ArcLibrary
L1入门L2进阶L3生态

HTTP/3 & QUIC

HTTP on top of UDP — 0-RTT setup, connection migration, and finally killing TCP head-of-line blocking.

HTTP3QUICUDP
核心 · Key Idea

In one line: QUIC is a transport-layer protocol (originally from Google, now an IETF standard) that runs on UDP and bundles TLS 1.3 + multiplexing + flow control + congestion control. HTTP/3 = HTTP over QUIC — 1-RTT setup, connection migration, no TCP HoL.

What it is#

HTTP/1.1: HTTP   over TCP
HTTP/2:   HTTP   over TLS over TCP
HTTP/3:   HTTP   over QUIC over UDP

QUIC redoes the transport layer. Because TCP is a kernel protocol that's hard to evolve, QUIC is implemented in user space on top of UDP — letting browsers and servers iterate freely.

Analogy#

打个比方 · Analogy

TCP is the old train — reliable, predictable, hard to upgrade. QUIC is a private high-speed rail — add features at will: new algorithms, connection migration, 0-RTT — ship today, deploy tomorrow.

Key concepts#

0-RTT resume0-RTT Resumption
When reconnecting to the same server, the very first packet can carry HTTP data — saves an RTT.
Connection migrationConnection Migration
QUIC uses a Connection ID, so swapping 4G ↔ Wi-Fi (different IP) **doesn't drop the connection**.
No TCP HoLNo HoL Blocking
QUIC's streams are independent at the transport layer; losing one packet only blocks one stream.
Built-in TLS 1.3Always Encrypted
QUIC has no plaintext mode — even handshake metadata is encrypted.
Userspace implUserspace
Servers handle QUIC in the application layer — fast iteration.

How it works#

QUIC merges the TLS handshake and the protocol handshake — the client ships data immediately.

Practical notes#

  • Verify a page uses H3: in Chrome DevTools the Protocol column shows h3-29 / h3.
  • Enable on servers:
    • nginx 1.25+ with the quic module;
    • Caddy 2 enables H3 by default;
    • Cloudflare / Fastly toggle it for you.
  • HTTPS-only: like H2, requires TLS (1.3 only).
  • Some networks block UDP. QUIC uses UDP 443; corporate / campus firewalls may drop it. Browsers auto-fallback to HTTP/2.
  • Alt-Svc header: server tells the browser "I also speak H3 — switch over next time".

Easy confusions#

HTTP/3 over QUIC
Runs on **UDP 443**.
1-RTT / 0-RTT setup.
Streams truly independent — packet loss confined.
HTTP/2 over TCP
Runs on **TCP 443**.
≥ 2-RTT setup (TCP + TLS).
Packet loss stalls all streams.

Further reading#