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.
[1] Happy Eyeballs is designed to address the problem that many IPv6 networks are unreachable from parts of the Internet,[2] and applications that try to reach those networks appear unresponsive, which frustrates users.
Happy Eyeballs solves this problem by determining which transport would be better used for a particular connection by trying them both in parallel.
Implementations of Happy Eyeballs stacks exist in Google's Chrome web browser, Opera 12.10, Firefox version 13[citation needed], OS X,[4] cURL[5] and OpenBSD.
[7] The Happy Eyeballs algorithm may be extended for choosing between types of transport protocols as well, such as TCP and SCTP, but development is still in an experimental phase.