See "Application Flow Control in YouTube Video Streams" - here.
ABSTRACT: This paper presents the results of an investigation into the application flow control technique utilised by YouTube. We reveal and describe the basic properties of YouTube application flow control, which we term block sending, and show that it is widely used by YouTube servers. We also examine how the block sending algorithm interacts with the flow control provided by TCP and reveal that the block sending approach was responsible for over 40% of packet loss events in YouTube flows in a residential DSL dataset and the re-transmission of over 1% of all YouTube data sent after the application flow control began.
We conclude by suggesting that changing YouTube block sending to be less bursty would improve the performance and reduce the bandwidth usage of YouTube video streams.
The authors say that "We have presented these results to engineers at YouTube and their parent company, Google. They have acknowledged that this is a legitimate problem and are currently working on modifying the block sending algorithm to be less bursty. We believe that this will offer improved YouTube performance for users and reduce YouTube’s bandwidth requirements. The largest improvements will be seen by YouTube clients using congested connections, but well-connected clients should also see some benefit".