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 prevent caching effects on the browsers side. However, the web server is only assigned a single IP address. Clients that cache connection information on a per-address basis rather than a per-domain basis may therefore produce skewed results in subsequent test runs.