Migration Guide: HTTP/2 to HTTP/3

Migrating from HTTP/2 to HTTP/3

Adding HTTP/3 support alongside your existing HTTP/2 setup is straightforward with most CDNs and modern servers. The key difference is infrastructure: QUIC uses UDP instead of TCP.

Step 1: Verify UDP Port 443

QUIC uses UDP port 443. Ensure your infrastructure allows UDP traffic:

# Check if UDP 443 is open
sudo ufw allow 443/udp
# Or with iptables
sudo iptables -A INPUT -p udp --dport 443 -j ACCEPT

Many enterprise firewalls block UDP by default. Coordinate with your network team.

Step 2: Enable QUIC on Your Server

Using a CDN (easiest):

• Cloudflare: Enabled by default
• AWS CloudFront: Enable in distribution settings
• Google Cloud CDN: Enable in load balancer config

Caddy:

example.com {
    # HTTP/3 is enabled by default in Caddy 2.6+
}

Nginx (experimental):

server {
    listen 443 ssl http2;
    listen 443 quic reuseport;
    http3 on;
    add_header Alt-Svc 'h3=":443"; ma=86400';
}

Step 3: Advertise HTTP/3 Support

Browsers discover HTTP/3 via two mechanisms:

Alt-Svc header (primary):

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

HTTPS DNS record (faster discovery):

example.com. 300 IN HTTPS 1 . alpn="h3,h2"

Step 4: Test

1. Chrome DevTools > Network > Protocol column shows "h3"
2. Firefox: about:networking#dns shows HTTPS record
3. curl: curl --http3 https://example.com
4. Online: HTTP/3 Check

Step 5: Monitor

Track the percentage of requests using each protocol:

• h3 (HTTP/3)
• h2 (HTTP/2)
• http/1.1

Monitor for fallback rates — if QUIC is blocked, users automatically fall back to HTTP/2. High fallback rates may indicate firewall or middlebox issues.

Key Difference from HTTP/2 Migration

Unlike HTTP/1.1 to HTTP/2, the HTTP/2 to HTTP/3 migration requires no application changes. All optimizations for HTTP/2 (multiplexing-friendly asset strategy, removing domain sharding) also benefit HTTP/3.