Bob Snyder wrote in a message to Mike Bilow:
> I'm also not clear how NTP would cope in the situation where the MAC layer
> could delay the packet's transmission by many seconds.
BS> I know NTP is designed to cope with fair amounts of network
BS> delay; it's built into the protocol. Of course, I don't
BS> remember how, as I haven't looked at the protocol details in
BS> many years, but it's in there. Dunno if it would be robust
BS> enough to handle the huge delays you can see on a 1200 baud
BS> packet radio connection. (Look! I almost made the thread
BS> relavent to the group! :-) )
The absolute magnitude of the delay is not really relevant, and NTP can cope
with that. Even non-reciprocal routing can be handled to a large extent
because of the use of timestamps on each transaction. The real problem is that
NTP cannot acceptably cope with wide variation in round-trip time, as this
introduces inherent uncertainty that cannot be eliminated statistically.
BS> NTP is designed to keep your time within milliseconds of
BS> it's source, and can handle silly things like leap seconds,
BS> where a minute is declared to have 61 seconds or 59 seconds
BS> to deal with minute changes in the Earth's orbit. It can
BS> also deal with the loss of a clock and a "false ticker," a
BS> clock that reporting incorrect data, given enough alternate
BS> inputs.
BS> I use xntpd on my home system, with it treating it's own
BS> clock as a stratum 7 time device, and when my Internet
BS> connection's up, using two stratum 2 time sources on the
BS> net. I like it a lot, although I realize it's more than some
BS> people want.
On the "real" Internet, where round-trip times are on the order of 10-20 ms, we
often see offsets within 1 ms to distant peers. However, for a machine which
is connected through a dial-up modem, the round-trip time to anywhere is going
to be an order of magnitude worse, 100-200 ms at best, and will be subject to
random variation due to error-correction reblocking in the modem. You would be
fairly lucky to be able to keep within 10 ms accuracy over a modem link without
some sort of external discipline.
-- Mike
