Does HTTP work on TCP or UDP? How does HTTP/3 ensure reliability over UDP?

HTTP traditionally operates over the TCP protocol, which provides reliable data delivery, ordering, and error checking.

HTTP/3, however, uses the QUIC protocol, which is built on top of UDP. UDP itself does not guarantee delivery, order, or integrity, but QUIC implements these functions at the protocol level:

• QUIC ensures connection establishment with minimal latency (0-RTT).
• Implements reliable data transfer with acknowledgments and retransmission of lost packets.
• Manages delivery order and data streams within a single connection.

Thus, HTTP/3 over UDP achieves reliability and efficiency by combining UDP's advantages (low latency, bypassing TCP restrictions) with the control mechanisms implemented in QUIC.