How it Works?
The fraud proof design
- Uses “interactive proofs”, specifically by splitting the dispute in halves recursively, the disputer picking one side, until we narrow down to a single step of execution that happens on L1.
- Benefit of this approach is most activity is happening off-chain: reduced gas usage on pessimistic case, support for a higher per-tx gas limit and contract sizes.
- However, takes up to 2 weeks to finish (realistically 1 week), and requires original claimer to be online and cooperative.
- “able to run unmodified EVM contracts and unmodified Ethereum transactions” (though that should now be true of optimism as well)
The Sequencer today: single one run by Offchain Labs’s official chain, controls insertion order into the Inbox (which is split into two: 1. regular inbox and 2. sequencer inbox)) up to ~10 minutes in the past. ArbOS will consume messages from the inboxes according to their block numbers, however sequencers impose a fixed subtraction to the block number causing it to go in before the other inbox.
it appears that today, if going via the sequencer it looks FIFO-style fairness (same as optimism).
Later, decentralized sequencer:
sequencer is a committee of servers, and as long as a quorum of more than two-thirds of the committee is honest, the sequencer will establish a fair ordering over transactions. see paper
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