[ntp:questions] NTP/clock behaviour/config
Patrik Arlos
Patrik.Arlos at bth.se
Wed Aug 31 10:43:07 UTC 2005
Hi,
I've been looking at the stability and accuracy that is obtained for a host
system clock. I've scanned the jun-aug archives and saw that a host should
have three servers in its list, too operate correctly.
I have a system with a Trimble Acutime 2000 GPS attached to it (gps1). The
NTP server on this machine had three national references in its config, as
well as three local servers (time1,time2 and time3) with unknown
configuration. Initially the local clients were configured to synchronize
to the gps1, time1, time2 and time3. What happened was that time1, time2 and
time3 decided to use gps1 as their time source. How does this affect the
accuracy of the local clients? Based on this, the local clients were
instructed to only use gps1 as their time source. Is it terribly wrong to
configure the clients to only use gps1 as their time source? The clients are
one router and two switches away from the gps1, making it their closest time
source. And should the gps1 have other time sources than its GPS antenna?
I did some measurements, where a host (running the NTP daemon) compared its
system time with the gps1's time every hour with 'ntpdate -d gps1'. And got
quite a strange behaviour (see table below), there were no errors reported
in the logs, which makes this behaviour even more strange. Apparently the
clock drifts, my question is why the there seems to be a (roughly) 6 hour
period, were the offset jumps a second? Both client and gps1 were running
Linux with a 2.4 kernel.
Any Ideas?
Best regards,
Patrik
Table column description: Sample is a running number, indirectly indicating
how many hours that has elapsed since start. Host time at the issuing of
ntpdate. Type and offset are reported from the ntpdate command.
Sample:Host time:Type:Offset
1:1115278882:adjust:-0.00759
2:1115282482:adjust:-0.052178
3:1115286083:adjust:-0.104167
4:1115289683:step:0.952238
5:1115293283:step:0.835014
6:1115296883:step:0.759389
7:1115300483:step:0.699821
8:1115304083:step:0.646394
9:1115307683:step:0.596673
10:1115311283:step:1.6063
11:1115314884:step:1.51765
12:1115318484:step:1.4533
13:1115322084:step:1.39814
14:1115325684:step:1.34639
15:1115329284:step:2.38936
16:1115332884:step:2.28085
17:1115336484:step:2.20906
18:1115340084:step:2.1511
19:1115343685:step:2.09836
20:1115347285:step:3.13514
21:1115350885:step:3.05059
22:1115354485:step:2.96719
23:1115358085:step:2.90464
24:1115361685:step:2.85022
25:1115365285:step:2.79883
26:1115368885:step:4.23343
27:1115372485:step:3.70664
28:1115376086:step:3.65113
29:1115379686:step:3.59919
30:1115383286:step:3.54857
31:1115386886:step:4.6173
32:1115390486:step:4.4957
33:1115394086:step:4.41666
34:1115397686:step:4.35592
35:1115401286:step:4.30216
36:1115404887:step:4.25115
37:1115408487:step:5.27246
38:1115412087:step:5.17749
39:1115415687:step:5.11069
Patrik Arlos
Tech. Lic Telecommunications systems
PhD Candidate Telecommunications
Blekinge Institute of Technology
School of Engineering, Telecommunications Group
371 79 KARLSKRONA
SWEDEN
+46 (0)455 385654 Office
+46 (0)733 800312 Mobile
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