You can try this: http://forum.mikrotik.com/t/script-implementing-active-congestion-control/115302/1
That is not a typical feature for QoS, but it looks like someone has implemented it.
You can try this: http://forum.mikrotik.com/t/script-implementing-active-congestion-control/115302/1
That is not a typical feature for QoS, but it looks like someone has implemented it.