Erlay
Also: BIP-330
network · advanced
A bandwidth-efficient transaction-relay protocol. Instead of announcing every transaction to every peer, nodes reconcile sets periodically.
Stock transaction relay is wasteful: a typical mempool transaction is announced to every peer of every node, even though most peers already saw it from somewhere else. With ~10 peers per node, ~90% of "I have this tx" announcements are redundant.
Erlay (Naumenko, Maxwell, Chaikovskii, 2019) replaces redundant per-peer announcements with periodic set-reconciliation. Each node maintains a sketch of its mempool; pairs of peers periodically exchange compact set-difference encodings instead of full inventory lists. Result: bandwidth use scales as O(log n) with the number of peers instead of O(n).
Erlay matters less for high-bandwidth datacenter nodes and more for nodes running on low-bandwidth or metered connections — Tor, satellite, mobile, embedded. Partial deployment landed in Bitcoin Core; full activation depends on enough peers also speaking it.