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Networking Fundamentals · Intermediate

QUIC

A UDP-based, encrypted, multiplexed transport protocol (RFC 9000) that underlies HTTP/3.

Definition

QUIC moves reliability, congestion control and stream multiplexing into user space on top of UDP, and it builds TLS 1.3 encryption into the handshake. It avoids TCP head-of-line blocking between streams and supports connection migration when a client's IP address changes. It typically uses UDP port 443.

Example

Modern browsers reach many large websites with HTTP/3 over QUIC on UDP 443.

Exam tip

Firewalls that allow only TCP 443 will force browsers to fall back from HTTP/3 to HTTP/2 over TCP.

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