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Glossary

HTTP/3

The third major version of HTTP, layered on the QUIC transport for lower latency and better resilience to packet loss.

Sitecheck Team

HTTP/3 is the latest major version of HTTP, standardised in RFC 9114. Instead of TCP, it runs over QUIC — a UDP-based transport defined in RFC 9000 that combines transport, encryption, and stream multiplexing into a single handshake. QUIC removes the head-of-line blocking that limits HTTP/2, so a single lost packet no longer stalls every other request on the connection.

Why it matters

HTTP/3 shines on lossy or high-latency networks: mobile users on patchy 4G, Wi-Fi roaming, or international visitors far from your origin. Faster connection setup and resilient streams cut TTFB and improve Core Web Vitals — particularly LCP — without any application changes. Connection migration also keeps sessions alive when a device switches networks.

How to enable it

  • Enable HTTP/3 at the edge: Cloudflare, Fastly, AWS CloudFront, and Google Cloud CDN all expose it as a single toggle.
  • Confirm your TLS configuration uses TLS 1.3, which QUIC requires.
  • Allow UDP/443 outbound and inbound on any firewalls in front of the origin or CDN.
  • Verify negotiation by checking the Alt-Svc: h3=":443" response header and the Protocol column in browser DevTools.
  • Keep HTTP/2 enabled as a fallback — clients that cannot use QUIC will negotiate down automatically.
  • Re-test Core Web Vitals after rollout to quantify the improvement.

See also