1. Introduction

Many communication protocols operating over the modern Internet use hostnames. These often resolve to multiple IP addresses, each of which may have different performance and connectivity characteristics. Since specific addresses or address families (IPv4 or IPv6) may be blocked, broken, or sub-optimal on a network, clients that attempt multiple connections in parallel have a chance of establishing a connection more quickly. This document specifies requirements for algorithms that reduce this user-visible delay and provides an example algorithm.

This document defines the algorithm for "Happy Eyeballs", a technique for reducing user-visible delays on dual-stack hosts. This definition obsoletes the original description in RFC 6555. Now that this approach has been deployed at scale and measured for several years, the algorithm specification can be refined to improve its reliability and general applicability.

The Happy Eyeballs algorithm of racing connections to resolved addresses has several stages to avoid delays to the user whenever possible, while preferring the use of IPv6. This document discusses how to handle DNS queries when starting a connection on a dual-stack client, how to create an ordered list of destination addresses to which to attempt connections, and how to race the connection attempts.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.