How To Find And Set Optimum Windows Values For MTU & TCP

[alister]

Windows® XP Version: Pro and Home

Windows Vista auto tunes your RWIN, there is nothing to do

Updated to include ADSL Max - September 12th, 2006

Title: How To Find And Set Optimum Windows Values For MTU & TCP Receive Window In DRTCP.

Description:

This article will show you how to find "your" own optimum values for MTU & TCP Receive Window which are the 2 most important values for your broadband too work at the fastest "stable" "downstream" & "upstream" speeds. These 2 values will then be put into DRTCP, which simply enables you too set these 2 values without having to do a Registry edit.


You need to use the Windows Ping Facility, this is run from the command prompt. We are first going to find out the MTU, the MTU setting controls the maximum ethernet packet size your PC will send. Unless otherwise set, windows defaults MTU to 1500, 1500 is ok unless you are running PPPoE, but it may not be the optimum. The best value for MTU is that value just before your packets get fragmented.

Finding the largest MTU, by EXPERIMENT:

Go to Start/ Programs/Accessories/Command Prompt, (or START/RUN and enter "cmd"/Return) and type....

• ping -f -l 1472 www.dslreports.com

(That is a dash-ell not a dash-one. Also note the spaces in-between the sections)
Press Enter. Then reduce 1472 by 10 until you no longer get the "packet needs to be fragmented" error message.
Then increase by 1 until you are 1 less from getting "packet need to be fragmented" message again.

Add 28 more to this (since you specified ping packet size, not including IP/ICMP header of 28 bytes) and this is your MaxMTU.

NOTE: If you can ping through with the number at 1472, you are done! Stop right there. Add 28 and your MaxMTU is 1500.

NOTE: There is an issue with DRTCP & Windows XP SP2 RWIN (TCP Receive Window) may not be changeable with SP2.
A “fix” is being prepared, however it does work for me.

Download DRTCP from here:

broadband help » DrTCP by Broadbandreports.com

Once you have downloaded DRTCP unzip too your desktop, it will look like this:



We are only going too change TCP Receive Window and Dial Up (RAS) MTU, ALL the other settings should be set at the "Default" values as shown in the copy of DRTCP above.

The RWIN should be a multiple of the MTU value, the following equation is the simplest way to find RWIN:

RWIN = n x ( MTU-40)

If you are using ADSL to connect to the internet, the value of “n” (ie the multiple) found by many people (including myself) to work best with an MTU is “9”.

*Update*
(For those of you connecting to the internet via ADSL Max, I have found that a higher value for “n” gives the best results, I use a value of “55”. When you put this higher TCP Receive Window value in, along with the MTU value, and click "Save", the "Window Scaling" box will become "Yes" and the "Time Stamping" box will become "No", the other boxes remain set to "Default")

Therefore in my case:

RWIN = 9 x (1500 – 40),
Which gives a RWIN value of 13140.

If you have a USB or PCI ADSL Modem that uses a dial-up adapter similar to a normal 56K dial-up then you only need to enter the new MTU value in the Dial Up (RAS) MTU box, as I have done.

However, if you have a Router, you only need to put the new MTU value in the box next to the Ethernet adapter as that is the one that needs it with your router.

Once you put your RWIN and MTU values in you click “Save”, then “OK”. You then have too reboot for the new values to be set in the registry.

Should you wish to re-set your original values, simply take a note of them before you put the new values in, then it is simply a case of removing the new values and inserting your original settings, click “Save”, “OK” and reboot.

Another Registry alternation that I have found that helps, especially in keeping stable "downstream" speeds when there is a lot of Internet traffic, is found here:

Navas Cable Modem/DSL Tuning Guide

Scroll down to, “Reduce DNS errors In Windows 2000/XP”, click on “Registry script” and save it. All you have too do to put these values into the Registry is simply Right click it and select “Merge”. Also should you wish too remove these new settings that you have put into the Registry simply click on “INF script” and simply run it.

The values you are putting into the Registry with the above "Registry" fix are:

• [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Dnscache\Parameters]
  "NegativeCacheTime"=dword:00000000
  "NetFailureCacheTime"=dword:00000000
  "NegativeSOACacheTime"=dword:00000000

More Information

Here is another link entitled "How to keep DNS Errors from slowing you down"

How to keep DNS Errors from slowing you down!

You might want too look at this link regarding Windows ping facility:

broadband help » Tweaking FAQ 4. MTU

Here is a link too DRTCP and many other FAQs:

broadband help » Tweaking FAQ

This is also a good article on RWIN, MTU and using DRTCP:

ADSL Tweaks Page

I hope that what I have found that works for me can maybe help you in some way.