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How to bridge from child chain to parent chain

This guide explains how to programmatically send messages and withdraw assets from an Arbitrum child chain to a parent chain (such as Ethereum). For conceptual information about the messaging protocol, see Child-to-parent chain messaging.

Prerequisites​

  • A child chain wallet with funds
  • Access to parent chain infrastructure (for executing the final step)
  • Awareness that child-to-parent messages require 6.4 days to finalize

Overview of the process​

Child-to-parent chain messaging follows these steps:

  1. Send message on child chain: Call ArbSys.sendTxToL1 to initiate the message
  2. Wait for finalization: The message enters a 6.4-day Challenge Period
  3. Execute on parent chain: After finalization, call Outbox.executeTransaction to complete the transfer

Sending a message from the child to the parent chain​

To send a message from the child chain to the parent chain, use the ArbSys precompile's sendTxToL1 method:

function sendTxToL1(
    address destination,
    bytes calldata data
) external payable returns (uint256)

Parameters​

  • destination: The parent chain address that will receive the message
  • data: Calldata to send to the destination address on the parent chain

Return value​

Returns a unique identifier for the message, used to track its status and construct the outbox proof for execution.

The ArbSys precompile is located at address 0x0000000000000000000000000000000000000064.

Example: Sending a simple message

const arbSys = new ethers.Contract('0x0000000000000000000000000000000000000064', arbSysABI, childChainSigner);

const tx = await arbSys.sendTxToL1(parentChainDestination, ethers.utils.toUtf8Bytes('Hello from L2!'));
const receipt = await tx.wait();

Executing the message on the parent chain​

After the 6.4-day challenge period, you can execute the message on the parent chain.

Step 1: Retrieve proof data​

Use the NodeInterface contract to get the Merkle proof:

function constructOutboxProof(
    uint64 size,
    uint64 leaf
) external view returns (
    bytes32[] memory proof,
    uint256 path,
    address l2Sender,
    address l1Dest,
    uint256 l2Block,
    uint256 l1Block,
    uint256 timestamp,
    uint256 amount,
    bytes memory calldataForL1
)

Parameters:

  • size: The batch number containing your message
  • leaf: The index of your message within the batch (0-indexed)

Example usage:

const nodeInterface = new ethers.Contract('0x00000000000000000000000000000000000000C8', nodeInterfaceABI, childChainProvider);

const proofData = await nodeInterface.constructOutboxProof(batchNumber, indexInBatch);
note

NodeInterface is a "virtual" contract accessible at 0x00000000000000000000000000000000000000C8. It isn't a true precompile but provides Arbitrum-specific data without requiring a custom RPC.

Step 2: Execute on the parent chain​

Call Outbox.executeTransaction with the proof data from Step 1:

function executeTransaction(
    bytes32[] calldata proof,
    uint256 path,
    address l2Sender,
    address l1Dest,
    uint256 l2Block,
    uint256 l1Block,
    uint256 timestamp,
    uint256 amount,
    bytes calldata calldataForL1
) external

All parameters come from the constructOutboxProof call. The method executes the message at the l1Dest address with the provided calldata and amount.

Example usage:

const outbox = new ethers.Contract(outboxAddress, outboxABI, parentChainSigner);

const tx = await outbox.executeTransaction(proofData.proof, proofData.path, proofData.l2Sender, proofData.l1Dest, proofData.l2Block, proofData.l1Block, proofData.timestamp, proofData.amount, proofData.calldataForL1);
await tx.wait();

Withdrawing ETH​

To withdraw ETH from the child chain, use the ArbSys precompile's withdrawEth method:

function withdrawEth(
    address destination
) external payable returns (uint256)

Parameters​

  • destination: The parent chain address that will receive the ETH
  • Value (msg.value): The amount of ETH to withdraw from the child chain

Return value​

Returns a unique identifier for the withdrawal message.

How ETH withdrawal works​

  1. On the child chain: The ETH balance is burned, and a message is created
  2. Challenge period: Wait 6.4 days for the assertion to finalize
  3. On the parent chain: Execute via Outbox.executeTransaction to claim your ETH

ArbSys.withdrawEth is equivalent to calling ArbSys.sendTxToL1 with an empty calldata argument. Like any child-to-parent message, it requires executing on the parent chain after the dispute period.

Example: withdrawing ETH​

const arbSys = new ethers.Contract('0x0000000000000000000000000000000000000064', arbSysABI, childChainSigner);

// Withdraw 0.1 ETH
const tx = await arbSys.withdrawEth(parentChainAddress, {
  value: ethers.utils.parseEther('0.1'),
});
const receipt = await tx.wait();

// After 6.4 days, execute on parent chain using the steps above

The withdrawal process:

Process that funds follow during a withdrawal operation

Withdrawing ERC-20 tokens​

For ERC-20 token withdrawals, see the dedicated Withdraw tokens guide, which provides detailed instructions for using Arbitrum's canonical token bridge.

Using the Arbitrum SDK​

The Arbitrum SDK simplifies child-to-parent messaging:

import { L2ToL1MessageStatus, L2TransactionReceipt } from '@arbitrum/sdk';

// Get the L2 transaction receipt
const l2Receipt = await childChainProvider.getTransactionReceipt(l2TxHash);
const l2TxReceipt = new L2TransactionReceipt(l2Receipt);

// Get L2ToL1 messages from the transaction
const messages = await l2TxReceipt.getL2ToL1Messages(parentChainSigner);

// Wait for the message to be executable
const message = messages[0];
await message.waitUntilReadyToExecute(childChainProvider);

// Execute the message on the parent chain
const executeResult = await message.execute(childChainProvider);
await executeResult.wait();

Message lifecycle​

Child-to-parent messages go through these stages:

StageDescription
Posted on child chainThe message is sent via ArbSys.sendTxToL1
Waiting for finalizationThe assertion containing the message is in the challenge period (6.4 days)
Confirmed and executable on parent chainThe assertion is confirmed, and the message can be executed in the outbox

Next steps​