@@ -16,7 +16,7 @@ Learn about the basics of the Rollup protocol used by Bedrock on the [Rollup Pro
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@@ -16,7 +16,7 @@ Learn about the basics of the Rollup protocol used by Bedrock on the [Rollup Pro
### Bedrock Explainer
### Bedrock Explainer
Learn all about the Bedrock release of the OP Stack by reading the [Bedrock Explainer](https://community.optimism.io/docs/developers/bedrock/explainer/).
Learn all about the Bedrock release of the OP Stack by reading the [Bedrock Explainer](../../understand/bedrock/).
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
## Opcode 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. [You can use an oracle for randomness](https://community.optimism.io/docs/useful-tools/oracles/#verifiable-randomness-function-vrf). |
| `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](#address-aliasing) 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](#address-aliasing) behavior applies.
::: tip `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](https://github.com/ethereum-optimism/optimism/blob/develop/packages/contracts-bedrock/contracts/L2/L1Block.sol).
This contract is a predeploy at address [`0x4200000000000000000000000000000000000015`](https://goerli-optimism.etherscan.io/address/0x4200000000000000000000000000000000000015).
You can use [the getter functions](https://docs.soliditylang.org/en/v0.8.12/contracts.html#getter-functions) 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
<details>
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:
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](https://docs.soliditylang.org/en/latest/security-considerations.html#tx-origin).
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.
1. Helena Hacker forks [Uniswap](https://uniswap.org/) 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](https://ethereum.org/en/developers/tutorials/uniswap-v2-annotated-code/).
1. 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](https://www.coindesk.com/price/dai/)to buy 1.1 [USDT](https://www.coindesk.com/price/tether/).
Both of those coins are supposed to be worth exactly $1.
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.
1. 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.
1. 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.
</details>
## Blocks
There are several differences in the way blocks are produced between L1 Ethereum and Optimism Bedrock.
| Parameter | L1 Ethereum | Optimism Bedrock |
| - | - | - |
| 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](https://ethereum.org/en/developers/docs/blocks/#block-time). The Optimism Bedrock values are taken from [the Optimism specs](https://github.com/ethereum-optimism/optimism/blob/develop/specs/guaranteed-gas-market.md#limiting-guaranteed-gas).
## Network specifications
### JSON-RPC differences
Optimism uses the same [JSON-RPC API](https://eth.wiki/json-rpc/API) as Ethereum.
Some additional Optimism specific methods have been introduced.
See the full list of [custom JSON-RPC methods](https://community.optimism.io/docs/developers/build/json-rpc/) for more information.
### Pre-EIP-155 support
[Pre-EIP-155](https://eips.ethereum.org/EIPS/eip-155) 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.
Starting in November 2022, pre-EIP-155 transactions are no longer supported on Optimism.
## Transaction costs
[Transaction costs on Optimism](https://community.optimism.io/docs/developers/build/transaction-fees/) include an [L2 execution fee](https://community.optimism.io/docs/developers/build/transaction-fees#the-l2-execution-fee) and an [L1 data fee](https://community.optimism.io/docs/developers/build/transaction-fees#the-l1-data-fee).
## Contract addresses
The addresses in which various infrastructure contracts are installed are different between L1 Ethereum and Optimism.
For example, [WETH9](https://github.com/gnosis/canonical-weth/blob/master/contracts/WETH9.sol) is installed on L1 Ethereum on [address `0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2`](https://etherscan.io/address/0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2).
On Optimism the same contract is installed on [address `0x4200000000000000000000000000000000000006`](https://explorer.optimism.io/address/0x4200000000000000000000000000000000000006).
