Parameters
Before migrating to Kailua, you'll need to decide on a few setup parameters and note them down for later use during the migration process.
Starting Block Number
You'll need to pick a block number for Kailua to start sequencing from.
The sequencing state according to your op-node at the block you pick will be immediately finalized.
When you choose to enable withdrawal against Kailua sequencing proposals, your users will be able to start withdrawals
using this finalized state.
You can postpone enabling withdrawals using Kailua at any later point in time after successful migration.
Proposal Output Count / Output Block Span
Each sequencing proposal in Kailua must cover a fixed number of L2 blocks, which will determine how much data must be published per proposal. Consequently, configuring how many output commitments are published per proposal and how many L2 blocks are covered per commitment will determine your proposer's DA costs for using Kailua and your validator's proving costs during dispute.
These commitments are published as Blobs, which means you should optimize your block span S to be S = B * 4096 + 1,
where B is the number of blobs required for a single proposal transaction (Ethereum currently limits a single block to
at most 6 blobs, i.e. B < 7).
Subsequently, combining S with your rollup's block time determines how often your sequencer has to publish a proposal
to ensure the liveness of your chain.
Consider Optimism Mainnnet as an example, which has a block time of 2 seconds. To keep its current average sequencing frequency of ~55 minutes, it only needs to publish ~1650 commitments per proposal. To maximize the utilization of the extra blob published when proposing, OP Mainnet can relax its proposal rate to once per 2 hours and 15 minutes.
Proposal Time Gap
This value can be safely set to zero when relying only on validity proving using a very large dispute timeout.
Because Kailua is designed for permissionless sequencing, it has an extra safety mechanism that can prevent sequencing proposals from being made eagerly before the parent chain data supporting that state is finalized.
This mechanism comes in the form of a forced delay between the timestamp of the L2 block being proposed, and the current timestamp on the parent chain (ethereum).
While the Kailua proposer agent won't publish a sequencing proposal until it is considered safe by op-node, the Kailua contracts allow you to enforce this requirement so that even an eager (potentially dishonest) proposer cannot have a head start!
The minimum value for the time gap is equal to the length of your rollup's sequencing window.
Consider a Rollup on Ethereum with a 50 block sequencing window.
At the time of writing, the Ethereum block time is 12 seconds.
Consequently, the minimum proposal time gap value should be 50 × 12 = 600 seconds (10 minutes).
At the time of writing, Ethereum finalizes each block in approximately 15 minutes.
Consequently, we recommend you add an extra 15 × 60 = 900 seconds to your gap to match.
Collateral Amount
The collateral requirements for a sequencer in Kailua come in the form of a fixed amount to be deposited, independent of how many sequencing proposals are in flight. This is because a malicious Kailua proposer, and any faulty sequencing proposals it has published, is eliminated using only a single fault proof.
The prover who submitted that fault proof consequently gets compensated with the faulty sequencer's staked collateral. This collateral should at least cover the proving cost, but should also include a sizeable tip for the prover to incentivize proving priority. Our estimates put a worst-case proving cost using Bonsai for a single (OP Mainnet) block at $100 USD.
Currently, OP Mainnet requires 0.08 ETH (~$300) of collateral per proposal, and finalizes a proposal after at least 3.5
days if it is undisputed.
This means, at an average hourly rate of proposing, the proposer has 84 * 0.08 = 6.72 ETH (~$3700 USD) on average
locked up as collateral in the best case where no disputes take place.
Using Kailua, 0.08 ETH would be sufficient as the total collateral locked up by the proposer, even under the same finality delay. This would cover the worst-case proving cost in case of dispute, and, discounting transaction costs, leave a $200 tip.
Challenge Timeout
The current implementation of Kailua does not yet have adaptive dispute periods based on congestion. Consequently, you should keep your existing challenge timeout period.
If you wish to practically operate Kailua using only validity proofs, set this value to 31536000000000000 seconds
(i.e. 1 billion calendar years).
Verifier Contract
RISC Zero maintains a set of pre-deployed verifier contracts for its ZK proving system. These contracts are regularly upgraded to support new releases of the prover, and also have a permissionless fail-safe mechanism that anyone who can produce a proof-of-exploit can trigger to halt the verifier.
You must ensure that the chosen verifier contract supports your RISC Zero zkVM version. Once a new zkVM version is released, there can be a delay in adding it to the router.
You have the choice of either using the already deployed verifier for your parent chain, or deploying and maintaining your own verifier contracts, as described in the later sections.
Vanguard Advantage
Kailua supports the designation of one proposer as a Vanguard, along with an advantage time for said proposer. When set, this designation prevents any other proposer from publishing a proposal for a certain block height until either the Vanguard has made a proposal for that height, or the advantage time granted to the Vanguard has passed. The advantage time is counted down until after the Vanguard was allowed to publish a proposal. The Vanguard still needs to lock up the necessary bond to make proposals.
Having a Vanguard that makes honest proposals within the advantage time guarantees the safety of the rollup.
If the advantage time is unreasonably long, Vanguard downtime delays the liveness of the rollup.
This middleground enables rollups transitioning from a permissioned scheme to enjoy permissionless fault proofs in Kailua with an added safety net.