not responded to messages during this time it is considered to be dead.
In other words, this is the length of time in seconds for which DPD-R-U-THERE messages will be
sent. If the other side of the tunnel has not sent a response to any messages then it is considered
to be dead (not reachable). The SA will then be placed in the dead cache.
This setting is used with IKEv1 only.
Default: 15 seconds
Chapter 9: VPN
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