HTTP/2 vs HTTP/3: what changes in transport

HTTP/2 multiplexes HTTP streams over one TCP connection; HTTP/3 maps HTTP semantics onto QUIC over UDP. QUIC provides independent streams and integrates TLS 1.3, changing loss recovery and connection establishment without changing HTTP methods or status codes.

What changes from HTTP/2 to HTTP/3

HTTP/2 runs over TCP, while HTTP/3 carries HTTP semantics over QUIC, which uses UDP. Packet loss in one QUIC stream therefore does not have to stall data in other streams in the same way that a lost TCP packet can delay multiple HTTP/2 streams sharing one connection.

How to check which protocol is in use

Open the browser developer tools and inspect the protocol used for a request: h2 indicates HTTP/2 and h3 HTTP/3. If h3 is absent, check browser/server support, UDP/443 and Alt-Svc. A site should normally fall back to HTTP/2 when HTTP/3 is unavailable. For an external view of the site, add 2ip site information check to the browser diagnostics.

When HTTP/3 is unavailable

Technical references: RFC 9113 and RFC 9114.