Nortel Networks DiffServ mechanisms have been developed to support Quality of Service (QoS). However, the level of assurance that can be provided with DiffServ without substantial over-provisioning is limited. Pre- Congestion Notification (PCN) investigates the use of per-flow admission control to provide the required service guarantees for the admitted traffic. While admission control will protect the QoS under normal operating conditions, an additional flow pre-emption mechanism is necessary in the times of heavy congestion (e.g. caused by route changes due to link or node failure). This document provides a problem statement on the addition of flow admission control and flow pre-emption functionality to a DiffServ network, in particular for the support of real time services such as voice and video.