TCP Holepunch (Bitcoin Core Dev Tech, May 2026)

With NAT-PMP seeming to do little for inbound-slot availability on home networks, this session explored TCP hole punching — which succeeds roughly 60% of the time between two NATed peers coordinated by a third node — to increase the network's reachable inbound capacity. Several protocol sketches were discussed: a coordinator triggering simultaneous SYNs so the NAT reuses a mapping, a connection-handoff scheme when a node's inbound slots are full, and an ADDR-based advertisement of hole-punch reachability.

A key concern is that a coordinator could Sybil the network by claiming to control many addresses. The proposed mitigation treats all peers reachable via a given coordinator as a single address for peer-selection purposes, so the scheme adds real reachable capacity without increasing the number of distinct parties in the network's 1-of-N trust assumption. Next steps are gathering data, via a Python script, on how well hole punching works across different hardware and networks (with a related Delving Bitcoin discussion linked in the notes).