Blockchain, the underlying technology of Bitcoin, refers to the public ledger used in a distributed network. Because blockchain does not rely on a central authority, peers have to agree on the state of the ledger among themselves, i.e., they have to reach a consensus on the state of the transactions. The way nodes reach that consensus has gained incredible attention in the literature. Bitcoin uses the Proof-of-Work (PoW) mechanism, as did Ethereum at first. The latter decided to move from PoW to Proof-of-Stake (PoS) because of the high energy consumption required by PoW. To date, many other consensus protocols have been proposed to address the limitations of the seminal ones.

In this paper, we inform researchers and practitioners about the current state of consensus protocols research. The aim is to provide an analysis of the research introducing new consensus protocols in order to enable a more unified treatment. To that end, we review 28 new consensus protocols and we propose a four-category classification framework: Origin, Design, Performance and Security. We demonstrate the applicability of the framework by classifying the 28 protocols. Many surveys have already been proposed in the literature and some of them will be discussed later in the paper. Yet, we believe that this work is relevant and important for two reasons. Firstly, blockchain being a fast evolving topic, new consensus protocols emerge regularly and improvements are also put forward on a regular basis. Hence, this work aims at reflecting the latest state-of-the-art in terms of consensus protocols. Secondly, we aim to propose a comprehensive classification framework, integrating knowledge from multiple works in the literature, as well as introducing classification dimensions that have not been proposed before.

This work demonstrates that multiple consensus have been proposed in a short period of time, and highlights the differences between these protocols. Furthermore, it is suggested that researchers and practitioners who aim to propose consensus protocols in the future should pay attention to all the dimensions presented in the classification framework.

区块链是比特币的基础技术，是指分布式网络中使用的公共分类帐。因为区块链不依赖中央机构，所以同行之间必须就账本的状态达成共识，即，他们必须就交易状态达成共识。节点达成共识的方式在文献中得到了令人难以置信的关注。比特币像最初的以太坊一样使用工作量证明（PoW）机制。后者决定从PoW迁移到权益证明（PoS），因为PoW需要消耗大量能源。迄今为止，已经提出了许多其他共识协议来解决开创性协议的局限性。

在本文中，我们将共识协议研究的当前状态告知研究人员和从业人员。目的是对引入新的共识协议的研究进行分析，以实现更统一的治疗。为此，我们回顾了28个新的共识协议，并提出了四类分类框架：起源，设计，性能和安全性。我们通过对28种协议进行分类来证明该框架的适用性。文献中已经提出了许多调查，其中一些将在本文后面进行讨论。但是，我们认为这项工作是有意义和重要的，有两个原因。首先，区块链是一个快速发展的话题，新的共识协议会定期出现，并且还会定期提出改进意见。因此，这项工作旨在反映共识协议方面的最新技术。其次，我们旨在提出一个全面的分类框架，整合文献中来自多本著作的知识，并介绍以前未曾提出过的分类维度。

这项工作表明在短时间内提出了多个共识，并突出了这些协议之间的差异。此外，建议将来打算提出共识协议的研究人员和从业人员应注意分类框架中提出的所有方面。