1 c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al. 2 SPDX-License-Identifier: curl 3 Long: happy-eyeballs-timeout-ms 4 Arg: <milliseconds> 5 Help: Time for IPv6 before trying IPv4 6 Added: 7.59.0 7 Category: connection 8 Example: --happy-eyeballs-timeout-ms 500 $URL 9 See-also: max-time connect-timeout 10 Multi: single 11 --- 12 Happy Eyeballs is an algorithm that attempts to connect to both IPv4 and IPv6 13 addresses for dual-stack hosts, giving IPv6 a head-start of the specified 14 number of milliseconds. If the IPv6 address cannot be connected to within that 15 time, then a connection attempt is made to the IPv4 address in parallel. The 16 first connection to be established is the one that is used. 17 18 The range of suggested useful values is limited. Happy Eyeballs RFC 6555 says 19 "It is RECOMMENDED that connection attempts be paced 150-250 ms apart to 20 balance human factors against network load." libcurl currently defaults to 21 200 ms. Firefox and Chrome currently default to 300 ms. 22