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From: Peter Jeremy <peter.jeremy@alcatel.com.au>
Subject: Re: ntpd configuration and strange time "jumps"
In-reply-to: <200001310018.SAA07833@alecto.physics.uiuc.edu>; from
 igor@physics.uiuc.edu on Mon, Jan 31, 2000 at 11:18:59AM +1100
To: Igor Roshchin <igor@physics.uiuc.edu>
Cc: security@FreeBSD.ORG
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Date: Mon, 31 Jan 2000 15:39:15 +1100
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On 2000-Jan-31 11:18:59 +1100, Igor Roshchin <igor@physics.uiuc.edu> wrote:
>1. I've noticed that there were a few rather strange time steps:
>(it's the first time I see it changing back and force without any 
>visible reason)
>Jan 28 19:58:45 <ntp.notice> myhost xntpd[144]: time reset (step) -0.244614 s
>Jan 28 21:04:09 <ntp.notice> myhost xntpd[144]: time reset (step) 0.353294 s
...

NTP assumes that the path delays between your daemon and it's peers (or
servers) are symmetric - it halves the RTT to determine the peer delay.
This is a flaw in the protocol, but I don't believe there's any way
around it.

If all your peers share a common bottleneck, which has a large traffic
asymmetry, it is possible for NTP to see this asymmetry as a peer offset
and adjust the local time to suit.  When the traffic asymmetry goes away,
xntpd will then skew the time the other way.

Many years ago, I added a fudge to detect (or try to) and ignore this
situation.  At the time Dave Mills wasn't interested in the patches.

Peter


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