HTTP/3 Protocol Specification

HTTP/3 is the third major version of the Hypertext Transfer Protocol, standardized as RFC 9114 by the IETF in June 2022. Unlike HTTP/1.1 and HTTP/2 which run over TCP, HTTP/3 runs over QUIC (Quick UDP Internet Connections, RFC 9000), a transport protocol built on UDP. Key improvements over HTTP/2: QUIC transport (replaces TCP with UDP-based QUIC protocol providing built-in encryption, stream multiplexing, and connection migration). Zero RTT connection setup (1-RTT and 0-RTT connection establishment compared to TCP plus TLS requiring 2-3 RTTs, dramatically reducing latency for returning clients). No head-of-line blocking (independent streams mean packet loss in one stream does not block others, unlike HTTP/2 where TCP-level packet loss blocks all streams). Connection migration (QUIC connection IDs allow connections to survive IP address changes from WiFi to cellular or NAT rebinding without reconnection). Integrated TLS 1.3 (encryption is mandatory and built into QUIC handshake, eliminating cleartext HTTP/3 and improving privacy by default). Stream multiplexing (multiple independent bidirectional and unidirectional streams over single QUIC connection with per-stream flow control). Fast handshakes (QUIC handshake combines transport and crypto negotiation in a single round trip). Improved mobile experience (connection migration is particularly beneficial for mobile devices switching between networks). Deployment status: supported by major browsers including Chrome, Firefox, Edge, and Safari, and servers including Cloudflare, Google, Facebook, nginx (via QUIC modules), and Akamai. Used by approximately 30 percent of websites as of 2026.