Table of Contents
- Proposing L2 Output Commitments
- L2 Output Commitment Construction
- L2 Output Oracle Smart Contract
- Security Considerations
- Summary of Definitions
After processing one or more blocks the outputs will need to be synchronized with L1 for trustless execution of L2-to-L1 messaging, such as withdrawals. Outputs are hashed in a tree-structured form which minimizes the cost of proving any piece of data captured by the outputs. Proposers submit the output roots to L1 and can be contested with a fault proof, with a bond at stake if the proof is wrong.
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.
The proposer's role is to construct and submit output roots, which are commitments made on a configurable interval,
to the L2OutputOracle contract running on L1. It does this by running the L2 output proposer,
a service which periodically queries the rollup node's
optimism_outputAtBlock rpc method for the latest output root derived
from the latest finalized L1 block. The construction of this output root is
described below.
If there is no newly finalized output, the service continues querying until it receives one. It then submits this
output, and the appropriate timestamp, to the L2 Output Root contract's
proposeL2Output() function. The timestamp block number must correspond to the startingBlockNumber plus the next
multiple of the SUBMISSION_INTERVAL value.
The proposer may also 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.
Note regarding future work: In the initial version of the system, the proposer will be the same entity as the sequencer, which is a trusted role. In the future proposers may need to submit a bond in order to post L2 output roots, and some or all of this bond may be taken in the event of a faulty proposal.
The output_root is a 32 byte string, which is derived based on the a versioned scheme:
output_root = keccak256(version_byte || payload)
where:
-
version_byte(bytes32) a simple version string which increments anytime the construction of the output root is changed. -
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:
-
The
latest_block_hash(bytes32) is the block hash for the latest L2 block. -
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 thelatest_block_hash. This reduces the merkle proof depth and cost of accessing the L2 state root on L1. -
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.
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 getNextBlockNumber() public view returns (uint256)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
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.
| Name | Value | Unit |
|---|---|---|
SUBMISSION_INTERVAL |
TBD | blocks |
L2_BLOCK_TIME |
2 |
seconds |