Keep hole punched theory

Author: Emilia Hane

discv5/discv5-theory.md

@@ -405,6 +405,22 @@ 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
+
+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
+keep the hole for Alice punched. If Alice too is behind a NAT, it must do the same for Bob.
+Implementations must ensure, when a node behind a NAT does not send a packet to a peer
+within the entry lifetime then an empty packet is sent that is dropped by the peer.
+
+NAT hole punching unavoidably creates overhead for all nodes in the network but once a
+hole is punched, keeping it punched requires no interaction between peers. The incentive
+for nodes behind a NAT to keep holes for its peers punched is to avoid reestablishing a
+session. If there is no hole for Alice in Bob's NAT when Alice carries out a
+[liveness check], Bob is considered offline and the session to Bob useless. If the node
+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.
+
 [EIP-778]: ../enr.md
 [identity scheme]: ../enr.md#record-structure
 [message packet]: ./discv5-wire.md#ordinary-message-packet-flag--0
@@ -420,6 +436,7 @@ hence of its ability to relay.
 
 [UDP communication]: ./discv5-wire.md#udp-communication
 [Sessions]: ./discv5-theory.md#sessions
+[liveness check]: ./discv5-theory.md#table-maintenance-in-practice
 
 [^1]: https://pdos.csail.mit.edu/papers/p2pnat.pdf
 [^2]: https://datatracker.ietf.org/doc/html/rfc4787