Simple definition
HTTP/3 is the version of HTTP that uses QUIC over UDP as its transport instead of running directly over TCP.
Technical definition
QUIC integrates features such as TLS 1.3 and independent multiplexed streams into the transport. A packet loss affecting one stream therefore does not automatically block unrelated streams in the same way that losses can affect multiplexed HTTP traffic over one TCP connection.
How it works / role
The client discovers or negotiates HTTP/3 support and establishes a QUIC connection to the server, commonly over UDP 443. HTTP exchanges are carried in QUIC streams, and clients can fall back to HTTP/2 or HTTP/1.1 when HTTP/3 is unavailable.
What is it used for?
Reduce some connection latency and improve web-application behavior on networks with loss, mobility, or changing paths.
Practical example
A browser connects to a compatible CDN over UDP 443 and uses HTTP/3. If a firewall blocks QUIC, the browser falls back to HTTP/2 over TCP.
Common issues
- UDP 443 is blocked by a firewall or proxy
- Network inspection equipment does not properly support QUIC
- Fallback hides the fact that HTTP/3 is not actually being used
- Troubleshooting confuses HTTP/3, QUIC, and TLS
Key takeaway: HTTP/3 uses QUIC over UDP; verify network-device compatibility and confirm which protocol was actually negotiated.