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feat: add new legacy section, move l2OO
Adds a new legacy section and moves the L2 Output Oracle spec into the new section. Replaces current Proposals doc with a temporary pointer to the fault proofs doc.
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| # L2 Output Root Proposals Specification (LEGACY) | ||
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| <!-- START doctoc generated TOC please keep comment here to allow auto update --> | ||
| <!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE --> | ||
| **Table of Contents** | ||
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| - [LEGACY CONTENT](#legacy-content) | ||
| - [Overview](#overview) | ||
| - [Proposing L2 Output Commitments](#proposing-l2-output-commitments) | ||
| - [L2OutputOracle v1.0.0](#l2outputoracle-v100) | ||
| - [L2 Output Commitment Construction](#l2-output-commitment-construction) | ||
| - [L2 Output Oracle Smart Contract](#l2-output-oracle-smart-contract) | ||
| - [Configuration](#configuration) | ||
| - [Security Considerations](#security-considerations) | ||
| - [L1 Reorgs](#l1-reorgs) | ||
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| <!-- END doctoc generated TOC please keep comment here to allow auto update --> | ||
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| <!-- All glossary references in this file. --> | ||
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| [g-rollup-node]: ../glossary.md#rollup-node | ||
| [g-mpt]: ../glossary.md#merkle-patricia-trie | ||
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| ## LEGACY CONTENT | ||
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| This is legacy content for a previous version of the OP Stack and is no longer current. | ||
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| ## Overview | ||
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| 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][op-proposer] in one such implementation of a proposer. | ||
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| [op-proposer]: https:/ethereum-optimism/optimism/tree/d48b45954c381f75a13e61312da68d84e9b41418/op-proposer | ||
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| _Note_: Fault proofs on Optimism are not fully specified at this time. Although fault proof | ||
| construction and verification [is implemented in Cannon][cannon], | ||
| the fault proof game specification and integration of a output-root challenger into the [rollup-node][g-rollup-node] | ||
| are part of later specification milestones. | ||
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| [cannon]: https:/ethereum-optimism/cannon | ||
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| ## Proposing L2 Output Commitments | ||
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| 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](../protocol/rollup-node.md) for the latest output root derived from the latest | ||
| [finalized](../protocol/rollup-node.md#finalization-guarantees) L1 block. It then takes the output root and | ||
| submits it to the `L2OutputOracle` contract on the settlement layer (L1). | ||
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| ### L2OutputOracle v1.0.0 | ||
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| 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. | ||
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| The [L2 output proposer][op-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. | ||
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| 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. | ||
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| ```python | ||
| import time | ||
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| 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) | ||
| ``` | ||
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| 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. | ||
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| ## L2 Output Commitment Construction | ||
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| The `output_root` is a 32 byte string, which is derived based on the a versioned scheme: | ||
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| ```pseudocode | ||
| output_root = keccak256(version_byte || payload) | ||
| ``` | ||
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| where: | ||
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| 1. `version_byte` (`bytes32`) a simple version string which increments anytime the construction of the output root | ||
| is changed. | ||
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| 2. `payload` (`bytes`) is a byte string of arbitrary length. | ||
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| In the initial version of the output commitment construction, the version is `bytes32(0)`, and the payload is defined | ||
| as: | ||
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| ```pseudocode | ||
| payload = state_root || withdrawal_storage_root || latest_block_hash | ||
| ``` | ||
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| where: | ||
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| 1. The `latest_block_hash` (`bytes32`) is the block hash for the latest L2 block. | ||
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| 1. The `state_root` (`bytes32`) is the Merkle-Patricia-Trie ([MPT][g-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. | ||
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| 1. The `withdrawal_storage_root` (`bytes32`) elevates the Merkle-Patricia-Trie ([MPT][g-mpt]) root of the [Message | ||
| Passer contract](../protocol/withdrawals.md#the-l2tol1messagepasser-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. | ||
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| ## L2 Output Oracle Smart Contract | ||
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| 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. | ||
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| The L2 Output Oracle contract implements the following interface: | ||
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| ```solidity | ||
| /** | ||
| * @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) | ||
| ``` | ||
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| ### Configuration | ||
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| 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. | ||
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| The first `outputRoot` proposed will thus be at height `startingBlockNumber + SUBMISSION_INTERVAL` | ||
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| ## Security Considerations | ||
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| ### L1 Reorgs | ||
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| 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. |
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