[ntp:questions] frequency adjusting only

Greg Dowd GDowd at symmetricom.com
Wed Apr 30 20:38:30 UTC 2008


PTP default profile operation is 1 way sync transmissions from the
grandmaster to all slaves (via multicast) with an implicit "occasional"
delay request/response for ranging from each slave.  The work we are
doing in the telecom space (through ITU and IETF) defines a new
application profile for PTP which is unicast based and has a 1:1
correspondence between sync and delay request/response.  It also allows
higher sync and delay request rates.  

So, while NTP and PTP essentially have a like set of timestamps and
fundamental assumptions, I wouldn't say they do the same thing.  The
small LAN is where PTP default profile is optimized for operation.
While everything from a single LAN segment to the big, hairy, scary
Internet is the target for NTP (along with lousy oscillators).

On a small LAN, with light traffic, it is likely all moot.  

I'm not sure we agree on the effectiveness of the clock servo in ntp.
First, there is no protocol requirement to only use 8 taps, it could be
changed.  I've checked :-) IIRC, there are 20 taps in the ACTS reference
clock driver.  However, since the network client filter is 8 entries
deep, and the poll interval can climb to 1024 seconds, I wonder if you
feel like the frequency stability of a pc is useful out at observation
intervals in the 10k seconds range?  My guess would be that
environmentals would stomp on those samples.  Also, wander can be
introduced by the LRD characteristics of network traffic.

However, moving closer in, I still think the higher update rate has
value.  If you start using higher quality oscillators and hardware
timestamping, the dominant noise source becomes the delay variation in
the network.  Since the remote clock can't average this (it's not
uniform or Gaussian), it needs to use some intelligent filtering.
Higher packet rates mean that there are more samples to pick from.  

Also, one thing a lot of these discussions miss is the natural tradeoff
between trying to be the most accurate vs trying to be the most stable.
These tradeoffs, as well as differences in noise processes, mean there
is no one "correct" servo.   Having spent some time studying networks
with multiple network load generators connected through and across
network where sync is transferred (mostly for wireless backhaul), I've
developed a healthy respect for many of the various sampling and
filtering functions in ntp. 




  
Greg Dowd
gdowd at symmetricom dot com (antispam format)
Symmetricom, Inc.
www.symmetricom.com
"Everything should be made as simple as possible, but no simpler" Albert
Einstein

-----Original Message-----
From: Bill Unruh [mailto:unruh at physics.ubc.ca] 
Sent: Wednesday, April 30, 2008 3:21 PM
To: Greg Dowd
Cc: questions at lists.ntp.org
Subject: RE: [ntp:questions] frequency adjusting only

On Wed, 30 Apr 2008, Greg Dowd wrote:

> As noted, these are really stability measurements of the difference 
> between two clocks.  The absolute accuracies, particularly once you 
> reach the submillisecond domain, are impacted by the sum of all biases

> in the measurement system, os, stack, driver, dma controller, bus, 
> mac, phy, physical layer, switching/routing matrix and protocols
> (ARP/STP/QoS) and phy,mac,bus,driver,stack,os,app on the other end.  
> Not just jitter and delay variation, but biases. Sometimes the biases 
> are complentary and cancel and sometimes they don't.

Agreed. However, ntp and PTP is software do almost the same thing
(unless ptp really uses broadcast in which case it is much worse than
ntp-- broadcast is horrible since it cannot see those sudden increases
in delays due to congestion, etc. NTP is far to aggressive in throwing
away packets-- keeping only about 1/8 of the packets due to the clock
filter algorithm But ptp is if what you say is correct, much worst,
since broadcast mode is really only good to ms due to those variable
delays.


>
> There is a real difference available which is the followup message.  
> It is possible to have the system record the timestamp of actual 
> transmission and send it in a followup in ptp.  I did some testing 
> with this a few years ago and achieved the same results in timestamp 
> transmission with both protocols.  Having said that, I presume that 
> one REAL benefit for time transfer is that PTP can, and does, run at a

> higher sync rate than ntp.  It is also synchronizing to a single
clock.

The higher sync rate can be a benefit. It can also be bad because the
Markovian clock discipline means that no use can be made of long time
baselines to get better clock frequency accuracy (one of the great
advantages of chrony in situations where the phase noise dominates).
ntp's handling is a kludge.


> Also, the default ptp app is using multicast "broadcasts" with ttl 1 
> and the client uses a slightly funky "point to point" multicast 
> transmission as a ranging request to calculate propagation delay.  The

> delay is then added to sync to arrive at value for local clock 
> comparison.  However, I don't think that there is a multi tap filter.

> In fact, in the open source ptp, I think the servo is just pretty much

> a jam hack.  The point was to show the protocol.

It looked like it. But both ntp and ptp use simply markovian response
filters. They preserve no memory, which is silly.


>
> All of this is good dialogue but it is VERY important to note that 
> what you test in a small LAN has very little bearing on the 
> performance possible in various types of real networks of greater
scale..

Agreed. 
But the OP wanted to use it in a small lan.

>
>
> Greg Dowd
> gdowd at symmetricom dot com (antispam format) Symmetricom, Inc.
> www.symmetricom.com
> "Everything should be made as simple as possible, but no simpler" 
> Albert Einstein
>
> -----Original Message-----
> From: questions-bounces+gdowd=symmetricom.com at lists.ntp.org
> [mailto:questions-bounces+gdowd=symmetricom.com at lists.ntp.org] On 
> Behalf Of Bill Unruh
> Sent: Wednesday, April 30, 2008 1:20 PM
> To: questions at lists.ntp.org
> Subject: Re: [ntp:questions] frequency adjusting only
>
> m.louvel at gmail.com (maxime louvel) writes:
>
>> On Tue, Apr 29, 2008 at 6:27 PM, Unruh <unruh-spam at physics.ubc.ca>
> wrote:
>
>>> m.louvel at gmail.com (maxime louvel) writes:
>>>
>>>> Hi,
>>>
>>>> I have know run a lot of tests.
>>>> Just to let you know what I've got so far.
>>>> I have tried NTP, and NTP + PTP (Precision Time Protocol).
>>>> I haven't tried Chrony nor TSClock.
>>>> I have used the software only implementation of PTP (ptpd).
>>>
>>>> With NTP only I have got an accuracy around 1ms,
>
> Actually, I have no idea what the difference is between the "software 
> implimentation" of PTP and standard NTP is. The advantage of PTP is 
> the HARDWARE timestamping of the packets as they come into the 
> ethernet card (special purpose ethernet cards with clocks on board) 
> and possibly PTP aware switches which race through the PTP packets
without delay.
> Software only means
> that PTP uses exactly the same kernel routines, etc. to read the 
> computer clock as does ntp I assume. I cannot see how it can be better

> unless there are some severe bugs in NTP.
> What version of NTP are you running?
>
>
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-- 
William G. Unruh   |  Canadian Institute for|     Tel: +1(604)822-3273
Physics&Astronomy  |     Advanced Research  |     Fax: +1(604)822-5324
UBC, Vancouver,BC  |   Program in Cosmology |     unruh at physics.ubc.ca
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