[time-nuts] Re: how much is my router influencing time-keeping over the network

[Bill Notfaded]

Since you brought up the speed of light and fiber.  One interesting thing
I've noted about white rabbit time sync is the use of BIDI fiber
transceivers.  With WR often these are used with one fiber and
bidirectional traffic on it.  I assume testing showed this is best using
same path both directions?  It's exciting more new network interfaces from
Nvidia and Intel support PTP and making more accurate time stamps.  WR is
compatible with IEEE 802.3z Ethernet protocol and IEEE1588v2

https://opg.optica.org/oe/fulltext.cfm?uri=oe-29-8-11693&id=449744

The new Nvidia DPU tech:
https://www.nvidia.com/content/dam/en-zz/Solutions/gtcf21/networking/data-processing-unit/gtc-fall-21-networking-overall-dpu-technical-overview-firefly.pdf

Seems the future is bright with these new DPU's.  I'm a network guy but
also a timenut.  I wish Cisco included support on more of their new
switches.  Hopefully that will change soon.  I'm excited today about more
25G connected servers using fan out cables from 100G ports and multigig aka
mGig switches with SFP28 transceivers.  Seems to me the time stamping
should be on all ports but alas it's not yet.

Best Regards,

Bill

On Thu, Sep 15, 2022, 3:14 AM folkert via time-nuts <
time-nuts at lists.febo.com> wrote:

> > > For fun I'm developing a router for HAM packet networks. What it does
> is
> > > route AX.25 packets between radios and tunnels (it can also bridge- and
> > > filter them).
> >
> > > What I would like to measure now is, how bad does it influence
> time-keeping
> > > when syncing time takes place over a network. I could of course just
> setup
> > > tcp/ip and let two ntp instances sync over it and then calculate an
> allan
> > > deviation plot.
> >
> > After an exchange of NTP packets, the client has 4 time stamps.
> >   The time the request packet left the client
> >   The time the request packet arrived at the server
> >   The time the reply packet left the server
> >   The time the reply packet arrived at the client
> >
> > There are 3 unknowns:
> >   Transit time client to server
> >   Transit time server to client
> >   Clock offset between client and server
> >
> > With the 4 time stamps, you can setup 2 equations.  You need one more.
> > NTP assumes the transit times are equal.
> >
> > If you have good clocks at both ends, you can assume the clocks are
> equal and
> > compute transit times in each direction.
>
> Indeed, but I also would like measure how good the path between the two
> points is. I mean: if the network randomly delays packets (or so), then
> that would influence the syncing I think?
>
> In any case, I've done a little experiment with that: I invoke a ping
> 100k times and then write down the time when the reply was received.
> I've explained it further at: https://vanheusden.com/time/ping-test/
>
> > I would like to see some graphs of network transit times over radio
> links.
> > How noisy is yor radio link?  It will be interesting to see if if you
> can get
> > a decent ADEV graph.
>
> I'm currently working on that. The two radios are close to each other, with
> signal levels of -57dB and SNR of 10.5. Ping times of around 8.3 seconds
> (yes, seconds).
>
> > Timings will depend a lot on network traffic.  It would be neat if you
> can get
> > data under both light load and heavy load.  Do you know about
> bufferbloat?  ...
>
> I do know about bufferbloat yes.
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