Gas token deposits

Table of Contents

• Design
• Existing rollups
• Open questions

Rollup users need the L2 gas token to send transactions. Native rollups cannot mint it with a special transaction type, since L1 has none. Instead, the gas token is pre-minted on L2 and released by L1 to L2 messages.

Design

A predeployed L2 contract holds a pre-minted supply of the gas token. When the messenger processes a deposit message from L1, the contract releases the corresponding amount to the recipient. Withdrawals lock tokens back into the contract and send an L2 to L1 message.

Claiming a deposit is an L2 transaction, and all L2 ETH starts in the pre-minted supply, so the claimed ETH has to pay for its own claim. EIP-8141 frame transactions allow this: a DEFAULT frame claims the message, then a VERIFY frame of the recipient approves execution and payment. EIP-8141 collects the fee from the payer only when it approves, and by then the recipient holds the deposit, so later frames can already use the rest. Deposits to contracts and other addresses that cannot approve payment are claimed by others, for the fee the message carries (see Messaging).

The claim frame runs before any frame approves payment, so a failing claim costs the operator its execution and pays nothing, and such transactions fall outside the public mempool’s validation rules. The operator therefore admits a claim only if it succeeds on its head state, and only one claim per message, so a pending claim cannot keep a message from being claimed.

Because the L2 contract only reacts to messages, the L1 side decides what backs the gas token. Custom gas tokens are therefore only a matter of L1 contract design:

• ETH: the L1 contract escrows the ETH sent with each message, as Linea does.
• ERC20: the L1 contract transfers an ERC20 into escrow instead. Tokens with non-standard decimals or transfer logic need special care.
• ETH burn: the L1 contract burns the ETH instead of escrowing it.
• NFT-gated credits: NFT holders can claim a fixed amount of gas token once per NFT.

Alternatives that were considered and dropped:

• A transaction type that mints the gas token, as in the OP and Orbit stacks. L1 has no such transaction type.
• Minting through the withdrawals field of the payload. The rollup contract could fill it from its own deposit queue, but withdrawals are only processed at the end of a block, so deposits could not be used within the same block. The reference design keeps withdrawals empty.

Existing rollups

Open questions

• Claims before EIP-8141: native rollups only have frame transactions once L1 does. Until then, nobody can pay for the first claim, since all L2 ETH starts in the pre-minted supply. Funding an account at genesis would work around it, but L1 would not back that ETH.