ETH bridging

Ether (ETH) is the native currency of Ethereum and all Arbitrum chains. It is used to pay the necessary fees to execute a transaction in those chains. Bridging ETH from Ethereum (Layer 1, or L1) to an Arbitrum chain (Layer 2, or L2) follows, thus, a different process than the one followed when bridging other types of asset.

Depositing ether

To move ETH from L1 to L2, you execute a deposit transaction via Inbox.depositEth. This transfers funds to the Bridge contract on the L1 and credits the same funds to you inside the Arbitrum chain at the specified address.

function depositEth(address destAddr) external payable override returns (uint256)

The following diagram depicts the process that funds follow during a deposit operation.

flowchart TB
  subgraph L1 ["L1 - Ethereum"]
    %% Nodes
    USERL1["#128100;"]
    INBOX("Inbox\ncontract")
    BRIDGE("Bridge\ncontract")
    WAIT{{"
      Wait for the transaction to be
      justified on Ethereum, meaning that
      2 epochs have passed
    "}}
    ESCROWNOTE["
      Deposited funds are escrowed in
      the Bridge contract
    "]:::noteBox

    %% Links
    USERL1 --->|" depositEth() "| INBOX
    INBOX --->|" enqueueDelayedMessage() "| BRIDGE
    BRIDGE -.- ESCROWNOTE
    BRIDGE --> WAIT
  end
  L1 --> L2
  subgraph L2 ["L2 - Arbitrum"]
    %% Nodes
    NODE[(Node)]
    NODENOTE["
      Nodes pick up the transaction
      from the Bridge's emitted events and
      add it to a block
    "]:::noteBox
    CREDITUSER{{"
      Funds equal to the deposited
      amount are credited to the user
    "}}
    USERL2["#128100;"]

    %% Links
    NODE -.- NODENOTE
    CREDITUSER --> USERL2
  end

%% Styling
classDef noteBox fill:#ffffcc,stroke:#000000,color:#000000

As far as the L1 knows, all deposited funds are held by Arbitrum's Bridge contract.

Withdrawing ether

Withdrawing ether can be done using the ArbSys precompile's withdrawEth method:

ArbSys(100).withdrawEth{ value: 2300000 }(destAddress)

Upon withdrawing, the Ether balance is burnt on the Arbitrum side, and will later be made available on the Ethereum side.

ArbSys.withdrawEth is actually a convenience function which is equivalent to calling ArbSys.sendTxToL1 with empty calldataForL1. Like any other sendTxToL1 call, it will require an additional call to Outbox.executeTransaction on L1 after the dispute period elapses for the user to finalize claiming their funds on L1 (see "L2 to L1 Messages"). Once the withdrawal is executed from the Outbox, the user's Ether balance will be credited on L1.

The following diagram depicts the process that funds follow during a withdraw operation.

flowchart TB
  subgraph L2 ["L2 - Arbitrum"]
    %% Nodes
    USERL2["#128100;"]
    ARBSYS("ArbSys\nprecompile")
    BURNNOTE["
      Ether is burnt and
      a withdrawEth event is emitted
    "]:::noteBox
    WAIT{{"
      Wait for a challenge period to elapse
    "}}

    %% Links
    USERL2 --->|" withdrawEth() "| ARBSYS
    ARBSYS -.- BURNNOTE
    ARBSYS --> WAIT
  end
  L2 --> L1
  subgraph L1 ["L1 - Ethereum"]
    %% Nodes
    USERL1["#128100;"]
    OUTBOX("Outbox\ncontract")
    BRIDGE("Bridge\ncontract")

    %% Links
    USERL1 --->|" executeTransaction() "| OUTBOX
    OUTBOX -->|" triggers ETH transfer "| BRIDGE
    BRIDGE ---> USERL1
  end

%% Styling
classDef noteBox fill:#ffffcc,stroke:#000000,color:#000000