: 
: Let's take the extreme case where the cluster and the peer synchronize 
: the SA, then there is no IKE traffic, and then they synchronize the SA 
: again. If we don't do anything special, the second pair of synch 
: messages will be identical to the first, i.e. will look like a replay. 
: So we have to prevent this case. One trivial solution is to mandate one 
: liveness test (initiated by the peer) immediately after the synch.
: 
Note that draft recommends that the sync is done only when the IKE SA is 
actually needed.  If there is not IKE traffic then the sync would have 
never been done in the first place.

...
   Since there can be a large number of sessions at the standby member,
   and sending synchronization exchanges for all of them may result in
   overload, the standby member can choose to initiate the exchange in a
   "lazy" fashion: only when it has to send or receive the request.  In
   general, the standby member is free to initiate this exchange at its
   discretion.
...

I've implemented the lazy sync, it's seems the best way to do it.

        Pekka
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