Ido Michael <michael.ido@gmail.com> added the comment: Thanks Julien, I forgot about this part, will create a new PR. I thought referencing the abstract of the algorithm + a link to the IETF RFC: Happy Eyeballs: Success with Dual-Stack Hosts When a server's IPv4 path and protocol are working, but the server's IPv6 path and protocol are not working, a dual-stack client application experiences significant connection delay compared to an IPv4-only client. This is undesirable because it causes the dual- stack client to have a worse user experience. This document specifies requirements for algorithms that reduce this user-visible delay and provides an algorithm. https://tools.ietf.org/html/rfc6555 ---------- _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue39128> _______________________________________