A transaction may include a transaction fee, to encourage miners to include the transaction in the blockchain. When a transaction is included in a block, miners can collect the included fees. Transactions fees may supplement, and must eventually replace, the diminishing block reward paid to miners.
Most transactions require a mandatory fee when they spend recently received bitcoins. The minimum fee rules are enforced by relay nodes and miners to discourage spam-like or attack transactions that may degrade network performance or unnecessarily bloat the blockchain; without a minimum fee, the transaction will be discarded.
A minimum fee is automatically calculated when sending a payment, but for preferential treatment, a larger optional fee can be included with a transaction. As the Bitcoin network becomes more utilized, a fee higher than default should be specified for timely transaction processing, and future software clients may recommend the best additional fee for your type of transaction.
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