L2 Output Root Proposals Specification

Table of Contents

Overview

After processing one or more blocks the outputs will need to be synchronized with the settlement layer (L1) for trustless execution of L2-to-L1 messaging, such as withdrawals. These output proposals act as the bridge's view into the L2 state. Actors called "Proposers" submit the output roots to the settlement layer (L1) and can be contested with a fault proof, with a bond at stake if the proof is wrong. The op-proposer in one such implementation of a proposer.

Note: Fault proofs on Optimism are not fully specified at this time. Although fault proof construction and verification is implemented in Cannon, the fault proof game specification and integration of a output-root challenger into the rollup-node are part of later specification milestones.

Proposing L2 Output Commitments

The proposer's role is to construct and submit output roots, which are commitments to the L2's state, to the L2OutputOracle contract on L1 (the settlement layer). To do this, the proposer periodically queries the rollup node for the latest output root derived from the latest finalized L1 block. It then takes the output root and submits it to the L2OutputOracle contract on the settlement layer (L1).

L2OutputOracle v1.0.0

The submission of output proposals is permissioned to a single account. It is expected that this account will continue to submit output proposals over time to ensure that user withdrawals do not halt.

The L2 output proposer is expected to submit output roots on a deterministic interval based on the configured SUBMISSION_INTERVAL in the L2OutputOracle. The larger the SUBMISSION_INTERVAL, the less often L1 transactions need to be sent to the L2OutputOracle contract, but L2 users will need to wait a bit longer for an output root to be included in L1 (the settlement layer) that includes their intention to withdraw from the system.

The honest op-proposer algorithm assumes a connection to the L2OutputOracle contract to know the L2 block number that corresponds to the next output proposal that must be submitted. It also assumes a connection to an op-node to be able to query the optimism_syncStatus RPC endpoint.

import time

while True:
    next_checkpoint_block = L2OutputOracle.nextBlockNumber()
    rollup_status = op_node_client.sync_status()
    if rollup_status.finalized_l2.number >= next_checkpoint_block:
        output = op_node_client.output_at_block(next_checkpoint_block)
        tx = send_transaction(output)
    time.sleep(poll_interval)

A CHALLENGER account can delete multiple output roots by calling the deleteL2Outputs() function and specifying the index of the first output to delete, this will also delete all subsequent outputs.

L2 Output Commitment Construction

The output_root is a 32 byte string, which is derived based on the a versioned scheme:

output_root = keccak256(version_byte || payload)

where:

  1. version_byte (bytes32) a simple version string which increments anytime the construction of the output root is changed.

  2. payload (bytes) is a byte string of arbitrary length.

In the initial version of the output commitment construction, the version is bytes32(0), and the payload is defined as:

payload = state_root || withdrawal_storage_root || latest_block_hash

where:

  1. The latest_block_hash (bytes32) is the block hash for the latest L2 block.

  2. The state_root (bytes32) is the Merkle-Patricia-Trie (MPT) root of all execution-layer accounts. This value is frequently used and thus elevated closer to the L2 output root, which removes the need to prove its inclusion in the pre-image of the latest_block_hash. This reduces the merkle proof depth and cost of accessing the L2 state root on L1.

  3. The withdrawal_storage_root (bytes32) elevates the Merkle-Patricia-Trie (MPT) root of the Message Passer contract storage. Instead of making an MPT proof for a withdrawal against the state root (proving first the storage root of the L2toL1MessagePasser against the state root, then the withdrawal against that storage root), we can prove against the L2toL1MessagePasser's storage root directly, thus reducing the verification cost of withdrawals on L1.

    After Isthmus hard fork, the withdrawal_storage_root is present in the block header as withdrawalsRoot and can be used directly, instead of computing the storage root of the L2toL1MessagePasser contract.

    Similarly, if Isthmus hard fork is active at the genesis block, the withdrawal_storage_root is present in the block header as withdrawalsRoot.

L2 Output Oracle Smart Contract

L2 blocks are produced at a constant rate of L2_BLOCK_TIME (2 seconds). A new L2 output MUST be appended to the chain once per SUBMISSION_INTERVAL which is based on a number of blocks. The exact number is yet to be determined, and will depend on the design of the fault proving game.

The L2 Output Oracle contract implements the following interface:

/**
 * @notice The number of the first L2 block recorded in this contract.
 */
uint256 public startingBlockNumber;

/**
 * @notice The timestamp of the first L2 block recorded in this contract.
 */
uint256 public startingTimestamp;

/**
 * @notice Accepts an L2 outputRoot and the timestamp of the corresponding L2 block. The
 * timestamp must be equal to the current value returned by `nextTimestamp()` in order to be
 * accepted.
 * This function may only be called by the Proposer.
 *
 * @param _l2Output      The L2 output of the checkpoint block.
 * @param _l2BlockNumber The L2 block number that resulted in _l2Output.
 * @param _l1Blockhash   A block hash which must be included in the current chain.
 * @param _l1BlockNumber The block number with the specified block hash.
*/
  function proposeL2Output(
      bytes32 _l2Output,
      uint256 _l2BlockNumber,
      bytes32 _l1Blockhash,
      uint256 _l1BlockNumber
  )

/**
 * @notice Deletes all output proposals after and including the proposal that corresponds to
 *         the given output index. Only the challenger address can delete outputs.
 *
 * @param _l2OutputIndex Index of the first L2 output to be deleted. All outputs after this
 *                       output will also be deleted.
 */
function deleteL2Outputs(uint256 _l2OutputIndex) external

/**
 * @notice Computes the block number of the next L2 block that needs to be checkpointed.
 */
function nextBlockNumber() public view returns (uint256)

Configuration

The startingBlockNumber must be at least the number of the first Bedrock block. The startingTimestamp MUST be the same as the timestamp of the start block.

The first outputRoot proposed will thus be at height startingBlockNumber + SUBMISSION_INTERVAL

Security Considerations

L1 Reorgs

If the L1 has a reorg after an output has been generated and submitted, the L2 state and correct output may change leading to a faulty proposal. This is mitigated against by allowing the proposer to submit an L1 block number and hash to the Output Oracle when appending a new output; in the event of a reorg, the block hash will not match that of the block with that number and the call will revert.