DSL Tweaking

Alexander Dalloz alexander.dalloz at uni-bielefeld.de
Mon Apr 19 21:23:36 UTC 2004


Am Mo, den 19.04.2004 schrieb Luc Bouchard um 20:07:

> > What you changed in details would then be good to know and how it really
> > affected the upstream.
> 
> I'll post the changes tonight.  We also did a few changes to Apache, I'll try to dig
> those up.

That will be nice.

> > So your ISP uses non common values or technique? To which value did you
> > have to change the MTU? Are you referring to the mssclamping?

> We had to change the MTU to 1452 from 1492 (this is again off the top of my head), the
> problem that we had was some network names could not be resolved and by changing the
> maximum transmission unit we were once again able to resolve all addresses.  This seems
> to be a known issue with Windows XP and in some rare cases Linux (has something to do
> with upstream equipment from our modem).

I hardly doubt you speak of MTU. It is the MSS - so I previously spoke
about MSSCLAMPING - you need to change to 1452 if the PPPoE host acts as
a NAT router. This is specific for PPPoE because of the header size.

You do not need to change any script for that. Either you set that in
the options file for the DSL setup: CLAMPMSS=1452 in
/etc/sysconfig/network-scripts/ifcfg-ppp0. Or you alternatively use
iptables for that.

See for instance my pppd/pppoe running:
root      1743  0.0  0.2  1808  740 ?        S    Apr18   0:00
/usr/sbin/pppd pty /usr/sbin/pppoe -p /var/run/pppoe-adsl.pid.pppoe -I
eth1 -T 80 -U  -m 1452    ipparam ppp0 linkname ppp0 noipdefault noauth
default-asyncmap defaultroute hide-password nodetach mtu 1492 mru 1492
noaccomp noccp [ ... ]
The -m 1452 means the clamping of MSS, you see too the values for MTU
and MRU at 1492.

But to conclude, that is not any DSL "tweaking", in sense of performance
or else improvements. It is just a necessary setting on any PPPoE router
to avoid needed configuration of the MTU on any client behind the
NAT/masq DSL router.

> Luc Bouchard

Alexander


-- 
Alexander Dalloz | Enger, Germany | GPG key 1024D/ED695653 1999-07-13
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