Review of Online Music and Video Downloading

Basically, online music, video, TV show or movie could be delivered in two approaches, by file or in streaming.

File based approach is for browser to firstly download that media file from server to local computer, then to launch media player to playback. To play the music or video, user has to wait until tie file download is completed, which may last from several seconds to hours or days, depending on file size, network latency and server workload.

Adobe proprietary Flash Video is an innovated solution to the approach. Its encoding format would allow media player to play downloaded content while downloading rest of file. In this way, it could greatly reduce user's waiting period to watch or listen online media. This technology has been widely adopted by most online service providers such as MySpace, YouTube, and Google.

By applying Adobe technology, browser will dynamically create a temp file in a special folder to store downloaded media. In the meantime, flash video player will read stored media to play. The file will be removed once the browser window close or another new media file starting downloading.

To save flash video, some Internet site will simulate flash player process to get flash video content to be buffered and saved. Once server changes exchange protocol for flash video delivery, the solution may not work. In addition, this approach may not work on TV series or chunked movie files.

Other software will plug a NT LSP protocol driver into Windows in order to intercept flash video packet from network level. It needs to search 'FLV' signature from each packet as starting point. In fact, it proves to be error prone solution for either missed or false signature.

For more information about free flash video forensic and player, checking those open source projects, and show your salute to those pioneer.

• http://www.osflash.com
• http://www.videolan.org

Streaming mode is an expensive solution but do give user prompt response upon request. Streaming server will pack content of media data into streaming packet and send out. Those packed media will be buffered in the sequence of timestamp for player to decode and display. Its limitation comes from streaming server capability in preparing and handling streaming data. Typically, a streaming server could only support 1024 stream simutaneously.

Most commonly used streaming protocols are RTMP and RTSP/RTP format. MMS and HTTP with flv or wmv format are the least selection to streaming data.

There is several ways to download or record streaming media. One solution is to use network monitor to watch those streaming ports of RTMP (port 1935), RTSP (port 554) or MMS (port 1755) and all packets in a connection. This turns out a very challenge approach as it requires both protocol parser and decoder to support the exploit of content contained in the packet.

Unfornately, Adobe RTMP is still a proprietary technology and no public document could be referred for forensic analysis.

Microsoft uses RTSP/RTP to deliver media in wmv/wma or VC1 format. MMS was used widely before, and now, is gradually replaced by RTSP/RTP streaming protocol.