BIP 431 — Topology Restrictions for Pinning

BIP 431 (Topology Restrictions for Pinning) documents transaction-pinning attacks — where an adversary exploits mempool replacement and package rules to block fee-bumping of time-sensitive transactions — and specifies TRUC (v3) transactions: an opt-in, restricted-topology regime, paired with concepts like package relay and ephemeral anchors, that limits pinning surface. It targets relay policy, not consensus. Repository status: Draft; implementations have shipped in Bitcoin Core's relay policy.

Debate relevance: Directly relevant to Lightning-security debates: pinning is a practical attack on HTLC settlement, and TRUC/anchor discussions are the current mitigation story. Common misstatements: Calling TRUC/v3 a consensus change — it is mempool/relay policy. Also, 'Draft' here describes the document's repository status; policy features can ship in node software while the informational BIP remains Draft.