IETF Pre-Congestion Notification (PCN) working group is working on mechanisms to ensure quality of service of voice and video traffic within a DiffServ domain in the Internet. These mechanisms operate at the domain boundary based on congestion information from within the domain. The reaction mechanisms at the boundary consist of flow admission and flow termination. In this paper, we present a simple analytical model for PCN. Closed form expressions for flow rejection and termination probabilities are provided. If the acceptance and rejection thresholds are set close to each other, thrashing may happen, that is, some flows will be terminated while others are being admitted. We study the sensitivity of the thrashing probability to various system parameters. This will help network service providers to set the parameters accordingly.