On the Optimality of Optimistic Responsiveness

Abstract

Synchronous consensus protocols, by definition, have a worst-case commit latency that depends on the bounded network delay. The notion of optimistic responsiveness was recently introduced to allow synchronous protocols to commit instantaneously when some optimistic conditions are met. In this work, we revisit this notion of optimistic responsiveness and present optimal latency results. We present a lower bound for Byzantine Broadcast that relates the latency of optimistic and synchronous commits when the designated sender is honest and while the optimistic commit can tolerate some faults. We then present two matching upper bounds for tolerating f faults out of n = 2f + 1 parties. Our first upper bound result achieves optimal optimistic and synchronous commit latency when the designated sender is honest and the optimistic commit can tolerate at least one fault. We experimentally evaluate this protocol and show that it achieves throughput comparable to state-of-the-art synchronous and partially synchronous protocols and under optimistic conditions achieves latency better than the state-of-the-art. Our second upper bound result achieves optimal optimistic and synchronous commit latency when the designated sender is honest but the optimistic commit does not tolerate any faults. The presence of matching lower and upper bound results make both of the results tight for n = 2f + 1. Our upper bound results are presented in a state machine replication setting with a steady-state leader who is replaced with a view-change protocol when they do not make progress. For this setting, we also present an optimistically responsive protocol where the view-change protocol is optimistically responsive too. CCS CONCEPTS • Security and privacy → Distributed systems security.

Good-case Latency of Byzantine Broadcast

“…We adopt the separation between the conservative worst-case bound Δ and the actual (unknown) bound ≤ Δ as suggested in [3,4,20,27,31]. Moreover, our categorization highlights the importance of the assumption on the synchronization of when the protocol starts at each party.…”

“…Another line of research aims at developing BFT protocols with small latencies when certain optimistic conditions are met [2,22,28,32]. Common examples of such optimistic conditions include: more than 3 /4 parties are honest in synchrony [2,28,31] or all parties vote under partial synchrony [19,22]. Note that these conditions are much more demanding than our definition of good-case, which only requires an honest leader.…”

Brief Announcement

“…A recent line of work investigates synchronous BFT protocol with an asynchronous fallback, focusing on achieving optimal resilience tradeoffs between synchrony and asynchrony [5,6] or optimal communication complexity [27]. Another related line of work on optimistic BFT protocols focuses on including a fast path in BFT protocols to improve the performance such as communication complexity and latency under the optimistic scenarios, for asynchronous protocols [19][20][21]27], partially synchronous protocols [1,15,18] and synchronous protocols [10,[24][25][26].…”

Brief Announcement

“…However, the first rule requires a party to wait for at least Δ time after burying C ( ). Protocols like OptSync [10] are designed to commit faster and decide as soon as a unique certificate for a value is formed (and achieve responsiveness). Modifying this protocol to meet above constraints does make the protocol secure in the presence of mobile-sluggish faults at the expense of slower fast commits.…”

“…We address these questions by analyzing many of the existing synchronous protocols, namely, Dfinity [8], Streamlet [4], Opt-Sync [10], Sync HotStuff [2] and the optimal latency BFT protocol [3], and presenting their mobile sluggish counterparts. We identify a common theme to make these protocols secure under the weaker synchrony model.…”