I presented Firefox’s Happy Eyeballs v3 implementation in the HAPPY working group at IETF 125 in Shenzhen.
At that point the implementation had landed in Firefox Nightly, still off by default and reachable by flipping network.http.happy_eyeballs_enabled in about:config. It followed the draft with no major deviation.
I introduced mozilla/happy-eyeballs, the Rust library behind it: deterministic, free of side effects, abstract over I/O and time, and with no dependency on Firefox. Each scenario is a plain unit test, which makes the algorithm easy to formalize. I offered those tests to the working group as a way to pin the algorithm down.
I showed the first public telemetry on DNS lookups and connection establishment latency, where the tail is long: a median of 73 ms, but 537 ms at the 95th percentile and 1.7 s at the 99th. Those numbers are what we planned to tune the resolution and connection attempt delays against.
I closed on the plan: enable Happy Eyeballs v3 in Nightly within weeks, ship it to release within months, and bring more telemetry back to IETF 126.