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