I would broaden your experiment to try many different remote servers.
Maybe using different chunks of the global NTP pool
(https://www.ntppool.org/en/use.html)
It could be traffic shaping on your ISP , an ISP in the middle, or one
on the remote end.
It could be traffic prioritization from one of the above that lowers
priority for ports > 1024
It could be weirdness from the network stack on your Windows XP clients.
You might considering using something more modern.
K5ROE Mike
On 5/26/19 4:26 AM, Peter Martinez via time-nuts wrote:
Greetings, Time Nuts, from a new member.
I have two old Windows XP laptops on which I can lock the timing to
GPS, which means I can read the time at which things happen to a few
microseconds. I thought I would modify some of my old NTP software,
both client and server, to make use of this and see how well the ntp
system performs.
It's all working fine, but in the course of trying to decide what to
set for the "local port address", I discovered a strange effect. If I
set the local port address of my ntp client to one value (somewhere
between 49152 and 65535 for example), then query an ntp server on the
internet, then change the local port to another value and do it again,
the Time Offset and Round Trip Delay readings come back different.
Change the port back and the offset/delay values go back to the
original. Same on the other PC. But ONLY on some distant servers.
Most of them don't show the effect.
I have seen jumps of about 6.2msec in delay and 3.1msec in offset, but
the offset might be positive or negative. This leads me to think that
this wierd effect is a propagation delay occuring in one of the two
paths, either the path from me to the server or from the server back
to me. On some servers I have seen the delay jump by 12.4msec with no
jump in the offset. This must be a 6.2 msec. delay in BOTH propagation
delays. In this case, four different values of local port address can
give rise to 4 different delay/offset combinations. A scatter plot of
delay versus offset, with random port address, shows four dots in a
diamond shape. Different delay values give different-sized diamonds.
Routes with more than one such effect show even prettier patterns of
superimposed diamonds. The effect is stable over time, at least for
the length of time (weeks) I have been studying it.
If this is real (and I am fairly sure it's not a bug at my end or at
the servers), then it will impact on the accuracy which can be
achieved with NTP. I ask myself "Why does the network do this?". Is
there a valid reason for it, or is it a side-effect of something
else? Has anyone else seen this effect? Is there anyone out there
reading this who could modify an NTP client program so that the loal
port address can be changed manually, and see if this is a widespread
feature of the internet. If this effect didn't occur, NTP could be a
lot better than it is now.
Regards
Peter Martinez G3PLX
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