[ntp:hackers] NIC knacks
David L. Mills
mills at udel.edu
Sun Feb 10 21:08:13 UTC 2008
Guys,
Here is a snip from a memo I am writing. I read it again and am puzzled
about the apparent conclusion that the incidental delay of each
transceiver - transmit and receive - is 50 us. That sound really high
and implies a huge number of machine cycles. Is there a driver/NIC
expert onboard that has insight?
As an example of current practice, NTP servers macabre and mort are
identical Intel Pentium clones running FreeBSD and synchronized to a GPS
receiver. The servers share a lightly loaded 100-Mb switch; the GPS
receiver is connected by fiber repeaters and another 100-Mb switch. Each
server shows offsets of about 25 us relative to the other and the GPS
receiver. Each server shows roundtrip delays of about 140 us with the
other. Note that only 40 us (four LAN hops) can be accounted for on the
wire, with the remaining 100 us split between the two transceivers.
Dave
More information about the hackers
mailing list