[time-nuts] Network Time Puzzle
peter.martinez at btinternet.com
Sun May 26 08:26:24 UTC 2019
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
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.
Peter Martinez G3PLX
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