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Last update: July 19, 2022

Configure IBFT consensus

GoQuorum implements the IBFT proof of authority consensus protocol. IBFT is supported for existing private networks, but QBFT is the recommended enterprise-grade consensus protocol for private networks. You can migrate an IBFT network to QBFT.

You can create a private network using IBFT.

In IBFT networks, approved accounts known as validators validate transactions and blocks. Validators take turns to create the next block. Before inserting a block onto the chain, a super-majority (greater than or equal to ⅔) of validators must first sign the block.

Existing validators propose and vote to add or remove validators. Adding or removing a validator requires a majority vote (greater than 50%) of validators.

Important

Configure your network to ensure you never lose more than ⅓ of your validators. If more than ⅓ of validators stop participating, new blocks are no longer created, and the network stalls. It may take significant time to recover once nodes are restarted.

Blocks in IBFT protocol are final, meaning there are no forks, and valid blocks must be in the main chain.

To prevent a faulty node from generating a different chain from the main chain, each validator appends ceil(2N/3) of received COMMIT signatures to the extraData field in a block’s header before inserting it into the chain. Therefore, all blocks are self-verifiable.

Add and remove validators

IBFT provides two methods to manage validators:

You can use transitions to swap between block header validator selection and contract validator selection in an existing network.

For block header validator selection, initial validators are configured in the genesis file’s extraData property, whereas the initial validators when using the contract validator selection method are configured in the genesis file’s validatorcontractaddress section.

Minimum number of validators

IBFT requires four validators to be Byzantine fault tolerant. Byzantine fault tolerance is the ability for a blockchain network to function correctly and reach consensus despite nodes failing or propagating incorrect information to peers.

Add and remove validators using a smart contract

Users can create their own smart contracts to add or remove validators based on their organizational requirements. View the example smart contract for more information on how to create and deploy the smart contract.

You can pre-deploy the validator smart contract in a new QBFT network by specifying the contract details in the genesis file.

Important

You can’t use the JSON-RPC methods to add or remove validators when using a smart contract to manage nodes. You must interact with the contract functions using transactions.

Genesis file

To use IBFT, GoQuorum requires a genesis file. The genesis file defines properties specific to IBFT and to your specific network.

Example IBFT genesis file

  {
    "config": {
      "chainId": 1337,
      "homesteadBlock": 0,
      "eip150Block": 0,
      "eip150Hash": "0x0000000000000000000000000000000000000000000000000000000000000000",
      "eip155Block": 0,
      "eip158Block": 0,
      "byzantiumBlock": 0,
      "constantinopleBlock": 0,
      "ibft": {
        "epochlength": 30000,
        "blockperiodseconds": 2
        "requesttimeoutseconds": 4
        "policy": 0,
        "ceil2Nby3Block": 0,
        "validatorcontractaddress": "0x0000000000000000000000000000000000007777"
      },
      "txnSizeLimit": 64,
      "maxCodeSize": 0,
      "isQuorum": true
    },
    "nonce": "0x0",
    "timestamp": "0x5f1663fc",
    "extraData": "0x0000000000000000000000000000000000000000000000000000000000000000f89af8549493917cadbace5dfce132b991732c6cda9bcc5b8a9427a97c9aaf04f18f3014c32e036dd0ac76da5f1894ce412f988377e31f4d0ff12d74df73b51c42d0ca9498c1334496614aed49d2e81526d089f7264fed9cb8410000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000c0",
    "gasLimit" : "0xf7b760",
    "difficulty": "0x1",
    "mixHash": "0x63746963616c2062797a616e74696e65206661756c7420746f6c6572616e6365",
    "coinbase": "0x0000000000000000000000000000000000000000",
    "alloc": {},
    "number": "0x0",
    "gasUsed": "0x0",
    "parentHash": "0x0000000000000000000000000000000000000000000000000000000000000000"
  }

The properties specific to IBFT are in the ibft section:

