[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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