Head-of-line blocking (HOL blocking) in computer networking is a performance-limiting phenomenon that occurs when a line of packets is held up in a queue by a first packet. Solving HOL blocking was also one of the main motivations behind not just HTTP/3 and QUIC but also HTTP/2.

The HOL blocking problem

It’s difficult to give you a single technical definition of HOL blocking, as this blogpost alone describes four different variations of it. A simple definition however would be:

When a single (slow) object prevents other/following objects from making progress

A good real-life metaphor is a grocery store with just a single check-out counter. One customer buying a lot of items can end up delaying everyone behind them, as customers are served in a First In, First Out manner. Another example is a highway with just a single lane. One car crash on this road can end up jamming the entire passage for a long time. As such, even a single issue at the “head” can “block” the entire “line”^[1].

This concept has been one of the hardest Web performance problems to solve.

HOL blocking in HTTP/1.1

↑ https://calendar.perfplanet.com/2020/head-of-line-blocking-in-quic-and-http-3-the-details/

[1] ttps://calendar.perfplanet.com/2020/head-of-line-blocking-in-quic-and-http-3-the-details/

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