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Networking

HTTP3 over QUIC Revisited

Why the newest HTTP version runs on UDP instead of TCP.

6 min read · advanced · beat Platinum to climb

A New Transport

HTTP3 carries the same requests and responses as before, but over QUIC, a transport built on UDP rather than TCP. QUIC absorbs the jobs that TCP and TLS used to do separately.

What QUIC Brings

  • Independent streams: each request is its own stream, so a lost packet stalls only that stream, not all of them.
  • Built in encryption: TLS 1.3 is part of QUIC, so there is no separate handshake stacked on TCP.
  • Faster setup: connection and crypto handshakes combine, and resumed sessions can use zero round trip data.
  • Connection migration: a connection ID lets a session survive an address change, such as moving from wifi to cellular.

Why Move Off TCP

TCP delivers a single ordered byte stream, so one lost segment blocks everything behind it even across unrelated requests. Because TCP is baked into operating systems, QUIC was built in user space on UDP to evolve faster.

Key idea

HTTP3 runs on QUIC over UDP, folding in TLS, giving independent streams that avoid cross request blocking, and supporting connection migration across networks.

Check yourself

Answer to earn rating on the learn ladder.

1. Which transport does HTTP3 use?

2. What does QUIC connection migration enable?

3. Why is QUIC built in user space on UDP?