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Date: Thu, 4 May 2006 17:05:38 +0400
From: Solar Designer <>
Subject: Re: TCP-Tuning

On Thu, May 04, 2006 at 01:07:18PM +0200, Bernhard Fischer wrote:
> - transfering 10MB of data within 10 seconds
> so far, so good!
> 2. scenario: gateway
> - two lan-interfaces (WAN: 10Mbit, LAN: 100Mbit)
> - transfering 10MB of data through the gateway within 122 seconds ?!?

What you describe sounds like a problem with path MTU discovery, perhaps
related to your ADSL modem (is one involved?) or your ISP.

The documentation for rp-pppoe suggests setting CLAMPMSS=1412 in
pppoe.conf - so just do that if you're using this package.  Otherwise,
you can try adding:

[0:0] -A FORWARD -p tcp --syn -j TCPMSS --set-mss 1412

to /etc/sysconfig/iptables on the gateway.  Alternatively, you can try
reducing the interface MTU on all hosts in your LAN to 1452.

This is kind of "TCP anti-tuning" - these settings are not optimal - but
they should work around the problem - if it is in fact MTU-related.

> How do i have to configure both lan-interfaces for best throughput?

That's not what you need.  What you have is not just suboptimal
throughput.  Things just aren't working right.

> When i look at the traffic-statistiks, there is a gap of 200ms silence
> followed by 300ms transmission. Why?

I've provided a guess above.  If it doesn't help, you'll need to post
excerpts of tcpdump output which demonstrate the periods of silence.

Hope this helps and does not annoy other owl-users too much - although
it's not specific to Owl...

Alexander Peslyak <solar at>
GPG key ID: B35D3598  fp: 6429 0D7E F130 C13E C929  6447 73C3 A290 B35D 3598 - bringing security into open computing environments

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