[ntp:questions] Compensating for asymmetric delay on a per-peer/server basis?
William Unruh
unruh at invalid.ca
Thu Sep 11 18:29:53 UTC 2014
On 2014-09-11, Rob <nomail at example.com> wrote:
> 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.
Nope. You could have a local network in which each computer has its own
public IP addess, but the connection to that subnetwork is assymetric.
I doubt that NAT would add much assymetry. An adsl connection might well
since they advertise very different rates up from down.
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