JOURNAL OF L A T E X CLASS FILES, VOL. XX, NO. X, DECEMBER 2020 1 Speeding up Block Propagation in Blockchain Network: Uncoded and Coded Designs Lihao Zhang, Taotao Wang, Member, IEEE, and Soung Chang Liew, Fellow, IEEE Abstract—We design and validate new block propagation protocols for the peer-to-peer (P2P) network of the Bitcoin blockchain. Despite its strong protection for security and privacy, the current Bitcoin blockchain can only support a low number of transactions per second (TPS). In this work, we redesign the current Bitcoin’s networking protocol to increase TPS without changing vital components in its consensus-building protocol. In particular, we improve the compact-block relaying protocol to enable the propagation of blocks containing a massive number of transactions without inducing extra propagation latencies. Our improvements consist of (i) replacing the existing store-and-forward compact-block relaying scheme with a cut-through compact-block relaying scheme; (ii) exploiting rateless erasure codes for P2P networks to increase block-propagation efficiency. Since our protocols only need to rework the current Bitcoin’s networking protocol and does not modify the data structures and crypto-functional components, they can be seamlessly incorporated into the existing Bitcoin blockchain. To validate our designs, we perform analysis on our protocols and implement a Bitcoin network simulator on NS3 to run different block propagation protocols. The analysis and experimental results confirm that our new block propagation protocols could increase the TPS of the Bitcoin blockchain by 100x without compromising security and consensus-building. Index Terms—Blockchain, Networking Protocol, Cut-through Forwarding, Rateless Coding. ✦ 1 I NTRODUCTION B LOCKCHAIN was proposed as a supporting technol- ogy for Bitcoin [1], the first decentralized cryptocur- rency. After Bitcoin, other decentralized cryptocurrencies (e.g., Litecoin [2], Ethereum [3]) quickly emerged. The blockchains of these cryptocurrencies use the Nakamoto’s proof-of-work (PoW) protocol to build consensus among distributed nodes. Blockchain has, by now, become a cutting-edge technology in the fields of FinTech, Internet of Things (IoT), and supply chains [4], [5], [6], thanks to its ability to enable Byzantine agreement over a permission-less decentralized network [7]. A weakness of the current blockchains is the low on- chain transaction throughput. For example, the throughput of Bitcoin is around 5 ∼ 7 transactions per second (TPS), and that of Ethereum is around 40 TPS [8]. Both are ex- tremely low compared to around 110 TPS of PayPal and 1700 TPS of Visa. Its low transaction throughput hampers the widespread adoption of today’s blockchain technology. A straightforward method to increase TPS is to enlarge the block size so that a block can carry more transactions. However, the propagation of large blocks in the network may incur huge delays that compromise the blockchains’ security and integrity [9], and thus it is not a good idea to increase the TPS by merely increasing the block size. Consequently, new consensus-building protocols and new specially deployed networking infrastructures have been • Lihao Zhang and Soung Chang Liew are with the Department of Informa- tion Engineering, ,The Chinese University of Hong Kong, Hong Kong. E-mail: zl018@ie.cuhk.edu.hk, soung@ie.cuhk.edu.hk • Taotao Wang is with the College of Electronics and Information Engineering, Shenzhen University, Shenzhen 518060, China. E-mail: ttwang@szu.edu.cn Manuscript received xxx, xxx; revised xxx, xxx. proposed as solutions to increase the TPS of blockchains (see discussions of these related works in Section 2). In this work, we put forth a new block propagation pro- tocol to propagate large blocks containing a large number of transactions without increasing the block relay delay. Unlike the existing solution that requires changing the consensus- building protocol or deploying new network infrastructures, we build our block propagation protocol upon the compact- block relaying protocol that is already adopted by the cur- rent Bitcoin network [10]. Compact-block relaying reduces the block relay delay by compressing blocks that contain transactions (around 250 Bytes each in Bitcoin) into compact blocks that contain transaction hashes (6 Bytes each). There- fore, compact-block relaying can include more transactions into each compact block while maintaining the same relay delay, thus increasing the TPS without compromising the blockchain’s security. In this paper, we further boost TPS by further increas- ing compact-block size including even more transaction hashes into each compact block. However, simply increasing compact-block size induces extra propagation delays that may compromise blockchain’s security. We devise methods to keep the propagation delays at bay while increasing the compact-block size. We adopt a two-pronged approach: 1) we replace the store-and-forward compact-block relaying scheme with a cut-through forwarding scheme; 2) we apply rateless erasure codes to increase the efficiency of block propagation. The contributions of this work are listed as follows. 1) We put forth a new block propagation protocol that replaces the store-and-forward compact-block relaying scheme with a cut-through forwarding scheme. Our new cut-through compact-block re- arXiv:2101.00378v1 [cs.NI] 2 Jan 2021