QA
Intermediate LevelREST APIs & Web Services

What is HTTP/3 (QUIC) and how does UDP multiplexing improve API latency?

❓ Interview Prompt: Explain HTTP/3, QUIC protocol over UDP, eliminating TCP Head-of-Line Blocking, 0-RTT connection establishment, and mobile network connection migration.

HTTP/3 replaces TCP with QUIC (a UDP-based transport protocol originally developed by Google):

Key Benefits for APIs:

1
No TCP Head-of-Line (HoL) Blocking: In HTTP/2 over TCP, a single lost packet freezes ALL multiplexed streams. In HTTP/3 QUIC, streams are independent over UDP; losing one packet delays ONLY that specific stream!
2
Fast 0-RTT Connection Setup: Combines transport and TLS 1.3 handshake into 1 round-trip.
3
Connection Migration: Mobile devices switching from Wi-Fi to 5G preserve active QUIC connections seamlessly using Connection IDs without re-authenticating!

Code Implementation Example

http
// HTTP/3 Alt-Svc Header informing clients to upgrade to HTTP/3 QUIC over UDP:
HTTP/1.1 200 OK
Alt-Svc: h3=":443"; ma=86400
💡Alt-Svc header advertising HTTP/3 QUIC availability.
🎯 Interview Key Takeaway

HTTP/3 uses QUIC over UDP to eliminate TCP head-of-line blocking and enable seamless mobile connection migration.

#HTTP/3#QUIC#UDP#Performance#Intermediate
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