ntpd supports an interleaved mode to allow the protocol to exchange transmit timestamps that were captured after the packet was sent in symmetric associations and broadcast modes. It can be enabled in the configuration file, but it's also enabled automatically when a packet received from the source is detected to be in the interleaved mode. The detection compares the origin timestamp in the packet to the previous local receive timestamp. The interleaved mode is enabled even in client associations, even though it makes no sense there. http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2016-1548
