Copyright (C) 2016 Internet Systems Consortium, Inc. ("ISC")

This Source Code Form is subject to the terms of the Mozilla Public

License, v. 2.0. If a copy of the MPL was not distributed with this

file, You can obtain one at http://mozilla.org/MPL/2.0/.

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.