[ntp:questions] Compensating for asymmetric delay on a per-peer/server basis?

Rob nomail at example.com
Thu Sep 11 14:36:14 UTC 2014


Martin Burnicki <martin.burnicki at meinberg.de> wrote:
> This is also what Rob has mentioned in another post of this thread, and 
> I agree with Rob that a one approach could be to specify (and configure 
> for ntpd) the systematic error due to asymmetry of your internet connection.
>
> However, this can also be pretty tricky if you have several NTP nodes on 
> your home network, if all nodes and the inet router are connected to the 
> same switch.
>
> For different nodes on you home net there is no asymmetry (thus no time 
> error), but for each of them who contacts also an external server there 
> is. And often a specific machine contacts the other internal devices as 
> well as the external ones via the same own LAN interface.
>
> So for your internal operation this means:
>
> - If you specify a fudge time for a specific interface this may be OK 
> for external servers but yield an error for internal servers, i.e. 
> exactly the other way round as without compensation.
>
> - You had indeed to specify a fudge time for servers of which you know 
> they are outside on the internet, e.g. other pool servers
>
>
> On the other hand, if your local NTP server shall be accessible both for 
> external pool clients, and local clients, how should you know where a 
> specific request comes from? Based on the IP address? Only if the local 
> network and the internet interface are connected via different interfaces?
>
> So even though it would be good to be able to specify some compensation 
> values, there should be different ways to do it, and putting all 
> together in a way that there is no error is tricky.

Well, in my own system I have a different IP address for the internet
than I have for my local network.  In the bug report I asked for a
fudge time1 that could be specified per local IP addres.  This would
work OK in my case.  When you use the same address on a LAN and on
internet it is more difficult.  I guess this only happens in cases
where there is a NAT router that translates requests from internet to
a local address.  Not a configuration I would recommend when being
in the pool anyway.



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