Istanbul Byzantine Fault Tolerance (IBFT)

Istanbul Byzantine Fault Tolerance (IBFT) is a blockchain adaptation of the PBFT paper. BFT consensus algorithms support a rate of failure of F validators, where F has to be less than 1/3 of the validators in the network. Failures here include crashes, slow network or any malicious behaviours. Therefore, given a maximum F faulty validators the network size must be of size 3F+1 (More details can be found in the Byzantine General Problem paper).

Blocks in IBFT are final, ie no forks can occur, validators reach consensus on the current block by appending COMMIT signatures to the extraData field of the header. Each validator appends 2F+1 COMMIT signatures received to the block header before inserting the block into the chain. Therefore, blocks can be easily verified and the light client is also supported.

Consensus State Transitions

The algorithm chooses a validator (the proposer), in a round robin fashion, which proposes a new block to the validators along with a PRE-PREPARE message. Upon reception of this new block and the PRE-PREPARE message, a validator verifies whether the proposal is valid and enters the PRE-PREPARED state, the validator broadcasts a PREPARE message. The PREPARE message is to ensure that validators are working on the same block and round. Upon receiving 2F+1 PREPARE message the validator enters the PREPARED state and broadcasts a COMMIT message. The COMMIT message informs the other peers that it has accepted the proposed block and is ready to insert the block into the chain. After receiving 2F+1 COMMIT messages the validator enters the COMMITTED state and attempts to insert the block into the chain, if successful, the validators enters FINAL COMMITTED state and moves to the next sequence.


A sequence represents the current block on which validators are trying to reach consensus on, while a round consists of the states mentioned above in reaching consensus on the block. A sequence can contain multiple rounds, however, there is only one sequence per block. A round change can happen if there is a timeout, the proposal is invalid or the block insertion fails, upon these events the validators enter the ROUND CHANGE state and waits for 2F+1 ROUND CHANGE messages before moving into a new round.