[time-nuts] Network Time Puzzle

[Steven Sommars]

See figures 7 & 8 in
http://leapsecond.com/ntp/NTP_Paper_Sommars_PTTI2017.pdf

When a router forwards an NTP packet multiple potential egress links may
have equal cost.  In order to distribute the traffic across the egress
links the router can use the IP addresses and UDP source/destination port
numbers to select a link.  Doing this reduces the likelihood of
out-of-sequence packets.

Instead of having N x 1 Gbps links between two adjacent routers it is
preferable to have a single N Gbps link.  The latter may not be available
or cost effective.

Similar behavior can be found at network layer 2 for link aggregation
groups, see https://en.wikipedia.org/wiki/Link_aggregation .

Steve Sommars


On Sun, May 26, 2019 at 7:16 AM Peter Martinez via time-nuts <
time-nuts at lists.febo.com> 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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