title: Necessity for separate VirtualService and DestinationRule configs in Istio
body: <p>In Istio, I was wondering why VirtualService and DestinationRule are separated into two separate config files? AFAICT, the DestinationRule defines the subset and the VirtualService routes to the subset. Obviously, they both do more than just that, but my question is, what could have been the design thought behind separating the two. Why couldn't we have defined the subset in the VirtualService yaml itself?</p>
  
  <p>A follow-on question is: Why is the circuit-breaker rules configured in the destination rule, while the timeouts and retries are configured in the VirtualService? Again, I guess, I am unable to comprehend the reasoning behind this design decision. If that is clear, both questions may be answered simultaneously. Any help in understanding this conceptually will be much appreciated. </p>