Breakpoint resume download: In-depth understanding of the Range and Content-Range parameters in the HTTP Header

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introduction

We often experience network instability or other interruptions while file downloads are in progress. In order to provide a better user experience and save bandwidth resources, the technology of resuming uploading from breakpoints came into being. The HTTP protocol supports resumed uploads by using the Range and Content-Range parameters. This article will discuss these two key HTTP Header parameters in depth.

Range parameter

The Range parameter is a request header parameter sent by the client to the server, which is used to specify the byte range of the file to be downloaded. It has the format Range: bytes=start-endwhere startand endrepresent offsets in bytes. By using the Range parameter, the client can continue downloading the file from a specified position after the part already downloaded, rather than starting from the beginning.

After the server receives the request with the Range parameter, it will return partial content in the response, and the status code is 206 Partial Content. In this way, the client can combine the downloaded part with the data returned by the server to achieve the effect of resuming the download.

Here is an example using the Range parameter:

GET /example-file.txt HTTP/1.1
Host: example.com
Range: bytes=500-999

In the example above, the client requests to download example-file.txta file in the byte range of 500 to 999 bytes.

Content-Range parameter

The Content-Range parameter is a response header parameter used by the server when returning a response to indicate the byte range of the data sent. It has the format Content-Range: bytes start-end/totalwhere startand endrepresent the byte range of the returned data and totalrepresent the total number of bytes for the complete file.

The Content-Range parameter allows the server to specify in the response the byte range of partial data sent, as well as the total number of bytes for the complete file. In this way, after receiving the response, the client can know the position of the data returned by the server in the entire file, which is convenient for merging and managing the downloaded parts.

Here is an example using the Content-Range parameter:

HTTP/1.1 206 Partial Content
Content-Range: bytes 500-999/2000
Content-Length: 500

In the above example, the data returned by the server is a file with a byte range of 500 to 999 bytes, for a total of 2000 bytes.

Implementation process of resuming uploads from breakpoints

The general process for implementing resumable uploads from breakpoints is as follows:

The client sends a request with a Range parameter specifying the range of bytes to download.
After receiving the request, the server returns partial content according to the Range parameter, and the status code is 206 Partial Content.
After the client receives the server's response, it merges the downloaded part and the newly downloaded content according to the Content-Range parameter.
During the download process, the client and server continue to interact until the file is completely downloaded.

Advantages and Applications

Breakpoint resume technology brings the following advantages and applications:

Provide better user experience : Resumable downloading allows users to continue downloading after the download is interrupted without restarting, saving time and bandwidth resources.
Reduce server load : By returning only part of the requested content, the server can reduce the use of network transmission and computing resources and improve overall performance.
Large file download optimization : For large file downloads, resuming uploads can avoid repeated downloads due to interruptions and improve download efficiency and stability.

sample application

The following is a sample application using the resumed upload technique:

GET /big-file.mp4 HTTP/1.1
Host: example.com
Range: bytes=0-999999

In the above example, the client requests to download a large file big-file.mp4named and specifies a byte range of 0 to 999999 bytes. If the download is interrupted, the client can resume the download by sending a request with the appropriate Range parameter.

Precautions

When implementing resumable uploads from breakpoints, you need to pay attention to the following:

Server Support : The server needs to correctly handle requests with the Range parameter and return the correct Content-Range response header.
File Consistency : Since resuming downloads are part of a file based on byte ranges, it is important to ensure that the downloaded part is consistent with the content of the file on the server.
Concurrent download limit : Resuming downloads from breakpoints may result in a large number of concurrent requests, and the server needs to be properly limited to avoid excessive load.

in conclusion

The Range and Content-Range parameters are the key HTTP Header parameters for resumable upload. By making reasonable use of these two parameters, you can continue to download the specified part of the file after the download is interrupted, providing a better user experience and optimizing the use of network resources. Understanding the working principle and correct use of these parameters can help developers better implement the resume function.