Ntpd: receive: Unexpected origin timestamp from

three months since I see these warnings in syslog about ntpd, is this normal?

ntpd: receive: Unexpected origin timestamp from 200.160.7.193
ntpd: receive: Unexpected origin timestamp from 200.160.0.8
ntpd: receive: Unexpected origin timestamp from 200.189.40.8
ntpd: receive: Unexpected origin timestamp from 91.189.89.199
ntpd: receive: Unexpected origin timestamp from 133.243.238.164
ntpd: receive: Unexpected origin timestamp from 181.119.30.63

those are random ip, sometimes from ukraine etc.
ubuntu mate 16.04.5

I’m worried. http://www.cs.bu.edu/~goldbe/papers/NTPattacks.html