IPv6

Happy Eyeballs Races IPv6 and IPv4 Connections

A device on a dual-stack network can often reach the same service over both IPv6 and IPv4. DNS may return AAAA records for IPv6 addresses and A records for IPv4 addresses, leaving the client with several possible routes to the destination. Preferring IPv6 and waiting for a complete failure before trying IPv4 sounds orderly, but it can create a visible pause when the IPv6 path is broken or unusually slow. The reverse ordering can hide IPv6 even when it offers a healthy path. Happy Eyeballs avoids both extremes by giving preferred connection attempts a short head start while allowing another address family to compete soon afterward.