On 2021-06-25, Jim Pennino <ji...@gonzo.specsol.net
> William Unruh <un...@invalid.ca
>> On 2021-06-25, Jim Pennino <ji...@gonzo.specsol.net
><snip old stuff>
>>> Or for $14/machine I could use a USB GPS, my machine with PPS GPS, and A
>>> public server that does NOT request use of DNS which yields 3 sources of
>>> time without using a pool or DNS lookups.
>> Not at all sure what you are suggesting. DNS is a way of translating
>> names to IP addresses, which your machine MUST use to talk to a remote
>> machine not on your network. The remote machine has nothing to do with
>> this. Now some remote machines will as for the name associated with the
>> IP address of machines sending the remote machine a query, to try to see
>> if someone is spoofing the IP address, but as far as I know ntpd does
>> not do that. Takes too much time and would make the time responses
>> really bad.
> This is not quite correct.
> If a program has an IP address, as in put the IP address in ntp.conf,
> then the program already has the IP address and does NOT need to do a
> DNS query ever.
Apparently I was not clear. Some systems are set up to do an inverse
DNS, to see if the source of the packet really is the machine it claims
to be, or is from a site that they want to allow in.
> Using a IP address for ntp pools is a bad idea as someone else has said.
ntp pool is a "DNS" that shoves out addresses-- semi randomized from the
pool list. If you have the address, then you can use that. Now, that
puts a burden on the person who is supplying that pool server, in that
your requests always hit that particular server. Does this matter for
one person? No. Does it matter if everyone starts doing it-- yes.
Also, people can drop their servers out of, and into the pool without
warning, and thus hardcoding the IP can leave you high and dry.
However, If DNS is a problem for someone, using hard coded IP is a way
out of the problem.
> However, there are lists of publicly available ntp servers which list
> the owners preference for DNS usage. Some servers want you to use the
> fully qualified domain name and some servers don't care if you use the
> IP address.
>>> Or for $28/machine I could use 2 USB GPS receivers and my machine with
>>> PPS GPS, which also provides 3 sources of time without any network
>>>e access at all.
>> Sure. The problem of course is that that $28 onlybuys you a pretty bad
>> time source (pretty bad meaning milliseconds rather than microseconds or
>> nanoseconds), which for most of man's history on this earth is
>> absolutely astonishingly, and inconceivably good.
> Except you are forgetting a few things:
> 1. I have a ntp server with a real PPS GPS attached which is good to
Fine, except that still leaves you reliant on one machine.
> 2. My actual real time requirement is in the 10s of millisecond range.
Which is why I phrased it as I did.
> 3. Any accuracy past the requirements of number 2 is purely out of
>> Note that hanging all three off of one machine can lead to conflict
>> between them as to interrupt processing, leading to degraded time
>> performance. But again that is at the microsecond level, not milli or
>> second level.
> As each will go into a separate plug, that is HIGHLY unlikely to happen.
The plugs are not the problem. Those plugs, I assume, are all attached
to the same machine, with the same CPU(s). Interrupt conflicts are thus
possible, not matter how many plugs there are.
Note again, that interrupt conflict is a problem at microsecond
accuracy. At millisecond, it is not (unless someone coded ntpd really
really badly). So this is for you curiosity, not for your need.
> I never said anything about hanging three receivers on one machine,
> as two receivers are more than sufficient for normal, i.e. WWIII isn't
> happening, times.
Two receivers can also produce interrupt conflicts. Again, microsecond,
>> Of course if you machine is at the bottom of a mineshaft in mountains,
>> gps receivers are pretty useless. Or in the basement of a highrise
>> without windows.
> At one place I worked at where the computer room was in the basement and
> they did care about accurate time, they bought a commercial ntp server
> black box that cost several thousands of dollars and ran a cable to the
> roof for the antenna.
No idea why they would need a thousands of dollar box. On the other
hand, if the roof is 5 stories up (50m up), that a microsecond delay in
the antenna signal getting to the box.
There was a huge kerfuffle a few years ago, caused by signal delay in
fibre optic cable from outside to a received in a mine. There was a
neutrino detector in the mine, and the delay in the time signal meant
that the time at the neutrino detector was delayed from that of the
source at a particle accelerator, meaning that the apparent time it took
from source to detector was smaller than it should be. They published a
paper claiming neutrinos travelled faster than light. Although they
thought they had compensated, it turned out there was a bad fibre optics
connection which introduced an extra delay.
This was however at the nano second level, which of course you are not
interested in. But an uncompensated 50m is at the microsecond level,
which they (or your curiosity) might be interested in.