Head-of-Line Blocking

ELI5 — The Vibe Check

Head-of-Line Blocking is when one slow request blocks everything behind it, like a slow car in a single-lane tunnel. In HTTP/1.1, one slow response blocks the entire connection. HTTP/2 fixes it at the HTTP level, and HTTP/3 fixes it at the transport level too. It's the traffic jam of the internet.

Real Talk

Head-of-line (HOL) blocking occurs when a single delayed packet or request prevents subsequent items from being processed. HTTP/1.1 suffers at the application layer (pipelining). HTTP/2 solves application-layer HOL blocking but TCP still has transport-layer HOL blocking. HTTP/3 (QUIC/UDP) eliminates both.

When You'll Hear This

"HTTP/2 solved HOL blocking at the HTTP layer, but one lost TCP packet still stalls all streams." / "HTTP/3 uses QUIC to eliminate HOL blocking entirely — each stream is independent."