> For the complete documentation index, see [llms.txt](https://docs.useicaria.xyz/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.useicaria.xyz/risk/sequencer-and-chain.md).

# Sequencer and chain risk

The venue's response to the chain beneath it going wrong.

Robinhood operates the single sequencer on Robinhood Chain, an Arbitrum Nitro rollup. For an L2 this early that arrangement is standard, and it carries particular failure modes that a venue trading around the clock has to plan for instead of wishing away.

## The things that can break

| Scenario                   | Effect on users                                                                                                                         |
| -------------------------- | --------------------------------------------------------------------------------------------------------------------------------------- |
| Sequencer outage           | Nothing is included on the L2 while it lasts, and the underlying market goes on moving regardless.                                      |
| Sequencer censorship       | Specific transactions get omitted, an LP's withdrawal or a maker's cancellation among them.                                             |
| Delayed L1 finality        | Withdrawals to Ethereum still take the canonical 7 days; trading on the L2 is untouched.                                                |
| Chain configuration change | Altering DA mode or the validator set, or switching on Timeboost or BoLD, would change what can be assumed about finality and ordering. |

## Mitigations

### A grace period on trading once an outage ends

The Chainlink L2 Sequencer Uptime Feed is read by `OracleRouter`. Once that feed indicates the sequencer has only just come back, every market stays `HALTED` for a grace period of **one hour**. Through the outage the underlying market kept moving while LPs could not withdraw and parameters could not react; the grace period exists so oracle rounds can refresh and participants can act before anyone gets to trade against a vault frozen mid-move. Withdrawals and cancellations work the instant recovery happens, and quoting is the only thing that waits.

### The L1 delayed inbox reaches everything

Arbitrum's delayed inbox on Ethereum accepts any state-changing function Icaria exposes, and a transaction the sequencer has ignored for 24 hours can be forced in. What that means concretely:

* no censoring sequencer can prevent an LP from withdrawing,
* no censoring sequencer can prevent a maker from cancelling quotes by nonce,
* the guardian's pause and the timelock's parameter changes are just as uncensorable.

Swaps themselves gain little from forced inclusion, since a quote dies long before 24 hours elapse, but nothing that protects funds needs the sequencer's cooperation.

### Timestamps rather than block numbers

`block.number` on Arbitrum-stack chains reports something derived from L1. Quote expiries, staleness bounds, grace periods and the timelock consequently use `block.timestamp` and nothing else. Where an actual L2 block height is needed, the protocol reaches for `ArbSys.arbBlockNumber()`.

### Calldata cost

Compressed calldata size is what the L1 data portion of gas is priced on. RFQ quote structs are packed tightly and signatures travel in the compact 64-byte form wherever that is possible, which holds settlement under a cent even through a blob-price spike.

## Keeping watch on ordering policy

First-come-first-served is today's sequencer policy, with preconfirmations near 100 ms. Turning on Timeboost (express-lane ordering) or BoLD (permissionless validation) would change how quote competition and forced inclusion behave, and both are things Icaria tracks. Favourable ordering is already something the design assumes it will not get, since fills are bounded by the signed slippage limit and the oracle band wherever they sit in a block, and any configuration change prompts a review published in the governance log.


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