Quality of Service in Virtual Cut-through Networks

This thesis explores the possibility of achieving class level and flow level Quality of Service guarantees in a Virtual Cut-Through network with a class based Quality of Service mechanism in conjunction with admission control. There is an increasing number of System Area Network technologies based on the Virtual Cut-Through principle. Many of these support Quality of Service mechanisms, but little work has been done on performing admission control in Virtual Cut-Through networks. Three different admission control algorithms for use in Virtual Cut-Through networks are proposed in this thesis. All three algorithms operate in accordance with the DiffServ philosophy, but the basis for their admission control decisions differ. The first relies on apriori knowledge of the capacity of each link, and has information about the load on each link in the network. Its decision is based on whether the links can support more traffic. The second method performs measurements at the egress of the network to ascertain whether the network can tolerate an increase in traffic with a given latency requirement. The third and final method for admission control measures the jitter of special probe packets as the basis for its decision. An evaluation of the proposed algorithms is presented through extensive simulation results. The Quality of Service properties that are studied are the ability to give bandwidth guarantees to each individual flow, and to the service class as a whole, and the latency and jitter characteristics that the traffic displays with the different admission control algorithms. Through these simulations the apparent limits of the admission control algorithms are discovered, and the range of QoS guarantees that may be achieved in Virtual Cut-Through networks becomes clear. The simulations show that throughput guarantees on the class level and the flow level are achievable, but that latency and jitter in VCT networks are hard to control. Finally, packet dropping is investigated as a method for reducing packet jitter. The results show that this method is able to reduce the jitter perceived by the network traffic, but it does not outperform some of the admission control algorithms