Head-of-line blocking.
also: HOL blocking · head of line blockingWhen one lost or slow item at the front holds up everything queued behind it, such as packets waiting for a retransmission.
the two-minute refresher
- TCP delivers bytes in order, so packets 3, 4 and 5 wait while packet 2 is retransmitted.
- HTTP/2 multiplexes many streams over one TCP connection, so one lost packet stalls them all.
- HTTP/3 runs over QUIC on UDP: a loss only stalls its own stream.