HTTP/3 and QUIC

HTTP/3 keeps the same HTTP semantics—methods, status codes, fields, content, caching, and authentication—but maps them onto a different transport.

HTTP/1.1 and HTTP/2 normally use TCP. TCP gives the application one reliable, ordered byte stream. HTTP/2 creates logical streams inside that byte stream, but TCP cannot see those boundaries. If one TCP byte is missing, delivery of all later bytes waits, even when those bytes belong to unrelated HTTP/2 streams.

HTTP/3 runs over QUIC, a secure multiplexed transport built on UDP. QUIC makes streams part of the transport itself. It preserves reliable, ordered delivery within each stream without imposing one delivery order across every stream in the connection.

This is the central design change:

QUIC also combines connection establishment, encryption, loss recovery, congestion control, flow control, and connection migration in one transport. HTTP/3 can therefore reuse transport features instead of rebuilding them in its own framing layer.