Understanding Head-of-Line Blocking: HTTP/2 vs. HTTP/3 (QUIC) in Production

Introduction

You’re streaming a 4K video on YouTube when suddenly your Wi-Fi hiccups. A single lost packet freezes all 127 video chunks in transit—not because they’re damaged, but because TCP won’t deliver chunk #43 until it retransmits the missing packet #42. Meanwhile, chunks #44 through #127 sit idle in kernel buffers, perfectly intact but artificially stalled. This is head-of-line blocking, and it’s why Google spent five years rebuilding the internet on UDP.