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

Richard B. Gilbert rgilbert88 at comcast.net
Fri Oct 28 13:01:22 UTC 2005

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.
> If possible, please reply to both my work email (David.Hagood at 
> aeroflex com) as well to the list (yes, I hate it too when somebody 
> asks that) - something between the list and my desk seems to be eating 
> the list mails.
> David Hagood
> Principal Software Engineer
> Aeroflex Wichita
> David (period) Hagood at Aeroflex (period) com
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NTP does not, typically, synchronize multiple systems to "hundreds of 
nanoseconds".   Hundreds of microseconds might be doable on a fast LAN.

Linux has been known to have timekeeping problems due to losing clock 
interrupts.   Each lost interrupt can introduce an error of  one or ten 
milliseconds.  If the kernel parameter HZ is set to 1000, the clock 
ticks 1000 time a second and the system is likeliest to lose 
interrupts.  It works much better if HZ is set to 100.

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