Rollup Protocol


The big idea that makes Optimism possible is the Optimistic Rollup. We'll go through a brief explainer of how Optimistic Rollups work at a high level. Then we'll explain why Optimism is built as an Optimistic Rollup and why we believe it's the best option for a system that addresses all of our design goals.

# Optimistic Rollups TL;DR

Optimism is an "Optimistic Rollup," which is basically just a fancy way of describing a blockchain that piggy-backs off of the security of another "parent" blockchain. Specifically, Optimistic Rollups take advantage of the consensus mechanism (like PoW or PoS) of their parent chain instead of providing their own. In Optimism's case this parent blockchain is Ethereum.

# Block storage

Pre-Bedrock (current version)

All Optimism blocks are stored within a special smart contract on Ethereum called the CanonicalTransactionChain (opens new window) (or CTC for short). Optimism blocks are held within an append-only list inside of the CTC (we'll explain exactly how blocks are added to this list in the next section). This append-only list forms the Optimism blockchain.

The CanonicalTransactionChain includes code that guarantees that the existing list of blocks cannot be modified by new Ethereum transactions. However, this guarantee can be broken if the Ethereum blockchain itself is reorganized and the ordering of past Ethereum transactions is changed. The Optimism mainnet is configured to be robust against block reorganizations of up to 50 Ethereum blocks. If Ethereum experiences a reorg larger than this, Optimism will reorg as well.

Of course, it's a key security goal of Ethereum to not experience these sort of significant block reorganizations. Optimism is therefore secure against large block reorganizations as long as the Ethereum consensus mechanism is too. It's through this relationship (in part, at least) that Optimism derives its security properties from Ethereum.

Bedrock (coming Q1 2023)

In Bedrock L2 blocks are saved to the Ethereum blockchain using a non-contract address (0xDeadDeAddeAddEAddeadDEaDDEAdDeaDDeAD0001 (opens new window)), to minimize the L1 gas expense. As these blocks are submitted as transaction calldata on Ethereum, there is no way to modify or censor them after the "transaction" is included in a block that has enough attestations. This is the way that Optimism inherits the availability and integrity guarantees of Ethereum.

Blocks are written to L1 in a compressed format (opens new window) to reduce costs. This is important because writing to L1 is the major cost of Optimism transactions.

# Block production

Optimism block production is primarily managed by a single party, called the "sequencer," which helps the network by providing the following services:

  • Providing transaction confirmations and state updates.
  • Constructing and executing L2 blocks.
  • Submitting user transactions to L1.
Pre-Bedrock (current version)

The sequencer has no mempool and transactions are immediately accepted or rejected in the order they were received. When a user sends their transaction to the sequencer, the sequencer checks that the transaction is valid (i.e. pays a sufficient fee) and then applies the transaction to its local state as a pending block. These pending blocks are periodically submitted in large batches to Ethereum for finalization. This batching process significantly reduces overall transaction fees by spreading fixed costs over all of the transactions within a given batch. The sequencer also applies some basic compression techniques to minimize the amount of data published to Ethereum.

Because the sequencer is given priority write access to the L2 chain, the sequencer can provide a strong guarantee of what state will be finalized as soon as it decides on a new pending block. In other words, it is precisely known what will be the impact of the transaction. As a result, the L2 state can be reliably updated extremely quickly. Benefits of this include a snappy, instant user experience, with things like near-real-time Uniswap price updates.

Alternatively, users can skip the sequencer entirely and submit their transactions directly to the CanonicalTransactionChain via an Ethereum transaction. This is typically more expensive because the fixed cost of submitting this transaction is paid entirely by the user and is not amortized over many different transactions. However, this alternative submission method has the advantage of being resistant to censorship by the sequencer. Even if the sequencer is actively censoring a user, the user can always continue to use Optimism and recover any funds through this mechanism.

