Towards Single Slot Finality: Evaluating Consensus Mechanisms and Methods for Faster Ethereum Finality

Abstract: Ethereum's current Gasper consensus mechanism, which combines the Latest Message Driven Greediest Heaviest Observed SubTree (LMD-GHOST) fork choice rule with the probabilistic Casper the Friendly Finality Gadget (FFG) finality overlay, finalizes transactions in 64 to 95 blocks, an approximate 15-minute delay. This finalization latency impacts user experience and exposes the network to short-term chain reorganization risks, potentially enabling transaction censorship or frontrunning by validators without severe penalties. As the ecosystem pursues a rollup-centric roadmap to scale Ethereum into a secure global settlement layer, faster finality allows cross-layer and inter-rollup communication with greater immediacy, reducing capital inefficiencies. Single slot finality (SSF), wherein transactions are finalized within the same slot they are proposed, promises to advance the Ethereum protocol and enable better user experiences by enabling near-instant economic finality. This thesis systematically studies distributed consensus protocols through propose-vote-merge, PBFT-inspired, and graded agreement families - scrutinizing their capacities to enhance or replace LMD-GHOST. The analysis delves into the intricate tradeoffs between safety, liveness, and finality, shedding light on the challenges and opportunities in designing an optimal consensus protocol for Ethereum. It also explores different design decisions and mechanisms by which single slot or fast finality can be enabled, including cumulative finality, subsampling, and application-layer fast finality. Furthermore, this work introduces SSF-enabled and streamlined fast finality constructions based on a single-vote total order broadcast protocol. The insights and recommendations in this thesis provide a solid foundation for the Ethereum community to make informed decisions regarding the future direction of the protocol's consensus.