QUIC is a transport protocol designed by Google and standardized by the IETF to run over UDP; it is the foundation of HTTP/3.
It builds TLS 1.3 encryption into the protocol itself, removes TCP's Head-of-Line blocking via independent per-stream delivery, and makes connection setup fast.
Why over UDP? TCP is baked deep into OS kernels and middleboxes, making new features hard to deploy, and its strict ordering structurally causes transport-layer HoL blocking. QUIC reimplements reliability, congestion control, multiple streams, and encryption in user space atop the thin UDP layer, recovering loss independently per stream — a lost packet on one stream doesn't stall the others.
As a bonus, integrating TLS 1.3 folds handshake and crypto negotiation together, so connection setup is 1-RTT (0-RTT on resumption). It also identifies connections by a 'Connection ID' rather than IP/port, enabling connection migration so a switch from Wi-Fi to cellular doesn't drop the connection. Downsides include networks that block/throttle UDP, firewall/middlebox compatibility, and higher CPU usage, so a TCP fallback is usually kept alongside.