#savol
Why HTTP/3 uses UDP instead of TCP?

#javob
I think the main reason why HTTP/3 (which uses QUIC) is built on top of UDP instead of TCP is to solve the problem with lost packets in HTTP/2.

One of the biggest problems with h2 is that streams (a logical communication for a single request) are not fully independent from each other. Since the TCP retransmission works on layer 4 of the OSI model but h2 itself is implemented on layer 7, the TCP connection has no knowledge of h2 streams. So if a single TCP packet is missing in one of the streams, TCP blocks the entire connection (thus, all the streams on it) till the missing packet is retransmitted successfully.
That means, if I try to send 10 request at the same time over h2 and if any of them fails to reach, all 10 requests are blocked. This is called HOL blocking, but on the transport layer.

HTTP/3 solves this problem by using QUIC, which is built on top of UDP. Since UDP does not guarantee delivery of data, it has no retransmission mechanism. QUIC has its own recovery mechanism that guarantees delivery. Unlike TCP, this recovery mechanism works on the same layer as the QUIC streams and knows which packet belongs to which stream. That's why if any packet is lost, the recovery mechanism blocks only the stream that packet belongs to. It does not affect other streams.