[1] http://www.cttl.cn/tecm/dxwjs/200903/t20090312_573809.htm [2] https://segmentfault.com/a/1190000005337116 [3] http://rhelblog.redhat.com/2016/07/20/combining-ptp-with-ntp-to-get-the-best-of-both-worlds/