Arbitrum

Resources

• Inside Arbitrum

Another [layer-2-scaling] solution based on Optimistic Rollups (OR) technology (like [optimism]).

How it Works?

See [optimism] and [layer-2-scaling] for background on how optimistic rollups work. The key differences with Arbitrum are,

1. The fraud proof design
   1. 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.
   2. 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.
   3. However, takes up to 2 weeks to finish (realistically 1 week), and requires original claimer to be online and cooperative.
2. "able to run unmodified EVM contracts and unmodified Ethereum transactions" (though that should now be true of [optimism] as well)
3. ...

Sequencing

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