In-depth parse of HTML5<video></video> Tags: Preload and segmentation requests

This article will analyze HTML5 in detail<video></video> The video preloading time control of the tag and the segmented download mechanism of MP4 videos answer the questions about how the browser can control the preloading time and realize video segmented download. The article will combine HTTP Range request header and video file index information to deeply explore the operating principles behind the browser.

The core problem is: the browser<video></video> The control mechanism of label preload time and how to calculate the byte range specified in the Range request header to achieve video segment loading. It is not accurate to directly divide the file size by the duration to calculate the number of bytes per second, and the actual method adopted by the browser is more complicated.

Preload duration control: Currently, there is no direct API that can accurately control it<video></video> The preload duration of the tag. The browser will dynamically adjust the preload amount according to factors such as network conditions, video file size, etc., and it is not a fixed time. This is closely related to the browser's caching mechanism and resource management strategy, and its internal logic is relatively complex and opaque.

Segment download and Range request header: The browser uses the Range request header of the HTTP protocol to realize video segment download. The Range request header allows the client to request only part of the content of the video file. Instead of simply dividing the file size by duration to calculate the byte range, browsers rely on the encoding format and index information of the video (such as the moov atoms of MP4 files).

When the user operates (such as dragging a progress bar), the browser calculates the corresponding byte offset based on the target time period, combined with the video index information, and writes this range to the Range request header to send it to the server. The server returns the data in the specified range and uses the status code 206 Partial Content to indicate the partial content response. The server may even return data in multiple byte ranges, identifying using multipart/byteranges Content-Type, thereby obtaining multiple video clips at once.

Summarize:<video></video> The preloading mechanism of tags is complex and lacks direct API control; while segmented downloads rely on the index information of HTTP Range request headers and video files, and are not simple linear calculations. The browser can achieve efficient video streaming by cleverly using HTTP protocol and video file structure information.

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