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Date: Mon, 6 Dec 2004 18:13:35 +0200
From: Giorgos Keramidas <keramida@ceid.upatras.gr>
To: Gerald Heinig <gheinig@syskonnect.de>
Message-ID: <20041206161335.GA91583@orion.daedalusnetworks.priv>
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cc: freebsd-net@freebsd.org
Subject: Re: ping counter overflow
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On 2004-12-06 15:32, Gerald Heinig <gheinig@syskonnect.de> wrote:
> Hi all,
>
> I've run into a possible bug in ping(8) while investigating a strange
> duplicate packet effect in our driver.
> The effect shows up on other drivers and I believe it's due to a counter
> overflow in ping.
> Basically, leave ping -f on a gigabit link for a few days (for medium to
> large values of "a few", ie. at least 3 or 4). Stop the ping and get
> lots of duplicate packets.
> I changed the 5 counter variables (nmissedmax, npackets, nreceived,
> nrepeats, ntransmitted) to short (from long) to reduce the time taken to
> overflow and managed to get the effect very easily - it takes about 3
> seconds on a gigabit link.
> Changing these variables to their unsigned counterparts solved the problem.
> My question: shouldn't these counters be unsigned? (ie. unsigned long,
> instead of long)

They can probably be switched to uint64_t too.  I'm not sure how much
time this will buy us though on links with so very high speeds.

A quick calculation with a link that can push 8 Mpps indicates it will
take a few dozen thousands of years to overflow a 64-bit counter, unless
my Python-foo has failed me:

>>> (2**64) / (365*24*3600*8000000)
73117L