Bedrock (coming Q1 2023)

In Bedrock the sequencer does have a mempool, similar to L1 Ethereum, but the mempool is private to avoid opening opportunities for MEV. Blocks are produced every two seconds, regardless of whether they are empty (no transactions), filled up to the block gas limit with transactions, or anything in between.

Transactions get to the sequencer in two ways:

  1. Transactions submitted on L1 (called deposits whether they have assets attached or not) are included in the chain in the appropriate L2 block. Every L2 block is identified by the "epoch" (the L1 block to which it corresponds, which typically has happened a few minutes before the L2 block) and its serial number within that epoch. The first block of the epoch includes all the deposits that happened in the L1 block to which it corresponds. If the sequencer attempts to ignore a legitimate L1 transaction it ends up with a state that is inconsistent with the verifiers, same as if the sequencer tried to fake the state by other means. This provides Optimism with L1 Ethereum level censorship resistance. You can read more about this mechanism is the protocol specifications (opens new window).

  2. Transactions submitted directly to the sequnecer. These transactions are a lot cheaper to submit (because you do not need the expense of a separate L1 transaction), but of course they cannot be made censorship resistant, because the sequencer is the only entity that knows about them.

For the moment, The Optimism Foundation (opens new window) runs the only block producer. Refer to Protocol specs section for more information about how we plan to decentralize the Sequencer role in the future.

# Block execution

Pre-Bedrock (current version)

Ethereum nodes download blocks from Ethereum's p2p network. Optimism nodes instead download blocks directly from the append-only list of blocks held within the CanonicalTransactionChain contract. See the above section regarding block storage for more information about how blocks are stored within this contract.

Optimism nodes are made up of two primary components, the Ethereum data indexer and the Optimism client software. The Ethereum data indexer, also called the "data transport layer" (opens new window) (or DTL), reconstructs the Optimism blockchain from blocks published to the CanonicalTransactionChain contract. The DTL searches for events emitted by the CanonicalTransactionChain that signal that new Optimism blocks have been published. It then inspects the transactions that emitted these events to reconstruct the published blocks in the standard Ethereum block format (opens new window).

The second part of the Optimism node, the Optimism client software, is an almost completely vanilla version of Geth (opens new window). This means Optimism is close to identical to Ethereum under the hood. In particular, Optimism shares the same Ethereum Virtual Machine (opens new window), the same account and state structure (opens new window), and the same gas metering mechanism and fee schedule (opens new window). We refer to this architecture as "EVM Equivalence" (opens new window) and it means that most Ethereum tools (even the most complex ones) "just work" with Optimism.

The Optimism client software continuously monitors the DTL for newly indexed blocks. When a new block is indexed, the client software will download it and execute the transactions included within it. The process of executing a transaction on Optimism is the same as on Ethereum: we load the Optimism state, apply the transaction against that state, and then record the resulting state changes. This process is then repeated for each new block indexed by the DTL.

Bedrock (coming Q1 2023)

The execution engine (implemented as the op-geth component) receive blocks using two mechanisms:

  1. The execution engine can update itself using peer to peer network with other execution engines. This operates the same way that the L1 execution clients synchronize the state across the network. You can read more about it in the specs (opens new window).

  2. The rollup node (implemented as the op-node component) derives the L2 blocks from L1. This mechanism is slower, but censorship resistant. You can read more about it in the specs (opens new window).

# Bridging assets between layers

Optimism is designed so that users can send arbitrary messages between smart contracts on Optimism and Ethereum. This makes it possible to transfer assets, including ERC20 tokens, between the two networks. The exact mechanism by which this communication occurs differs depending on the direction in which messages are being sent.

Optimism uses this functionality in the Standard bridge to allow users to deposit assets (ERC20s and ETH) from Ethereum to Optimism and also allow withdrawals of the same from Optimism back to Ethereum. See the developer documentation and examples on details on the inner workings of the Standard bridge.

# Moving from Ethereum to Optimism

In Optimism terminology, transactions going from Ethereum (L1) to Optimism (L2) are called deposits, even if they do not have any assets attached to them.

