PPPOE, MTU, and boom.

Wed Jul 18 11:38:52 UTC 2001

On Wed, 18 Jul 2001, Alex Rubenstein wrote:

> 
> 
> On Wed, 18 Jul 2001, Alex Rubenstein wrote:
> 
> > It allows you to tinker with the Windows settings of TCP. I adjusted MTU
> > to 1400, wham-o, all the sites I listed started working; including, what I
> > didn't list before, the downloading of transactions via Quicken. Seems to
> > be a fixall.
> 
> I can tell it's getting late, when I reply to myself.
> 
> In the scenerio where you have a router be the PPPOE client (in my test
> case, it's a Cisco 827 running 12.1(3)XG4), things are still somewhat
> broken. The 827 is routing between an ethernet and a PPPOE session, and
> even with the 827 having the MTUs at 1492 or 1400, windoze boxen are still
> stuck up there at 1500 and those sites don't work. I then use Mr. TCP and
> set the ethernet card of the windows box to a MTU of 1400, and voila, it
> works.
> 
> This really sucks for those folks who will have many machines bechind said
> router; it will require a Mr. TCP on each and every one of them.

You do not need to use a third party app. for this.  You can set MTU on 9x
and NT/2k with a registry setting.  

Under 9x : HKLM -> System -> CurrentControlSet -> Services -> Class ->
NetTrans -> 00x (where x is the number of the adapter whose MTU you wish
to change), you create a new String Value called MaxMTU and set it to the
byte size you wish to use.

Under NT/2k : HKLM -> System -> CurrentControlSet -> Services ->
<adapterX> -> Parameters -> TCPIP (where adapterX is (in my case) E100B),
you create a new DWORD called MTU and set it to the MTU you wish to use.

The easy way to find where to create these is just enter regedit and
search on the machines IP address.  If you have multiple hardware profiles
in NT/2k you may end up in ControlSet00x.  Another option is the use DHCP,
which has a setting for MTU if I recall (but you would want to read the
docs on this).  Doing a quick look through the registry on my laptop under
Win2k shows the possibilities are 3, with CurrentControlSet,
ControlSet001, and ControlSet002 all have the current IP parameters in
them.  Trial and error would reveal which section actually needs the MTU
dword (maybe all 3).

I can't believe I know this much about MS's TCP stack, someone please
shoot me.

Jamie Bowden
-- 
"It was half way to Rivendell when the drugs began to take hold"
Hunter S Tolkien "Fear and Loathing in Barad Dur"
Iain Bowen <alaric at alaric.org.uk>