A NAT that drops an idle mapping leaves the server socket looking
connected while the server can no longer reach us, and nothing errors:
peers stop finding us and searches go unanswered until a restart. The
server socket now carries TCP keepalive (idle 10s, interval 2s, ten
probes), which keeps the mapping alive and turns a dead path into a read
error within about half a minute, as Nicotine+ does. The server stopped
answering ServerPing (code 32) long ago, so an application ping is no
substitute. Windows gets a no-op until the daemon ships there.
Michel de Graaf
662d40b79c