[ntp:questions] NTP request retry?

William Unruh unruh at invalid.ca
Mon Jan 27 20:16:59 UTC 2014


On 2014-01-27, Rob <nomail at example.com> wrote:
> Marco Marongiu <brontolinux at gmail.com> wrote:
>> On 01/26/2014 08:08 PM, Rob wrote:
>>> My hypothesis is that the ARP entry for the NTP server has timed out,
>>> and when ARP has to resolve an entry in some implementations the first
>>> packet is always lost (it is not cached pending a reply).
>>> When the cycle is 1024 seconds, the ARP entry has again timed out the
>>> next poll cycle and the issue is the same.
>>
>> If you believe that it is the problem, and you own the servers you're
>> polling, then you may set maxpoll so that the polling interval is always
>> smaller then the ARP cache timeout.
>
> Despite lots of tracing I still cannot really pinpoint the problem.
>
> The only thing I see is that ping has absolutely zero loss and all
> usual protocols work fine, but ntp indicates a high loss when there
> is no other network activity.

No idea what operating system you use. On linux use tcpdump to try to
see what is happening and if there is a problem with arp. 

>
> When the network is kept busy, ntp works fine as well.
> Very strange.
>
> However, I do not like to "fix" it by increasing frequency, it may
> not even help.  I'd like to have some "burst until there is a reply,
> up to X tries" option.
> It appears to be a function of other NTP clients, but not of ntpd.

Really really bad idea. You are relying on the generaosity of others
because you are too lazy or incopentent to fix your own problem.
You are asking that the poll level be set to zero when the other end
does not respond. There was a router manufacturer about 10 years ago
that did that with a hard wired server address. It took the server down
which was getting millions of ntp packets per second. Not nice. 

Fix your problem or set up your own server and then you can poll it as
often as you like. 



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