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Is my MTU / MSS fixed?

DrSeeAll
Grafter
Posts: 51
Registered: 30-07-2007

Is my MTU / MSS fixed?

I've been trying various tweak tests and want to set MTU to 1500. I've set this using TCPOptimiser (for some reason DrTCP will not run on my PC) and also confirmed the changes in the registry. I've also rebooted.

However, when I do an online tweak test I still get the MTU reported as 1372 and MSS as 1332.

Are these values fixed by my hardware - Actiontec ADSL/Ethernet/USB modem and a Dell TrueMobile 1184 Wireless Router? I cannot find anywhere to change these setting in either of the h/w setup menus and cannot find info on their websites.

I've got two PCs using WinXP on a wireless LAN. I've only made changes in one. Do I have to set MTU to 1500 in both for it to work?

TIA
5 REPLIES
N/A

Is my MTU / MSS fixed?

Firstly you need DrTCP021 to work with XP.
If you unzip and open the file, you'll see two
boxes, one is 'Dial up RAS MTU' and the other
'Adapter settings'.( the drop down list should list
all your network cards including any wireless)
If the values against all these boxes are blank
then this indicates the windows default of MTU 1500.
To make sure then save a blank value against
all these boxes or (or put in the value 1500) and re-
boot each pc.
If your still getting the value of 1372 on DSLreports
then it's as you say the router maybe setting MTU
and that will need further investigation, there may be
a firmware fix or a command line solution.
Alec
DrSeeAll
Grafter
Posts: 51
Registered: 30-07-2007

Is my MTU / MSS fixed?

Hi Alec
I've set MTU tp 1500 on all adapters on both PCs, rebooted both and rebooted modem and router. On-line tweak tests still report MTU as 1372.
So it must be the hardware (Actiontec ADSL/Ethernet/USB modem and/or a Dell TrueMobile 1184 Wireless Router) that is setting this value.
How do I go about changing this? Anybody know if it is possible?
Thanks
DrSeeAll
DrSeeAll
Grafter
Posts: 51
Registered: 30-07-2007

Actiontec's respons

I emailed Actiontec's help desk about changing the MTU value in the modem.
This was their reply:-

============

You would have to check the settings in the advanced setup of the router.
There is no way that I know of to adjust the MTU value in the modem at this
time.

============
So it seems that there is no way to change this setting!
In that case why is it set at 1372 and not 1500 or even 1472?
Anyone.....?
Cheers
N/A

Is my MTU / MSS fixed?

I couldn't find any information at all on this modem.
1) Some modem / routers can't cope with the
higher throughput or work more efficiently at
lower MTU values.
2) Some VPN software / set ups require lower
MTU values to work properly.
3) There could be a fault with the firmware.
Since you can't change it, I would suggest
setting all your pc's, network cards, wireless network cards etc, to operate at the same value
and see if you notice any improvement in performance.
Alec
DrSeeAll
Grafter
Posts: 51
Registered: 30-07-2007

Is my MTU / MSS fixed?

Hi Alec
I've now set everything to MTU 1372. I've not noticed any appreciable difference.
Speed may perhaps be marginally better.
Thanks for your time.
Cheers