• Tweaking is changing your MTU & RWIN settings on your PC to maximise your connection and achieve better download speeds.
 • MTU (Maximum Transfer Unit) is the default packet size (amount of data) that a system uses to send and receive information over a network.
 • RWIN (Receive Window) is the maximum amount of data that you can receive without having to send an acknowledgement to the sender.

Changing these settings can frequently improve your connection speeds, by decreasing the risk of fragmentation on the broadband network.

The History behind tweaking adsl settings.

Although it has always been possible to tweak your MTU and RWIN settings, MTU tweaking came into its own during the period of 2002/2003 when adsl in the UK was still in its infancy. At that time the default MTU setting of 1500 caused processing overheads on the exchange equipment, the effect of which was an overall deterioration in service and speed. Therefore many users found that their connection was not as efficient as it could have been.

In January 2003, BTw declared that the optimum MTU setting was 1458 and made a tool available to change your settings.  The tool and more information were available from here.
Quote from BTW

Fragmentation is the result of IP packets, larger than the 1500 byte maximum attempting to the pass across the network. The MTU tool enables users of Microsoft Windows based operating systems, to improve their network performance. By modifying the MTU to a setting suitable for the network, the MTU can be lowered to stop fragmentation, thereby improving the perceived performance of the network. This setting is nominally 1458 bytes. The change is made to the ADSL interface whether it's via a USB modem or an Ethernet Network Interface Card.

In July 2003 BTw undertook to carry out work on all their exchanges to set the adsl network with higher MTU values to stop the default windows MTU of 1500 causing a problem. More information about this work can be found on adslguides archives here.
By this time many users were now familiar with tweaking and happy to explore its benefits further.

The Need for Speed

By July 2003 many early adopters to adsl discovered that although the BT network problems may have been fixed, tweaking could bring additional benefits such as ekeing out a better speed on your connection.
Many users on adsl-guide spent literally hours finding and trying new settings to increase their speed.*
Various users undertook testing which did indeed prove that by tweaking both MTU and RWIN you could improve a 512kbps by around 30kbps or more.

With the advent of (1Mb) higher speeds the same tweakers also found that doubling your RWIN on your previous best settings gave the optimum performance and speeds for their lines. The same theory was recommended for 2Mb connections.

* Yes rather sadly I was one of those that spent many a happy hour or so trying to eke out a few extra kbps :/
Quote from me at the time "My speed (478/250kbps) increased by about 30kb after tweaking"

Tweaking Today

So where does that leave us today - with the advent of maxdsl the increase of the extra speed is not quite so important when talking 8Mb compared to gaining an extra 30kbps when on a 512kbps line. Some naysayers say that tweaking makes no difference.

My own opinion is that it does.... on a 2Mb connection I get an extra 40+ kbps when tweaked against an untweaked connection. Not that much now in the grand scheme of things - but I'm more than happy to spend 2 mins setting my MTU and RWIN just because I can. ;o)

Tweaking can also resolve some strange happenings on the internet - particularly when viewing sites such as ebay and M$ - So much so that I have devoted an MTU problems page to it.

I recommend that you use DrTCP for tweaking, and Ive also written a page about How to tweak MTU and which settings to use.

Read the next page to find out more about tweaking.

|| How to tweak and suggested settings || Understanding MTU and RWIN || MTU Problems||

Finally, I cant complete this page without giving due credit to the very early tweakers -
In particular Pengbo and Tam for starting me on the path to tweaking. Also thanks to guest_again for his input in sharing his findings.

If you want an more in depth explanation try Pengbo's website who gives a step by step guide on running Dr TCP.
More information about Dr TCP can be found at DSL Reports and you can also use their Tweak Test to see if your connection is optimised correctly.