[time-nuts] Network Time Puzzle

[K5ROE Mike]

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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