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8. Summary of Configurable Values

The values that may be configured as defaults on a client for use in Happy Eyeballs are as follows:

  • Resolution Delay (Section 3): The time to wait for a AAAA response after receiving an A response. Recommended to be 50 milliseconds.

  • First Address Family Count (Section 4): The number of addresses belonging to the first address family (such as IPv6) that should be attempted before attempting another address family. Recommended to be 1; 2 may be used to more aggressively favor a particular address family.

  • Connection Attempt Delay (Section 5): The time to wait between connection attempts in the absence of RTT data. Recommended to be 250 milliseconds.

  • Minimum Connection Attempt Delay (Section 5): The minimum time to wait between connection attempts. Recommended to be 100 milliseconds. MUST NOT be less than 10 milliseconds.

  • Maximum Connection Attempt Delay (Section 5): The maximum time to wait between connection attempts. Recommended to be 2 seconds.

  • Last Resort Local Synthesis Delay (Section 7.2): The time to wait after starting the last IPv6 attempt and before sending the A query. Recommended to be 2 seconds.

The delay values described in this section were determined empirically by measuring the timing of connections on a very wide set of production devices. They were picked to reduce wait times noticed by users while minimizing load on the network. As time passes, it is expected that the properties of networks will evolve. For that reason, it is expected that these values will change over time. Implementors should feel welcome to use different values without changing this specification. Since IPv6 issues are expected to be less common, the delays SHOULD be increased with time as client software is updated.

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