Let's try this again, having posted earlier from an address that isn't
subscribed. For those wondering why I stick with Turnpike despite the
occasional PGP crash, that's why. The excellent mailing list handling
that wouldn't have let me make that mistake...
On Mon, 6 Feb 2006, Nelson Minar <[EMAIL PROTECTED]> wrote
It's surprising ntp doesn't take into account network delays in some
way...
NTP does try to measure and account for network delay; otherwise our
clocks would be 70ms or more off! But my understanding is it assumes
that network delay is symmetric: ie delay from A to B is the same as
delay from B to A. That hasn't been true on the Internet for 10+ years,
but it's really not true if your NTP server is on the end of an ADSL
link. Particularly one where the slow upstream pipe gets full.
What I don't know is how much error we get from the reality of
asymmetric network routing. Has there been anything written about that?
I was a little surprised ADSL servers are in the pool at all; I'd
naively assumed their time would be bad enough to not be worth it. But
better to have some uncertainty than no server at all.
ash.oak-wood.co.uk is on an ADSL link. A graph of its perceived offsets
to
various servers can be seen at http://ntp.raggedstaff.net/offsets2.php.
Its primary sync source is an MSF receiver, but that has only ever been
calibrated
by comparison to nearby NTP servers
rupert.raggedstaff.net is in a well connected data centre in London. A
graph of
its perceived offsets to various servers is available at
http://rupert.raggedstaff.net/status/offsets.php, including its
perceived
offset to ash.oak-wood.co.uk
Offset of ash measured at rupert is rarely more than 2ms out from the
bulk of
other sync sources, usually rather less. Given that a couple of these
sources
are stratum 1 and less than 2ms delay away this gives you some idea of
just how
bad ADSL is for NTP. That is, surprisingly, not very bad at all. Or at
least
not in this particular case, it seems.
--
Chris Hastie
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