[ntp:questions] Asymmetric Delay and NTP

Joe Gwinn joegwinn at comcast.net
Fri Mar 21 16:04:04 UTC 2014


Magnus,

In article <532B5AB9.4070502 at rubidium.dyndns.org>, Magnus Danielson
<magnus at rubidium.dyndns.org> wrote:

> Joe,
> 
> On 19/03/14 11:55, Joe Gwinn wrote:
> > In article <5328AAA6.70200 at rubidium.dyndns.org>, Magnus Danielson
> > <magnus at rubidium.dyndns.org> wrote:
> >
> >> On 18/03/14 01:36, Joe Gwinn wrote:
> >>> In article <5327757E.5040504 at rubidium.dyndns.org>, Magnus Danielson
> >>> <magnus at rubidium.dyndns.org> wrote:
> >>>
> >>>> Is that formal enough for you?
> >>>
> >>> It may be.  This I did know, and would seem to suffice, but I recall a
> >>> triumphant comment from Dr. Mills in one of his documentation pieces.
> >>> Which I cannot recall well enough to find.  It may be the above
> >>> analysis that was being referred to, or something else.
> >>
> >> I can't recall. The above I came up with myself some 10 years ago or so.
> >
> >
> > When I awoke the day after writing the above, I saw two problems with
> > the above analysis.
> >
> > First is that with added message-exchange volleys, one does not get
> > added variables and equations, one instead gets repeats of the
> > equations one already has.  If there is no noise, the added volleys
> > convey no new information.  If there is noise, multiple volleys allows
> > one to average random noise out.
> 
> True. What does happen over time is:
> 1) Clocks drift away from each other due to systematics and noises
> 2) The path delay shifts, sometimes because of physical distance shifts,
> but also due to shift of day and season.
> 
> These require continuous tracking to handle

Yes.


> > Second is that what is proven is that a specific message-exchange
> > protocol cannot work, not that there is no possible protocol that can
> > work.
> 
> The above analysis only assumes a way to measure some form of signal.
> The same equations is valid for TWTFTT as for NTP, PTP or whatever uses 
> the two-way time-transfer. What will differ is they way they convey the 
> information and the noise-sources they see.

It's certainly true that the two-way time transfer (including
bouncing-packet) approach works well, and is widely used, but all are
sensitive to asymmetry, and the overarching question was if this
limitation is fundamental and thus unavoidable.

As discussed later, it appears that the limitation is fundamental.


> >> Will see if I can find Dave's reference.
> >
> > I hit pay dirt yesterday, while searching for data on outliers in 1588
> > systems.   Dave's reference may well be in the references of the
> > following article.
> >
> > "Fundamental Limits on Synchronizing Clocks Over Networks", Nikolaos M.
> > Freris, Scott R. Graham, and P. R. Kumar, IEEE Trans on Automatic
> > Control, v.56, n.6, June 2011, pages 1352-1364.
> 
> Sounds like an interesting article. Always interesting to see different 
> peoples' view of fundamental limits.

It is interesting.  I've now read it reasonably closely.  

The basic approach is to express each packet flight in a one-line
equation (a row) in a linear-system matrix equation, where the system
matrix (the A in the traditional y=Ax+b formulation, where b is zero in
the absence of noise), where A is 4 columns wide by a variable number
of rows long (one row to a packet flight), and show that one column of
A can always be computed from the two other columns that describe who
is added and subtracted from who.  In other words, these three columns
are linearly dependent on one another.  The forth column contains
measured data.

This dependency means that A is always rank-deficient no matter how
many packets (including infinity) and no matter the order, so the
linear system cannot be solved.

The "no matter the order" part comes from the property of linear
systems that permuting the rows and/or columns has no effect, so long
as one is self-consistent.

So far, I have not come up with a refutation of this approach.  Nor
have the automatic control folk - this proof was first published in
2004 into a community that knows their linear systems, and one would
think that someone would have published a critique by now.

The key mathematical issue is if there are message exchange patterns
that cannot be described by a matrix of the assumed pattern.  If not,
the proof is complete.  If yes, more work is required.  So far, I have
not come up with a counter-example.  It takes only one to refute the
proof.


> >>> I also took the next step, which is to treat d_AB and d_BA as random
> >>> variables with differing means and variances (due to interference from
> >>> asymmetrical background traffic), and trace this to the effect on clock
> >>> sync.  It isn't pretty on anything like a nanosecond scale.  The
> >>> required level of isolation between PTP traffic and background traffic
> >>> is quite stringent.
> >>
> >> It's even worse when you get into packet networks, as the delays contain
> >> noise sources of variable mean and variable deviation, besides being
> >> asymmetrical. NTP combats some of that, but doesn't get deep enough due
> >> to too low packet rate. PTP may do it, but it's not in the standard so
> >> it will be propritary algorithms. The PTP standard is a protocol
> >> framework. ITU have spent time to fill in more of the empty spots.
> >
> > Yes.  In closed networks, the biggest cause of asymmetry I've found is
> > interference between NTP traffic and heavy background traffic in the
> > operating system kernels of the hosts running application code.
> > Another big hitter was background backups via NFS (Network File
> > System).  The network switches were not the problem.  What greatly
> > helps is to have a LAN for the heavy applications traffic, and a
> > different LAN for NTP and the like, forcing different paths in the OS
> > kernel to be taken.
> 
> If you can get your NIC to hardware time-stamp your NTP, you will clean 
> things up a lot.

True, but these were never much available, and the rise of PTP may
obviate the need.  

Although, if one goes to the trouble to make a NIC PTP-capable, it
wouldn't be so hard to have it recognize and timestamp passing NTP
packets.  The hard part would be figuring out how to transfer this
timestamp data from collection in the NIC to point of use in the NTP
daemon, and standardizing the answer.


Joe Gwinn



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