvantages, and their applicability in IoTs. We enlist the categorical use of consensus algorithms in blockchains and other applications. Finally, we present several research trends and open issues emphasizing for consensus protocols emphasizing on IoTs. Compared to the other surveys in the field, our present survey objectifies to provide a more thorough summary of the most relevant protocols and application issues; this survey helps the researchers and the application developers to obtain an insight on the current status of the consensus protocols’ suitability to deliver the desired functionalities in IoTs. The notified disadvantages of each of the protocol provide future scope for the industries and academia. To the best of our knowledge, such a comprehensive and summarized survey of consensus protocols including DAG-based protocols is unavailable in the literature and thus, our contribution claims are significant.

The properties of decentralization, security, and immutability have made the blockchain technology beneficial for various Internet of Things (IoT) applications. Consensus algorithms enable the peers in IoT-based networks to reach a common agreement on the current state of the network stored in a distributed ledger. The consensus protocols such as Proof of Work (PoW) and Proof of Stake (PoS) face centrality failure problem in terms of high concentration of computational power and the amount of tokens respectively. Thus, the scope exists to enhance the existing consensus protocols and address the centralization problem maintaining low complexity and resource consumption. We propose a novel consensus protocol based on Proof of Work (PoW) and Proof of Stake (PoS). We name our proposed consensus protocol as Proof of SCore (PoSC). PoSC provides fairness in decentralized network by calculating the combination score from all the nodes. This combination score relates to the stake and reputation score. This score helps the IoT nodes to maintain trust and ensure security. We have also performed the experimental and logical interpretation of PoSC. We compare the performance of PoSC with the existing consensus protocols including PoW, PoS, Byzantine Fault Tolerance, Raft, and Paxos. Our proposed PoSC protocol is efficient in terms of reduced memory consumption by 35%, reduced time consumption by 30%, and energy consumption reduction by 25% as compared to the mentioned existing protocols on average. Moreover, PoSC avoids centrality failure suffered by PoW and PoS. Thus, PoSC is suitable for resource constrained applications of IoTs.