Optimistic rollup

The sequencer posts batches and the resulting states to layer 1 without a validity proof. A challenge period then opens, canonically seven days on Arbitrum and Optimism, during which any verifier can prove a transition invalid and revert it. If someone detects an error, they submit a fraud proof. Both protocols use an interactive dispute game that narrows the disagreement, via bisection, down to a single computation step that layer 1 re-executes and arbitrates.

The implementations differ in technical detail. Arbitrum compiles its virtual machine to WASM and has deployed BoLD, a permissionless validation protocol whose base challenge period is roughly 6.4 days and is adjustable by governance. Optimism uses Cannon, a fault proof game over the MIPS instruction set. Until the window expires, direct withdrawals to layer 1 are not final, hence the seven-day wait to withdraw without using liquidity bridges.

The trade-off is that finality latency plus the assumption that at least one honest verifier is watching. In return it offers high EVM compatibility and low costs, without the complexity of generating zero-knowledge proofs. Data posting became far cheaper with the EIP-4844 blobs (March 2024) and the expansion in EIP-7691 in the Pectra upgrade (7 May 2025). Production examples include Arbitrum One (Offchain Labs), OP Mainnet (Optimism) and Base (Coinbase, on OP Stack), where stablecoins and tokenized real-world assets are deployed.