[ntp:questions] Leap second bug?

Unruh unruh-spam at physics.ubc.ca
Wed Jan 2 20:30:40 UTC 2008


"Richard B. Gilbert" <rgilbert88 at comcast.net> writes:

...
>A good way to learn what's good and bad is to use a hardware reference 
>clock such as a GPS receiver.  A timing receiver with PPS output will 
>generally keep the leading edge of the PPS within 50 microseconds of the
>"top of the second".  The very best internet servers will agree with the 
>GPS within two or three milliseconds.  A very poorly chosen server might 
>be out by 100 milliseconds or more.  By very poorly chosen, I mean 
>something like someone in New York City configuring ntpd to use a server 
>in Tokyo! A server with a GPS reference will open your eyes to the 
>atrocities committed by typical internet connections!

A good gps receiver should be good to 1usec, not 50. And if you interrupt
drive the computer, the computer i timestamp should also be good to 2-3usec or so. 
That is the kind of jitter I get in getting the gps time from a gps PPS
receiver. 
And yes, even for computers in the same building, the best I can get using
that ntp server as the source is about 50 usec, with .2 msec best delay (
but sometimes the delays are many many times that). The biggest problem
seems to be the computer itself getting the packet out the network card
onto the net. 

Astonishingly a connection across the country 1000 miles away has a round
trip of 40ms, but the time offset is usually about .2msec-- ie the two legs
of the trip are amazingly repeatable. 





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