Choosing the Best Provider

The next component is to select the best provider for a destination at a given time based on past measurement samples from monitoring provider links. The core issue here is whether, and how, historical data about ISP performance to a given destination should be used at all. The performance of an ISP link to a destination can be tracked by keeping a smoothed, time-weighted estimate of the performance, for example an exponentially-weighted moving average (EWMA). If performance of using an ISP $P$ to reach destination $D$ at time $t_i$ is $s_{t_i}$ (as obtained from active or passive measurement) and the previous performance sample was from time $t_{i-1}$, then the EWMA metric at time $t_i$ is:

$$\begin{eqnarray*} EWMA_{t_i}(P,D) & = &(1-e^{-(t_i-t_{i-1})/\alpha}) s_{t_i} \\ & & \mbox{} + e^{-(t_i-t_{i-1})/\alpha}EWMA_{t_{i-1}}(P,D) \end{eqnarray*}$$

where $\alpha>0$ is a constant. A smaller value of $\alpha$ attaches less weight to historical samples. A value of $\alpha = 0$ implies no reliance on history. At any time, the provider with the best performance as calculated above could be chosen for a transfer. When no history is employed ($\alpha = 0$), only the most recent performance sample is used to evaluate the providers and select the best.