The Consensus Mechanism Upon Dag-Based Distributed Ledger

The advent of Bitcoin in 2008 has opened up the era of blockchain technology,which is essentially a distributed ledger maintained by multiple parties in a permissionless setting.With the characteristic of being decentralized,immutable and programmable,blockchain shows great potential in digital finance,supply chain traceability,anti-counterfeit data services,etc.On the other hand,however,blockchain exhibits poor scalability in terms of throughput and latency.Taking Bitcoin as an example,it can only process up to 7 transactions per second,and it takes tens of minutes for each transaction to be settled.The scalability bottleneck severely impedes the further development of blockchain technology and the landing of related applications.To face these issues,an alternative distributed ledger structure based on Directed Acyclic Graph(DAG)was proposed to achieve high performance with its native support for parallel processing of transactions,and is now a promising solution for blockchain scalability.While extensive efforts have been made in designing DAG-based distributed ledgers in recent years,there still lacks a systematic research work in this field.Moreover,existing DAGbased ledger schemes are far from satisfactory regarding security,efficiency,and functional scalability.Given the above problems,we conducted a systematic and in-depth study,and attained the following achievements:Firstly,we propose a research framework for DAG-based ledger.According to the differences in the form of the ledger and its consensus mechanism,we classify DAG-based ledgers into three categorizes: DAG-based ledger with a backbone chain,DAG-based ledger of parallel chains,and naive DAG-based ledger.Besides,we categorize DAG-based ledgers into block-based ones and transaction-based ones on the basis of data organization methods.Secondly,we conduct a comparative analysis of different types of DAG ledgers.From the aspects of design intuition,security analysis,and the support of the smart contract,we carry out the study of different forms of DAG ledgers.And in view of network utilization,ledger permission,and the support of the light client,we compare DAG-based ledgers with different data organization methods.Furthermore,we present an efficient and scalable DAG-based distributed ledger named Guava.By adopting a two-tier architecture,and combining the proof-of-work mechanism in the traditional blockchain with the classic Byzantine consensus protocol,it achieves high throughput and instant transaction confirmation.Finally,we analyze the security of the Guava protocol.Based on the well-recognized blockchain security analysis framework,we derive the computational security bound of Guava,and finally prove that the Guava protocol has achieved two critical security guarantees: consistency and liveness.