Streaming and buffering

When you stream a video, the network does not deliver it at the steady rate the screen plays it. The network is bursty: fast for a moment, slow the next, sometimes gone. Playback is the opposite: it needs the next second of video every second, without fail. The buffer is the cushion between these two rhythms, a small reserve of downloaded-but-unplayed video that absorbs the mismatch.

Filling faster than draining

While the network delivers faster than playback consumes, the buffer grows: you are downloading ahead. This is why a video often plays smoothly through brief slowdowns you never notice, the dip was paid for out of the reserve, not out of your viewing.

The stall

Drop the network below the playback rate and the arithmetic turns: the buffer drains, second by second, until it hits zero. At that moment there is nothing left to play, and playback stalls. This is the buffering wheel. Note what it means: a stall is not caused by the moment the network got slow, it is caused by the reserve running out afterward. The slowdown happened earlier; the buffer told you the stall was coming before it arrived.

Recovery

The network does not need to be fast for playback to resume, it only needs to refill a small reserve first. That is why the wheel spins for a few seconds even after the connection recovers: the player is deliberately rebuilding a cushion before it risks playing again, rather than resuming instantly and stalling right back.

What the demo shows

Press play and the buffer fills from the network at the rate you set on the slider, while playback drains it at a steady rate. Drag the network speed below the playback rate mid-play and watch the reserve shrink, hit zero, and stall; restore the speed and watch it refill to the resume threshold before playback continues. Every state you see falls out of one piece of arithmetic, what came in minus what played out, see the note below the figure for what this toy leaves out.