Fairness Measurements about TCP Flows in DS Networks: Comparison of Per-flow and Aggregate Marking Schemes

Traffic marking is an essential component in any Differentiated Services (DS) architecture. Traffic marking can be done with respect to a single flow or a flow aggregate. In a DS network, traffic marking is typically performed at aggregate levels, rather than on the level of individual flows. The use of aggregation raises a number of concerns, such as fairness concern. When we deal with aggregated sources in such networks, we need to consider not only the fairness issues among aggregates but also the fairness issues among individual flows within an aggregate. We have carried out measurements in a test network to examine the effect of DS mechanisms on the fairness among individual TCP flows. The aim of the measurements was to evaluate how per-flow marking and aggregate marking affect the performance of individual TCP connections when competing with each other within the same Assured Service class. In order to eliminate the effects of different RTTs (Round Trip Times) and lengths of the flows, we used homogeneous TCP flows, i.e., flows with equal RTT and duration. Our measurements showed that per-flow marking not only provides much fairer access to the assured bandwidth, but also provides higher throughputs seen by individual flows. Key-Words: -. QoS and service differentiation, network measurements, network access, traffic control