TCP as an Implementation of Age-Based Scheduling: Fairness and Performance

We show that different flavors of TCP may be viewed as implementations of age-based scheduling disciplines. By parameterizing the scheduling disciplines of interest we are able to position variants of TCP in a wide spectrum having FCFS (first-come first-served) and LAS (least attained service first) as extremal policies, and including PS (processor sharing) as an intermediate case. We argue that for highly loaded systems, providing a fair bandwidth allocation among all users is secondary to ensuring network stability. So as to isolate protocol fairness from congestion effects, we therefore focus on scenarios with infinite buffers. This way, asymmetries in capacity shares are the consequences of the protocol only, and not affected by the packet loss process. The model, however, is flexible enough to include finite buffers with random packet loss as a special case (for example to capture Active Queue Management). The results are helpful in studying fairness and performance concerned with transmission protocols in communication networks. For persistent HTTP connections we study the distributions of the transmission rates and the relative fairness index under various assumptions on the file size distributions and scheduling disciplines. For infinite file sizes, we show that protocols that increase priority more than linearly with the attained service asymptotically behave similar to FCFS. In contrast, protocols with at most linearly increasing (or even decreasing) priority, with TCP´s Congestion Avoidance mechanism as the most prominent example, converge to PS scheduling (even in absence of losses). When the priority is exactly linear in the attained service, such as for scalable TCP and TCP´s slow start phase, the shares remain constant in between file initiations and completions.