If the client's network goes down for a while but the same IP address
is assigned later, it won't be aware if the server killed the IKE_SA
while it wasn't reachable. This way, a DPD is triggered and the client
can reestablish the SA if necessary. When roaming to a different IP,
a MOBIKE update is triggered with the same effect.
References strongswan/strongswan#2696
Tobias Brunner
8e97e20642