[ntp:questions] NTP request retry?

detha detha at foad.co.za
Wed Jan 29 12:37:46 UTC 2014

On Wed, 29 Jan 2014 08:28:25 +0000, Rob wrote:

> detha <detha at foad.co.za> wrote:
>> On Mon, 27 Jan 2014 21:45:22 +0000, Rob wrote:
>> [...]
>>> I can ping it as much as I like, no loss: 1571 packets transmitted,
>>> 1571 received, 0% packet loss, time 20468ms rtt min/avg/max/mdev =
>>> 0.702/0.845/1.168/0.090 ms
>>> But when ntpd is allowed to climb to 1024-second polls, it gets almost
>>> no replies.
>> Simple way to test if the problem could be ARP-related: set ntp for 1024
>> poll, and keep a ping running. The ping will keep the ARP caches (and
>> anything else in the path) fresh.
> Yes, but that is not a test if the problem could be ARP-related.
> It is apparent that the problem does not occur when the link is busy, but
> I still don't know the cause.
> It may also be some power-saving mechanism, for example.

First step is to prove that the problem goes away when the link is kept
busy with totally unrelated traffic, to be sure it has nothing to do with
the ntp client or server. That is what running a ping from client to
server was supposed to accomplish.

If keeping the link open with a ping helps, second step is to determine
what times out. Run a ping from somewhere else to the server, or from the
client to somewhere else. If running a ping from the client to somewhere
else makes the problem go away, it could be some power-saving thing
getting in the way. If not, it /might/ be ARP; time to start running
tcpdump on both client and server, and compare the captures.

When troubleshooting this type of thing, don't forget switches. I've
recently encountered so-called 'green' switches that shut down ports with
no traffic, and occasionally forget to turn them on again.


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