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Date:      Tue, 22 Nov 2011 22:58:14 +0100
From:      Kees Jan Koster <>
To:        Gary Gatten <>
Cc:        "''" <>
Subject:   Re: Diagnosing packet loss
Message-ID:  <>
In-Reply-To: <>
References:  <>

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Dear Gary,

Thank you for your reply. Your comment about dupe IP triggered something =
that I failed to mention: the interface is aliased. It has two IP =
addresses. IP address a and it has an alias IP address b. I just tested =
binding mtr to each of these interfaces separately to measure packet =

If I use mtr to measure packet loss from saffron (the stricken machine) =
to cumin (another machine in a different data center) I see the =

  saffron (ip address a) -> cumin: packet loss
  saffron (ip address b) -> cumin: no packet loss

  cumin -> saffron (ip address a): packet loss
  cumin -> saffron (ip address b): no packet loss

This is consistent from running mtr for 5 minutes straight. This to me =
shows that the hardware is fine. Using the alias IP address I can run =
with no packet loss for as long as I like.

Hum.... Could it be that my switch does not support IP aliasing? Then =
why is there packet loss only on one IP and not on both?

This is getting weirder and weirder.

Kees Jan

On 22 Nov 2011, at 22:15, Gary Gatten wrote:

> Well, 1% is not good but I've seen worse for sure!  Sounds like you =
tried the obvious.  I would recommend a different IP to rule out a dupe =
ip; else it must be NIC related - either hardware or driver.  Also, =
perhaps swap cables and ports with a working machine and see if the =
problem follows or stays put.
> ----- Original Message -----
> From: Kees Jan Koster []
> Sent: Tuesday, November 22, 2011 02:33 PM
> To: <>
> Subject: Diagnosing packet loss
> Dear All,
> I am stuck with a machine that shows serious packet loss (about 1% of =
all traffic is dropped). I tried the obvious (new network cable, =
different switch port, different ethernet interface on the machine), but =
the problems remain.
> Another machine that sits in the same rack and is hooked up to the =
same switch shows no such packet loss issues. The problematic machine is =
a dual Opteron with FreeBSD 8.2-STABLE from Thu Aug 11 14:05:47 CEST =
> The machine is lightly loaded. A MySQL slave is running, but the =
machine is not serving queries. Plus a Munin server process.
> I am at a loss where to start diagnosing this. Can you advise me where =
to look? Are there network buffers that may be overflowing?
> --
> Kees Jan
> +31651838192
> Change is good. Granted, it is good in retrospect, but change is good.
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Kees Jan

I hate unit tests; I much prefer the illusion that there are no errors =
in my code.
                                                                 -- =
Hendrik Muller

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