A Survey on Consensus Mechanisms and Mining Strategy Management in Blockchain Networks <s> VI. RELAXED AND PARALLEL CONSENSUS PROTOCOLS FOR PERFORMANCE SCALABILITY <s> The Bitcoin system only provides eventual consistency. For everyday life, the time to confirm a Bitcoin transaction is prohibitively slow. In this paper we propose a new system, built on the Bitcoin blockchain, which enables strong consistency. Our system, PeerCensus, acts as a certification authority, manages peer identities in a peer-to-peer network, and ultimately enhances Bitcoin and similar systems with strong consistency. Our extensive analysis shows that PeerCensus is in a secure state with high probability. We also show how Discoin, a Bitcoin variant that decouples block creation and transaction confirmation, can be built on top of PeerCensus, enabling real-time payments. Unlike Bitcoin, once transactions in Discoin are committed, they stay committed. <s> BIB001 </s> A Survey on Consensus Mechanisms and Mining Strategy Management in Blockchain Networks <s> VI. RELAXED AND PARALLEL CONSENSUS PROTOCOLS FOR PERFORMANCE SCALABILITY <s> Consensus, or state machine replication is a foundational building block of distributed systems and modern cryptography. Consensus in the classical, "permissioned" setting has been extensively studied in the 30 years of distributed systems literature. Recent developments in Bitcoin and other decentralized cryptocurrencies popularized a new form of consensus in a "permissionless" setting, where anyone can join and leave dynamically, and there is no a-priori knowledge of the number of consensus nodes. So far, however, all known permissionless consensus protocols assume network synchrony, i.e., the protocol must know an upper bound of the network's delay, and transactions confirm slower than this a-priori upper bound. ::: ::: We initiate the study of the feasibilities and infeasibilities of achieving responsiveness in permissionless consensus. In a responsive protocol, the transaction confirmation time depends only on the actual network delay, but not on any a-priori known upper bound such as a synchronous round. Classical protocols in the partial synchronous and asynchronous models naturally achieve responsiveness, since the protocol does not even know any delay upper bound. Unfortunately, we show that in the permissionless setting, consensus is impossible in the asynchronous or partially synchronous models. ::: ::: On the positive side, we construct a protocol called Hybrid Consensus by combining classical-style and blockchain-style consensus. Hybrid Consensus shows that responsiveness is nonetheless possible to achieve in permissionless consensus (assuming proof-of-work) when 1) the protocol knows an upper bound on the network delay; 2) we allow a non-responsive warmup period after which transaction confirmation can become responsive; 3) honesty has some stickiness, i.e., it takes a short while for an adversary to corrupt a node or put it to sleep; and 4) less than 1/3 of the nodes are corrupt. We show that all these conditions are in fact necessary - if only one of them is violated, responsiveness would have been impossible. Our work makes a step forward in our understanding of the permissionless model and its differences and relations to classical consensus. <s> BIB002 </s> A Survey on Consensus Mechanisms and Mining Strategy Management in Blockchain Networks <s> VI. RELAXED AND PARALLEL CONSENSUS PROTOCOLS FOR PERFORMANCE SCALABILITY <s> Technological innovation and consequential decentralisation are driving forces in the ongoing evolution and increasing openness of digital infrastructures and services. One of the most discussed and allegedly disruptive innovations is the distributed database technology referred to as blockchain. Although it is still in its technological infancy, experimental adoption and customization seem to be in full progress in various potential fields of application ranging from decentralized grids for computation and storage to global financial services. However, the technology and its path of development still entail a lot of common unknowns for practitioners and researchers alike. Especially regarding the question how the technology could amend or be incorporated into the existing landscape of digital services, processes and infrastructures. Hence, in this article we develop an ontology that (1) clearly delineates common terminology, core concepts and components, their relationships as well as innovative features of blockchain technology. It further (2) connects these insights with implications for relevant types of digital market models. Our framework is of high theoretical and practical value as it provides researchers and practitioners a common basis for communication and means for guided analysis of blockchain applicability. <s> BIB003
So far, we have surveyed the design methodologies of various consensus protocols, especially for permissionless blockchains. As our discussion indicates, the BFT-based consensus mechanisms achieve high transaction throughput with immediate finality at the cost of high message complexity. Thus, they are restricted to small numbers of replicas and offer limited network scalability in terms of the number of consensus nodes. In contrast, the permissionless protocols surveyed in Sections III and V provide good network scalability with low message complexity. However, most of the Nakamoto-like protocols (except the hybrid protocols guaranteeing immediate finality BIB001 , BIB002 ) provide only probabilistic consensus finality. As a result, consistency of replicas across the entire network (cf. the consistency condition for the PoW-based protocol in BIB003 ) is maintained at the cost of low transaction throughput and high latency. Figure 20 provides a descriptive illustration of the scalability levels of different protocol families with respect to both performance and network size. For the protocols surveyed in our previous sections, network scalability and transaction throughput are generally considered as two performance indices that can only be attained at the cost of each other. In this section, we aim to review the solutions that scale out the throughput of a permissionless blockchain as the size of the network increases.