The Future of Package Relay in Bitcoin (Bitcoin Core Dev Tech, May 2026)

Package relay lets nodes relay a low-fee parent transaction together with a high-fee child that pays for it (child-pays-for-parent). Bitcoin Core today has 1-parent-1-child (1p1c) opportunistic relay, which uses the orphanage to hold a child until its parent arrives. The session detailed a "package relay jam" failure mode: because inv messages carry no feerate claim, an attacker can cheaply induce orphanage activity with low-feerate transactions, and the current DoS limits (3,000 announcements, per-peer trimming) only partly help.

With several contributors moving on, the pragmatic focus is on narrow wins — a testonlysubmitpackage RPC, and private transaction relay — while larger goals (generalized or batch-CPFP relay for Ark-style constructions, sender-initiated topological hints along the lines of BIP 331) are unlikely to land soon. The session questioned whether BIP 331 is even the right design now that cluster mempool enables richer constructions (not just ancestors but siblings and uncles, perhaps a "cluster prefix relay"). 1p1c alone was estimated to cover roughly 90% of what is wanted.