[Paper Review] Practical Synchronous Byzantine Consensus

Ling Ren, Kartik Nayak+2 more|arXiv (Cornell University)|Apr 7, 2017

Distributed systems and fault tolerance20 citations

TL;DR

This paper presents practical Byzantine state machine replication and agreement protocols that achieve $2f+1$ replica tolerance under synchrony, reducing the $3f+1$ replica requirement of PBFT. By leveraging synchrony to form a post-commit quorum intersecting all pre-commit quorums at $f+1$ honest replicas, it enables faster consensus with fewer rounds than prior work.

ABSTRACT

We present new protocols for Byzantine state machine replication and Byzantine agreement in the synchronous and authenticated setting. The celebrated PBFT state machine replication protocol tolerates $f$ Byzantine faults in an asynchronous setting using $3f+1$ replicas, and has since been studied or deployed by numerous works. In this work, we improve the Byzantine fault tolerance to $n=2f+1$ by utilizing the synchrony assumption. The key challenge is to ensure a quorum intersection at one \emph{honest} replica. Our solution is to rely on the synchrony assumption to form a \emph{post-commit} quorum of size $2f+1$, which intersects at $f+1$ replicas with any \emph{pre-commit} quorums of size $f+1$. Our protocol also solves synchronous authenticated Byzantine agreement in fewer rounds than the best existing solution (Katz and Koo, 2006). A challenge in this direction is to handle non-simultaneous termination, which we solve by introducing a notion of \emph{virtual} participation after termination. Our protocols may be applied to build practical synchronous Byzantine fault tolerant systems and improve cryptographic protocols such as secure multiparty computation and cryptocurrencies when synchrony can be assumed.

Motivation & Objective

• To reduce the replica count required for Byzantine fault tolerance from $3f+1$ to $2f+1$ by exploiting the synchrony assumption.
• To solve synchronous authenticated Byzantine agreement in fewer rounds than existing protocols, such as Katz and Koo (2006).
• To address non-simultaneous termination in synchronous consensus by introducing virtual participation after termination.
• To enable practical deployment of Byzantine fault-tolerant systems under synchrony assumptions.
• To improve the efficiency of cryptographic protocols like secure multiparty computation and cryptocurrencies when synchrony can be assumed.

Proposed method

• Leverages the synchrony assumption to form a post-commit quorum of size $2f+1$ that intersects all pre-commit quorums of size $f+1$ at $f+1$ honest replicas.
• Uses quorum intersection guarantees under synchrony to ensure consistency and liveness in the presence of $f$ Byzantine faults.
• Introduces virtual participation to handle non-simultaneous termination, allowing replicas to complete consensus after termination without blocking.
• Designs a new authenticated Byzantine agreement protocol that reduces the number of rounds compared to prior solutions.
• Relies on authenticated communication and timing assumptions to ensure safety and liveness in the synchronous model.
• Constructs a state machine replication protocol based on the new agreement protocol, achieving $2f+1$ replica tolerance.

Experimental results

Research questions

• RQ1 Can Byzantine state machine replication be made more efficient by reducing the required number of replicas from $3f+1$ to $2f+1$ under synchrony?
• RQ2 How can quorum intersection be guaranteed at honest replicas in a synchronous Byzantine consensus protocol?
• RQ3 What mechanisms can ensure fast termination and liveness in synchronous Byzantine agreement despite non-simultaneous termination?
• RQ4 Can the number of rounds in synchronous authenticated Byzantine agreement be reduced below existing solutions?
• RQ5 How can virtual participation be used to resolve non-simultaneous termination in a way that preserves correctness?

Key findings

• The proposed protocol achieves Byzantine fault tolerance with only $2f+1$ replicas, improving upon the $3f+1$ requirement of PBFT.
• The protocol ensures quorum intersection at $f+1$ honest replicas by relying on the synchrony assumption to form a post-commit quorum of size $2f+1$.
• It solves synchronous authenticated Byzantine agreement in fewer rounds than the best prior solution (Katz and Koo, 2006).
• The introduction of virtual participation enables correct handling of non-simultaneous termination without compromising safety or liveness.
• The protocols are practical and applicable to real-world systems, including secure multiparty computation and cryptocurrencies, when synchrony can be assumed.
• The approach maintains strong consistency and fault tolerance under authenticated, synchronous communication.