[ntp:questions] Re: IEEE 1588 support in NTP?

David L. Mills mills at udel.edu
Mon Oct 31 02:44:37 UTC 2005


Martin,

I have seen the 1588 document. I hear newsflash from your competitor 
Syummetricom on the same issues. However, affordable PTP interface 
devices will need to be available for various networks (10/100/1000 
Ethernets, FDDI, etc.) for the technology to propser.

While 1588 provides a precision timestamp, it doesn't say what to do 
with them to discipline the system clock, if that is required. The NTP 
clock discipline algorithm might be a good candidate for that. This is 
not a trivial algorithm, along with kernel support, but would be 
necessary if the overhead of frequent PTP exchanges becomes a problem.

An interesting project would be to write a driver for the PTP device 
that passes the timestamps and data on to NTP and modifiy the protocol 
module to use them. This needs to be done carefully, as the I/O cacheing 
on Solaris, for example, seems to be ever increasing.

I suspect your product, like the Symetricom product, includes both 1588 
and NTP. I would assume also that 1588 could serve as a reference clock 
for NTP and possibly the other way around, like for instance to 
synchronize time among 1588 islands.

Dave

Martin Burnicki wrote:
> David,
> 
> Our company Meinberg is offering a NTP time server which also works as a PTP 
> grandmaster clock with a built-in hardware time stamp unit. We've recently 
> participated in the IEEE-1588 meeting in Winterthur near Zurich/Switzerland. 
> Part of that meeting was a plug-fest where PTP capable devices from different 
> manufacturers were interconnected in order to see that they could work 
> together. For more information please see our news article:
> http://www.meinberg.de/english/news/news_051015_001.htm
> 
> David D. Hagood wrote:
> 
>>Has any consideration been given to adding support for IEEE-1588
>>Precision Time Protocol (PTP) into NTP?
>>
>>PTP (http://ieee1588.nist.gov/) is a standard for time synchronization
>>over LAN for instrumentation - it is a part of the LAN extensions for
>>Instrumentation (LXI - http://www.lxistandard.org/), and provides for
>>time sync to tens of nanoseconds (YMMV - routers screw that up). The
>>intended use is in a stack of instruments in a lab, connected to each
>>other and to a PTP source via a local hub.
>>
>>The overall goals and methods of PTP are very much the same as NTP, and
>>I'd hate to have to put both NTPD and PTPD
>>(http://ptpd.sourceforge.net/) on the same machine as they would fight
>>like cats and dogs over who was master - ideally you'd have one daemon
>>doing both, and picking the "best" source.
>>
>>Where I work we are building gear that is Linux based (and so can have
>>NTPD trivially) but I'd like to be fully LXI compliant which means
>>supporting PTP as well (plus some of the stuff we do we need time sync
>>to at worst hundreds of nanoseconds), and being a lazy cuss I'd rather
>>see something like that in the mainstream NTPD rather than having to put
>>it in myself on our own branch.
> 
> 
> Please keep in mind that PTP yields an accuracy in the nanosecond range *only* 
> if special network adapters with PTP support are used on all PTP nodes. 
> 
> PTP support for network adapters means that the adapters include a hardware 
> timestamp unit which takes a time stamp whenever a PTP packet goes on the 
> wire or is coming in from the wire. 
> 
> Without that hardware support PTP doesn't offer significantly more precision 
> than NTP.
> 
> 
> Martin




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