Clock tracking

Gary E. Miller gem at
Mon Mar 22 21:38:45 UTC 2021

Yo Hal!

On Mon, 22 Mar 2021 14:24:50 -0700
Hal Murray <hmurray at> wrote:

> Gary said:
> > You could try PTP.  The linuxptp project is active.  With ethernet
> > cards that support hardware time stamps you can get 1 micro second
> > offsets, maybe a lot better.   
> Thanks.  Could you say a bit more.
> If I start with 4 PCs plugged into a low end 5 port switch, what else
> do I need to make a working PTP setup?

Linuxptp and ntpd, that is it.  linuxptp writes a SHM that ntpd
reads.  The net will select one of its own as a grandmaster, or you can
force that.

There web site has instructions to build, install and run:

Source code:

The people on their email list are helpfulL

> Do I have to buy a fancy GrandMaster or can one of the PCs serve that
> function?

Nah.  The "best" is slected automagically.  Or force your best GPS
PPS steered host as "grandmaster".

> Low cost switches aren't going to update any packets.  Will that
> screw things up if the reason I want the clocks in sync is to measure
> network queuing delays when there is a lot of network traffic?

Well, anything that uses the network to mesaure your network has that
probblem.  If you are worried, turn it upside down: set every host
from GPS PPS, and use PTP to measure their offsets.

> Is there a HOWTO for getting started?


> Since you mentioned PTP, can we use the PTP (IEEE 1588) time stamping
> stuff to get better time stamps for NTP packets? (without dragging in
> any/much PTP stuff)

Yes, but not easily.  Some ethernic nics that support hardware PTP
have a PPS input.  So they can sync to PPS in hardwre.  An advanced
topic best discussed on the linuxptp list.

Most consumer nics do not support any hardware stamping.  The middle and
upper range Intel nics do.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at  Tel:+1 541 382 8588

	    Veritas liberabit vos. -- Quid est veritas?
    "If you can't measure it, you can't improve it." - Lord Kelvin
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