# Rollups explained: optimistic vs zero-knowledge

*Written by the RakeMint team.*

Rollups are the most common kind of layer 2 network on Ethereum. They bundle many transactions together, process them off the main chain, and post the results back to Ethereum. There are two main families, optimistic and zero-knowledge, and they differ in how they prove that the results are correct.

## Why rollups exist

Ethereum's block space is limited, so fees rise when demand is high. Rollups move execution elsewhere and use Ethereum mainly for data and settlement. ethereum.org's [layer 2 overview](https://ethereum.org/en/layer-2/) explains that this lets them offer lower fees while relying on Ethereum's security.

## Optimistic rollups

Optimistic rollups assume transactions are valid by default and post them to Ethereum. There's a **challenge window** during which anyone can submit a "fraud proof" showing an invalid result. If nobody does, the result stands.

* **Example:** Arbitrum One.
* **Everyday experience:** fast confirmations from the sequencer, with low fees.
* **Trade-off:** withdrawing directly to Ethereum through the official bridge can take days because of the challenge window. Third-party services can move funds faster, with their own risks and fees.

Arbitrum's [finality documentation](https://docs.arbitrum.io/how-arbitrum-works/deep-dives/finality) separates soft finality (sequencer feed, under a second) from hard finality (once data is final on Ethereum).

## Zero-knowledge (ZK) rollups

ZK rollups generate a cryptographic **validity proof** for each batch, showing the new state is correct. Ethereum verifies the proof, so no challenge window is needed.

* **Everyday experience:** low fees; withdrawals to Ethereum can settle once proofs are verified.
* **Trade-off:** generating proofs is computationally heavy and the technology is more complex.

## Side by side

|  | Optimistic | Zero-knowledge |
| --- | --- | --- |
| Correctness | Assumed, can be challenged | Proven cryptographically |
| Withdrawal to Ethereum via native bridge | Typically days | Typically faster once proven |
| Maturity | Widely used | Rapidly developing |
| Fees for users | Low | Low |

## The sequencer

Most rollups today use a **sequencer** to order transactions. It gives you fast confirmations, but it's often run by a single operator. If it goes down, transactions can be delayed. Many rollups have mechanisms that let users force transactions onto Ethereum if the sequencer misbehaves, but these are slower.

## What it means for everyday transfers

* **Fees include Ethereum data costs**, so they can rise when Ethereum is busy.
* **Gas is usually paid in ETH** on Ethereum rollups.
* **Each rollup is its own network** with its own token contracts, so match the network when sending.
* **Bridging out** to Ethereum or another chain takes extra steps; it's often simpler to use a service that supports the rollup directly.

## Questions to ask about any rollup

1. Is it optimistic or ZK?
2. Who runs the sequencer, and what happens if it stops?
3. How long does a withdrawal to Ethereum take through the official bridge?
4. Do the services you use support this network for both deposits and withdrawals?

## Key terms

* **Batch:** a group of rollup transactions posted together.
* **Fraud proof:** evidence that an optimistic rollup batch is invalid.
* **Validity proof:** a cryptographic proof that a ZK rollup batch is correct.
* **Challenge window:** the period in which fraud proofs can be submitted.
* **Sequencer:** the operator that orders transactions on the rollup.

## The short version

Rollups make Ethereum transactions cheaper by processing them off-chain. Optimistic rollups assume correctness and allow challenges; ZK rollups prove correctness upfront. For everyday use, both are fast and cheap. The main practical issues are matching networks and knowing how long it takes to move funds back to Ethereum.

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