HTTP/3 and QUIC
HTTP/3 runs over QUIC, a UDP-based transport with per-stream loss recovery and built-in TLS 1.3, eliminating the TCP-level head-of-line blocking that HTTP/2 cannot fix.
Definition
HTTP/3 is the third HTTP version, transported over QUIC instead of TCP. QUIC is a UDP-based protocol that implements streams, flow control, congestion control, and TLS 1.3 encryption in user space. Each QUIC stream recovers packet loss independently, so a lost packet only stalls its own stream.
Context and scope
Adopted by browsers and large CDNs to improve performance on lossy mobile networks, to cut handshake latency with 0-RTT resumption, and to make connection migration across network changes practical.
Common misconceptions
- QUIC replaces TLS — QUIC integrates TLS 1.3 for its handshake and encryption, not a substitute
- HTTP/3 is faster in every scenario — on clean low-loss networks the gain over HTTP/2 is often negligible
- UDP means unreliable delivery — QUIC reimplements reliability and ordering above UDP
Anti-patterns
- Enabling HTTP/3 without a fallback path and breaking clients behind UDP-blocking middleboxes
- Assuming Alt-Svc advertisement alone upgrades clients — they still need a valid QUIC endpoint and certificate
- Comparing protocol versions on different routes and attributing latency gains to the protocol
Related concepts
Related examples
Authoritative source
What it is
HTTP/3 keeps HTTP semantics — methods, headers, status codes — but replaces the transport with QUIC, which multiplexes streams natively. Because each stream has independent loss recovery, one lost packet no longer blocks unrelated streams. TLS is integrated into the QUIC handshake rather than layered above TCP.
How it works
The client usually learns an HTTP/3 endpoint through the Alt-Svc header over HTTP/2, then attempts QUIC on UDP. The combined crypto and transport handshake establishes keys and connection state together, typically in one round trip. Connection identifiers survive IP address changes, enabling migration between Wi-Fi and cellular.
When to use it
Lossy or high-latency networks benefit most, because per-stream recovery avoids stalls and fewer handshakes shorten first-byte time. Video conferencing, large file transfer, and mobile web browsing are frequent winners. Servers need UDP capacity and a mature QUIC stack before enabling it.
Pitfalls
Some corporate firewalls throttle or block UDP, so always keep HTTP/2 as fallback. CPU cost per connection can be higher in software QUIC implementations than kernel TCP. Measure video rebuffer rates and connection error rates after rollout, since a broken Alt-Svc advertisement can silently strand clients on the old protocol.