What changed between HTTP/1.1, HTTP/2, and HTTP/3, and what problem does each solve?
What they are really testing: Whether you understand head-of-line blocking and why HTTP/3 moved to UDP. This shows depth beyond "HTTP/2 is faster".
A real interview question
What changed between HTTP/1.1, HTTP/2, and HTTP/3, and what problem does each solve?
What most people say
drag me
“Each version is faster than the last; HTTP/3 is the newest and quickest.”
It is a ranking, not an explanation. It misses multiplexing, head-of-line blocking, and the reason HTTP/3 abandoned TCP for QUIC/UDP, which is the actual content of the question.
The follow-ups they ask next
If HTTP/2 multiplexes over one connection, why is there still head-of-line blocking?
It is at the TCP layer: TCP guarantees in-order delivery, so one lost segment blocks all multiplexed streams until it is retransmitted. QUIC sidesteps this with independent streams over UDP.
Why did HTTP/3 build on UDP instead of TCP?
To control delivery per stream and avoid TCP head-of-line blocking, and to evolve the transport in user space. QUIC adds its own reliability, ordering per stream, and encryption on top of UDP.
What the interviewer is listening for
- Knows HTTP/2 multiplexing + header compression
- Explains TCP head-of-line blocking
- Knows HTTP/3 = QUIC over UDP and why
What sinks the answer
- "Each one is just faster"
- Unaware of head-of-line blocking
- Thinks HTTP/3 still runs on TCP
If you genuinely do not know
Say this instead of freezing. Reasoning out loud from what you do know beats silence every single time, and a good interviewer is listening for exactly that.
“HTTP/1.1 is [one request per connection, so browsers open many]. HTTP/2 [multiplexes streams over one TCP connection with header compression, but a lost packet stalls all streams, TCP head-of-line blocking]. HTTP/3 [runs over QUIC/UDP with independent streams, so loss only stalls its own stream], plus [a faster handshake and connection migration].”
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