[whatwg] Limiting the amount of downloaded but not watched video

Jeroen Wijering jeroen at longtailvideo.com
Mon Jan 17 08:41:13 PST 2011

Hello all, 

We are getting some questions from JW Player users that HTML5 video is quite wasteful on bandwidth for longer videos (think 10min+). This because browsers download the entire movie once playback starts, regardless of whether a user pauses the player. If throttling is used, it seems very conservative, which means a lot of unwatched video is in the buffer when a user unloads a video.

I did a simple test with a 10 minute video: playing it; pausing after 30 seconds and checking download progress after another 30 seconds. With all browsers (Firefox 4, Safari 5, Chrome 8, Opera 11, iOS 4.2), the video would indeed be fully downloaded after 60 seconds. Some throttling seems to be applied by Safari / iOS, but this could also be bandwidth fluctuations on my side. Either way, all browsers downloaded the 10min video while only 30 seconds were being watched. 

The HTML5 spec is a bit generic on this topic, allowing mechanisms such as stalling and throttling but not requiring them, or prescribing a scripting interface:


Are there people working on ways to trim down the amount of not-watched data for video playback? Any ideas on this, anything in the pipeline?


A suggestion would be to implement / expose a property called "downloadBufferTarget". It would be the amount of video in seconds the browser tries to keep in the download buffer.

When a user starts (or seeks in) a video, the browser would try to download "downloadBufferTarget" amount in seconds of video. When "downloaded > currentTime + downloadBufferTarget", downloading would get stalled, until a certain lower treshold is reached (e.g. 50%) and the browser would start downloading additional data. 

A good default value for "downloadBufferTarget" would be 60 seconds. Webdevelopers who have short clips / do not care about downloads can set "downloadBufferTarget" to a higher value (e.g. 300). Webdevelopers who have long videos (15min+) / want to keep their bandwidth bill low can set "downloadBufferTarget" to a lower value (e.g. 15). Webdevelopers might even change the value of "downloadBufferTarget" per visitor; visitors with little bandwidth get a sizeable buffer (to prevent stuttering) and visitors with a big pipe get a small download buffer (they don't need it). 

The "buffered" timeranges could be used to compare the actual download buffer to the buffer target, should a user-interface want to display this feedback.

Note that the download buffer is not the same as the playback buffer. A download buffer underrun should not result in pausing the video. The download buffer does also not apply to live streaming.  

Kind regards,

Jeroen Wijering
Longtail Video


PS: Having the "preload=none" property available in all browsers will also help in keeping the amount of downloaded but not watched data low. In our tests, only Firefox (4 b9) seems to honor this property at present. 

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