Paper 2019/360

SoK: Off The Chain Transactions

Lewis Gudgeon and Pedro Moreno-Sanchez and Stefanie Roos and Patrick McCorry and Arthur Gervais

Abstract

Blockchains have the potential to revolutionize markets and services, yet, currently exhibit high latencies and fail to handle loads comparable to those managed by traditional custodian financial systems. Layer-two protocols, built on top of (layer-one) blockchains, avoid disseminating every transaction to the whole network by sending transactions off-chain and instead utilize the blockchain only as a recourse for disputes. The promise of layer-two protocols is to complete transactions in sub-seconds, reduce fees, and allow blockchains to scale. With this Systematization of Knowledge, we are the first to structure the complete rich and multifaceted body of research on layer-two transactions. Categorizing the research into payment and state channels as well as commit-chains, we provide a comparison of the protocols and their properties. We contribute a systematization of the associated synchronization and routing protocols along with their privacy and security aspects. Contrary to common belief in the blockchain community, we show that layer-two can scale blockchains; that layer-two protocols are secure without full collateralization; that privacy of layer-two transaction is not granted by default; and that fees depend on the transmitted transaction value. The SoK clears the layer-two fog, highlights the potential of layer-two solutions and identifies their unsolved challenges and promising avenues of future work.