[ntp:questions] Re: NTP precision

Tom Smith smith at cag.zko.hp.com
Fri Sep 23 16:03:13 UTC 2005


Leandro Pfleger de Aguiar wrote:
> Hi 
> 
>     I´m using NTP to timestamp digital events in a specifit application. On my application i need to know what is the exact estimated error that my timestamp marks should consider. Some peaple frequently use offset to mean what is the real estimated difference between local and ref clock. Should i consider other parameters like jitter,  precision and accuracy manually ? Remember: i need to say how mutch can my timestamp be wrong.
> 
> Tanks Again !

Interesting question. I think the data needed, taking into account
all of the servers, is in the results of "ntpq -c rl 0":

$ ntpq -c rl 0
status=06f4 leap_none, sync_ntp, 15 events, event_peer/strat_chg,
version="ntpd 4.2.0 at 1.1161-r Tue Feb  8 16:04:28 EST 2005 (1)",
processor="alpha", system="OSF1/V5.1", leap=00, stratum=2,
precision=-20, rootdelay=88.901, rootdispersion=39.233, peer=33113,
                                  ^^^^^^^^^^^^^^^^^^^^^
refid=[IP address],
reftime=c6de8d00.31a79fec  Fri, Sep 23 2005 10:17:36.193, poll=10,
clock=c6de8f3b.4f435265  Fri, Sep 23 2005 10:27:07.309, state=4,
offset=0.231, frequency=-1.572, jitter=0.280, stability=0.001
^^^^^^^^^^^^                    ^^^^^^^^^^^^

However, I'm not sure of the proper way to put these together.

Using "estimated error":
"true" time = systemtime + offset +- jitter
      or
"true" time = systemtime +- jitter

Using "maximum error":
"true" time = systemtime + offset +- rootdispersion
      or
"true" time = systemtime +- rootdispersion

or something else?

-Tom




More information about the questions mailing list