[ntp:hackers] Server timestamps going backwards by up to 2 milliseconds

Dave Hart davehart at gmail.com
Thu Dec 24 18:50:28 UTC 2009


Dr. Mills,

I think you may be misunderstanding what David Taylor is reporting.
This is not a matter of non-monotonic updates comparing the local
clock to remote clocks.  Here ntpd is generating non-monotonic
timestamps itself.  When his system with about 1ms resolution system
clock (and no interpolation) sends a mode 4 response to a mode 3
request, the resulting packet regularly indicates the low-precision
ntpd received the mode 3 request after it sent the mode 4 response, by
up to 2ms, due to both timestamps being fuzzed below the 1ms
resolution.

For me, it raises a question in my mind if get_systime() should be
enforcing monotonicity of its result (except immediately after a
step).

Cheers,
Dave Hart


More information about the hackers mailing list