Glossary · Performance

HTTP/3 and QUIC

HTTP/3 runs HTTP over QUIC, a UDP-based transport with built-in TLS 1.3, avoiding TCP head-of-line blocking and speeding up connections on lossy networks.

Updated October 5, 2026

HTTP/3 (RFC 9114) is the third major HTTP version. Instead of TCP, it uses QUIC (RFC 9000), a transport built on UDP that integrates TLS 1.3 encryption, multiplexed streams and connection migration.

Why it matters

QUIC fixes TCP's head-of-line blocking: a lost packet delays only the stream it belongs to, not every request on the connection. Setup is faster because transport and encryption handshakes are combined, and a connection can survive a phone switching from Wi-Fi to cellular. The gains are most visible on high-latency or lossy mobile networks.

Example

Browsers discover HTTP/3 through an Alt-Svc header sent over HTTP/1.1 or HTTP/2:

Alt-Svc: h3=":443"; ma=86400

This tells the client that the same host supports HTTP/3 on UDP port 443 for 86,400 seconds. The client may then try QUIC and falls back to TCP if it fails. Test with curl --http3 -I https://example.com if your curl build supports it.

Common mistakes

  • Opening only TCP 443 in the firewall. HTTP/3 needs UDP 443 as well.
  • Assuming it is available because the CDN supports it. Check that it is enabled for your zone.
  • Disabling HTTP/2 in the belief HTTP/3 replaces it. Clients need the fallback.
  • Not monitoring over both protocols, since a UDP problem can degrade users silently while TCP checks pass.

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