A QUICk Introduction to HTTP/3

HTTP/3 is the newest iteration of the HTTP protocol that improves web security and performance. The first in a three-part series, this blog will discuss the HTTP protocol timeline, QUIC revolution, how HTTP/3 differs from HTTP/2, and the ways you can benefit from HTTP/3. 

The HTTP protocol

HTTP protocol (also known as web protocol), powers most websites, mobile apps, and videos. It was created by Tim Berners-Lee at CERN in 1989, and has been enhanced over the years to keep up with the ever-changing World Wide Web. Each new version of the protocol provides features that improve the performance, usability, and security of the web. A timeline of the protocol’s evolution is shown in Figure 1.

Currently, the web is a mixed bag of HTTP/1.1 and HTTP/2 adoption. Most well-known websites are running HTTP/2, while smaller websites and late adopters plan to migrate to HTTP/2 in the near future as it is relatively easy to implement. HTTP/2 is used by about 45% of websites and supported by all major web browsers. In contrast, HTTP/3 is only used by about 5% of websites now and not well-supported by web browsers yet.

HTTP/2 provides on average a 5% to 15% performance improvement on page load times. Moreover, the performance improvements of HTTP/2 are impacted by several factors that can be tweaked by developers and webmasters. For example, practices such as domain sharding, number of objects downloaded, and third-party domains. Other factors cannot be adjusted easily because they change with the client connection — such as packet loss or network latency — and are outside of your control.

In my research on HTTP/2, testing shows that the delivery of large objects over HTTP/2 can be slower than over HTTP/1.1 when there is packet loss. This is because HTTP/2 uses a single TCP connection, versus about six connections which most web browsers open over HTTP/1.1. In addition, the TCP congestion control algorithms reduce the TCP congestion window size, resulting in fewer bytes sent over the wire when using just one TCP connection. Read “Will HTTP/2 make my site faster?” for more details.

The solution to this problem is to use HTTP/2 over a different transport protocol that provides more efficient congestion control. That, in essence, is what HTTP/3 is: HTTP/2 over User Datagram Protocol (UDP)-based Quick UDP Internet Connections (QUIC). 

The QUIC revolution

QUIC is a transport layer protocol designed in 2012 by Jim Roskind at Google with the intention of reducing connection and transport latency as well as providing bandwidth estimation in each direction to avoid congestion. The initial QUIC implementation (now known as Google-QUIC, or gQUIC) garnered attention from the web performance community when Google shared performance results indicating that QUIC reduced latency of desktop Google Search responses by 8.0% and rebuffer rates of YouTube playbacks by 18.0%. 

The QUIC working group was established in 2016 to standardize the protocol within the Internet Engineering Task Force (IETF) and, in October 2018, the IETF's HTTP and QUIC Working Groups jointly decided to call the HTTP mapping over QUIC "HTTP/3" in advance of making it a worldwide standard. The IETF forked QUIC into what is known as the “IETF-QUIC (or iQUIC)” which bases its encryption and security on TLS 1.3 instead of the custom approach used by gQUIC.

Differences between HTTP/2 and HTTP/3

There are several differences between HTTP/2 and HTTP/3 (see “HTTP/3 Explained” for more details), but the main one is that HTTP/3 runs over a transport layer network protocol called QUIC that uses UDP as the transport layer protocol instead of TCP, resulting on performance and security improvements.

The table below shows some of the differences between HTTP/2 and HTTP/3: