Differences between Ethereum and Metal L2
Metal L2 is designed to be EVM equivalent (opens in a new tab) and introduces as few changes as possible to the Ethereum protocol. However, there are some minor differences between the behavior of Ethereum and Metal L2 that developers should be aware of.
Opcodes
| Opcode | Solidity Equivalent | Behavior |
|---|---|---|
COINBASE | block.coinbase | Returns the address of the current Sequencer's fee wallet. Effectively the same as Ethereum with the caveat the value typically does not change from block to block. |
PREVRANDAO | block.prevrandao | Set pseudorandomly for each block by the Sequencer as opposed to the stronger guarantees provided by RANDAO (opens in a new tab) on Ethereum. |
ORIGIN | tx.origin | If the transaction is an L1 ⇒ L2 transaction triggered by a smart contract on L1, then tx.origin is set to the aliased address of the address that triggered the L1 ⇒ L2 transaction. Otherwise, this opcode behaves normally. |
CALLER | msg.sender | If the transaction is an L1 ⇒ L2 transaction triggered by a smart contract on L1, and this is the first call frame (rather than an internal transaction from one contract to another), the same address aliasing behavior applies. |
Address Aliasing
Address aliasing is an important security feature that impacts the behavior of transactions sent from L1 to L2 by smart contracts.
Make sure to read this section carefully if you are working with cross-chain transactions.
Note that the CrossChainMessenger contracts will handle address aliasing internally on your behalf.
When transactions are sent from L1 to L2 by an Externally Owned Account, the address of the sender of the transaction on L2 will be set to the address of the sender of the transaction on L1. However, the address of the sender of a transaction on L2 will be different if the transaction was triggered by a smart contract on L1.
Because of the behavior of the CREATE opcode, it is possible to create a contract on both L1 and on L2 that share the same address but have different bytecode.
Even though these contracts share the same address, they are fundamentally two different smart contracts and cannot be treated as the same contract.
As a result, the sender of a transaction sent from L1 to L2 by a smart contract cannot be the address of the smart contract on L1 or the smart contract on L1 could act as if it were the smart contract on L2 (because the two contracts share the same address).
To prevent this sort of impersonation, the sender of a transaction is slightly modified when a transaction is sent from L1 to L2 by a smart contract. Instead of appearing to be sent from the actual L1 contract address, the L2 transaction appears to be sent from an "aliased" version of the L1 contract address. This aliased address is a constant offset from the actual L1 contract address such that the aliased address will never conflict with any other address on L2 and the original L1 address can easily be recovered from the aliased address.
This change in sender address is only applied to L2 transactions sent by L1 smart contracts. In all other cases, the transaction sender address is set according to the same rules used by Ethereum.
| Transaction Source | Sender Address |
|---|---|
| L2 user (Externally Owned Account) | The user's address (same as in Ethereum) |
| L1 user (Externally Owned Account) | The user's address (same as in Ethereum) |
L1 contract (using OptimismPortal.depositTransaction) | L1_contract_address + 0x1111000000000000000000000000000000001111 |
Transactions
Transaction Fees
Transactions on Metal L2 must pay for an L1 data fee on top of the standard execution gas fee you would expect on Ethereum. Refer to the guide on Metal L2 Transaction Fees for more information.
EIP-1559 Parameters
The base fee on Metal L2 is, like Ethereum, computed via the EIP-1559 (opens in a new tab) mechanism. The EIP-1559 parameters used by Metal L2 differ from those used by Ethereum as follows.
| Parameter | Metal L2 value | Ethereum value (for reference) |
|---|---|---|
| Block gas limit | 30,000,000 gas | 30,000,000 gas |
| Block gas target | 5,000,000 gas | 15,000,000 gas |
| EIP-1559 elasticity multiplier | 6 | 2 |
| EIP-1559 denominator | 250 | 8 |
| Maximum base fee increase (per block) | 2% | 12.5% |
| Maximum base fee decrease (per block) | 0.4% | 12.5% |
| Block time in seconds | 2 | 12 |
Mempool Rules
Unlike Ethereum, Metal L2 does not have a large public mempool. The Metal L2 Sequencer mempool is currently only visible to the Sequencer. The Sequencer executes transactions from the mempool in priority fee order (highest fee first).