Buffering is a decision the player made

A player picks a quality level every few seconds, based on a guess about how much bandwidth it has. Guess high and the buffer empties and the viewer sees a spinner. Guess low and the picture is soft on a connection that could have carried more. Neither is an error, so neither is in the error rate.

The guess is the whole game, and it is made harder by everything real networks do: a congested cell, a shared line at eight in the evening, a router that buffers. An algorithm tuned on a clean connection behaves badly on a real one.

The work was on that estimate and on what the player does around it, on the browsers where the media stack behaves least predictably.