Estimating the cost of interop messages

There are several factors that determine the cost of an interop transaction:

• How you pass the message. You can either use CrossL2Inbox (opens in a new tab) directly, or use the cross domain messenger, L2ToL2CrossDomainMessenger (opens in a new tab), which uses CrossL2Inbox internally.
• The transaction type. Every interop message has two transactions, an initiating message in a transaction to the source chain, and an executing message in the destination chain.

CrossL2Inbox

This is the low level protocol used by all interop protocols, including L2ToL2CrossDomainMessenger.

Initiating message

The initiating message is any log entry. A log entry emitted by Solidity code contains 1-4 topics (t) and unlimited unstructured data bytes (n). The gas cost is calculated as 375(t+1)+8n (opens in a new tab).

Executing message

The executing message cost has several components:

1. The cost of posting the transaction.
2. The cost of hashing the message.
3. The cost of CrossL2Inbox.validateMessage.
4. The cost of using the message.

The first and second components depend on the log entry. CrossL2Inbox.validateMessage only requires a 32 byte hash of the log entry, but actually using it typically requires the information that has been hashed.

Additionally, you must provide the CrossL2Inbox with the information needed to locate the log entry. This information is encoded in a five-member structure (opens in a new tab) that requires 160 bytes (32 bytes × 5 members). Lastly, you need to call a function which requires a 4-byte selector.

Therefore, the total bytes required is: 164 + 32t + n

Where:

• 164 = base overhead (160 bytes for the structure + 4 bytes for the function selector)
• t = number of topics in the log entry
• n = number of data bytes in the log entry

Every transaction posted costs at least 21,000 gas. The hashing operation costs approximately 30+0.2×<number of bytes> (opens in a new tab), which is negligible by comparison. We can usually ignore the memory expansion cost (opens in a new tab), unless the validating contract uses a really large amount of memory.

The cost of using the message is beyond the scope here, because it depends on your application.

The main cost drivers are the 21,000 gas transaction cost plus the cost of posting a 164+32t+n byte transaction.

Cross domain messenger

This higher level protocol adds some expenses, mostly because replay protection requires storage, and writing to storage (opens in a new tab) is a relatively expensive operation.

Initiating message

The initiating message is sent by L2ToL2CrossDomainMessenger.sendMessage (opens in a new tab). This function writes to storage twice.

It writes to specify that the hash has a sent message (opens in a new tab). This would typically be written to previously empty storage, so the cost is 22,100 gas.

Then it increments the nonce value (opens in a new tab). Overwriting previously used storage (which means storage where the present value is not zero) only costs 5,000 gas.

Hence, the gas cost for creating an initiating message is approximately 27,100 gas, plus minor overhead for log emission and contract operations. Note that this estimate excludes the 21,000 gas base transaction cost, which applies to all transactions.

Executing message

If autorelay is turned on in a blockchain, then you don't care about the cost of the executing message. The chain operator will bear the cost.

If autorelay is not turned on, the executing message is a call to L2ToL2CrossDomainMessenger.relayMessage (opens in a new tab). The only storage operation here is noting the hash has been used for a message already (opens in a new tab). This is previously unwritten storage, so we can expect to pay the full 22,100 in gas. Plus, of course, the 21,000 that any transaction costs. All the other gas costs are negligible.

Conclusion

Unless the message is extremely long, the cost of an interop message, taking both sides together, is unlikely to exceed 100,000 gas. At the time of writing, each gas unit costs approximately $3×10^-9, so it would take about thirty messages to add up to a full cent.

Next steps

• Build a revolutionary app that uses multiple blockchains within the Superchain
• Deploy a SuperchainERC20 to the Superchain
• Learn how messages get from one chain to another chain
• Watch this video (opens in a new tab), which gives an overview of Superchain interoperability.