3 ms·

I can't imagine how you could hope to reach nanosecond-precision over the internet without changing all the routing hardware in use today. With PTP you can alre

by simias 8y ago

I can't imagine how you could hope to reach nanosecond-precision over the internet without changing all the routing hardware in use today. With PTP you can already reach sub-microsecond synchronization but you need full hardware support for every network element on the route if you want to achieve that. The timestamping is done on the network PHY itself, which is how you can remove all jitter introduced by the kernel stack. Anything receiving and re-transmitting the packet along the way must update the timestamps to account for the processing delay. Huygens is a little more clever and uses a statistical approach to sample multiple clocks and correlate them (if I understand correctly) however it still seems designed to work within a datacenter, I assume that over the internet the signal-to-noise ratio for measurements would worsen very significantly and lower the precision dramatically. It could well still outperform NTP however.

deleted 8y ago
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makomk 8y ago
The signal-to-noise ratio would be a nuisance, naturally, but there's a more fundamental problem that's stopped anyone from trying this. Internet routing and latency is asymmetric - your request to the time server will, in general, go via a different route that takes a different amount of time compared to the message back. This introduces a bias that is indistinguishable from local clock error. Without precisely synchronised clocks at both ends, you can only determine the overall round-trip time and not the time each leg took, and therefore you cannot calculate the exact difference between your local clock and the server. This is an inhererent limit on how accurate NTP can be over the Internet. (The other reason not to bother is that GPS is cheap and can provide a very accurate synchronized clock source over long distances.)
gsich 8y ago
You usually have full hardware support, at least if you are using intel nics.

by simias 8y ago

deleted 8y ago

[deleted]

makomk 8y ago

The signal-to-noise ratio would be a nuisance, naturally, but there's a more fundamental problem that's stopped anyone from trying this. Internet routing and latency is asymmetric - your request to the time server will, in general, go via a different route that takes a different amount of time compared to the message back. This introduces a bias that is indistinguishable from local clock error. Without precisely synchronised clocks at both ends, you can only determine the overall round-trip time and not the time each leg took, and therefore you cannot calculate the exact difference between your local clock and the server. This is an inhererent limit on how accurate NTP can be over the Internet. (The other reason not to bother is that GPS is cheap and can provide a very accurate synchronized clock source over long distances.)

gsich 8y ago

You usually have full hardware support, at least if you are using intel nics.