What is playback buffers?
Look at the grey line below: packets that have been buffered but have not yet been played back to the user.
Principles of playback buffers
每个router都有queueing delay。为什么曲线长这样？因为each packet arrives in first come first serve order.
Finally, 从最后一个router到laptop的这条link上，由于有贷款的存在（不能高于XXMbps），所以反应到graph上就是cumulative bytes received的曲线有一个maximum gradient，它的导数不会高于这个带宽。
如果红色虚线代表playback（播放）的time和cumulative bytes的关系，那么图中黄色的线就代表at any time, how many packets are buffered. 要不卡的话，红色的虚线不能与曲线相交。
诶等等，是不是想起了信号控制？排队论？
信号控制里的曲线各部分含义如下：
- 曲线和直线（playback line）间的差额为排队的车辆数；
- 曲线下的累积量就是造成的延误值（time delayed * Number of vehicles）
右边灰色的是playback buffer里缓存的部分packets.
如果运气不佳，会发生什么事情？Then re-buffering happens.
The application (e.g. youtube) tries to estimate the delay, and set a buffer with reasonable size, othewise the user will get annoyed at the delay caused by a big buffer (视频迟迟不开始放）。