Gas token deposits
Table of Contents
Overview
Rollup users need a way to obtain the gas token to be able to send transactions on the L2. Existing solutions divide into two approaches: either an escrow contract contains preminted tokens that are unlocked through the L2 to L1 messaging channel, or a new transaction type that is able to mint the gas token is added to the STF. This page will also discuss two more approaches that are currently not used in any project.
Current approaches
OP stack
The custom DepositTransaction type allows to mint the gas token based on TransactionDeposited event fields. On L2, the gas token magically appears in the user's balance.
Linea
Linea uses preminted tokens in the L2MessageService contract, which are then unlocked when L1 to L2 messages are processed on the L2. No new transaction type that can mint gas token is added to the STF.
Taiko
Taiko uses preminted tokens in the L2 Bridge contract, which are then unlocked when L1 to L2 messages are processed on the L2. No new transaction type that can mint gas token is added to the STF.
Orbit stack
Orbit stack uses a custom transaction type that is able to mint the gas token based on the ArbitrumDepositTx type. On L2, the gas token magically appears in the user's balance.
Other approaches
Manual state manipulation
Before (and after) calling the EXECUTE precompile, projects are free to modify the L2 state root directly with custom execution, including dedicated proving systems. This can be used to touch balances, but it requires doing all updates either before or after block execution. This strategy cannot be used to support arbitrary intra-block gas-token deposits.
Beacon chain withdrawals
Another possible mechanism is to use the beacon chain withdrawal mechanisms which mints the gas token on L1. Withdrawals are processed at the end of a block, so they wouldn't be able to allow deposits to be processed intra-block. As of now, no existing project uses beacon chain withdrawals for gas token deposits, but the mechanism can be left open for use.
Proposed design
Following the design principles, it is preferred not to add a new transaction type that can mint gas tokens, as existing projects already handle gas token deposits through other means. The preferred approach is to use a predeployed contract that contains preminted tokens, which are then unlocked when L1 to L2 messages are processed on the L2. This design fully supports custom gas tokens as it is not opinionated on what type of message unlocks the gas token on L2, it being ETH, an ERC20, NFTs, or mining mechanisms.
The first deposit problem
WIP.
To be discussed:
- How can a user claim on L2 the first deposit if they don't have any gas token to pay for the transaction fees?