HTTP/2 and HTTP/3

The headers of HTTP requests and responses contain certain metadata. In HTTP/1.1, headers were stored in plain text, which resulted in a lot of overhead, especially when cookies and security tokens were sent along with them.

HTTP/2 compresses the headers using the HPACK compression algorithm, which helps reduce the size of the header by an average of 30%. The advantage is particularly great for HTTP requests, as these often do not contain a request body. Some studies even report a 53% reduction in incoming traffic.

An HTTP/2 connection provides multiple streams via which messages can be sent.

This means that a single TCP connection can be used to send multiple HTTP requests and responses simultaneously.

A web server that supports HTTP/2 therefore has to accept far fewer connections and remains much more stable.

Another advantage is that the Head-of-line blocking issue is also solved at the level of HTTP.

With HTTP/1.1, the various components of a website are received serially. The order in which the various resources are received completely depends on the browser.

Thanks to frame prioritisation, the browser determines which components it wants to receive first. Blocking resources such as CSS and JavaScript files located in the section are usually given the highest priority.

The order in which CSS files, JavaScript files, images and web fonts are loaded, however, differs from one browser to another, which can result in differences in user experience.

HTTP/2 also supports frame prioritisation at the level of the server. The browser sends multiple requests to the server in rapid succession, just as with HTTP/1.1. However, the server can choose to return the files in a different order. In addition, the files do not have to be sent in their entirety; they can also be fragmented.

When you visit a website, your browser first downloads an HTML file. It then checks what other files are required to display that HTML file properly. These files include images, web fonts and CSS or JavaScript files. These files are then requested by the web client via new HTTP requests.

Thanks to Server Push feature of the HTTP/2 protocol, the web server can also send other data with the HTML, such as all JavaScript and CSS files. This allows the browser to display the page directly, without having to send a new request to the server. This method allows web pages to load more quickly.

Developers can indicate whether additional content should be sent along with an HTML file. This is done via a "Link" response header, which can look like this:

Link </style.css>;rel=preload, </framework.js>; rel=preload

Although TLS (Transport Layer Security) encryption is not a specific part of the HTTP/2 protocol, browsers do require all HTTP/2 connections to be established using TLS.

When drafting the protocol specification of HTTP/2, the creators of several web browsers used their leverage to require HTTPS URLs to be sent using TLS version 1.2 or later. Old SSL encryption protocols via HTTP/2 are no longer supported.