Arbitrum¶
Arbitrum, built by Offchain Labs, is Ethereum's highest-volume optimistic rollup and this section's case study for Optimistic Rollups and Fraud Proofs in actual production. Its history includes an honest, worth-naming gap between the theoretical fraud-proof security model and what was actually deployed for its first several years.
Launch and the Nitro rewrite¶
Arbitrum One launched on Ethereum mainnet in beta in May 2021, opening to general use that August. In August 2022, Arbitrum migrated to Nitro, a substantial rewrite of its underlying technology stack that compiles Ethereum's own Geth client directly into the WebAssembly environment Arbitrum's fraud-proof system uses to interactively verify disputed execution steps (see Fraud Proofs), meaningfully improving both EVM compatibility and performance over the earlier, custom-built execution environment Arbitrum originally launched with.
The permissioned-validation gap, and BOLD¶
For years after launch, Arbitrum's fraud-proof dispute system, while designed the way Fraud Proofs describes generally, was only actually usable by a permissioned, Offchain-Labs-approved set of validators, not genuinely open to anyone, because the dispute protocol as originally built was vulnerable to denial-of-service attacks if opened to arbitrary participants. This is a real, honest gap worth naming directly: for a meaningful stretch of Arbitrum's history, the fraud-proof security model was closer to theoretical than fully operative in practice, since an unauthorized party couldn't actually submit a challenge even if they detected fraud. BOLD (Bounded Liquidity Delay), Offchain Labs' redesigned dispute protocol that resolves this specific denial-of-service vulnerability, went live on Arbitrum One's mainnet, enabling genuinely permissionless validation for the first time since launch.
Common misconceptions¶
Arbitrum's fraud-proof security has not been uniformly "permissionless" throughout its entire history. Genuinely open, permissionless validation is a comparatively recent addition (via BOLD); anyone evaluating Arbitrum's security model at a specific point in time should check whether that period predates or postdates this specific upgrade, rather than assuming the fully decentralized dispute model described in Fraud Proofs was always actually operative.
"Arbitrum" does not refer to one single chain. Offchain Labs operates multiple Arbitrum chains (Arbitrum One and Arbitrum Nova, with different data-availability tradeoffs), and Arbitrum's underlying technology stack is also used by numerous independently operated "Arbitrum Orbit" chains, similar in spirit to the OP Stack ecosystem covered in Optimism.
Further reading¶
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