Byzantine Fault Tolerant Consensus (PBFT)

Network

Total nodes N 7

Byzantine (faulty) nodes f 2

Round

Phase: idle

Quorum

Quorum needed (2f+1)

Fault bound ⌊(N-1)/3⌋

PREPARE votes (honest)

COMMIT votes (honest)

How it works

PBFT (Practical Byzantine Fault Tolerance, Castro & Liskov 1999) lets a group of N replicas agree on a value even if up to f of them are arbitrarily malicious ("Byzantine"), as long as:

N ≥ 3f + 1   (fault bound: f ≤ ⌊(N-1)/3⌋)
Quorum size Q = 2f + 1

Each round runs three phases relayed through a primary replica:

Pre-prepare — the primary broadcasts a proposed value with a sequence number.
Prepare — every replica broadcasts PREPARE to every other replica; a replica moves on once it collects Q matching PREPARE messages (itself included).
Commit — every replica broadcasts COMMIT once prepared; a replica finalizes once it collects Q matching COMMIT messages. Two quorums of size 2f+1 out of N ≥ 3f+1 always intersect in at least one honest node, which is what stops faulty replicas from forcing two different honest replicas to commit two different values.

This simulator's Byzantine nodes equivocate: instead of relaying one consistent vote, they send PREPARE/COMMIT for a different value to a random subset of honest peers — the classic Byzantine attack this protocol was built to survive. Toggle "Equivocate" to compare honest-crash faults against active equivocation, and push f above the ⌊(N-1)/3⌋ bound to watch the quorum intersection guarantee break down and the round fail to reach safe consensus.

Primary Honest replica Byzantine replica Committed

⚠ Couldn't load the 3D engineThree.js failed to load from the CDN. Check your connection and reload.