In case somebody still has issue with LA monitoring station (my story) - During last few weeks (end of March 2019) i’ve had 2 issues involving UDP transmission from US to Europe (Poland actually):
-
the first one involved UDP DNS packet drop in transit from dnsviz(dot)net (DNS-OARC) to NASK (.pl registrar)
-
the second one involved UDP NTP packet drop in transit from LA monitoring station to OVH datacenter in Warsaw (also PL) which occurs “from time to time” … and transmission through ipv6 was OK, as can be seen/compared here:
https://www.ntppool.org/scores/54.38.193.17
https://www.ntppool.org/scores/2001:41d0:602:1811::
(it is the same machine)
In both cases the issue was UDP related and in both cases - from the feedback provided to me - packets were dropping somewhere in/or on the edge of “Hurricane Electric LLC” (AS6939). So if somebody has similar problems (with NTP or generally UDP transit), plz check both ipv4 and ipv6 with “spell” (thanks OVH ! 
):
mtr -o “J M X LSR NA B W V” -wbzc 100 TARGET_IP
If the second (ipv6) travels (almost normally) through AS6939 and the first one (ipv4) has “black holes” where AS6939 normally should be, then you probably should contact your AS operator (or ISP) and send him this mtr report. You can also provide backward information from monitoring network at:
dev(dot)ntppool(dot)org(slash)monitoring(slash)network-debugging