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← all concepts

Head-of-line blocking.

also: HOL blocking · head of line blocking

When 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.

Where the course teaches it

W00 · Part 4Networking Fundamentals for System Designlearn it →

Related concepts

HTTP/2
QUIC