L1 to L2 messaging

In the Transaction Lifecycle section, we covered how users interact with L2 contracts. They submit transactions by putting messages into the chain’s inbox or having a full node Sequencer or aggregator do so on their behalf.

L1 and L2 chains run asynchronously from each other, so it is not possible to make a cross-chain call that produces a result within the same transaction as the caller. Instead, cross-chain calls must be asynchronous, meaning that the caller submits the call at some point in time, and the call runs later. Consequently, a cross-chain contract-to-contract call can never produce a result available to the calling contract (except for acknowledgment of a successful call submitted for later execution).

In this section, we will discuss how contracts interact between L1 and L2, how an L1 contract is called an L2 contract, and vice versa.

Retryable Tickets

Retryable tickets are Arbitrum's canonical method for creating L1 to L2 messages, i.e., L1 transactions that initiate a message to be executed on L2. A retryable can be submitted for a fixed cost (dependent only on its calldata size) paid at L1; its submission on L1 is separable / asynchronous with its execution on L2. Retryables provide atomicity between the cross chain operations; if the L1 transaction to request submission succeeds (i.e. does not revert) then the execution of the Retryable on L2 has a strong guarantee to ultimately succeed as well.

Retryable Tickets Lifecycle

Here we walk through the different stages of the lifecycle of a retryable ticket; (1) submission, (2) auto-redemption, and (3) manual redemption.

Submission

1. Creating a retryable ticket is initiated with a call (direct or internal) to the createRetryableTicket function of the inbox contract. A ticket is guaranteed to be created if this call succeeds. Here, we describe parameters that need to be carefully set. Note that, this function forces the sender to provide a reasonable amount of funds (at least enough to submitting, and attempting to executing the ticket), but that doesn't guarantee a successful auto-redemption.

Parameter	Description
l1CallValue (also referred to as deposit)	Not a real function parameter, it is rather the callValue that is sent along with the transaction
address to	The destination L2 address
uint256 l2CallValue	The callvalue for retryable L2 message that is supplied within the deposit (l1CallValue)
uint256 maxSubmissionCost	The maximum amount of ETH to be paid for submitting the ticket. This amount is (1) supplied within the deposit (l1CallValue) to be later deducted from sender's L2 balance and is (2) directly proportional to the size of the retryable’s data and L1 basefee
address excessFeeRefundAddress	The unused gas cost and submssion cost will deposit to this address, formula is: (gasLimit x maxFeePerGas - execution cost) + (maxSubmission - (autoredeem ? 0 : submission cost)). (Note: excess deposit will transfer to the alias address of the parent chain tx's msg.sender rather than this address)
address callValueRefundAddress	The L2 address to which the l2CallValue is credited if the ticket times out or gets cancelled (this is also called the beneficiary, who's got a critical permission to cancel the ticket)
uint256 gasLimit	Maximum amount of gas used to cover L2 execution of the ticket
uint256 maxFeePerGas	The gas price bid for L2 execution of the ticket that is supplied within the deposit (l1CallValue)
bytes calldata data	The calldata to the destination L2 address

2. Sender's deposit must be enough to make the L1 submission succeed and for the L2 execution to be attempted. If provided correctly, a new ticket with a unique TicketID is created and added to retryable buffer. Also, funds (submissionCost + l2CallValue) are deducted from the sender and placed into the escrow for later use in redeeming the ticket.

3. Ticket creation causes the ArbRetryableTx precompile to emit a TicketCreated event containing the TicketID on L2.

--- title: Ticket Submission --- graph TB 1("🧍") -.-> 2("Initiating an L1-L2 message") 2 -.-> 3("Enough deposit?") 3 -. no .-> 4("Ticket creation fails") 3 -. yes .-> 5("Ticket is created")

🧍 The user who initiates an L1-L2 message

Initiating an L1-L2 message A call to inbox.createRetryableTicket function that puts the message in the L2 inbox that can be re-executed for some fixed amount of time if it reverts

Check user's deposit Logic that checks if the user have enough funds to create a ticket. This is done by checking if the msg.value provided by the user is greater than or equal to maxSubmissionCost + l2CallValue + gasLimit * maxFeePerGas

Ticket creation fails Ticket creation fails and no funds are deducted from the user

Ticket is created A ticket is created and added to the retryable buffer on L2 Funds (l2CallValue + submissionCost) are deducted to cover the callvalue from the user and placed into escrow (on L2) for later use in redeeming the ticket

Automatic Redemption

4. It is very important to note that the submission of a ticket on L1 is separable / asynchronous from its execution on L2, i.e., a successful L1 ticket creation does not guarantee a successful redemption. Once the ticket is successfully created, the two following conditions are checked: (1) if the user's L2 balance is greater than (or equal to) maxFeePerGas * gasLimit and (2) if the maxFeePerGas (provided by the user in the ticket submission process) is greater than (or equal to) the l2Basefee. If these conditions are both met, ticket's submission is followed by an attempt to execute it on L2 (i.e., an auto-redeem using the supplied gas, as if the redeem method of the ArbRetryableTx precompile had been called). Depending on how much gas the sender has provided in step 1, ticket's redemption can either (1) immediately succeed or (2) fail. We explain both situations here:

