NTp sources that work in a datacenter (was Re: Is latency equivalent to RTT?)

Valdis.Kletnieks at vt.edu Valdis.Kletnieks at vt.edu
Wed May 14 18:26:29 UTC 2003

On Wed, 14 May 2003 13:38:32 EDT, "N. Richard Solis" said:
> IIRC, NTP actually sends a packet with a local timestamp, sends it to a
> remote location for a remote timestamp, and then checks the local
> timestamp when the packet is received to calculate RTT.

Chicken-and-egg problem - even WITH the timestamps, you can't calculate the
actual offset of clocks without making some assumptions regarding RTT.  The
assumption NTP makes is that the remote timestamp happened exactly halfway
between your own send and receive timestamps (i.e. that the RTT is symmetric)

I've personally seen our 2 stratum-2 servers diverge *wildly* - through sheer
dumb luck and routing updates, ntp-1 had 6 peers all of which were routed
via our NetworkVirginia link, and ntp-2 had 6 peers all of which went via
our Abeline link.  So of *course* one night I get a phone call about the
fact that the two were about 7.5 entire seconds  out of sync.  Run 'ntpq -p'
on each of them, and see that each is reporting millisecond offsets, but
ntp-2 reported huge delay values.

Seems that this particular night, we were seeing high packet drops and a
consistent 15-second (yes, second) delay *outbound* through some ATM fabric.
I never did get a better explanation than "poltergeists"...

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