The liveness check for IKEv2 is similar to Dead Peer
Detection (DPD), which IKEv1 uses as the way to determine whether
a peer is still available.
In IKEv2, the liveness check is achieved by any IKEv2 packet
transmission or an empty informational message that the gateway
sends to the peer at a configurable interval, five seconds by default.
If necessary, the sender attempts the retransmission up to ten times.
If it doesn’t get a response, the sender closes and deletes the IKE_SA
and corresponding CHILD_SAs. The sender will start over by sending
out another IKE_SA_INIT message.