Would it be possible for the NTP server to detect what type of device/OS is sending the request and block it (ie: could au/nz.pool.ntp.org servers block all TP-Link requests to teach them a lesson)?
If they can't do that maybe they can just detect IPs that are making requests every 5 seconds as the TP-Link products are doing and block those since they're in violation of the once-every-10-minutes-maximum rule for the NTP servers)?
If they can't do that maybe they can just detect IPs that are making requests every 5 seconds as the TP-Link products are doing and block those since they're in violation of the once-every-10-minutes-maximum rule for the NTP servers)?