Mempool Policy

The most familiar policy rule is the minimum relay fee. A node declines to relay a transaction whose fee rate is below a threshold it sets, which protects it from being used to propagate transactions that have no prospect of confirmation. The threshold is a policy choice, not a consensus rule, and a miner is free to include a lower-fee transaction if it wants to.

A second family of rules concerns standardness. A transaction can be valid under consensus and still be non-standard, meaning that the default policy will not relay it. Examples include transactions with unusual script patterns, very large numbers of signature operations, or outputs below the dust threshold. These rules exist to limit the resources a transaction can consume and to keep the set of relayed transactions predictable.

A third family concerns replacement. BIP 125 defines the rules a replacement transaction must satisfy for a node to accept it in place of one already in its mempool, including a higher fee and a limit on how many replacements are permitted. The RBF and CPFP page covers the fee-bumping strategies these rules enable.

Because policy is local, two nodes can hold different views of which transactions are pending. A node with a higher minimum relay fee will have a smaller mempool than one with a lower threshold, and a node that has been running longer may hold transactions that a freshly started node has never seen. The mempool is not a single global pool; it is a collection of overlapping local views.

This is why a transaction can appear confirmed to one observer and absent to another, and why a wallet that queries a single node may report a different pending balance than one that queries several. The difference is not an error; it is the expected consequence of local policy. The mempool page covers the structure and eviction behaviour in more detail.

The practical implication for an operator is that policy settings are a trade-off between relaying more of what the network produces and limiting the resources the node spends on transactions that may never confirm. The default settings are a reasonable starting point, and changing them is a decision worth making deliberately rather than by accident.