[ntp:questions] Criteria for evaluating upstream server quality

Pierre Dubuc pldubuc at yahoo.ca
Sat Dec 22 15:07:05 UTC 2007


I'd like to get opinions as to the weight of specific criteria for 
choosing upstream servers to get time from.

For example, which of these is more important than the others, and why?

- round-trip delay;
- stratum;
- server's own configuration;
- and so on...

For now, I've got the following billboard (refid's trimmed):

mac> ntpq -pn | sort -k8.1,8.6n
      remote          refid st t when poll reach   delay   offset  jitter
=========================================================================
+132.205.7.81    132.205... 3 u  131 1024  377    5.897   -0.615   0.149
-24.200.225.157  10.23.12.. 4 u  390 1024  377    5.776   -1.854   0.190
-216.113.122.37  207.253... 3 u   77 1024  377    5.107    1.838   0.721
+192.26.210.193  172.22.... 2 u  199 1024  377    6.229   -0.214   0.439
-132.208.250.3   204.123... 2 u    7 1024  377    6.855    1.577   1.506
*132.246.168.148 132.246... 2 u   70 1024  377    8.820   -0.307   0.764
-199.212.17.21   142.135... 2 u   32 1024  377    9.314   -7.228   0.411
-132.214.200.120 18.26.4... 2 u   34 1024  377   17.752   -2.384   1.486
-209.87.233.53   209.87.... 2 u  205 1024  377   36.763    3.240   0.631

I concede this may a bit too many servers for domestic use, but how do I 
choose which ones to remove?

For example, the closest servers to me are actually my ISP's cisco routers 
(24.200.225.157 and 216.113.122.37). Although the round-trip is short 
(about 5-6ms), the routers are polling two time sources, which is a no-no. 
Furthermore, the one at 216 has lost connection to one of its servers for 
close to 4 months, indicating it's probably not monitored. Are those 
trustworthy time sources?

Some stratum 2 servers I get time from are actually polling stratum 1's 
that have a pretty significant round-trip (more than 70ms), so is this any 
better than having my ntp getting its time from a stratum 3 or 4 that's 
closer to its own upstream sources?

I already read the standard "best practice" of choosing servers that are 
close-by in the network, but I'd like to learn a bit more about how to 
choose a good-quality time source.

Opinions are suggestions are welcome...

-- 
Pierre Dubuc
pldubuc at yahoo.ca




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