Glossary / HTTP/3

What is HTTP/3

HTTP/3 is the third major version of HTTP, built on a new transport called QUIC instead of the usual TCP. Its main idea is to set up connections faster and avoid stalling when some packets are lost. The difference shows most on mobile internet and unstable networks.

Earlier HTTP versions ran over TCP, where losing a single packet blocks the whole queue (head-of-line blocking). HTTP/3 uses QUIC over UDP, where streams are independent: a loss in one does not stall the others. QUIC also merges connection setup and encryption into fewer steps, so the first connection is quicker.

HTTP/3 is supported by modern browsers and major CDNs (Cloudflare, Google, Fastly). It usually runs alongside HTTP/2: the browser picks the best available option through the Alt-Svc header.

For an ordinary visitor the gain is clearest where the network is poor: mobile links, Wi-Fi on the move, distant servers. On a stable wired connection the difference from HTTP/2 is smaller.

Example

alt-svc: h3=":443"

Common mistakes

• Thinking HTTP/3 simply replaces HTTP/2, when they usually run together and the browser picks the better one.
• Confusing the transport: HTTP/3 runs over UDP and QUIC, not TCP like earlier versions.
• Expecting a big speedup on fast wired internet, when the largest gain is on unstable networks.
• Assuming you configure it in the site's code, when it is a server and CDN setting.

Related terms

• TTFB What is TTFB (Time To First Byte)
• CDN What is a CDN (Content Delivery Network)
• HTTP/2 What is HTTP/2
• gzip / Brotli What is gzip and Brotli compression
• DNSBL What is a DNSBL (blacklist)