Core-Stateless Fair Queuing

This paper describes a method to implement fair queuing across a network without requiring exceptional state tracking from the most routers. Rather than compute a priority for packets, the primary mechanism used to control sending rates is probabilistic dropping based on the rate of the flow the packet is in. Computing the flow rate involves tracking substantial per-flow state, but such per-flow state can be tracked only at network edges and then attached to outgoing packets (and modified as necessary within). To determine how often to drop, it is also necessary to estimate the fair share deserved by each flow, which is done by comparing the observed overall drop and arrival rates at each router.

The authors evaluate this mechanism over several simulated topologies with variously behaved competing flows. They show that the performance of CSFQ is often competitive to a full per-flow state implementation of fair queuing (Deficient Fair Queuing). Its aboslute performance is usually slightly worse than DFQ (with the exception of performing better in cases where flows may suffer from not being able to temporarily exceed their "fair" buffer to accommodate burstiness).

CSFQ has some parameters which seem to be set arbitrarily and some which are not clear how to set in general. Arbitrary parameters include thresholds used to adjust for buffering effects. Exponential smoothing parameters the authors recommend setting substantially larger than delay-jitter and shorter than the average duration of a flow. Since with things like web pages which are in fact short-lived flows, we probably have a relatively short average flow duration; how should these parameters be set when flows may traverse the globe (with presumably high delay jitter) before exiting the network or traverse only to the local metropolis's peering point (with hopefully very low delay jitter)?