Ebb-and-Flow Protocols: A Resolution of the Availability-Finality Dilemma

TL;DR

This paper introduces ebb-and-flow protocols that enable a blockchain to maintain both full availability and finality, resolving the traditional trade-off under network partitions by allowing the finalized ledger to catch up after healing.

Contribution

The paper formulates a new class of flexible consensus protocols that support both available and finalized ledgers, providing provably secure constructions with optimal resilience.

Findings

Identified a vulnerability in Gasper's finality gadget under synchronous networks.

Proposed a secure construction combining available and BFT protocols for finality.

Improved the state-of-the-art in flexible BFT protocols.

Abstract

The CAP theorem says that no blockchain can be live under dynamic participation and safe under temporary network partitions. To resolve this availability-finality dilemma, we formulate a new class of flexible consensus protocols, ebb-and-flow protocols, which support a full dynamically available ledger in conjunction with a finalized prefix ledger. The finalized ledger falls behind the full ledger when the network partitions but catches up when the network heals. Gasper, the current candidate protocol for Ethereum 2.0's beacon chain, combines the finality gadget Casper FFG with the LMD GHOST fork choice rule and aims to achieve this property. However, we discovered an attack in the standard synchronous network model, highlighting a general difficulty with existing finality-gadget-based designs. We present a construction of provably secure ebb-and-flow protocols with optimal resilience. Nodes run an off-the-shelf dynamically available protocol, take snapshots of the growing available ledger, and input them into a separate off-the-shelf BFT protocol to finalize a prefix. We explore connections with flexible BFT and improve upon the state-of-the-art for that problem.