The playback buffer is the holding tank of decoded video a streaming player keeps ahead of the playhead, ranging from a few seconds to a few tens of seconds, filled from the network while the player plays out of it. As long as it refills at least as fast as it drains, playback is smooth and the viewer never knows it exists; when it empties to zero the stream stalls, which is rebuffering. Its depth is a deliberate trade-off. A player will not start until it holds a set start buffer threshold as a cushion against an immediate stall, so a deeper threshold protects against rebuffering but lengthens startup time, while a shallower one starts faster but is more fragile. This is why startup and rebuffering are tuned together rather than in isolation. The buffer is also the shock absorber an ABR algorithm manages, filling a cushion before allowing an upswitch; buffer length is reported per request in CMCD as the field that signals an imminent stall.
The handbook · 7 volumes
Fora Soft Video Engineering Handbook
The playback buffer trades startup speed against stalls: a deeper cushion survives network dips but makes viewers wait longer at the start. We wrote seven volumes covering the whole video stack, and for the playback buffer start with Volume 3: players, buffer-based ABR, stall recovery and QoE, with Volume 2 on HLS and DASH segments. Written by Nikolay Sapunov, CEO at Fora Soft.
Tuning the playback buffer and stuck between slow starts and rebuffering?
Share your player config and QoE data: we model playback buffer settings for your audience and give you values with expected startup and stall changes. 250+ video products shipped since 2005.
