Head-of-Line Blocking

A performance problem where a slow or lost packet blocks the processing of all subsequent packets in the same ordered stream, even if those later packets have already arrived. In HTTP/1.1, a single slow request on a connection blocks all queued requests (pipelining head-of-line blocking). HTTP/2 solved this at the HTTP layer with multiplexed streams but still suffers from TCP-level head-of-line blocking when a packet is lost. HTTP/3 over QUIC solves TCP head-of-line blocking by providing independent stream multiplexing at the transport layer.