A method for streaming non-live video to the user for immediate playback. Supported in the user's media player, progressive download employs HTTP, the protocol used to download everything from the Web. Unlike a regular download, progressive download lets users view the video as soon as a small amount of content has been received. The video is stored in a temporary folder and can be played again within the same user session but cannot be saved unless the media player offers that option. See HTTP. A huge number of websites favor progressive downloads, including YouTube. These sites use free, open source Web server software, rather than proprietary streaming servers, such as Windows Media Services and Flash Media Server. As long as the video describes its content structure in the front of the file (see metadata), modern content delivery networks (CDNs) enable fast forwarding into the middle of the video without downloading the preceding content. Progressive Download Vs. Real-Time Streaming Although most users cannot tell the difference, progressive download stores the entire file, while "real-time streaming," sometimes called "true streaming," is analogous to broadcast radio and TV. Although there are always more video frames in the memory buffer at each moment, real-time streaming content essentially passes through the computer. In addition, real-time streaming uses the UDP protocol, which may ignore lost packets and accept packets with errors. Progressive download uses TCP to ensure that each network packet arrives intact and will request retransmission if not. This can temporarily stop the video when only a momentary blip in viewing might have been observed. See streaming video and TCP/IP.