HTTP/3 & QUIC Reference
Core concepts behind HTTP/3 and QUIC, one short paragraph each.
| Concept | Description |
|---|---|
| QUIC | Transport protocol built on UDP that combines TLS 1.3, congestion control and stream multiplexing. |
| HTTP/3 | HTTP semantics over QUIC. Uses QPACK header compression instead of HPACK. |
| 0-RTT | Resume a previous session and send data in the very first packet — avoids the connection-setup round trip. |
| 1-RTT | Standard new-connection flow: full handshake then application data. |
| Connection ID | Opaque identifier in every QUIC packet; survives client IP/port changes (good for mobile). |
| Stream | Independent ordered byte sequence within a connection. Streams are unidirectional or bidirectional. |
| Frame | QUIC packets carry one or more frames (STREAM, ACK, CRYPTO, PING, MAX_DATA, etc.). |
| HoL blocking | Head-of-line blocking — QUIC eliminates per-connection HOL by keeping streams independent at the transport layer. |
| QPACK | HTTP/3's header compression algorithm; like HPACK but tolerates out-of-order streams. |
| Alt-Svc | HTTP response header that advertises an HTTP/3 endpoint to a client that connected via HTTP/1.1 or 2. |
| Service identity | Combination of QUIC version + ALPN protocol ID 'h3' that identifies an HTTP/3 service. |
| Port 443/udp | HTTP/3 listens on the same port as HTTPS but over UDP. |
| Spin bit | Optional bit in QUIC headers that lets passive observers estimate RTT. |
About this tool
Reference for HTTP/3 over QUIC: connections, streams, frames, 0-RTT, QPACK, Alt-Svc, port 443/udp and more.