An Overview of HTTP/2 Server Push and How to Check for Support

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Published·Nov 23, 2019Modified·Nov 23, 2019

HTTP/2 (Hypertext Transfer Protocol version 2, originally named HTTP 2.0), abbreviated as h2 (encrypted connections based on TLS/1.2 or higher) or h2c (unencrypted connections), is the second major version of the HTTP protocol used on the World Wide Web. Initially, Google developed SPDY to improve HTTP performance, which later evolved into the standard for HTTP/2. HTTP/2 is compatible with HTTP/1.1, including HTTP Methods, Status Codes, URIs, and most Header Fields.

HTTP/2 Features

Multiplexed requests;
Request prioritization;
Compressed HTTP headers;
Server Push streams (Server Push technology).

HTTP/2 Server Push Mechanism

In HTTP/1.1, within the same TCP connection, the server can only send the next response after the previous one is fully sent. The following image illustrates the HTTP/1.1 transmission process.

In Server Push mode, when requesting an HTML file, if the HTML contains CSS files, the server pushes them to the client simultaneously. The following image shows the HTTP/2.0 transmission process.

Assume an HTML page index.html includes a CSS file style.css and an image file example.png. In HTTP/1.1, the client needs to send three separate requests to the server to handle these three files. With Server Push, the server can send index.html, style.css, and example.png all at once. This requires only one round of HTTP communication, allowing the client to receive all resources immediately, significantly improving transmission efficiency.

Checking HTTP/2.0 Support in Chrome

If your server has enabled HTTP/2 support (note that HTTP/2.0 features are available only over HTTPS), you can check this using Chrome Developer Tools. The shortcut is F12. By default, you will see the following requests, but it is not possible to determine if HTTP/2 is supported.

Right-click on the tab bar and check the "Protocol" option, as shown below.

After refreshing, you will see the "Protocol" column display "h2", indicating that HTTP/2 features are supported.

Summary

Simply put, HTTP/2.0 makes transmission more efficient. However, enabling HTTPS does not guarantee HTTP/2 support; you must add the corresponding parameters in your web server. Currently, domestic CDNs such as Upyun, Tencent Cloud, and Aliyun allow you to enable HTTP/2 with a single click in the backend. Since HTTP/3 has already been released, if you haven't enabled HTTP/2 yet, you might be a bit outdated.

Some content in this article is referenced from: In-depth Understanding: HTTP/2