Mempool Policy

Mempool policy is the body of local, node-configurable rules that govern which unconfirmed transactions a Bitcoin Core node will accept into its memory pool, retain, and relay to peers. Unlike consensus rules — which every node must enforce identically to agree on the blockchain — policy is advisory and adjustable per node. It exists to manage finite resources (bandwidth, CPU, memory) and to deter abuse, while still converging in practice because most nodes ship the same defaults.

What Policy Governs

Policy encompasses standardness checks, minimum relay feerate, the dust limit, replace-by-fee handling, package and ancestor/descendant limits, and eviction behavior when the pool fills. Operators tune it through configuration flags such as -maxmempool, -minrelaytxfee, -limitancestorcount, and -datacarriersize.

Policy Versus Consensus

The separation is a deliberate safety boundary. Because policy is not consensus-critical, a node can change it unilaterally without risking a fork, which makes the mempool a low-stakes proving ground for behavior that might later be formalized. Bitcoin Core 31.0 introduced a notable shift: it replaced the older ancestor/descendant size and count enforcement with cluster-based limits, capping connected transaction clusters at 64 transactions and 101 kvB.

Mempool policy is the umbrella over most relay-layer entries in this glossary. For specific mechanisms see Fee Estimation, which reads the mempool's recent history, and Transaction Pinning Attacks, which exploit policy limits.