Fast Leaderless Byzantine Total Order Broadcast

This paper presents the Byzantine fault-tolerant agreement protocols Flutter and Blink. Both algorithms are deterministic, leaderless and signature-free; both assume partial synchrony and at least $(5f + 1)$ servers, where $f$ bounds the number of faults. The main contribution, Flutter, is a Total-Order Broadcast implementation that achieves faster broadcast-to-delivery latency by removing the extra message delay associated with serializing messages through a leader. In the "good case" where all processes are correct, the network is synchronous, and local clocks are well-synchronized, Flutter delivers client requests in $(2\Delta + \epsilon)$ time units, $\Delta$ being the message delay and $\epsilon$ an arbitrarily small constant. Under the same conditions, state-of-the-art protocols require $3\Delta$ time units. Flutter's good-case latency is quasi-optimal, meaning it cannot be improved upon by any finite amount. Under the hood, Flutter builds upon Blink, a (Representative) Binary Consensus implementation whose fast path enables decisions in $\Delta$ time units when all correct servers propose the same value. Blink generalizes the existing Binary Consensus solution Bosco from the $(7f + 1)$ to the $(5f + 1)$ setting.