HTTP/3 over QUIC
TechniqueNetworking and OSSeniorStaffUpdated
HTTP 3 runs over QUIC, a UDP based transport with built-in TLS 1.3, per-stream flow control, and connection migration across IP changes.
When to Use HTTP/3 over QUIC
Mobile clients on flaky networks, or any service hit by HTTP/2's TCP head-of-line blocking. QUIC streams are independent at the transport layer, so one lost packet only stalls one stream.
In Practice
YouTube, Facebook, and Cloudflare serve HTTP/3 to ~30 percent of traffic. A phone moving from Wi-Fi to LTE keeps the same QUIC connection (via connection ID) instead of redoing the TLS handshake.
How HTTP/3 over QUIC Comes Up in Interviews
Scenario
Your mobile app's long-lived stream drops every time the phone switches from Wi-Fi to LTE because the source IP changes. What transport handles this natively?
QUIC identifies connections by a 64-bit connection ID, not the (src ip, src port, dst ip, dst port) 4-tuple. The session survives an IP change, so HTTP 3 keeps streaming without a reconnect or TLS handshake.
HTTP/2 over TCP cannot do this: the kernel keys the TCP socket by 4-tuple. Some apps fake it with quick reconnect plus resume tokens; QUIC bakes it in.