bpo-39128: Added algorithm description (GH-18624)

# [bpo-39128](https://bugs.python.org/issue39128): happy eyeballs description

# [3.9] 39128 - happy eyeballs description (GH-18624)
# [3.8] 39128 - happy eyeballs description (GH-18624)

https://bugs.python.org/issue39128

diff --git a/Doc/library/asyncio-eventloop.rst b/Doc/library/asyncio-eventloop.rst
index 3acd79d283580089a9bfb6a24b52bbd2520cdce8..d60a6ce95cdd87c0a318bb3dcfede61e164d6ec2 100644 (file)
--- a/Doc/library/asyncio-eventloop.rst
+++ b/Doc/library/asyncio-eventloop.rst
@@ -451,6 +451,17 @@ Opening network connections
 
       Added the *happy_eyeballs_delay* and *interleave* parameters.
 
+      Happy Eyeballs Algorithm: 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.
+
+      For more information: https://tools.ietf.org/html/rfc6555
+
    .. versionadded:: 3.7
 
       The *ssl_handshake_timeout* parameter.