• If the ticket is successfully auto-redeemed, it will execute with the sender, destination, callvalue, and calldata of the original submission. The submission fee is refunded to the user on L2 (excessFeeRefundAddress). Note that to ensure successful auto-redeem of the ticket, one could use the Arbitrum SDK which provides a convenience function that returns the desired gas parameters when sending L1-L2 messages.

• If a redeem is not done at submission or the submission's initial redeem fails (for example, because the L2 gas price has increased unexpectedly), the submission fee is collected on L2 to cover the resources required to temporarily keep the ticket in memory for a fixed period (one week), and only in this case, a manual redemption of the ticket is required (see next section).

--- title: Automatic Redemption of the Ticket --- graph TB 1("Auto-redeem succeeds?") -. yes .-> 2("Ticket is executed") 2 -.-> 3("Ticket is deleted") 1 -. no .-> 4("callValueRefundAddress gets refunded")

Does the auto-redeem succeed? Logic that determines if the user's L2 Balance is greater than (or equal to) maxFeePerGas * gasLimit && maxFeePerGas is greater than (or equal to) the l2Basefee

Ticket is executed Ticket is executed, the actual submissionFee is refunded to the excessFeeRefundAddress since the ticket was not kept in the buffer on L2

Ticket is deleted Ticket gets deleted from the L2 retryable buffer

callValueRefundAddress gets refunded callValueRefundAddress gets refunded with (maxGas - gasUsed) * gasPrice. Note that this amount is capped by l1CallValue in the auto-redeem

Manual Redemption

5. At this point, anyone can attempt to manually redeem the ticket again by calling ArbRetryableTx's redeem precompile method, which donates the call's gas to the next attempt. Note that the amount of gas is NOT limited by the original gasLimit set during the ticket creation. ArbOS will enqueue the redeem, which is its own special ArbitrumRetryTx type, to its list of redeems that ArbOS guarantees to exhaust before moving on to the next non-redeem transaction in the block its forming. In this manner redeems are scheduled to happen as soon as possible, and will always be in the same block as the tx that scheduled it. Note that the redeem attempt's gas comes from the call to redeem, so there's no chance the block's gas limit is reached before execution.

6. If the fixed period (one week) elapses without a successful redeem, the ticket expires and will be automatically discarded, unless some party has paid a fee to keep the ticket alive for another full period. A ticket can live indefinitely as long as it is renewed each time before it expires.

--- title: Manual Redemption of the Ticket --- graph TB 1("Ticket manually cancelled or not redeemed in 7 days?") -. yes .-> 2("callValueRefundAddress gets refunded") 2 -.-> 3("Ticket is deleted") 1 -. no .-> 4("Ticket manually redeemed?") 4 -. yes .-> 3 4 -. no .-> 1

Is the ticket manually cancelled or not redeemed within 7 days? Logic that determines if the ticket is manually cancelled or not redeemed within 7 days (i.e., is expired)

callValueRefundAddress gets refunded callValueRefundAddress is refunded with the l2CallValue

Ticket is deleted Ticket gets deleted from the L2 retryable buffer

Is the ticket manually redeemed Logic that determines if the ticket is manually redeemed

Avoid Losing Funds!

If a ticket expires after 7 days without being redeemed or re-scheduled to a future date, any message and value (other than the escrowed callvalue) it carries could be lost without possibility of being recovered.

On success, the To address receives the escrowed callvalue, and any unused gas is returned to ArbOS's gas pools. On failure, the callvalue is returned to the escrow for the future redeem attempt. In either case, the network fee was paid during the scheduling tx, so no fees are charged and no refunds are made.

Note that during redemption of a ticket, attempts to cancel the same ticket, or to schedule another redeem of the same ticket, will revert. In this manner retryable tickets are not self-modifying.

If a ticket with a callvalue is eventually discarded (cancelled or expired), having never successfully run, the escrowed callvalue will be paid out to a callValueRefundAddress account that was specified in the initial submission (step 1).

Important Notes:

If a redeem is not done at submission or the submission's initial redeem fails, anyone can attempt to redeem the retryable again by calling ArbRetryableTx's redeem precompile method, which donates the call's gas to the next attempt. ArbOS will enqueue the redeem, which is its own special ArbitrumRetryTx type, to its list of redeems that ArbOS guarantees to exhaust before moving on to the next non-redeem transaction in the block its forming. In this manner redeems are scheduled to happen as soon as possible, and will always be in the same block as the transaction that scheduled it. Note that the redeem attempt's gas comes from the call to redeem, so there's no chance the block's gas limit is reached before execution.

