Okhttp introduced fastFallback in version 4 and then in version 5, they have introduced that again naming it as HappyEyeballs, can share some share the difference between the two implementations ?
In Okhttp4: https://square.github.io/okhttp/4.x/okhttp/okhttp3/-ok-http-client/fast-fallback/
In Okhttp5: https://square.github.io/okhttp/changelogs/changelog/#version-500-alpha4
It's simpler than that. The URL for the docs is wrong; it should say 5.x and we OkHttp maintainers need to fix that.