CCTP Express Architecture

CCTP Express adopts a Hub-and-Spoke architecture, it can be broken down into a 3-layered system: a request for quote mechanism to obtain users' bridging intent, enabling a filler network to claim and fill those orders, and lastly a settlement layer periodically repay fillers through CCTP and utilizing attestation service from Iris.

CCTP Express aligns with the industry standards, the CrossChainOrder struct follows ERC-7683 standard.

Below is a diagram of the architecture of CCTP Express:

Order initiation

1. User signs an off-chain message defining the parameters of an order.

2. The order is disseminated to Fillers. The Filler calls initiate on the source chain SpokePool. A CrossChainOrder will be created and the user's funds are transferred to the SpokePool for escrow.

3. The SpokePool on source chain submits a Deposit message to Circle's off-chain attestation service, Iris, for attestation and subsequently a DepositAttestation will be generated.

Filler Network Fills Order

4. Fillers calls fillOrder on the destination SpokePool with their own assets which are then transferred to the user from the SpokePool.

5. The SpokePoolon destination chain submits a Fill message to Iris and a FillAttestation will be generated.

Settlement

6. A permissionless Datadaemon retrieves the DepositAttestation and FillAttestation from Iris and relay to the Hub Pool on the Settlement Chain.

7. Periodically, a permissionless Datadaemon calls repayFunds and rebalanceFunds at the Hub Pool, which would collect all the attestations and perform the following steps:

   1. Iterate through a list of attestations, a valid filled order is supported by both Deposit and Fill attestation.

   2. Determine the aggregate settlement sum from all valid fills for each filler.

   3. If there is sufficient fund on SpokePool to repay filler, a repayFunds message in the form of merkle root hash is sent to Iris.

   4. For the remaining outstanding payment, the Hub Pool will send a rebalanceFunds message in the form of merkle root hash to Iris, which indicates how much a SpokePool with surplus funds would send to another pool in deficit to fulfill the need for repayment.

8. Once the repayFunds and rebalanceFunds messages got attested by Iris, they are sent to respective SpokePools. Datamaemon will call repayFunds and rebalanceFunds on SpokePools with merkle root hash and their respective transaction details. Accordingly, funds would be repaid to fillers and sent to other SpokePools to ensure sufficient funds for handling repayments.

9. Repay funds to fillers from the SpokePool on destination chain, and rebalance funds across SpokePools on difference chains via CCTP.

Cctp Fill Settlement

10. In case of order initiated by Fillers not being filled, the Datadaemon can call cctpFill and mark the order status on destination chain SpokePool to RequestCctpFill and block any filler from filling it. At the same time, the SpokePool will emit a CctpFillmessage to Iris for attestation.

11. The CctpFillAttestation will be used to replace the FillAttestation mentioned in 5. and allow the user fund to be transferred via the CCTP route.