Cause and countermeasure of overtermination for PCN-based flow termination

Pre-congestion notification (PCN) is a packet marking mechanism for DiffServ domains to facilitate feedback-based admission control (AC) and flow termination (FT) for the Internet to achieve quality of service (QoS) for inelastic flows. While AC blocks new flows to prevent an overly high load on network links, FT removes traffic to protect the QoS of the remaining PCN-flows when traffic is rerouted due to any kind of failure. We focus on one of the PCN methods, the controlled load (CL) method, whose edge behavior is now under standardization in the PCN working group of the Internet Engineering Task Force. The CL method terminates a sufficient number of flows in one shot and seems to have accurate control. However, we discovered that more termination than was needed, which we call overtermination, occurred in the CL method. We found that the CL method causes overtermination when not all the ingress-egress pairs have the same round-trip time. A countermeasure is presented to remove this overtermination.