[ntp:questions] Using different timebase for ntpd

Danny Mayer mayer at ntp.org
Sun Mar 29 16:47:52 UTC 2009


Patrick Loschmidt wrote:
> Hi!
> 
> Thanks a lot for your detailed answer!
> 
> Greg Dowd schrieb:
>> Most if not all commercial ntp appliance manufacturers have some sort of
>> hardware support, either RTOS, custom clock, better oscillator or even
>> modified Ethernet devices to support hardware time stamping.
> 
> Well I'm mostly interested in systems doing hardware time stamping on 
> the Ethernet interface. My intention to just use another timebase are 
> just an intermediate step to ease my development.
> 
>> I tried  modifying one
>> of our SyncServers and gave a presentation at ietf tictoc bof on
>> precision frequency transfer over packet based networks. Dave Mills also
>> did some experimentation with this.  While I used mode 3/4 with a
>> lagging timestamp for the followup function, Dave restricted that
>> operation to symmetric modes. I have another presentation from a sync
>> conference last year comparing the two protocols.  Let me know if you
>> are interested and I'll dig them up.
> 
> Yes, please, I'm very much interested, since I did an extensive search 
> via IEEE Explore, ACM Library, google, etc. and couldn't really find any 
> results or even products explicitly stating that they use hardware support.
> 
>> In your case, you are not talking about hardware timestamping but
>> software timestamping using a hardware clock.  It is a good first step
>> but os/stack jitter is still likely to be dominant.
> 
> Thanks for all the further hints. I have to admit, that I'm working in 
> the IEEE 1588 community for quite a while. Since I'm currently compiling 
> my PhD, I asked myself the basic questions like, why didn't we just 
> enhance NTP with HW timestamping to reach the accuracy?
> So I came up with that idea, searched for implementations or scientific 
> papers giving results, reasons, etc. but had not much luck.
> 
> I found "A brief history of NTP time: Confessions of an Internet Time 
> Keeper" by David Mills, mentioning the Nanokernel and some fancy 
> synchronization results with 50ns RMS but I couldn't dig out the cited 
> reference, since the cite is incorrect ... Also proceedings of the PTTI 
> didn't help much. :-(
> 
> So, what I still would like to have is either documentation of someone 
> who already did NTP with hardware timestamps on Ethernet or can I 
> somewhere find a documentation of the NTP-Daemon to kernel clock 
> interface, in order to replace all the necessary system calls?
> 
> Anyway, I'll dig through the code an all your hints. Thank's a lot for 
> all the helpful answers in this newsgroup.
> 
> Regards,
> Patrick

There is work going on in the TICTOC working group of the IETF to define
standards and protocols to take advantage of of the IEEE 1588-2 standard
but it's early days yet. Dave Mills himself has come up with ideas of
how NTP can take advantage of 1588 to improve NTP results. Don't forget
that 1588 assumes a strict tree and you need 1588 everywhere and I
believe it applies only to Ethernet. Dave Mills himself has some ideas
on implementing this in NTP and has discussed this either here on in the
hackers mailing list. Dave's research documents can be found on his NTP
research web site: http://www.eecis.udel.edu/~mills/ntp.html and you
will find a reference on the left hand side to "NTP Interleaved On-Wire
Protocol" which is what I believe you are looking for.

If you are finding some of the references are wrong, please ask Dave and
he will correct them. In addition if you are interested in getting this
implemented in NTP I am sure that Dave would be more than happy to take
you on to get it implemented for general use. From the look of that
reference he has implemented a simulator (lev.c) for testing scenarios
as well as in the NTP reference implementation but I don't think that he
has made that available yet.

Dave will say more and is very approachable in these matters.

Danny

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