[bux93]

I listened to a fascinating podcast from Jane Street on their solution. Pasted the relevant part

https://signalsandthreads.com/clock-synchronization/ --quote "So, we’re trying to build a proof of concept. At the end of the day, we sort of figured, “All right. We have these GPS appliances.” We talked about hardware timestamping before on the GPS appliances, and how they can’t hardware timestamp the NTP packets, so that’s problematic. We thought, “How can we move time from the GPS appliances off into the rest of the network?” And so we decided that we could use PTP to move time from the GPS appliances to a set of Linux machines, and then on those Linux machines we could leverage things, like hardware timestamping, and the NTP interleaved mode to move the time from those machines onto machines further downstream.

The NTP interleaved mode, just to give a short overview of what that means… when you send a packet if you get it hardware timestamps on transmission the way you use that hardware timestamp is you get it kind of looped back to you as an application. So I transmit a packet, I get that hardware timestamp after the packet’s already gone out the door. That’s not super useful from an NTP point of view, because really you wanted the other side to receive that hardware timestamp, and so the interleaved mode is sort of a special way in which you can run NTP and that when you transmit your next NTP packet you send that hardware timestamp that you got for the previous transmission, and then the other side, each side can use those. I don’t want to get into too much of the details of how that works, but it allows you to get more accuracy and to leverage those hardware timestamps on transmission." -- end quote

[mgaunard]

In the ultra-low-latency space people just use PPS, either one pulse per second or 10Mhz.

WhiteRabbit and the like allow you to transmit such a signal with great accuracy.

There are timestamping layer 1 switches and NICs that support taking it as input.

[luma]

Same boat, major exchange backend and we just use 10MHz over 50ohm coax straight into the NIC which does the timestamping.

[willis936]

It's been folded into IEEE 1588-2019, but no one supports it.