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Some systems (Linux, FreeBSD, OS X/macOS and Windows 10) support
the TCP Fast Open (RFC 7413) mechanism in their recent versions.

BIND 9 supports this on the server side.

When the TCP_FASTOPEN socket option is defined after the listen()
system call the socket code in the libisc set the option with
the half of the listen backlog (so the fast open maximum queue length
is the half of the pending connection queue length).
Any failure is logged and ignored.

System specific notes:
 - FreeBSD doesn't interpret the argument as a queue length but
  only as an on/off switch.

 - Using TCP Fast Open on FreeBSD, as of versions 10.3 and 11.0, requires
   compiling a custom kernel and setting the "net.inet.tcp.fastopen.enabled"
   sysctl to 1.

 - Apple OS X/macOS allows only 0 or 1 so the code puts 1 for this system.

 - Windows 10 uses a 0/1 char flag? Note that TCP_FASTOPEN is defined
  only in SDK 10.0.14393.0 or higher (Visual Studio 2015 requires
  extra setting of the "Target Platform Version" in all project
  properties).

 - the only other system known to support this is Linux.