The network traffic control strategies and algorithms are part of the feedback loop that allows such a complex system to operate at an optimal or near-optimal functioning point. Effective performance criteria are an important element for providing the feedback regarding the current state of the system; thus, they represent a key aspect of the design of network traffic control strategies and algorithms.

We introduce three synthetic factors of merit regarding the quality of service, the quantity of service and the ratio of the two. The figures of merit may take into account multiple QoS attributes. Each of these figures of merit can be calculated either for individual flows, the flows with a common characteristic, e.g., belong to the same class, or cross the same router, or for all the flows in a network. We also define a coefficient of fairness among the flows with a common characteristic, as the ratio of the variance to the expected value of a factor of merit.