What I would do is to hide the implemntation[connetion] behnd a generic interface. How to connect to th external system should be a moot point - assume some sort of an adapter in between.
The data required [synchronous processing] is only yes or no as mentioned previously in some
thread here.. so if u assume mutual authentication between servers, I guess all you would need to show is the innterface and the implementing class thta can be swaapped out as necessary.