Networking

Head-of-Line Blocking

One delayed item stalls everything queued behind it. In TCP, a lost packet blocks all bytes after it.

Why interviewers ask about it

Why HTTP/2 can be slower than HTTP/1.1 on lossy networks, and the reason QUIC moved reliability above UDP so streams are independently ordered.

A UDP-based transport with per-stream reliability, a merged transport/crypto handshake, and connection migration by connection ID.

This term is part of the free System Design interview preparation module - browse the full glossary for every definition.