NTP packets travelling back in time?
Friday, November 11, 2011 11:57:57 AM
Yes, negative delay in ntpd. Packets travelling back in time when going from the server to your client. Fasten your seat belts people, Captain Kirk is back 
OK, seriously now. As you can expect, I was quite surprised to see this, but it appears to be expected in certain conditions.
Here's the full story.
One day I ran ntpq -c pe -c as on one of our clients, and I found out that the delay was negative for three out of fours servers (real IPs concealed):
$ ntpq -c pe -c as
remote refid st t when poll reach delay offset jitter
==============================================================================
-10.100.100.142 22.222.222.6 3 m 66 64 376 -3.302 1.517 0.022
+10.100.100.141 22.222.222.7 3 m 63 64 376 -6.828 0.083 0.025
+10.100.100.161 222.2.22.84 3 m 54 64 376 -7.786 -0.756 0.035
*10.100.100.162 22.222.222.3 3 m 50 64 376 0.314 0.609 0.031
ind assid status conf reach auth condition last_event cnt
===========================================================
1 61991 731a no yes ok outlyer sys_peer 1
2 61992 741a no yes ok candidate sys_peer 1
3 61993 741a no yes ok candidate sys_peer 1
4 61994 761a no yes ok sys.peer sys_peer 1
As you can imagine, I was a lot surprised... how could it be?
I posted my question to the relevant list, asking if it was a bug.
I got some contrasting answers. Some think I actually hit a bug. Others are less sure. But the bottom line seems clear: it is... well, "normal", it could happen in a multicast setting.
We'll wait and see if 4.2.8 fixes it.







Francesco «Oha» Rivettiohait # Tuesday, December 6, 2011 9:08:51 AM
Marco Marongiumarcomarongiu # Tuesday, December 6, 2011 9:22:22 AM