+alive. The default is 30 seconds for active dynamic peers, and 5
+minutes for passive peers.
+.IP
+The period for dynamic peers should be no longer than
+.I timeout
+\(mu
+.RI ( retries
+\- 1). Consider an idle mobile peer which has its IP address changed
+just before its passive peer begins pinging. The static peer's pings
+will go to the old address until it receives a ping back from the mobile
+peer. Therefore, the static peer has to keep pinging until it would
+definitely have received an unsolicited ping from the mobile peer, and
+therefore be informed of the change of address. And it's no use
+learning about the change of address just after sending the last ping to
+the old address, so the last retry doesn't count for the purposes of
+this calculation.
+.IP
+Besides, the consequences of failed pinging differ between dynamic and
+passive peers. In the former case, a failure provokes a reconnection
+attempt, after which (hopefully) things will work again: it's probably a
+good thing to check frequently and fail fast. In the latter case, the
+dynamic peer will certainly have to notice that it's been abandoned and
+arrange to retry, causing a communication failure where maybe there
+wasn't really one before.