[ntp:questions] Re: NTP4 has 3 different time formats! Namly (32, 64, 128) bits wide
Richard B. Gilbert
rgilbert88 at comcast.net
Sun Jul 16 00:40:18 UTC 2006
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)
>>>>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
> 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.
I'm sure that it's possible to integrate a refclock and a computer in
such a way the that time down to the picosecond level is gated directly
onto the computers data bus and so that computer will know what the nano
and pico seconds are. It still will not be able to convey that
information to another computer without loss of precision. The more
distant the two computers, the more difficult the problem becomes.
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