Package Relay (BIP-331)

Package relay is a proposed enhancement to Bitcoin's peer-to-peer transaction relay, formalized as BIP-331, that lets nodes announce and evaluate a group of related transactions as a single unit rather than judging each one in isolation. The goal is to make child-pays-for-parent (CPFP) fee bumping reliable across the whole network.

The problem it solves

A node's mempool normally rejects any transaction whose own feerate falls below its minimum. That breaks CPFP at the relay layer: if a low-fee parent is rejected on arrival, the high-fee child meant to bump it never gets a chance to be considered, because the parent it depends on is not in the mempool. The child is dropped too, and the package never propagates to miners.

How packages help

With package relay, a node can offer the parent and child together and assess them by their combined feerate. If the package as a whole pays enough, both are accepted and forwarded. The initial deployment targets the common 1-parent-1-child shape, which covers most Lightning and contract fee-bumping needs. Bitcoin Core has long used ancestor-aware logic internally for block template selection; package relay extends that awareness to mempool acceptance and propagation.

Package relay is closely tied to modern second-layer safety, where a counterparty must be able to fee-bump a stuck commitment transaction. It pairs naturally with ephemeral anchors and the channel designs enabled by AnyPrevOut. D-Central tracks these mempool-policy developments as part of an educational reference.