I agree with the suggestion to have all four physical servers use your internet sources directly, preferably with default minimum and maximum poll intervals so the software can tune the poll intervals to the conditions as they change.

I also agree with creating a single domain name which has A/AAAA records for all four servers (one or the other, either 4 As or 4 AAAAs, as the clients should not treat the same server via IPv4 and IPv6 as distinct servers unless the network paths are actually distinct, which I doubt they are).

Further, I would suggest the four use each other as additional sources, using the same pool directive as the clients, e.g.

pool ntp.example.com iburst

They will refuse to use their own IP as a source and add the remaining three. Once all servers are referencing each other, I would also suggest enabling orphan mode. Just use “tos orphan 10” in ntp.conf to set the orphan stratum (higher than any normal stratum they operate at, low enough to avoid any downstream clients reaching stratum 15). In case your internet connection is lost, the four servers should then agree on one functioning server the remaining will sync to, providing a single freewheeling clock your entire network will use, rather than 4 different freewheeling clocks which would slowly diverge.

Leap seconds cause network turmoil. Meta wants to end them before the next one. //

Meta’s call to action might not be the first, but it could end up being great timing.

Spec revision № 1.0

Time Card is the heart of the Open Time Server Project.
This spec can be accessed using http://www.timingcard.com

Time Master is a critical part of a PTP enabled network. It provides accurate time via GNSS while maintains the accuracy in case of GNSS failure via a high stability (and holdover) oscillator such as an atomic clock. Exisiting products in the market are often closed sourced an far from sufficient features. The Time Card project presents an open source solution via a PCIe card.
Form Factor

• Standard PCIe Stand-up Card
• Single Slot - Passive Cooling Solution

....

Repository content

• Bill of Materials (parts from Digikey)
• Schematic and PCB of the time card
• Driver (Kernel Module) CentOS 8
• CAD files for the custom PCIe bracket

Where can I get one?

You have all necessary source code, BOM, Gerber files and binaries to build it youself. However, we are currently working with several suppliers and will have their contact info soon available to allow you to puchase an out-of-the-box ready Time Card.

• Facebook engineers have built and open-sourced an Open Compute Time Appliance, an important component of the modern timing infrastructure.
• To make this possible, we came up with the Time Card — a PCI Express (PCIe) card that can turn almost any commodity server into a time appliance.
• With the help of the OCP community, we established the Open Compute Time Appliance Project and open-sourced every aspect of the Open Time Server.

In March 2020, we announced that we were in the process of switching over the servers in our data centers (together with our consumer products) to a new timekeeping service based on the Network Time Protocol (NTP). The new service, built in-house and later open-sourced, was more scalable and improved the accuracy of timekeeping in the Facebook infrastructure from 10 milliseconds to 100 microseconds. More accurate time keeping enables more advanced infrastructure management across our data centers, as well as faster performance of distributed databases.

The new NTP-based time architecture uses a Stratum 1 — an important component that is directly linked to an authoritative source of time, such as a global navigation satellite system (GNSS) or a cesium clock.

http://ocptap.com/

http://opentimeserver.com/

https://github.com/opencomputeproject/Time-Appliance-Project

It could mean a "negative" leap second.

Even time did not escape 2020 unscathed.

The 28 fastest days on record (since 1960) all occurred in 2020, with Earth completing its revolutions around its axis milliseconds quicker than average. That's not particularly alarming — the planet's rotation varies slightly all the time, driven by variations in atmospheric pressure, winds, ocean currents and the movement of the core. But it is inconvenient for international timekeepers, who use ultra-accurate atomic clocks to meter out the Coordinated Universal Time (UTC) by which everyone sets their clocks. When astronomical time, set by the time it takes the Earth to make one full rotation, deviates from UTC by more than 0.4 seconds, UTC gets an adjustment.

This is a structured list of my blogposts covering the network time protocol (NTP). It consists of the basic parts building, authenticating, and monitoring NTP servers. If you want to operate NTP in a secure manner, have a look!

