LayerZero Bridge Integration

USDT Bridging from TON to EVM Chains (Ethereum and TAC)

Overview

This document describes the integration of a cross-chain bridge that enables transferring USDT Jettons from the TON blockchain to EVM-compatible networks via LayerZero.
The system consists of two main components:

• Executor – an off-chain service running on TON that monitors treasury balances and triggers bridge operations.
• Treasury Contracts – smart contracts deployed on TON, responsible for receiving Jettons and initiating LayerZero/TAC calls to EVM networks.

Two bridge directions are implemented:

• TON → Ethereum
• TON → TAC

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Bridge Flow: TON → Ethereum

Process

1. A user sends Jetton USDT to the eth_usdt_treasury contract using a TON adapter from TAC.
2. The executor detects the balance change and initiates a LayerZero OFTSend message to the specified Ethereum address (e.g., a Veda pool).
3. Excess TON and Jettons (if any) are returned to the eth_usdt_treasury contract.
4. The executor can reuse this treasury balance for subsequent transactions, needing only to top up TON as gas.

Design Considerations

• The contract defines fixed and deliberately high values for native_fee, estimated_gas_cost, and jetton_transfer_gas_cost to ensure that no funds get stuck in case of incorrect fee estimation.
• Executor can optionally include additional fee data via the add_fee field if they wish to increase gas or forwarding fees.

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Bridge Flow: TON → TAC

Process

1. A user sends USDT Jettons to the tac_usdt_treasury contract from Ethereum using the LZ protocol.
2. The executor monitors this treasury contract and, upon detecting a balance change, triggers a transfer to TAC.
3. The payload includes:
   • Fee data (fee_data), specifying the TON amounts required for:
     • protocol operation,
     • TAC executor compensation,
     • TON executor gas,
     • and treasury margin.
   • EVM data (evm_data), which encodes the call to the TAC-side protocol:
     • the target address (e.g., a Veda pool) that should receive the funds,
     • and the list of valid executors permitted to execute the call.
4. The payload is constructed with isRoundTrip = true, allowing TAC to roll back the transaction in case of failure on the TAC side (though such failures are not expected).
5. All TON fees are fully consumed during execution — the contract does not rely on internal balance and requires full coverage of costs by the initiator.

Design Considerations

• This direction supports an optional gasless execution mode if funds are sent to pre-approved pools like Veda.
• Users can attach extra TON in the add_fee field for higher-value transfers.
• Treasury fees ensure operational independence from contract balance and prevent disruptions.

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License

MIT