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Date:      Mon, 27 Jul 2009 14:08:22 +0100
From:      Mark Sandford <>
To:        <>
Subject:   Using dummynet to restrict bandwidth with more than 2 active pipes / queues
Message-ID:  <COL110-W10D341161C0B631FE5239892140@phx.gbl>

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Hi all=2C


I've been using dummynet for a while to perform degraded network testing wh=
ich has been really useful.


Recently=2C we wanted to measure the performance limits of it on our
hardware. To do this we setup a machine with 8 interfaces paired into 4
ethernet bridges.


We are having throughput issues when more than 2 pipes are being used
simultaneously. These issues appear to be independent of the bandwidths

For example:
We set two traffic generators transmitting at 30Mbps across two of the brid=
ges (pipes)=2C sending 1000 byte UDP packets (1042 bytes on the wire) for a=
 20 second period.

These are passed through dummynet pipes set up to restrict the bandwidth to=
 20Mbps at the bridge and we can see from the ipfw counters that all the pa=
ckets hit the right rules and only the right rules.

We the capture on the far end and can see that bandwidth has been restricte=
d to 20Mbps as specified.  All good!  :o)

The problem comes when we add any extra flows.

The above example is repeated but with two extra traffic generators transmi=
tting at just one packet per second each across a further two pipes.

Again we can see from the counters that the packets all arrive at ipfw=2C h=
owever we only get 10Mbps at the receiving end (and we get a number of pack=
et_drops logged at dummynet).

We feel we must have missed something obvious but after over a week of read=
ing / testing we're running out of ideas.

Is anyone able / willing to help?

Mark Sandford

mob: 07990 565976


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