Happy Eyeballs (also called Fast Fallback) is an algorithm published by the IETF which can make dual-stack applications (those that understand both IPv4 and IPv6) more responsive to users. It is designed to address the problem that many IPv6 networks are unreachable from parts of the Internet, and applications trying to reach those networks will appear unresponsive, thus frustrating users. Happy Eyeballs solves this problem by determining which transport would be better used for a particular connection by trying them both in parallel. The requirements for the algorithm are described in RFC 6555, "Happy Eyeballs: Success with Dual-Stack Hosts". The name "happy eyeballs" derives from the term "eyeball" to describe endpoints which represent human Internet end-users, as opposed to servers.
An application that uses a Happy Eyeballs algorithm checks both IPv4 and IPv6 connectivity (with a preference for IPv6) and uses the first connection that is returned. The addresses are often chosen from the DNS with a round-robin algorithm. Implementations of Happy Eyeballs stacks exist in Google's Chrome web browser, Opera 12.10, Firefox version 13, and Mac OS X Lion (although Lion uses the fastest connection, rather than preferring IPv6). Happy Eyeball testing was part of World IPv6 Day in 2011.
- Dan Wing and Andrew Yourtchenko. "Happy Eyeballs: Improving User Experiences with IPv6 and SCTP". Internet Protocol Journal, vol.13 n.3. Retrieved 2012-01-15.
- Dan Wing and Andrew Yourtchenko (2012-04-12). "Happy Eyeballs: Success with Dual-Stack Hosts". RFC 6555.
- Daniel Stenberg. "getaddrinfo with round robin DNS and happy eyeballs". Retrieved 2012-01-15.
- "Lion and IPv6". Retrieved 2012-01-17.
- "Hampering Eyeballs - Observations on Two 'Happy Eyeballs' Implementations". Retrieved 2012-01-15.
- Mark Townsley. "Happy Eyeballs for World IPv6 Day". Retrieved 2012-01-15.