The value of tmnxIkePolicyDpd specifies whether DPD (dead peer
detection) is 'enable', 'disable' or in 'replyOnly' mode.
The DPD vendor ID is always advertised to the peer. To the extent
that the peer advertises DPD support as well, the service-router will
always reply to the peer's 'Are-You-There' messages.
If tmnxIkePolicyDpd object is set to 'enable' the service-router will
also send its own 'Are-You-There' message to the peer at the interval
specified by tmnxIkePolicyDpdInterval.
If tmnxIkePolicyDpd object is set to 'disable' the service-router will
never send its own 'Are-You-There' message to the peer.
If tmnxIkePolicyDpd object is set to 'replyOnly' the service-router
will take the peer's 'Are-You-There' message as proof of 'liveliness'
and will suppress the sending of its own 'Are-You-There' messages.
Once it stops receiving 'Are-You-There' messages from the peer, it
will start sending its own to determine if the peer is dead. The
service-router will only send an 'Are-You-There' message when the
other side has been idle (no traffic was forwarded through it) since
the last tmnxIkePolicyDpdInterval. If the other side is active
(as determined by its traffic counters) it is assumed the peer is
alive and the 'Are-You-There' message is suppressed.