Head-of-Line Blocking

Head-of-line (HOL) blocking occurs when the first item in a queue is stuck, so every item behind it must wait even if it is ready. It shows up at more than one layer of web traffic.

Why it matters

It explains why a single slow resource can hold up a whole page, and why newer protocols exist. Knowing which layer is blocking tells you whether tuning, multiplexing, or a protocol upgrade can help.

Example

• HTTP/1.1: each connection handles one request at a time. A slow response blocks the next request on that connection, which is why browsers open about six connections per host.
• HTTP/2: multiplexes many requests over one connection, removing the blocking at the HTTP layer. But all streams share one TCP connection, so one lost packet stalls every stream until it is retransmitted. That is TCP-level blocking.
• HTTP/3: runs over QUIC on UDP, with independent streams, so a lost packet only delays the stream it belongs to.

You can check which protocol a site negotiates with the HTTP/2 and HTTP/3 checker.

Common mistakes

• Assuming HTTP/2 eliminates all blocking. It moves the problem to TCP on lossy networks.
• Keeping old domain-sharding tricks, which helped HTTP/1.1 but hurt HTTP/2.
• Blaming the server for slow loads that are really packet loss on a mobile link.

Related

• HTTP/2
• HTTP/3 and QUIC
• Packet loss
• Waterfall chart

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