[chrony-users] Improving accuracy

[Stephen Satchell <list@xxxxxxxxxxxx>]

When I was analyzing the results of my little 2-server 2-client testing, 
I started wondering why the chronyd server's accuracy was not as good as 
I expected when syncing to the GPS NTP Appliance TM1000A.  I found the
"presend" option for a server declaration, which reads in part:

> If the timing measurements being made by chronyd are the only network
> data passing between two computers, you might find that some
> measurements are badly skewed due to either the client or the server
> having to do an ARP lookup on the other party prior to transmitting a
> packet. This is more of a problem with long sampling intervals, which
> might be similar in duration to the lifetime of entries in the ARP
> caches of the machines.

Local LAN:  check.
Long sample times:  check.
Only network traffic between the server and appliance:  check.

For the local appliance setting in my firewall, I set up this server line:

> server 10.1.2.15 minpoll 4 maxpoll 6 iburst prefer presend 4

* 10.1.2.x is the separate VLAN I set up for the appliance, so that the 
embedded TCP/IP stack sees a minimum of traffic.
* Set the minimum polling time to 16 seconds (default is 64)
* Set the maximum polling time to 64 seconds (default is 1024)
* Use iburst to obtain a useful set of measurements at startup
* Prefer the local appliance when it is available
* Use presend to exchange a packet (and set up ARP in both devices) 
before taking a measurement.

Note that this should be done ONLY for a locally-connected NTP stratum 1 
server, and not with public servers.

The resulting information, after making the above change:
> chronyc> ntpdata 10.1.2.15 (edited for brevity)
> Leap status     : Normal
> Version         : 4
> Mode            : Server
> Stratum         : 1
> Poll interval   : 6 (64 seconds)
> Precision       : -10 (0.000976562 seconds)
> Root delay      : 0.000000 seconds
> Root dispersion : 0.000000 seconds
> Reference ID    : 47505300 (GPS)
> Reference time  : Sun May 13 02:00:22 2018
> Offset          : +0.000211241 seconds
> Peer delay      : 0.000976595 seconds
> Peer dispersion : 0.000976596 seconds
> Response time   : 0.000000000 seconds
> Jitter asymmetry: +0.00
> NTP tests       : 111 111 1111

> chronyc> tracking                 (edited for brevity)
> Stratum         : 2
> System time     : 0.000073854 seconds fast of NTP time
> Last offset     : +0.000098993 seconds
> RMS offset      : 0.000139127 seconds
> Frequency       : 11.131 ppm slow
> Residual freq   : +0.023 ppm
> Skew            : 0.615 ppm
> Root delay      : 0.000976595 seconds
> Root dispersion : 0.000997213 seconds
> Update interval : 64.2 seconds
> Leap status     : Normal



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