usingcurl/connections/happy.md: describe happy eyeballs

SUMMARY.md

@@ -107,6 +107,7 @@
   * [Connections](usingcurl/connections/README.md)
     * [Name resolve tricks](usingcurl/connections/name.md)
     * [Connection timeout](usingcurl/connections/timeout.md)
+    * [Happy Eyeballs](usingcurl/connections/happy.md)
     * [Network interface](usingcurl/connections/interface.md)
     * [Local port number](usingcurl/connections/local-port.md)
     * [Keep alive](usingcurl/connections/keepalive.md)

usingcurl/connections/README.md

@@ -11,6 +11,7 @@ you might find yourself wanting to tweak the specifics…
 
 * [Name resolve tricks](name.md)
 * [Connection timeout](timeout.md)
+* [Happy Eyeballs](happy.md)
 * [Network interface](interface.md)
 * [Local port number](local-port.md)
 * [Keep alive](keepalive.md)

usingcurl/connections/happy.md

@@ -0,0 +1,46 @@
+# Happy Eyeballs
+
+When curl is about to connect to a given host, it has a list of addresses to
+try. The address list is separated into two lists: one list with IPv4
+addresses and one list with IPv6 addresses.
+
+curl starts by initiating a connect attempt to the first address in the IPv6
+list. If the connect attempt fails, it moves on to the next address in the
+list iterating through them all one by one until one works.
+
+If, after two hundred milliseconds curl still tries to connect to IPv6, it
+starts a second series of attempts over IPv4 in parallel. In the same style it
+iterates over the list of IPv4 addresses on connect errors. A connection race
+if you will.
+
+The first connect that works wins, and all the other attempts are then
+discarded.
+
+## HTTP/3 racing
+
+When asked to attempt HTTP/3 (with `--http3`), curl ups the connection racing
+game one level.
+
+curl first starts the Happy Eyeballs connect attempt for QUIC as described
+above. First it tries IPv6 QUIC and then IPv4 QUIC after a timeout. If there
+is no successful QUIC connection within the first two hundred milliseconds (or
+no UDP package received at all within half the timeout window, one hundred
+milliseconds by default), curl starts a *second* Happy Eyeballs scenario for
+TCP - first IPv6 TCP and then IPv4 TCP after the timeout.
+
+The first connect attempt that succeeds out of the up to four parallel ones
+that are running, wins. The "losers" are then simply discarded.
+
+## Tweak
+
+The two hundred milliseconds may delay your connects longer than you might
+want. For example when TCP packets over IPv6 are never getting any response on
+a certain network path or server.
+
+Or vice versa, you may need to communicate to a server with high latency, in
+which case the default timeout would strike too early.
+
+For moments and scenarios like that, curl provides the
+`--happy-eyeballs-timeout-ms` command line option. It changes the default two
+hundred milliseconds to whatever you specify. Setting it to zero makes curl
+start the connection attempts at exactly the same time.