Performance and efficiency tradeoffs in blockchain overlay networks

Parikshit Hegde, Gustavo de Veciana

Abstract: Underlying blockchain's scalability and performance is a Peer-to-Peer (P2P) overlay network and protocols for relaying blocks and transactions among participating nodes. In this work, we model and perform a systematic analysis of blockchain communication protocols. We begin by introducing the performance metric of interest for blockchains, the sequence of ordered-completion-times, which captures the progress distributed nodes are making in jointly constructing a consistent blockchain. We then study the characteristics of block relaying protocols. In particular, we show that when nodes cannot perform cut-through relaying, there is no optimal causal block relaying protocol if block relaying times are deterministic. We propose a simple age-based block relaying protocol that is provably near-optimal in terms of minimizing ordered-completion-times when the P2P overlay network is a tree. This analysis is relevant to blockchain relay networks such as Bitcoin-Fibre, Bloxroute etc., where the core part of the overlay is a tree. Finally, using the insights derived for tree overlays, we explore the interplay between transaction and block relaying and prioritization (age, size, FCFS based) on both tree and fully connected overlays via simulation. We observe that policies that relay blocks and incorporate transactions into blocks based on 'age' outperform other natural policies. We also explore a fundamental tradeoff between mining and computational efficiencies and performance (in terms of ordered completion times).