Prefetching Video
As we learned in lecture 2, client-side bu ering can be used to mitigate the e ects of varying end-
to-end delays and varying available bandwidth. In our earlier example in Figure 1 (lecture 2), the
server transmits video at the rate at which the video is to be played out. However, for streaming
stored video, the client can attempt to download the video at a rate higher than the consumption
rate, thereby prefetching video frames that are to be consumed in the future. This prefetched video
is naturally stored in the client application bu er. Such prefetching occurs naturally with TCP
streaming, since TCP s congestion avoidance mechanism will attempt to use all of the available
bandwidth between server and client. To gain some insight into prefetching, let s take a look at
a simple example. Suppose the video consumption rate is 1 Mbps but the network is capable of
delivering the video from server to client at a constant rate of 1.5 Mbps. Then the client will not
only be able to play out the video with a very small playout delay, but will also be able to increase
the amount of bu ered video data by 500 Kbits every second. In this manner, if in the future the
client receives data at a rate of less than 1 Mbps for a brief period of time, the client will be able
to continue to provide continuous playback due to the reserve in its bu er.
Client Application Bu er and TCP Bu ers
Figure 1 illustrates the interaction between client and server for HTTP streaming. At the server
side, the portion of the video le in white has already been sent into the server s socket, while the
darkened portion is what remains to be sent. After \passing through the socket door," the bytes
are placed in the TCP send bu er before being transmitted into the Internet. In Figure 1, because
the TCP send bu er at the server side is shown to be full, the server is momentarily prevented from
sending more bytes from the video le into the socket. On the client side, the client application
(media player) reads bytes from the TCP receive bu er (through its client socket) and places the
bytes into the client application bu er. At the same time, the client application periodically grabs
video frames from the client application bu er, decompresses the frames, and displays them on the
user s screen. Note that if the client application bu er is larger than the video le, then the whole
process of moving bytes from the server s storage to the client s application bu er is equivalent to
an ordinary le download over HTTP|the client simply pulls the video o the server as fast as
TCP will allow.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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