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NIST was 5 μs off UTC after last week's power cut

jeffgeerling.com

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Re: NIST was 5 μs off UTC after last week's power cut

#21

Post by zamadatix »

V__ wrote: post #10

Has anyone here ever needed microsecond precision? Would love to hear about it.

(Assuming "precision" really meant "accuracy") The network equipment I work on requires sub microsecond time sync on the network for 5G providers and financial trading customers. Ideally they'd just get it from GPS direct, but that can be difficult to do for a rack full of servers. Most of the other PTP use cases I work with seem to be fine with multiples of microseconds, e.g. Audio/Video over the network or factory floor things like PLCs tend to be find with a few us over the network.

Perhaps a bit more boring than one might assume :).

Re: NIST was 5 μs off UTC after last week's power cut

#23

Post by ticoombs »

eddyg wrote: post #19
evanriley wrote: post #5

Earlier quoted context omitted.

> Gah, just when you think you can trust time.nist.gov You still can... If you're that considered about 5 microseconds: Build your own Stratum 1 time server https://github.com/geerlingguy/time-pi or just use ntppool https://www.ntppool.org/en/

Be aware that there are members of the NTP pool with less-than-honorable intentions and you don't get to pick-and-choose. Yes, they all should provide the time, but they also get your IP address. For example: unlike the IPv4 space, the IPv6 space is too big too scan, so a number of "researchers" (if you want to call them that) put v6-capable NTP servers in the NTP pool to gather information about active v6 blocks to…

Do you have any acticles or references about this? That would be great research (pun intended) to find out

Re: NIST was 5 μs off UTC after last week's power cut

#24

Post by edoceo »

eddyg wrote: post #19
evanriley wrote: post #5

Earlier quoted context omitted.

> Gah, just when you think you can trust time.nist.gov You still can... If you're that considered about 5 microseconds: Build your own Stratum 1 time server https://github.com/geerlingguy/time-pi or just use ntppool https://www.ntppool.org/en/

Be aware that there are members of the NTP pool with less-than-honorable intentions and you don't get to pick-and-choose. Yes, they all should provide the time, but they also get your IP address. For example: unlike the IPv4 space, the IPv6 space is too big too scan, so a number of "researchers" (if you want to call them that) put v6-capable NTP servers in the NTP pool to gather information about active v6 blocks to…

Is this one of those extraordinary claims that requires evidence? Or is it generally true that there are homey-pots in many of these services (NTP, mirrors, etc)

Re: NIST was 5 μs off UTC after last week's power cut

#26

Post by wpollock »

sgillen wrote: post #20
V__ wrote: post #10

Has anyone here ever needed microsecond precision? Would love to hear about it.

We don't use NTP, but for robotics, stereo camera synchronization we often want the two frames to be within ~10us of eachother. For sensor fusion we then also need a lidar on PTP time to be translated to the same clock domain as cameras, for which we also need We actually disable NTP entirely (run it once per day or at boot) to avoid clocks jumping while recording data.

> We actually disable NTP entirely (run it once per day or at boot) to avoid clocks jumping while recording data.

This doesn't seem right to me. NTP with default settings should be monotonic. So no jumps. If you disable it Linux enters 11-minute mode, IIRC, and that may not be monotonic.

Re: NIST was 5 μs off UTC after last week's power cut

#28

Post by zahlman »

opello wrote: post #15
politelemon wrote: post #8

I'm missing the nuance or perhaps the difference between the first scenario where sending inaccurate time was worse than sending no time, versus the present where they are sending inaccurate time. Sorry if it's obvious.

The 5us inaccuracy is basically irrelevant to NTP users, from the second update to the Internet Time Service mailing list[1]: To put a deviation of a few microseconds in context, the NIST time scale usually performs about five thousand times better than this at the nanosecond scale by composing a special statistical average of many clocks. Such precision is important for scientific applications, telecommunications, c…

> Such precision is important for scientific applications, telecommunications, critical infrastructure, and integrity monitoring of positioning systems. But this precision is not achievable with time transfer over the public Internet

How do those other applications obtain the precise value they need without encountering the Internet issue?

Re: NIST was 5 μs off UTC after last week's power cut

#29

Post by LeoPanthera »

zahlman wrote: post #28
opello wrote: post #15

Earlier quoted context omitted.

The 5us inaccuracy is basically irrelevant to NTP users, from the second update to the Internet Time Service mailing list[1]: To put a deviation of a few microseconds in context, the NIST time scale usually performs about five thousand times better than this at the nanosecond scale by composing a special statistical average of many clocks. Such precision is important for scientific applications, telecommunications, c…

> Such precision is important for scientific applications, telecommunications, critical infrastructure, and integrity monitoring of positioning systems. But this precision is not achievable with time transfer over the public Internet How do those other applications obtain the precise value they need without encountering the Internet issue?

If you must use the internet, PTP gets closer.

Alternate sources include the GPS signal, and the WWVB radio signal, which has a 60kHz carrier wave accurate to less than 1 part in 10^12.

Re: NIST was 5 μs off UTC after last week's power cut

#30

Post by vel0city »

qmr wrote: post #2

Gah, just when you think you can trust time.nist.gov Suggestions from the community for more reliable alternatives?

Use the other servers as well: https://tf.nist.gov/tf-cgi/servers.cgi

For instance, time-a-wwv.nist.gov.

One should configure a number of different NTP sources instead of just a single host.

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