[ntp:questions] Re: NTP4 has 3 different time formats! Namly (32, 64, 128) bits wide

David L. Mills mills at udel.edu
Mon Jul 17 04:05:41 UTC 2006


I can't claim preconition, as the current NTP timestamp format was 
invented in 1978 when nominal accuracies were in the 16-ms range. 
However, the resolution limit of 232 picoseconds is likely to be 
exceeded when the CPU clock rate approaches 4 GHz, which might not be 
long off.


Danny Mayer wrote:

> Richard B. Gilbert wrote:
>>Danny Mayer wrote:
>>>Richard B. Gilbert wrote:
>>>>NTP could do worse than to adopt the VMS 64 bit time format.  IIRC it
>>>>was a count of 100 nanosecond "ticks" since some date in (I think)
>>>>November 1857.
>>>>The current 64 bit NTP timestamp wastes some bits in picosecond
>>>>precision.  I say "wastes" because even today's computers cannot
>>>>exchange time without an uncertainty of two or three microseconds and
>>>>those low order bits are meaningless noise.
>>>That's for today. In 10 years that may very well not be true.
>>The limitation, I believe, is in the round trip delay.  My Sun Ultra
>>10s, separated by a Cisco 1548M switch and a few feet of cable, show
>>delays of the order of 4 microseconds at 100mb full duplex.  Gigabit
>>Ethernet might shave a little off of that but not a whole lot.
>>If you are getting time over the internet you don't, and won't, have a
>>clue what the nanoseconds should be.  Even over a fast LAN, the delay is
>>a killer.
> On a network, that's true enough and is likely to be for a while,
> despite Internet 2 and other advances. However, we also have refclocks
> that are not limited in that way and I don't necessarily mean just GPS.
> There are other technologies coming along which have the potential for
> much improved accuracy. The current limitations there will be likely the
> path between the peripherals and the drivers, the O/S and NTP itself.
> Danny
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