| c: Copyright (C) 1998 - 2022, Daniel Stenberg, <daniel@haxx.se>, et al. |
| SPDX-License-Identifier: curl |
| Long: happy-eyeballs-timeout-ms |
| Arg: <milliseconds> |
| Help: Time for IPv6 before trying IPv4 |
| Added: 7.59.0 |
| Category: connection |
| Example: --happy-eyeballs-timeout-ms 500 $URL |
| See-also: max-time connect-timeout |
| Multi: single |
| --- |
| Happy Eyeballs is an algorithm that attempts to connect to both IPv4 and IPv6 |
| addresses for dual-stack hosts, giving IPv6 a head-start of the specified |
| number of milliseconds. If the IPv6 address cannot be connected to within that |
| time, then a connection attempt is made to the IPv4 address in parallel. The |
| first connection to be established is the one that is used. |
| |
| The range of suggested useful values is limited. Happy Eyeballs RFC 6555 says |
| "It is RECOMMENDED that connection attempts be paced 150-250 ms apart to |
| balance human factors against network load." libcurl currently defaults to |
| 200 ms. Firefox and Chrome currently default to 300 ms. |