HTTP/3

Protocol

The latest HTTP version, built on QUIC instead of TCP. Eliminates TCP head-of-line blocking, enables faster connection setup (0-RTT), and handles network switching gracefully.

Updated Mar 9, 2026

Full Explanation

HTTP/3 replaces TCP with QUIC as the transport layer. The biggest improvement: no more head-of-line blocking. In HTTP/2 over TCP, if one packet is lost, all streams on that connection stall until it's retransmitted. In HTTP/3, only the affected stream stalls. This makes a massive difference on lossy networks (mobile, WiFi).

Connection setup is faster too. QUIC bakes TLS 1.3 into the transport handshake—you get encryption and transport ready in one round trip instead of two. With 0-RTT resumption, returning visitors can send data immediately with zero handshake delay.

For CDNs, HTTP/3 adoption is growing fast. Cloudflare, Fastly, and Google serve it by default. Most CDNs advertise HTTP/3 support via the Alt-Svc header in HTTP/2 responses. Browsers then upgrade to QUIC on subsequent requests.

See the interactive Request Flow animation in the course to visualize how HTTP/3 differs from HTTP/2.

Examples 

# Check HTTP/3 support
$ curl --http3-only -sI https://example.com/ | head -1
HTTP/3 200

# Most CDNs advertise via Alt-Svc header
$ curl -sI https://example.com/ | grep alt-svc
Alt-Svc: h3=":443"; ma=86400

# Nginx: HTTP/3 support (1.25+)
server {
    listen 443 ssl;
    listen 443 quic;
    http2 on;
    http3 on;
    add_header Alt-Svc 'h3=":443"; ma=86400';
}

# Chrome: chrome://flags/#enable-quic
# DevTools Protocol column shows h3 for HTTP/3

Video Explanation

Frequently Asked Questions

The latest HTTP version, built on QUIC instead of TCP. Eliminates TCP head-of-line blocking, enables faster connection setup (0-RTT), and handles network switching gracefully.

# Check HTTP/3 support
$ curl --http3-only -sI https://example.com/ | head -1
HTTP/3 200

# Most CDNs advertise via Alt-Svc header
$ curl -sI https://example.com/ | grep alt-svc
Alt-Svc: h3=":443"; ma=86400

# Nginx: HTTP/3 support (1.25+)
server {
    listen 443 ssl;
    listen 443 quic;
    http2 on;
    http3 on;
    add_header Alt-Svc 'h3=":443"; ma=86400';
}

# Chrome: chrome://flags/#enable-quic
# DevTools Protocol column shows h3 for HTTP/3

Related CDN concepts include:

  • QUIC (QUIC) — A transport protocol built on UDP that replaces TCP for HTTP/3. Provides built-in encryption, eliminates …
  • TLS (Transport Layer Security) (TLS) — The encryption protocol that secures HTTPS connections. TLS encrypts data in transit between client and …
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