Introduction

Table of Contents

Foundations
Protocol Guarantees
Network Participants
Key Interaction Diagrams
- Depositing and Sending Transactions
- Withdrawing
Next Steps

Optimism is an EVM equivalent, optimistic rollup protocol designed to scale Ethereum while remaining maximally compatible with existing Ethereum infrastructure. This document provides an overview of the protocol to provide context for the rest of the specification.

Foundations

What is Ethereum scalability?

Scaling Ethereum means increasing the number of useful transactions the Ethereum network can process. Ethereum's limited resources, specifically bandwidth, computation, and storage, constrain the number of transactions which can be processed on the network. Of the three resources, computation and storage are currently the most significant bottlenecks. These bottlenecks limit the supply of transactions, leading to extremely high fees. Scaling ethereum and reducing fees can be achieved by better utilizing bandwidth, computation and storage.

What is an Optimistic Rollup?

Optimistic rollup is a layer 2 scalability technique which increases the computation & storage capacity of Ethereum without sacrificing security or decentralization. Transaction data is submitted on-chain but executed off-chain. If there is an error in the off-chain execution, a fault proof can be submitted on-chain to correct the error and protect user funds. In the same way you don't go to court unless there is a dispute, you don't execute transactions on on-chain unless there is an error.

What is EVM Equivalence?

EVM Equivalence is complete compliance with the state transition function described in the Ethereum yellow paper, the formal definition of the protocol. By conforming to the Ethereum standard across EVM equivalent rollups, smart contract developers can write once and deploy anywhere.

🎶 All together now 🎶

Optimism is an EVM equivalent, optimistic rollup protocol designed to scale Ethereum.

Protocol Guarantees

In order to scale Ethereum without sacrificing security, we must preserve 3 critical properties of Ethereum layer 1: liveness, availability, and validity.

Liveness - Anyone must be able to extend the rollup chain by sending transactions at any time.
- There are two ways transactions can be sent to the rollup chain: 1) via the sequencer, and 2) directly on layer 1. The sequencer provides low latency & low cost transactions, while sending transactions directly to layer 1 provides censorship resistance.
Availability - Anyone must be able to download the rollup chain.
- All information required to derive the chain is embedded into layer 1 blocks. That way as long as the layer 1 chain is available, so is the rollup.
Validity - All transactions must be correctly executed and all withdrawals correctly processed.
- The rollup state and withdrawals are managed on an L1 contract called the L2OutputOracle. This oracle is guaranteed to only finalize correct (ie. valid) rollup block hashes given a single honest verifier assumption. If there is ever an invalid block hash asserted on layer 1, an honest verifier will prove it is invalid and win a bond.

Footnote: There are two main ways to enforce validity of a rollup: fault proofs (optimistic rollup) and validity proofs (zkRollup). For the purposes of this spec we only focus on fault proofs but it is worth noting that validity proofs can also be plugged in once they have been made feasible.

Network Participants

There are three actors in Optimism: users, sequencers, and verifiers.

Users

At the heart of the network are users (us!). Users can:

Deposit or withdraw arbitrary transactions on L2 by sending data to a contract on Ethereum mainnet.
Use EVM smart contracts on layer 2 by sending transactions to the sequencers.
View the status of transactions using block explorers provided by network verifiers.

Sequencers

The sequencer is the primary block producer. There may be one sequencer or many using a consensus protocol. For 1.0.0, there is just one sequencer (currently operated under the oversight of the Optimism Foundation). In general, specifications may use "the sequencer" to be a stand-in term for the consensus protocol operated by multiple sequencers.

The sequencer:

Accepts user off-chain transactions
Observes on-chain transactions (primarily, deposit events coming from L1)
Consolidates both kinds of transactions into L2 blocks with a specific ordering.
Propagates consolidated L2 blocks to L1, by submitting two things as calldata to L1:
- The pending off-chain transactions accepted in step 1.
- Sufficient information about the ordering of the on-chain transactions to successfully reconstruct the blocks from step 3., purely by watching L1.

The sequencer also provides access to block data as early as step 3., so that users may access real-time state in advance of L1 confirmation if they so choose.

Verifiers

Verifiers serve two purposes:

Serving rollup data to users; and
Verifying rollup integrity and disputing invalid assertions.

In order for the network to remain secure there must be at least one honest verifier who is able to verify the integrity of the rollup chain & serve blockchain data to users.

Key Interaction Diagrams

The following diagrams demonstrate how protocol components are utilized during key user interactions in order to provide context when diving into any particular component specification.

Depositing and Sending Transactions

Users will often begin their L2 journey by depositing ETH from L1. Once they have ETH to pay fees, they'll start sending transactions on L2. The following diagram demonstrates this interaction and all key Optimism components which are or should be utilized:

Links to components mentioned in this diagram:

Withdrawing

Just as important as depositing, it is critical that users can withdraw from the rollup. Withdrawals are initiated by normal transactions on L2, but then completed using a transaction on L1 after the dispute period has elapsed.