Progressive download

A progressive download is the transfer of

protocol when initiated from a computer. The consumer may begin playback of the media before the download is complete. The key difference between streaming media and progressive download is in how the digital media data is received and stored by the end user

device that is accessing the digital media.

A

buffer of the digital media file as it is downloaded from a web server. At the point in which a specified amount of data becomes available to the local playback device, the media will begin to play. This specified amount of buffer is embedded into the file by the producer of the content in the encoder

settings and is reinforced by additional buffer settings imposed by the media player.

History

Initially the digital media file type known as

encoded digital media

content.

HTTP progressive download versus streaming media

The end user experience is similar to

temporary directory

of the associated web browser if the medium was embedded into a web page or is diverted to a storage directory that is set in the preferences of the media player used for playback. The file will stutter or stop playback if the rate of playback exceeds the rate at which the file is downloaded. The file will begin to play again after further download.

This fast start playback is the result of moving the meta data from the end of the digital media file to the front, this move of the meta data gave the media player all the information it required to begin playback as the file was still being downloaded. Prior to that change, the meta data summary was located at the end of a media file and the entire file would need to be downloaded in order for the meta data to be read and the player begin playback.[2]

HTTP Pseudo-streaming (or progressive download), similar to streaming media or HTTP Live Streaming, also supports adaptive bitrate streaming. The disadvantage of HTTP Pseudo-streaming over streaming media is reduced security, since HTTP is easier to sniff compared to RTMP, along with long loading times when seeking larger videos.^[3]

Optimization for HTTP Pseudo-streaming

The

HTML5 Video based clients can start playing the video. To optimize for HTTP Pseudo-streaming, it's important to move the moov atom to the beginning, so that web servers like Nginx optimize for faster pseudo-streaming resulting in playing of playbacks without having to wait for the entire file to be arrived or scanned.^[5]

ffmpeg -i sample_input.mp4 -movflags faststart -acodec copy -vcodec copy sample_output.mp4

-movflags faststart does the optimization by moving the index (moov atom) to the beginning of the file.[6]
-acodec copy extracts the audio from the input file unaltered.
-vcodec copy extracts the video from the input file unaltered.

By doing this optimization, we effectively prevent web servers, like

HTML5 Video based client.^[5]^[4]

Seeking

Initially, the file is played from the beginning. A user may wish to point to a part of the file which haven't been downloaded yet. This capability is called seeking [7] and it makes possible to download and start playing any part of the media file. That is often referred to as pseudo-streaming.

For

Flash video

seeking requires a list of seek points in the media file metadata. These points are offsets in the video (both in seconds and bytes) at which a new key frame starts. A web server or a media server which handles the download, must support seek points in query string of requests for downloading data.

For other types of media files such as

MP4 or MKV

, web servers must be capable of handling a special offset parameter. The offset parameter name differs for various servers so it must be specified in player settings.

Some servers support seeking via additional modules only, they are specified below. Seeking parameter names are written in italic.

Server	Flash seeking	MP4 seeking
Apache HTTP Server	mod_h264 start mod_flvx start	mod_h264 starttime
lighttpd	start	mod_h264 starttime
Nginx	http_flv_module start	http_mp4_module start
Nimble Streamer	start	start

References

^ QuickTime Streaming Server 4.1 (PDF), archived from the original (PDF) on 2011-07-06, retrieved 2010-09-21, Two methods are commonly used to deliver media over the web for real-time viewing: progressive download (Fast Start) and real-time streaming.
^ Understanding the MPEG-4 movie atom, retrieved 2015-07-17
^ Peter Luk. "Video Delivery: HTTP Pseudo-Streaming".
^ ^a ^b Billy Hoffman (2016). "Optimizing MP4 Video for faster streaming".
^ ^a ^b "Module ngx_http_mp4_module".
^ "FFmpeg Formats Documentation".
^ Pseudo Streaming in Flash, retrieved 2015-07-17

External links

Streaming vs Progressive Download, archived from the original on 2010-08-18
Web Server vs. Streaming Server, Microsoft, retrieved 2010-09-21
Flash Video: Progressive Download, retrieved 2010-09-21
Video Streaming Vs Progressive Download, archived from the original on 2015-05-29, retrieved 2015-05-29

[1] QuickTime Streaming Server 4.1 (PDF), archived from the original (PDF) on 2011-07-06, retrieved 2010-09-21, Two methods are commonly used to deliver media over the web for real-time viewing: progressive download (Fast Start) and real-time streaming.

[2] Understanding the MPEG-4 movie atom, retrieved 2015-07-17

[3] Peter Luk. "Video Delivery: HTTP Pseudo-Streaming".

[r1-4] Billy Hoffman (2016). "Optimizing MP4 Video for faster streaming".

[n1-5] "Module ngx_http_mp4_module".

[6] "FFmpeg Formats Documentation".

[7] Pseudo Streaming in Flash, retrieved 2015-07-17

[3]

[5]

[4]