update zero-fee document (#61)

content/tutorials/zero-fee-transactios.md

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+---
+title: How to Send Zero-Fee Transactions?
+weight: 10
+---
+
+## Preface
+
+Pactus supports zero-fee transactions starting from
+[Version 1.6.0 (Mumbai)](https://pactus.org/2024/11/14/pactus-1.6.0-mumbai-released/).
+However, for security reasons, there are restrictions on zero-fee transactions:
+
+- Each address can send approximately two zero-fee transactions daily.
+- Zero-fee transactions have longer confirmation times.
+
+## How Does It Work?
+
+Zero-fee transactions are based on the consumption-based fee model
+detailed in [PIP-31](https://pips.pactus.org/PIPs/pip-31).
+
+According to the Consumptional Fee model, each address has a daily limit for sending zero-fee transactions.
+This limit is measured in bytes and decreases whenever a transaction from the address
+is broadcast to the network or confirmed in a block.
+By default, each address is allowed 360 bytes of zero-fee transactions daily.
+
+It is important to note that the fee configuration in Pactus is not deterministic.
+Each validator sets its own preferences for accepting transactions, which means:
+
+1. A validator first checks whether the transaction's fee meets its minimum requirements.
+2. If the fee is sufficient, the transaction is added to the transaction pool and
+   processed when a validator proposes a new block.
+3. If the fee is below the threshold, the validator ignores it but
+   broadcasts the transaction to other validators, who may or may not accept it.
+
+This approach makes it impossible to guarantee that a transaction with will be accepted by the network.
+Users can monitor their transactions on the [Fee Tracker](https://pacviewer.com/fee) page in PacViewer.
+This page displays unconfirmed transactions in the pool and
+allows users to check whether their transaction is confirmed or not.
+
+Please note that every transaction in Pactus has a lifetime of one day.
+If a transaction is not included in a block within one day, it will be invalidated.
+
+## How to Send Zero-Fee Transactions?
+
+By default, the transaction fee is set to `0.01` PAC. To send a zero-fee transaction:
+
+- **In the GUI**: Set the fee to `0` PAC.
+- **In the Command Line**: Use the `--fee 0` option.
+
+## How to Accept Zero-Fee Transactions?
+
+By default, the consumptional fee is disabled in the
+[configuration file](https://github.com/pactus-project/pactus/blob/main/config/example_config.toml).
+If your validator node has sufficient resources,
+you can update the configuration to accept and process zero-fee transactions:
+
+1. Increase the transaction pool size to accommodate more transactions.
+2. Set `fixed_fee` to `0` to accept transactions with zero fee.
+3. Adjust the `daily_limit` and `unit_price` as needed. The recommended settings are:
+   - `daily_limit`: 360 bytes
+   - `unit_price`: 0.000005 PAC per byte
+
+Here is an example configuration for accepting zero-fee transactions:
+
+```toml
+# `tx_pool` contains configuration options for the transaction pool module.
+[tx_pool]
+
+  # `max_size` indicates the maximum number of unconfirmed transactions inside the pool.
+  # Default is `1000`.
+  max_size = 5000
+
+  # `tx_pool.fee` contains configuration to calculate the transaction fee.
+  [tx_pool.fee]
+
+    # The `fixed_fee` is a constant fee in PAC that applies to each transaction, regardless of its size or type.
+    # Default is `0.01` PAC.
+    fixed_fee = 0.00
+
+    # The `daily_limit` is the number of bytes an account can send each day without paying a fee.
+    # The `daily_limit` is part of the consumptional fee model.
+    # To understand how the consumptional fee model works, you can refer to
+    # PIP-31: Consumptional Fee Mode (https://pips.pactus.org/PIPs/pip-31)
+    # Default is `360` bytes.
+    daily_limit = 360
+
+    # The `unit_price` defines the fee per byte in PAC.
+    # The `unit_price` is part of the consumptional fee model.
+    # If it is zero, the consumptional fee will be ignored.
+    # To understand how the consumptional fee model works, you can refer to
+    # PIP-31: Consumptional Fee Mode (https://pips.pactus.org/PIPs/pip-31)
+    # Default is `0.0` PAC.
+    unit_price = 0.000005
+
+```
+
+After updating the configuration, restart your node to enable the consumptional fee model.