• One can redeem live tickets using the Arbitrum Retryables Transaction Panel
• The calldata of a ticket is saved on L2 until it is redeemed or expired
• Redeeming cost of a ticket will not increase over time, it only depends on the current gas price and gas required for execution

Receipts

In the lifecycle of a retryable ticket, two types of L2 transaction receipts will be emitted:

• Ticket Creation Receipt: This receipt indicates that a ticket was successfully created; any successful L1 call to the Inbox's createRetryableTicket method is guaranteed to create a ticket. The ticket creation receipt includes a TicketCreated event (from ArbRetryableTx), which includes a ticketId field. This ticketId is computable via RLP encoding and hashing the transaction; see calculateSubmitRetryableId.
• Redeem Attempt: A redeem attempt receipt represents the result of an attempted L2 execution of a ticket, i.e, success / failure of that specific redeem attempt. It includes a RedeemScheduled event from ArbRetryableTx, with a ticketId field. At most, one successful redeem attempt can ever exist for a given ticket; if, e.g., the auto-redeem upon initial creation succeeds, only the receipt from the auto-redeem will ever get emitted for that ticket. If the auto-redeem fails (or was never attempted — i.e., the provided L2 gas limit * L2 gas price = 0), each initial attempt will emit a redeem attempt receipt until one succeeds.

Alternative "unsafe" Retryable Ticket Creation

The Inbox.createRetryableTicket convenience method includes sanity checks to help minimize the risk of user error: the method will ensure that enough funds are provided directly from L1 to cover the current cost of ticket creation. It also will convert the provided callValueRefundAddress and excessFeeRefundAddress to their address alias (see below) if either is a contract (determined by if the address has code during the call), providing a path for the L1 contract to recover funds. A power-user may bypass these sanity-check measures via the Inbox's unsafeCreateRetryableTicket method; as the method's name desperately attempts to warn you, it should only be accessed by a user who truly knows what they're doing.

Eth deposits

A special message type exists for simple Eth deposits; i.e., sending Eth from L1 to L2. Eth can be deposited via a call to the Inbox's depositEth method. If the L1 caller is EOA, the Eth will be deposited to the same EOA address on L2; the L1 caller is a contract, the funds will deposited to the contract's aliased address (see below).

Note that depositing Eth via depositEth into a contract on L2 will not trigger the contract's fallback function.

In principle, retryable tickets can alternatively be used to deposit Ether; this could be preferable to the special eth-deposit message type if, e.g., more flexibility for the destination address is needed, or if one wants to trigger the fallback function on the L2 side.

Transacting via the Delayed Inbox

While retryables and Eth deposits must be submitted through the delayed inbox, in principle, any message can be included this way; this is a necessary recourse to ensure the Arbitrum chain preserves censorship resistance even if the Sequencer misbehaves (see The Sequencer and Censorship Resistance). However, under ordinary/happy circumstances, the expectation/recommendation is that clients use the delayed inbox only for Retryables and Eth deposits, and transact via the Sequencer for all other messages.

Address Aliasing

Unsigned messages submitted via the Delayed Inbox get their sender's addressed "aliased": when these messages are executed on L2, the sender's address —i.e., that which is returned by msg.sender — will not simply be the L1 address that sent the message; rather it will be the address's "L2 Alias." An address's L2 alias is its value increased by the hex value 0x1111000000000000000000000000000000001111:

L2_Alias = L1_Contract_Address + 0x1111000000000000000000000000000000001111

Try it out

The Arbitrum protocol's usage of L2 Aliases for L1-to-L2 messages prevents cross-chain exploits that would otherwise be possible if we simply reused the same L1 addresses as the L2 sender; i.e., tricking an L2 contract that expects a call from a given contract address by sending retryable ticket from the expected contract address on L1.

If for some reason you need to compute the L1 address from an L2 alias on chain, you can use our AddressAliasHelper library:

modifier onlyFromMyL1Contract() override {
    require(AddressAliasHelper.undoL1ToL2Alias(msg.sender) == myL1ContractAddress, "ONLY_COUNTERPART_CONTRACT");
    _;
}

Signed Messages

The delayed inbox can also accept messages that include a signature. In this case, the message will execute with the msg.sender address equal to the address that produced the included signature (i.e., not its alias). Intuitively, the signature proves that the sender address is not a contract, and thus is safe from cross-chain exploit concerns described above. Thus, it can safely execute from signer's address, similar to a transaction included in a Sequencer's batch. For these messages, the address of the L1 sender is effectively ignored at L2.

These signed messages submitted through the delayed inbox can be used to execute messages that bypass the Sequencer and require EOA authorization at L2, e.g., force-including an Ether withdrawal (see "withdraw eth tutorial").