Happy Eyeballs
Happy Eyeballs (also called Fast Fallback) is an algorithm published by the IETF that makes dual-stack applications (those that understand both IPv4 and IPv6) more responsive to users by attempting to connect using both IPv4 and IPv6 at the same time (preferring IPv6), thus minimizing IPv6 brokenness and DNS whitelisting experienced by users that have imperfect IPv6 connections or setups. The name "happy eyeballs" derives from the term "eyeball" to describe endpoints which represent human Internet end-users, as opposed to servers.