Happy Eyeballs
HTTP/3 Availability Test

We operate a web server with HTTP/3 (QUIC) enabled. A subset of the test domains pointing to this server publish an HTTPS record that includes the h3 protocol in the alpn parameter. Additionally, certain paths on the server respond with an Alt-Svc header advertising HTTP/3 availability. The different test scenarios combine these domains and paths in various permutations.

We randomize domains to try to prevent caching effects on the browser's side as much as possible. Furthermore, we aggressively close connections on the server side after every request, forcing the browser to perform a fresh handshake for each new request.