Discover nat topology theory

Author: Emilia Hane

discv5/discv5-theory.md

@@ -405,7 +405,7 @@ ENR as its potential relay, the longer time that has passed since a peer sent us
 the less guarantee we have that the peer is in fact connected to the owner of that ENR and
 hence of its ability to relay.
 
-### Job of keeping the hole punched falls on Bob and Bob's incentives to do so
+### Job of keeping the hole punched falls on Bob and Bob's incentive to do so
 
 UDP session table entry lifetimes are configurable, though a common lower bound is 20
 seconds[^1]. Bob must periodically reset the session table entry for Alice in its NAT to
@@ -421,6 +421,16 @@ session. If there is no hole for Alice in Bob's NAT when Alice carries out a
 behind a NAT intends to frequently communicate with a peer, reestablishing the session is
 more costly than managing the interval of sent packets to that peer.
 
+### Discovering if the local node is behind a NAT and must keep holes for peers punched
+
+A node may be assigned an externally reachable socket to advertise at start-up. If it is
+not, the [external socket used by the local node is discovered] at runtime and at start-up
+the node assumes it is behind a NAT and must activate the [mechanism for keeping holes punched].
+Once an externally reachable IP address is known, implementations will try to bind to the
+address at some number of ports, randomly selected from a given range of probably unused
+ports. If binding succeeds for any port, the node is not behind a NAT and the
+[mechanism for keeping holes punched] is deactivated.
+
 [EIP-778]: ../enr.md
 [identity scheme]: ../enr.md#record-structure
 [message packet]: ./discv5-wire.md#ordinary-message-packet-flag--0
@@ -437,6 +447,8 @@ more costly than managing the interval of sent packets to that peer.
 [UDP communication]: ./discv5-wire.md#udp-communication
 [Sessions]: ./discv5-theory.md#sessions
 [liveness check]: ./discv5-theory.md#table-maintenance-in-practice
+[external socket used by the local node is discovered]: ./discv5-theory.md#maintaining-the-local-node-record
+[mechanism for keeping holes punched]: ./discv5-theory.md#job-of-keeping-the-hole-punched-falls-on-bob-and-bob's-incentive-to-do-so
 
 [^1]: https://pdos.csail.mit.edu/papers/p2pnat.pdf
 [^2]: https://datatracker.ietf.org/doc/html/rfc4787