Pre-Bedrock (current version)

To send messages from Ethereum to Optimism, users simply need to trigger the CanonicalTransactionChain contract on Ethereum to create a new block on Optimism block. See the above section on block production for additional context. User-created blocks can include transactions that will appear to originate from the address that generated the block.

Bedrock (coming Q1 2023)

The contract interface for deposits is very similar, you use L1CrossDomainMessenger (opens new window) or L1StandardBridge (opens new window). Deposit transactions become part of the canonical blockchain in the first L2 block of the "epoch" corresponding to the L1 block where the deposits were made. This L2 block will usually be created a few minutes after the corresponding L1 block. You can read more about this in the specs (opens new window).

# Moving from Optimism to Ethereum

Pre-Bedrock (current version)

It's not possible for contracts on Optimism to easily generate transactions on Ethereum in the same way as Ethereum contracts can generate transactions on Optimism. As a result, the process of sending data from Optimism back to Ethereum is somewhat more involved. Instead of automatically generating authenticated transactions, we must instead be able to make provable statements about the state of Optimism to contracts sitting on Ethereum.

Making provable statements about the state of Optimism requires a cryptographic commitment (opens new window) in the form of the root of the Optimism's state trie (opens new window). Optimism's state is updated after each block, so this commitment will also change after every block. Commitments are regularly published (approximately once or twice per hour) to a smart contract on Ethereum called the StateCommitmentChain (opens new window).

Users can use these commitments to generate Merkle tree proofs (opens new window) about the state of Optimism. These proofs can be verified by smart contracts on Ethereum. Optimism maintains a convenient cross-chain communication contract, the L1CrossDomainMessenger (opens new window), which can verify these proofs, typically used for withdrawals, on behalf of other contracts.

These proofs can be used to make verifiable statements about the data within the storage of any contract on Optimism at a specific block height. This basic functionality can then be used to enable contracts on Optimism to send messages to contracts on Ethereum. The L2ToL1MessagePasser (opens new window) contract (predeployed to the Optimism network) can be used by contracts on Optimism to store a message in the Optimism state. Users can then prove to contracts on Ethereum that a given contract on Optimism did, in fact, mean to send some given message by showing that the hash of this message has been stored within the L2ToL1MessagePasser contract.

Bedrock (coming Q1 2023)

Withdrawals (the term is used for any Optimism to Ethereum message, regardless of whether it has attached assets or not) have three stages:

  1. You initialize withdrawals with an L2 transaction.

  2. Wait for the next output root to be submitted to L1 (you can see this on the SDK) and then submit the withdrawal proof using proveWithdrawalTransaction. This new step enables off-chain monitoring of the withdrawals, which makes it easier to identify incorrect withdrawals or output roots. This protects Optimism users against a whole class of potential bridge vulnerabilities.

  3. After the fault challenge period ends (a week on mainnet, less than that on the test network), finalize the withdrawal.

You can read the full withdrawal specifications here (opens new window)

# Fault proofs

In an Optimistic Rollup, state commitments are published to Ethereum without any direct proof of the validity of these commitments. Instead, these commitments are considered pending for a period of time (called the "challenge window"). If a proposed state commitment goes unchallenged for the duration of the challenge window (currently set to 7 days), then it is considered final. Once a commitment is considered final, smart contracts on Ethereum can safely accept withdrawal proofs about the state of Optimism based on that commitment.

When a state commitment is challenged, it can be invalidated through a "fault proof" (formerly known as a "fraud proof" (opens new window)) process. If the commitment is successfully challenged, then it is removed from the StateCommitmentChain to eventually be replaced by another proposed commitment. It's important to note that a successful challenge does not roll back Optimism itself, only the published commitments about the state of the chain. The ordering of transactions and the state of Optimism is unchanged by a fault proof challenge.

The fault proof process is currently undergoing major redevelopment as a side-effect of the November 11th EVM Equivalence (opens new window) update. You can read more about this process within the Protocol specs section of this website.