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Overview

The cross domain messengers are responsible for providing a higher level API for developers who are interested in sending cross domain messages. They allow for the ability to replay cross domain messages and sit directly on top of the lower level system contracts responsible for cross domain messaging on L1 and L2. The CrossDomainMessenger is extended to create both an L1CrossDomainMessenger as well as a L2CrossDomainMessenger. These contracts are then extended with their legacy APIs to provide backwards compatibility for applications that integrated before the Bedrock system upgrade. The L2CrossDomainMessenger is a predeploy contract located at 0x4200000000000000000000000000000000000007. The base CrossDomainMessenger interface is: 
interface CrossDomainMessenger {
    event FailedRelayedMessage(bytes32 indexed msgHash);
    event RelayedMessage(bytes32 indexed msgHash);
    event SentMessage(address indexed target, address sender, bytes message, uint256 messageNonce, uint256 gasLimit);
    event SentMessageExtension1(address indexed sender, uint256 value);

    function MESSAGE_VERSION() external view returns (uint16);
    function MIN_GAS_CALLDATA_OVERHEAD() external view returns (uint64);
    function MIN_GAS_CONSTANT_OVERHEAD() external view returns (uint64);
    function MIN_GAS_DYNAMIC_OVERHEAD_DENOMINATOR() external view returns (uint64);
    function MIN_GAS_DYNAMIC_OVERHEAD_NUMERATOR() external view returns (uint64);
    function OTHER_MESSENGER() external view returns (address);
    function baseGas(bytes memory _message, uint32 _minGasLimit) external pure returns (uint64);
    function failedMessages(bytes32) external view returns (bool);
    function messageNonce() external view returns (uint256);
    function relayMessage(
        uint256 _nonce,
        address _sender,
        address _target,
        uint256 _value,
        uint256 _minGasLimit,
        bytes memory _message
    ) external payable returns (bytes memory returnData_);
    function sendMessage(address _target, bytes memory _message, uint32 _minGasLimit) external payable;
    function successfulMessages(bytes32) external view returns (bool);
    function xDomainMessageSender() external view returns (address);
}

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Message Passing

The sendMessage function is used to send a cross domain message. To trigger the execution on the other side, the relayMessage function is called. Successful messages have their hash stored in the successfulMessages mapping while unsuccessful messages have their hash stored in the failedMessages mapping. The user experience when sending from L1 to L2 is a bit different than when sending a transaction from L2 to L1. When going from L1 into L2, the user does not need to call relayMessage on L2 themselves. The user pays for L2 gas on L1 and the transaction is automatically pulled into L2 where it is executed on L2. When going from L2 into L1, the user proves their withdrawal on OptimismPortal, then waits for the finalization window to pass, and then finalizes the withdrawal on the OptimismPortal, which calls relayMessage on the L1CrossDomainMessenger to finalize the withdrawal.

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Upgradability

The L1 and L2 cross domain messengers should be deployed behind upgradable proxies. This will allow for updating the message version.

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Message Versioning

Messages are versioned based on the first 2 bytes of their nonce. Depending on the version, messages can have a different serialization and hashing scheme. The first two bytes of the nonce are reserved for version metadata because a version field was not originally included in the messages themselves, but a uint256 nonce is so large that we can very easily pack additional data into that field.

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Message Version 0

abi.encodeWithSignature(
    "relayMessage(address,address,bytes,uint256)",
    _target,
    _sender,
    _message,
    _messageNonce
);

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Message Version 1

abi.encodeWithSignature(
    "relayMessage(uint256,address,address,uint256,uint256,bytes)",
    _nonce,
    _sender,
    _target,
    _value,
    _gasLimit,
    _data
);

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Backwards Compatibility Notes

An older version of the messenger contracts had the concept of blocked messages in a blockedMessages mapping. This functionality was removed from the messengers because a smart attacker could get around any message blocking attempts. It also saves gas on finalizing withdrawals. The concept of a “relay id” and the relayedMessages mapping was removed. It was built as a way to be able to fund third parties who relayed messages on the behalf of users, but it was improperly implemented as it was impossible to know if the relayed message actually succeeded.