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Date:      Wed, 23 May 2001 19:43:32 +1000 (EST)
From:      Bruce Evans <bde@zeta.org.au>
To:        Greg Lehey <grog@FreeBSD.ORG>
Cc:        arch@FreeBSD.ORG
Subject:   Re: Re: http://uptime.netcraft.com/up/accuracy.html#cycle
Message-ID:  <Pine.BSF.4.21.0105231936220.67212-100000@besplex.bde.org>
In-Reply-To: <20010523083342.E41189@wantadilla.lemis.com>

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On Wed, 23 May 2001, Greg Lehey wrote:

> Comments?
> 
> Greg
> 
> ----- Forwarded message from Richard Wendland <richard@starburst.demon.co.uk> -----
> 
> > Date: Tue, 22 May 2001 21:51:02 +0100 (BST)
> > From: Richard Wendland <richard@starburst.demon.co.uk>
> > To: grog@FreeBSD.org (Greg Lehey)
> > Cc: webmaster@netcraft.com
> > Subject: Re: http://uptime.netcraft.com/up/accuracy.html#cycle
> > X-Mailer: ELM [version 2.5 PL2]
> >
> >> At this link, you claim:
> >>
> >>   Additionally HP-UX, Linux, Solaris and recent releases of FreeBSD
> >>   cycle back to zero after 497 days, exactly as if the machine had
> >>   been rebooted at that precise point. Thus it is not possible to see
> >>   a HP-UX, Linux or Solaris system with an uptime measurement above
> >>   497 days.
> >>
> >> FreeBSD does not suffer from this problem.  You'll notice that you
> >> have a large number of FreeBSD systems with uptimes of over 497 days.
> >> I'd appreciate if you would correct this statement.
> >
> > Hi Greg,
> >
> > I think that statement is accurate.  Note that we're not talking about
> > the FreeBSD 'uptime' command, but our ability to ascertain uptime remotely
> > by decoding the TCP timestamp option.
> >
> > Prior to FreeBSD 3 the TCP timestamp option was incremented every 500ms,
> > as is traditional with BSD.  From FreeBSD 3 it was incremented every
> > 10ms, presumably to improve RTT measurement.  But it does have the
> > consequence that the 32-bit TCP timestamp wraps around at 497.1 days.
> > Hence, with our current method at least, we don't detect uptimes above
> > this for FreeBSD 3 and later.
> >
> > So the FreeBSD systems listed > 497 days are running FreeBSD 2.
> > Once everyone has upgraded from FreeBSD 2, FreeBSD will no longer get
> > in that top uptimes list!

The TCP timestamp is actually incremented every 1/hz seconds, so it
overflows after every 48.5 days on alphas (and on i386's with
"options HZ=1024").

Bruce


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