QUIC
Also known as:
HTTP/3 Transport
Quick UDP Internet Connections
Definition
QUIC is a transport protocol built on top of UDP that powers HTTP/3. It bundles connection setup and TLS encryption into a single handshake, multiplexes many independent streams over one connection, and gives each stream its own delivery so a lost packet only stalls that one stream. That last property removes the transport-layer head-of-line blocking that limits HTTP/2 over TCP, and connection migration lets a session survive a network change.
Key Takeaways
- QUIC runs over UDP and is the transport underneath HTTP/3.
- Each stream is delivered independently, so one lost packet does not stall the others (no transport-layer head-of-line blocking).
- It combines the transport and TLS handshakes, cutting connection setup to as little as one round trip, or zero on resumption.
- Connection migration keeps a session alive across network changes, such as Wi-Fi to cellular, using a connection ID instead of the IP and port.
How It Works
- A client opens a QUIC connection over UDP, negotiating transport parameters and TLS 1.3 keys in one handshake.
- Application data flows over multiple independent streams identified by stream IDs.
- Lost packets are retransmitted per stream, so unaffected streams keep delivering in order.
- A stable connection ID lets the connection move to a new IP address without a fresh handshake.
Where It Is Used
- HTTP/3 uses QUIC, and major browsers and CDNs like Cloudflare and Google serve traffic over it.
- WebTransport is built on HTTP/3 and QUIC to offer low-latency, multiplexed client-server messaging.
- Video and real-time apps favour QUIC because a single dropped packet does not freeze every stream.