Happy Eyeballs
Happy Eyeballs, specified in RFC 8305, is an algorithm a dual-stack client uses to connect to a destination that publishes both an IPv6 (AAAA) and an IPv4 (A) address, without waiting for a slow or broken path to time out before trying the other. The client starts an IPv6 connection attempt first and, if it hasn't succeeded within a short delay (RFC 8305 recommends around 250 milliseconds), starts an IPv4 attempt in parallel, keeping whichever connection completes first and abandoning the other. This replaced the original Happy Eyeballs described in RFC 6555, which RFC 8305 updates with refinements like sorting destination addresses and tuning the connection-attempt delay. Without it, a client that only tried IPv6 first and waited for a full timeout before falling back would make dual-stack connectivity noticeably slower whenever a network's IPv6 path was actually broken.
By AdminKit team, Netbase JSC · Updated