  • epochlength - Number of blocks that should pass before pending validator votes are reset.
  • blockperiodseconds - The minimum block time, in seconds.
  • requesttimeoutseconds - The timeout for each consensus round before a round change, in seconds.
  • policy - Proposer selection policy, which is 0 (Round Robin) or 1 (Sticky). ‘Round Robin’ is where validators take turns in proposing blocks, and ‘Sticky’ is where a single validator proposes blocks until they go offline or are unreachable.
  • ceil2Nby3Block - Sets the block number from which to use an updated formula for calculating the number of faulty nodes. For new networks, we recommended setting this to 0 to use the updated formula immediately.
  • validatorcontractaddress - Address of the validator smart contract. Required only if using a contract validator selection. This option can also be used in the transitions configuration item if swapping validator management methods in an existing network.
  • extraData - RLP encoded string with a list of validators. RLP encoding is a space-efficient object serialization scheme used in Ethereum.

Block time

When the protocol receives a new chain head, the block time (blockperiodseconds) timer starts. When blockperiodseconds expires, the round timeout (requesttimeoutseconds) timer starts and the protocol proposes a new block. The default is 1 second.

If requesttimeoutseconds expires before adding the proposed block, a round change occurs, with the block time and timeout timers reset. The timeout period for the new round is two times requesttimeoutseconds. The timeout period continues to double each time a round fails to add a block.

Usually, the protocol adds the proposed block before reaching requesttimeoutseconds. A new round then starts, resetting the block time and round timeout timers. When blockperiodseconds expires, the protocol proposes the next new block.

Important

If more than ⅓ of validators stop participating, new blocks can no longer be created and requesttimeoutseconds doubles with each round change. The quickest method to resume block production is to restart all validators, which resets requesttimeoutseconds to its genesis value.

Transitions

The transitions genesis configuration item allows you to specify a future block number at which to change IBFT network configuration in an existing network. For example, you can update the block time or validator management method.

Caution

Do not specify a transition block in the past. Specifying a transition block in the past could result in unexpected behavior, such as causing the network to fork.

Configure block time on an existing network

To update an existing network with a new blockperiodseconds:

  1. Stop all nodes in the network.
  2. In the genesis file, add the transitions configuration item where:

    • <FutureBlockNumber> is the upcoming block at which to change blockperiodseconds.
    • <NewValue> is the updated value for blockperiodseconds.

    Transitions configuration

    {
      "config": {
        ...
        "ibft": {
          "blockperiodseconds": 2,
          "epochlength": 30000,
          "requesttimeoutseconds": 4
        },
        "transitions": [{
          "block": <FutureBlockNumber>,
          "blockperiodseconds": <NewValue>
        }]
      },
      ...
    }
    
    {
      "config": {
        ...
        "ibft": {
          "blockperiodseconds": 2,
          "epochlength": 30000,
          "requesttimeoutseconds": 4
        },
        "transitions": [ {
          "block": 1240,
          "blockperiodseconds": 4
        }]
      },
      ...
    }
    
  3. Restart all nodes in the network using the updated genesis file.

  4. To verify the changes after the transition block, call istanbul_getValidators, specifying latest.

Swap validator management methods

To swap between block header validator selection and contract validator selection methods in an existing network:

  1. Stop all nodes in the network.
  2. In the genesis file, add the transitions configuration item where:

    • <FutureBlockNumber> is the upcoming block at which to change the validator selection method.
    • <SelectionMode> is the validator selection mode to switch to. Valid options are contract and blockheader.
    • <ContractAddress> is the smart contract address, if switching to the contract validator selection method.

    Transitions configuration

    {
      "config": {
        ...
        "ibft": {
          "blockperiodseconds": 5,
          "epochlength": 30000,
          "requesttimeoutseconds": 10
        },
        "transitions": [{
          "block": <FutureBlockNumber>,
          "validatorselectionmode": <SelectionMode>,
          "validatorcontractaddress": <ContractAddress>
        }]
      },
      ...
    }
    
    {
      "config": {
        ...
        "ibft": {
          "blockperiodseconds": 5,
          "epochlength": 30000,
          "requesttimeoutseconds": 10
        },
        "transitions": [ {
          "block": 102885,
          "validatorselectionmode": "contract",
          "validatorcontractaddress": "0x0000000000000000000000000000000000007777"
        }]
      },
      ...
    }
    
  3. Restart all nodes in the network using the updated genesis file.

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