Differences between OP Stack and L1 Ethereum

It's important to note that there are various minor discrepancies between the behavior of Optimism and Ethereum. You should be aware of these descrepancies when building apps on top of Optimism or the OP Stack codebase.

OP Code Differences

Opcode	Solidity equivalent	Behavior
`COINBASE`	`block.coinbase`	Undefined
`DIFFICULTY`	`block.difficulty`	Random value. As this value is set by the sequencer, it is not as reliably random as the L1 equivalent.
`NUMBER`	`block.number`	L2 block number
`TIMESTAMP`	`block.timestamp`	Timestamp of the L2 block
`ORIGIN`	`tx.origin`	If the transaction is an L1 ⇒ L2 transaction, 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, and this is the initial call (rather than an internal transaction from one contract to another), the same address aliasing behavior applies.

tx.origin == msg.sender

On L1 Ethereum tx.origin is equal to msg.sender only when the smart contract was called directly from an externally owned account (EOA). However, on Optimism tx.origin is the origin on Optimism. It could be an EOA. However, in the case of messages from L1, it is possible for a message from a smart contract on L1 to appear on L2 with tx.origin == msg.origin. This is unlikely to make a significant difference, because an L1 smart contract cannot directly manipulate the L2 state. However, there could be edge cases we did not think about where this matters.

Accessing L1 information

If you need the equivalent information from the latest L1 block, you can get it from the L1Block contract (opens in a new tab). This contract is a predeploy at address 0x4200000000000000000000000000000000000015 (opens in a new tab). You can use the getter functions (opens in a new tab) to get these parameters:

number: The latest L1 block number known to L2
timestamp: The timestamp of the latest L1 block
basefee: The base fee of the latest L1 block
hash: The hash of the latest L1 block
sequenceNumber: The number of the L2 block within the epoch (the epoch changes when there is a new L1 block)

Address Aliasing

Because of the behavior of the CREATE opcode, it is possible for a user to create a contract on L1 and on L2 that share the same address but have different bytecode. This can break trust assumptions, because one contract may be trusted and another be untrusted (see below). To prevent this problem the behavior of the ORIGIN and CALLER opcodes (tx.origin and msg.sender) differs slightly between L1 and L2.

The value of tx.origin is determined as follows:

Call source	`tx.origin`
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 `CanonicalTransactionChain.enqueue`)	`L1_contract_address + 0x1111000000000000000000000000000000001111`

The value of msg.sender at the top-level (the very first contract being called) is always equal to tx.origin. Therefore, if the value of tx.origin is affected by the rules defined above, the top-level value of msg.sender will also be impacted.

Note that in general, tx.origin should not be used for authorization (opens in a new tab). However, that is a separate issue from address aliasing because address aliasing also affects msg.sender.

Why is address aliasing an issue?

The problem with two identical source addresses (the L1 contract and the L2 contract) is that we extend trust based on the address. It is possible that we will want to trust one of the contracts, but not the other.

Helena Hacker forks Uniswap (opens in a new tab) to create her own exchange (on L2), called Hackswap.

Note: There are actually multiple contracts in Uniswap, so this explanation is a bit simplified. See here if you want additional details (opens in a new tab).
Helena Hacker provides Hackswap with liquidity that appears to allow for profitable arbitrage opportunities. For example, she can make it so that you can spend 1 DAI (opens in a new tab)to buy 1.1 USDT (opens in a new tab). Both of those coins are supposed to be worth exactly $1.
Nimrod Naive knows that if something looks too good to be true it probably is. However, he checks the Hackswap contract's bytecode and verifies it is 100% identical to Uniswap. He decides this means the contract can be trusted to behave exactly as Uniswap does.
Nimrod approves an allowance of 1000 DAI for the Hackswap contract. Nimrod expects to call the swap function on Hackswap and receive back nearly 1100 USDT.
Before Nimrod's swap transaction is sent to the blockchain, Helena Hacker sends a transaction from an L1 contract with the same address as Hackswap. This transaction transfers 1000 DAI from Nimrod's address to Helena Hacker's address. If this transaction were to come from the same address as Hackswap on L2, it would be able to transfer the 1000 DAI because of the allowance Nimrod had to give Hackswap in the previous step to swap tokens.

Nimrod, despite his naivete, is protected because Optimism modified the transaction's tx.origin (which is also the initial msg.sender). That transaction comes from a different address, one that does not have the allowance.

Note: It is simple to create two different contracts on the same address in different chains. But it is nearly impossible to create two that are different by a specified amount, so Helena Hacker can't do that.

Blocks

There are several differences in the way blocks are produced between L1 Ethereum and Optimism.

Parameter	L1 Ethereum	Optimism
Time between blocks	12 seconds(1)	2 seconds
Block target size	15,000,000 gas	to be determined
Block maximum size	30,000,000 gas	to be determined

(1) This is the ideal. If any blocks are missed it could be an integer multiple such as 24 seconds, 36 seconds, etc.

Note: The L1 Ethereum parameter values are taken from ethereum.org (opens in a new tab). The Optimism values are taken from the Optimism specs (opens in a new tab).

Network specifications

JSON-RPC differences

OP Stack codebase uses the same JSON-RPC API (opens in a new tab) as Ethereum. Some additional OP Stack specific methods have been introduced. See the full list of custom JSON-RPC methods for more information.

Pre-EIP-155 support

Pre-EIP-155 (opens in a new tab) transactions do not have a chain ID, which means a transaction on one Ethereum blockchain can be replayed on others. This is a security risk, so pre-EIP-155 transactions are not supported on OP Stack by default.

Transaction costs

By default, transaction costs on OP Stack chains include an L2 execution fee and an L1 data fee.

Getting Started: OP Stack Superchain Explainer