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This guide provides practical instructions for interacting with the Fee Refunder module, primarily for smart contract developers who need to include the appropriate fees when sending IBC packets.

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For Smart Contract Developers

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Including Fees in IBC Transfers

When using the MsgTransfer message from the IBC Transfer module, you need to include fees for relayers: 
// Example in CosmWasm

// Import necessary types
use neutron_sdk::bindings::msg::NeutronMsg;
use neutron_sdk::bindings::types::{Fee as IbcFee, Coin};

// Create the fee structure
let fee = IbcFee {
    // Must be zero for compatibility reasons
    recv_fee: vec![],
    
    // Fee paid to the relayer who delivers acknowledgment
    ack_fee: vec![Coin {
        denom: "untrn".to_string(),
        amount: "1000".to_string(),
    }],
    
    // Fee paid to the relayer who delivers timeout
    timeout_fee: vec![Coin {
        denom: "untrn".to_string(),
        amount: "1000".to_string(),
    }],
};

// Create the IBC transfer message with fee
let msg = NeutronMsg::IbcTransfer {
    source_port: "transfer".to_string(),
    source_channel: "channel-3".to_string(),
    token: Coin {
        denom: "untrn".to_string(), 
        amount: "1000000".to_string(),
    },
    receiver: "cosmos1...".to_string(),
    timeout_height: None,
    timeout_timestamp: Some(timeout_timestamp),
    fee: Some(fee),
    memo: None,
};

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Including Fees in Interchain Transactions

When using the MsgSubmitTx message for Interchain Transactions, you also need to include relayer fees: 
// Example in CosmWasm

// Import necessary types
use neutron_sdk::bindings::msg::NeutronMsg;
use neutron_sdk::bindings::types::{Fee as IbcFee, Coin};
use neutron_sdk::interchain_txs::helpers::create_submit_tx_msg;

// Create the fee structure
let fee = IbcFee {
    recv_fee: vec![],
    ack_fee: vec![Coin {
        denom: "untrn".to_string(),
        amount: "2000".to_string(),
    }],
    timeout_fee: vec![Coin {
        denom: "untrn".to_string(),
        amount: "1000".to_string(),
    }],
};

// Create the interchain transaction message with fee
let cosmos_msgs: Vec<cosmwasm_std::CosmosMsg> = vec![/* Your Cosmos messages here */];
let connection_id = "connection-0";
let interchain_account = "cosmos1...";

let msg = create_submit_tx_msg(
    cosmos_msgs,
    connection_id.to_string(),
    interchain_account.to_string(),
    Some(fee),
    None, // memo
    None, // timeout_timestamp
);

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Ensuring Your Fees Are Sufficient

The chain enforces minimum fees for IBC operations, which can be queried through the params endpoint: 
// Query the module parameters to get minimum required fees
let params: QueryParamsResponse = deps.querier.query(
    &QueryRequest::Custom(NeutronQuery::FeerefunderParams {})
)?;

// Use these values to ensure your fees are sufficient
let min_ack_fee = params.params.min_fee.ack_fee;
let min_timeout_fee = params.params.min_fee.timeout_fee;
Remember that:
  1. Fees must be equal to or higher than the minimum required
  2. You can only use denominations allowed in the minimum fee parameters
  3. recv_fee must always be empty (set to zero)

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For Relayers

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Claiming Fees

As a relayer, you don’t need to take any special actions to claim fees. The Fee Refunder module automatically:
  1. Detects when you deliver an acknowledgement or timeout packet
  2. Identifies the appropriate fee from its storage
  3. Transfers the fee to your address
The fees are distributed to the address that submits the acknowledgement or timeout transaction.

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For Governance Participants

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Viewing Current Parameters

To view the current Fee Refunder parameters, including minimum required fees: 
neutrond query feerefunder params
Example output: 
{
  "params": {
    "min_fee": {
      "recv_fee": [],
      "ack_fee": [
        {
          "denom": "untrn",
          "amount": "1000"
        }
      ],
      "timeout_fee": [
        {
          "denom": "untrn",
          "amount": "1000"
        }
      ]
    }
  }
}

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Updating Minimum Fees

To propose changing the minimum required fees through governance using MsgUpdateParams: 
{
  "@type": "/neutron.feerefunder.MsgUpdateParams",
  "authority": "neutron1...", // governance authority address
  "params": {
    "min_fee": {
      "ack_fee": [
        {
          "denom": "untrn",
          "amount": "2000"
        }
      ],
      "timeout_fee": [
        {
          "denom": "untrn",
          "amount": "1500"
        }
      ],
      "recv_fee": []
    }
  }
}
This message can only be executed through governance proposals, not directly by individual users.

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Querying Fee Information

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Checking Fees for a Specific Packet

To query the fees associated with a specific IBC packet: 
neutrond query feerefunder fee-info [port_id] [channel_id] [sequence]
Example: 
neutrond query feerefunder fee-info transfer channel-0 123

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Monitoring Fee Refunder Events

The module emits specific events that can be monitored:
  • lock_fees: Emitted when fees are locked for a new IBC packet
  • distribute_acknowledgement_fee: Emitted when acknowledgement fees are distributed
  • distribute_timeout_fee: Emitted when timeout fees are distributed
Example of querying for fee distribution events: 
neutrond query txs --events 'distribute_acknowledgement_fee.receiver=<relayer-address>'
This allows relayers to monitor their earned fees and developers to track their packet processing.
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