[time] problems with your ntp service

[Dag-Erling Smørgrav]

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
> between.
>
> 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:

  http://www.pool.ntp.org/scores/194.63.250.121

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.
>>

DES
-- 
Dag-Erling Sm?rgrav - des at des.no