QUIC is a UDP-based, encrypted transport protocol defined in RFC 9000 that serves as the foundation for HTTP/3, offering reduced latency and improved multiplexing over TCP.

What is QUIC? Definition, examples and how it works

QUIC (Quick UDP Internet Connections) is a transport layer network protocol initially designed by Google in 2012 and standardized as RFC 9000 in May 2021. It replaces the traditional TCP+TLS+HTTP/2 stack with a single, encrypted, multiplexed connection built directly on UDP. The protocol reduces connection establishment overhead by combining cryptographic handshake with transport parameters in a single round trip (or zero round trip for returning clients), eliminating the TCP and TLS handshake serialization.

QUIC integrates TLS 1.3 natively, encrypting both payload and most control signals, including packet headers. This prevents middlebox interference and ossification. Its multiplexing is connection-level rather than stream-level, meaning a lost packet for one stream does not block delivery on other streams (head-of-line blocking elimination). QUIC also provides connection migration: a client changes IP addresses or networks without renegotiating the connection, a feature critical for mobile devices.

In the protocol stack, QUIC sits between IP and application layers, operating as a transport protocol over UDP port 443 by convention. HTTP/3 maps HTTP semantics directly onto QUIC streams, replacing HTTP/1.1 and HTTP/2 over TCP. While primarily known for HTTP/3, QUIC is a general-purpose transport capable of carrying any application protocol, analogous to TCP but with built-in encryption and lower latency characteristics.

Key facts

Standardized in RFC 9000, with additional RFCs 9001 and 9002 for TLS usage and loss detection.
Runs over UDP port 443 by default, bypassing TCP-processing bottlenecks in middleboxes.
Supports 0-RTT handshake for returning clients, allowing immediate data transmission.
Eliminates TCP head-of-line blocking by treating packet loss per stream independently.
Connection migration allows continued session across IP and network interface changes.

How it works in practice

A user on a smartphone switches from Wi-Fi to cellular while watching a streaming video in Chrome. With QUIC, the video player does not rebuffer, because the QUIC connection migrates to the new network path without rehandshaking. The server sees the same connection ID, and data packets resume flowing on the new IP address within milliseconds.

Related terms

HTTP/3 TLS 1.3 UDP TCP HTTP/2 Multiplexing Connection Migration

References

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