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

William Unruh unruh at invalid.ca
Sat Sep 13 11:56:37 UTC 2014


On 2014-09-12, Martin Burnicki <martin.burnicki at meinberg.de> wrote:
> William Unruh wrote:
>> No idea why a fudge parameter would be complicated. If you wanted to use
>> ntpd itself to figure out the assymmetry, that could well be
>> complicated. But if it is a fixed offset, I cannot see how that would be
>> complicated and it ihas already been implimented in the refclock case.
>
> I tried to explain this in my earlier email.
>
> If you have a local (GPS controlled) NTP server plus some NTP clients 
> connected to the inet via an asymmetric connection you
>
> - need to apply a fudge time if your local server contacts external servers

Again, let us separate the two questions-- one is trying to make the
time on your computer equal UTC, the other is to deliver that UTC to
others. At present it is the first that is most important to me, and for
the solution to THAT problem that it seems to me to be simple-- exactly
the same as the fudge for refclock.
Also if you have not solved the first, then the second-- delivering
exact time to others, cannot be solved. 

One could argue that the second is the problem for the client, not the
server to solve.  Ie, if you connect to some source, it is up to you to
figure out if there is an assymetry on the path from them to you. Now
the server could help in this if it knows that there are some local
assymmeteries, but that is help, not duty.

>
> - need to apply the same fudge time with reversed sign if *you* try to 
> compensate the time error caused by *your* inet connection when external 
> clients try to get the time from your NTP server
>
> - need to apply no fudge time at all for connections on your local network
>
> I don't know how this is in other countries, but at least here in 
> Germany a typical home setup is a NAT router connected to the inet via 
> ADSL, and the router providing an internal switch with several ports to 
> which you can connect your devices, e.g. your NTP server and some NTP 
> clients.
>
> As has been stated in some other posts:
>
> - NAT doesn't hurt at all, unless you are trying to use NTP's authentication
>
> - the asymmetry of the ADSL connection causes a time error of a certain 
> range, the sign of which depends on whether you look from an external 
> node to your home NTP server, or from your home NTP server to some 
> remote NTP server

Agreed.
>
> So an overall solution might be:
>
> - if the source IP of incoming client requests is not on your local 
> subnet, apply a fudge time

No. That is their problem not yours. Otherwise you havee too many cooks. 


>
> - if the destination address of reply packets you send is not on your 
> local subnet, apply a fudge time with reversed sign
>
> - otherwise (source and destination on your local subnet) apply no fudge 
> time at all
>
>
> Martin



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