[time] problems with your ntp service
Sun Feb 10 23:12:16 UTC 2008
Jan Hoevers <ml.janhoevers at xs4all.nl> writes:
> That's correct, it is a network issue. The monitoring system is in
> California, for us here in europe there's an ocean and a continent in
> I did some simple tests on this a few months ago; my conclusion was that
> the monitoring system can be used to see if my server is reachable,
> nothing more. The inaccuracies introduced by the network are in the
> order of ten to fifty ms, with peaks far over 100 ms, while a carefully
> run stratum 2 server doesn't have to be more than a millisecond off.
Yet it somehow never has any trouble with my server:
Note that the offset is mostly within 2 ms, with occasional peaks
approaching 10 ms.
Assuming the measuring server correctly estimates and compensates for
network latency (using the very simple algorithm described in the NTP
and SNTP RFCs), it should not affect the offset it sees unless there is
considerable asymmetry between the outbound and inbound leg.
To calculate the roundtrip delay d and system clock offset t relative
to the server, the client sets the Transmit Timestamp field in the
request to the time of day according to the client clock in NTP
timestamp format. For this purpose, the clock need not be
synchronized. The server copies this field to the Originate
Timestamp in the reply and sets the Receive Timestamp and Transmit
Timestamp fields to the time of day according to the server clock in
NTP timestamp format.
When the server reply is received, the client determines a
Destination Timestamp variable as the time of arrival according to
its clock in NTP timestamp format. The following table summarizes
the four timestamps.
Timestamp Name ID When Generated
Originate Timestamp T1 time request sent by client
Receive Timestamp T2 time request received by server
Transmit Timestamp T3 time reply sent by server
Destination Timestamp T4 time reply received by client
The roundtrip delay d and system clock offset t are defined as:
d = (T4 - T1) - (T3 - T2) t = ((T2 - T1) + (T3 - T4)) / 2.
Dag-Erling Sm?rgrav - des at des.no
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