Building Stratum 1 NTP Servers

• Why should I run own NTP Servers?
• Packet Capture: Network Time Protocol (NTP)
• NTP Server via DCF77 on a Raspberry Pi
• NTP Server via GPS on a Raspberry Pi
• Updating NTP Servers
• NTP Appliance: Meinberg LANTIME & SyncFire

The table below lists the time servers used by the NIST Internet Time Service (ITS). The table lists each server's name, IP address, and location, organized geographically within the US from North to South and then from East to West. Please note that while we make every effort to ensure that the names of the servers are correct, we control the names of only the nist.gov servers. If you have difficulty using the name of a system, you can access a server using the IP address directly.

We need to sync all the servers across many data centers with sub-millisecond precision. For that we tested chrony, a modern NTP server implementation with interesting features. During testing, we found that chrony is significantly more accurate and scalable than the previously used service, ntpd, which made it an easy decision for us to replace ntpd in our infrastructure. Chrony also forms the foundation of our Facebook public NTP service, available from time.facebook.com. In this post, we will share our work to improve accuracy from 10 milliseconds to 100 microseconds and how we verified these results in our timing laboratory. //

In comparing ntpd with chrony, our measurements indicate that chrony is far more precise, which is why we’ve migrated our infra to chrony and launched a public NTP service. We’ve found that the effort to migrate is worth the immediate improvement in precision from tens of milliseconds to hundreds of microseconds.

Using hardware timestamps can further improve precision by two orders of magnitude. Despite its improvements, NTP has its own limitations, so evaluating PTP can take your precision to the next level.

Update pool.ntp.org address on a Mikrotik Router.Configure a Mikrotik router to use the pool.ntp.org public NTP server pool in its NTP client. So the clock will be set accurately.

If a simple copy-paste wasn’t going to work, a script which updates the IP addresses on a regular basis will.

Step-by-step guide time!

NTP Pool Servers

This is an GPS expansion board designed specifically for the both the version 1,2 and 3 Raspberry Pi+ Models.

This board is designed for applications that use a GPS connected via the serial ports to the Raspberry Pi such as timing applications or general applications that require GPS information. To facilitate PPS the time pulse output is connected to a GPIO so you can utilise this board to give NTP PPS discipline. PPS does not work without an antenna.

This is an article on making a network attached Stratum 1 NTP Server using a Raspberry Pi+ Model 2 or 3 and the Uputronics Raspberry Pi+ GPS Expansion Board. If you need a standalone ready to go solution or need more performance please consider the LeoNTP Networked Time server unit which is available hereThis solution will not give you a standalone off grid NTP server. NTP isn’t designed to work off grid using just the NMEA derived time and PPS to discipline it. You need an external server and then use PPS to discipline it.

NTP Server Online Tester

This tool is useful to check if a given Network Time Protocol server is reachable over the internet using IPv4 and IPv6 connectivity.

Verify that your server and NTP service are up and running.
Verify that the time sources of your server are working well.
Verify that your server UDP 123 port is publicly accessible.

LeoNTP is a Stratum 1 NTP time server with GPS synchronised reference clock source. This NTP server has a very unique custom design with three key features:

• highest performance while serving time with microsecond accuracy at speeds reaching 100% network port bandwidth of 100Mbps
• simple user interface. You can configure it without any user manual
• cost effectiveness. It is priced at a fraction of the cost of comparable performance NTP time servers

Powered by either USB or PoE (802.3af), it can provide accurate synchronised time for your LAN, WAN, CCTV, PLC, telephone systems or wherever accurate standalone time is required. Can be safely used on the open Internet, for example in NTP pool.

We have several publicly available units currently participating in NTP pool project. Its traffic report is available [here].

£300.- GBP

Live traffic of LeoNTP Stratum 1 public NTP servers
Nine identical LeoNTP servers are available for public use via NTP pool project.

All servers are capable of supporting continuous 100Mbps traffic (109,000 requests per second) and serve tens of millions of client computers throughout the world.
Actual traffic is defined by the capacity of internet provider connections and density of ntp pool servers in local zones.

Total number of NTP requests served to date by these servers is 12317.1 billion.
Total traffic is 1108.54 TBytes in each direction.