Guide To RWIN & MTU

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Note: This article applies only for Windows Operating Systems up to and including Windows XP. On Vista, Windows 7, and later systems the receive window is managed dynamically which generally does a reasonable job. Any tweaking carried out on these operating systems is for advanced users and beyond the scope of this article.

Over the years that broadband has been available many people have found that tweaking their PC settings gives them faster speeds.
Often people will find that they see very different results from using different speedtests - usually a higher speed on the BT Speedtest than other speedtests (e.g. www.mybroadbandspeed.co.uk or www.speedtest.bbmax.co.uk). This is often caused by a low RWIN value.

What is RWIN?

RWIN (or 'Receive Window') is the limit on the amount of data your PC will receive before it needs to send an acknowledgement (or 'ack packet') back to the server it is downloading from , to say that has been received.
Getting the right RWIN setting is a bit of an art form. Setting the RWIN too low means that your connection will be idle 50% of the time it is downloading, as it is waiting for 'ack packets' to be received, setting it too high will create a slowdown if there's any packet loss.
With a higher RWIN setting more data is downloaded and the connection will be more active more of the time. This will give a higher result on a speedtest which basically measures how long t takes to download a certain amount of data.

How to change your RWIN settings

1. To get your RWIN setting right first the first thing to do is run the DSL Reports test.  This will show a few important figures (such as the current RWIN value, the MTU and the MSS).

• MTU - (Maximum Transfer Unit): this is the largest packet size that can be transmitted by your PC.
• MSS - (Maximum Segment Size): this is calculated from the MTU (MTU - 40 bytes for IP and TCP headers).

The RWIN value is always calculated as an even multiple of the MSS:
EXAMPLE
n * MSS or n * (MTU - 40) - where 'n' is a whole number.
2. For the first test of getting the RWIN value correct you should use the following formula:
EXAMPLE
Connection throughout rate to the nearest Mbps (rounded up) * 10.
So for an 8Mbps connection the RWIN would be calculated as:
EXAMPLE
8 * 10 * (MTU - 40)
Using the example above this works out as 8 * 10 * (1458 - 40) = 113440
3. To change the RWIN setting you'll need to download and use DrTCP.
You can also use DrTCP to change the MTU, although unless you are having problems accessing certain websites, e.g. eBay, Amazon, London Congestion charging site, MSN and Hotmail, or the MTU is higher than 1500, this is usually unnecessary.
Once the changes have been made, make sure that you reboot your PC to allow the changes to take effect, before you try the speedtests again. It may take a bit of trial and error to find the best RWIN value by using different values for 'n'. Make sure you reboot after each RWIN change.