If there have been no RTT measurements for a server (e.g. the first ismaster command after the server becomes available), the average RTT MUST be set equal to the first RTT measurement.

After the first measurement, average RTT MUST be computed using an exponentially-weighted moving average formula, with a weighting factor (alpha) of 0.2. If the prior average is denoted old_rtt, then the new average (new_rtt) is computed from a new RTT measurement using the following formula:

alpha = 0.2
new_rtt = alpha * x + (1 - alpha) * old_rtt

A weighting factor of 0.2 was chosen to put about 85% of the weight of the average RTT on the 9 most recent observations.