Reading about internet protocols, one might encounter the term “head-of-line blocking.” What does it mean, and why is it important?
The following is by my understanding.
Head-of-line blocking is the idea that until one resource is fully received, the next one in the pipe has to wait.
The http1.1, protocol, the main internet protocol from 1997 to around 2015, was susceptible to head-of-line blocking. Then, with http2, multiplexing was introduced, meaning that over the same connection, several lines of communication could occur at once. This reduced head-of-line blocking, but didn’t eliminate it. The reason is that although separate files that will make up a page can load in parallel, the whole page still has to load, eventually, before the next task in the pipe can start. Therefore, the slowest resource that loads in parallel can eventually stall the connection from being available for its next task. (This may be a bit simplified, but conceptually is what can happen.)
The http3 protocol uses totally different ways to send information, and is more or less immune to head-of-line blocking.
As of June 1, 2026, http1.1 and the older http1.0 make up about 28 percent of internet traffic, http2, about 51 percent, and http3, 21 percent.
There is much more to discuss about internet protocols.
Source:
developer.mozilla.org: Evolution of HTTP
developer.mozilla.org: Head-of-line blocking
technologychecker.io: “We analyzed HTTP protocol adoption in 2026….”
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