Post-Dispatch Interchain Gas

During the post-dispatch hook, the InterchainGasPaymaster contract will revert if payment is insufficient to cover the relayer’s anticipated costs. The gas quote calculated at dispatch time must align with the relayer’s anticipated costs.

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Gas Limit

The gasLimit is set based on the cost of calling handle on the destination chain for a given message. This can vary depending on message content and the logic of the handler.

The default gasLimit for metering the handle call is a static default of 50_000 gas. This is sufficient for simple operations but may not be enough for more complex handle functions.

If your handle function performs complex operations or requires more gas, you must override the default gasLimit in metadata to avoid transaction reverts. Benchmark your handle implementations in unit tests to determine a reasonable gasLimit to use.

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Metadata

This hook expects metadata in a packed encoding of StandardHookMetadata. See the Mailbox dispatch overloads for how to pass metadata overrides.

struct StandardHookMetadata {
    uint16 variant;
    uint256 msgValue;
    uint256 gasLimit;
    address refundAddress;
}

The StandardHookMetadata struct defines the fields required for metadata encoding:

• variant: Specifies the metadata format version.
• msgValue: Amount of native token sent with the message.
• gasLimit: Gas limit for the handle function on the destination chain. Ensure this matches your simulation results.
• refundAddress: Address to refund unused gas payments.

To encode this metadata, use the StandardHookMetadata.formatMetadata library function. Direct encoding of the struct with abi.encodePacked is not supported in Solidity.

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Example Usage

// Example: Encoding metadata using StandardHookMetadata
bytes memory metadata = StandardHookMetadata.formatMetadata(
    0,               // ETH message value
    200000,          // Custom gas limit
    address(this),   // Refund address
    bytes("")        // Optional custom metadata
);

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Determine and Override the Gas Limit

1. Simulate and Benchmark Gas Usage:

• Use tools like Tenderly or Foundry to simulate the handle function on your message recipient. Ensure the from address is set to the Mailbox contract on your chain.
• If gas usage exceeds 50,000 gas, calculate an appropriate gasLimit and update your metadata.
• Use the call-to-action in the Hyperlane Explorer to simulate transactions from the message details.

2. Update Your Metadata:

• Calculate the required gasLimit based on the simulation.
• Pass the updated gasLimit in your metadata to ensure the relayer will deliver your message.

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Destination Gas Config

For each remote domain, the InterchainGasPaymaster sets the domain gas config.

struct DomainGasConfig {
    IGasOracle gasOracle;
    uint96 gasOverhead;
}

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Gas overhead

The gas overhead is set as part of the destination gas configuration. This corresponds to the operational cost of processing a message on the destination chain.

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Gas Oracles

Interchain Gas Payment requirements are calculated using oraclized gas prices and exchange rates between the origin and destination chains.

IGP contracts may be configured with gas oracles, which are responsible for tracking remote token gas prices and exchange rates. Developers should use the quoteDispatch function on the Mailbox contract to calculate gas fees. The quoteDispatch accounts for system-level overhead and ensures accurate fee calculation for the entire dispatch process.

Eventually, a relayer will be able to automatically update their gas oracles in order to ensure that their IGP always quotes a fair price for remote gas.

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getExchangeRateAndGasPrice

function getExchangeRateAndGasPrice(
    uint32 destinationDomain
) external view returns (uint128 tokenExchangeRate, uint128 gasPrice);

Parameters

• destinationDomain: The message’s destination domain

Returns

• tokenExchangeRate: The exchange rate between the origin and destination chain’s gas tokens
• gasPrice: The gas price for the destination chain

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quoteGasPayment

The quoteGasPayment function calculates fees for the relayer’s anticipated costs.

function quoteGasPayment(
    uint32 destinationDomain,
    uint256 gasLimit
) external view returns (uint256 fee);

Parameters

• destinationDomain: The message’s destination domain
• gasLimit: The gas limit to meter the handle call with

Returns

• fee: The payment required for the postDispatch to succeed

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Retrying

If the handle call consumes more gas than quoted, the relayer will not submit a process transaction. This issue often occurs due to insufficient gas payment during the initial dispatch.

In this case, additional gas can be paid for with the payForGas function.

function payForGas(
    bytes32 messageId,
    uint32 destinationDomain,
    uint256 gasAmount,
    address refundAddress
) external payable;

Parameters

• messageId: The message identifier returned from dispatch call
• destinationDomain: The message’s destination domain
• gasAmount: The additional gas amount to pay for
• refundAddress: The address to refund excess payment to

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Using the Hyperlane Explorer for Debugging

The Hyperlane Explorer is a powerful tool to debug cross-chain message issues, including gas payments and relayer behavior.

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Key Features

• Message Status: View the current status of your message (e.g., “Retry: GasPaymentRequirementNotMet”).
• Gas Payment Details: Check the gas amount paid (Origin IGP gas amount) and the amount required by the relayer.
• Simulate Calldata: Use the “View calldata details” option to simulate transactions on tools like Tenderly.

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Troubleshooting

This section covers common issues developers encounter with Interchain Gas Payments, along with potential solutions.

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GasPaymentRequirementNotMet Warning

• Reason: This occurs when the gas payment provided during dispatch does not meet the relayer’s calculated requirement.

• Solution:

  • Use quoteDispatch to calculate the aggregate fee required for a dispatch call to be successful.
  • Verify that msg.value in your metadata covers the relayer’s quoted fee.
  • Check the message status in the Hyperlane Explorer. Look for: Retry(GasPaymentRequirementNotMet).

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Fallback Routing and Overpayment Warning

• Reason: msg.value exceeds the required gas payment, triggering the fallback routing hook.

• Solution:

  • Verify that your quoting logic (quoteDispatch) matches the relayer’s anticipated fees.
  • Avoid overestimating gasLimit values without first benchmarking the handle function.
  • Simulate the transaction to confirm appropriate payment.

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Unexpectedly Large Gas Quotes

• Reason: A very high gasLimit was set, leading to excessively large gas quotes.

• Solution:

  • Double-check the gasLimit specified in your metadata.
  • Verify that your quoting logic (quoteDispatch) matches the relayer’s anticipated fees.
  • Adjust the gasLimit to match the estimated gas consumption of your handle function.