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       Friday, 17-Sep-2021 23:07:09 Central Daylight Time

Rwin Calculator Beta (Receive Window Calculator for MS Windows TCP/IP).

By BigAmp

You now have a choice. Rwin online or on your desktop.

We have the Rwin Calculator to set the Rwin based on your Latency here.

You can now download the Calculator to your desktop and get the Settings based on your MTU. Its still a beta version for the program itself and with your input in the Discussion Forum it can only get better.

It's been kept simple.

As you can see from this screen shot.

How to use....

Download the RWINCalcBeta2.zip

Compatible with WindowsXP / Windows2000 / WindowsME / Windows98 and SE / Windows95b or better.

Unzip the Archive to either your desktop or a safe place you can get to easy. The ZIP file includes the Calculator and a Simple help file. There is no install it resides ontop. If you need a ZIP utility you can get one here

Help File is in HTML format.

Simply open the README.HTML file in your browser for the simple to use instructions.

Or use the ones provided below.

Included on the help file.

You need to find your MTU that can be done one of two ways or both.

Either through this link  Speedguide Analyzer. Or using a ping described below. Your MTU(Maximum Transmission Unit),Will show up in the Analyzer near the top. If you have any questions on that we're more than happy to answer in the Discussion Forum.

Using the Calculator:

Select your MTU by clicking on the Radio buttons with the corresponding MTU value, and then enter the MSS multiplier value in to the MSS Multiplier Textbox and press the Calculate button.

Changing RWIN Value:

After getting you RWIN Simply input that into either the Cablenut program or Speedguide TCP/IP Optimizer Save & Reboot.

Advanced Mode:

The Advanced Mode RWIN Calculation is for nut cases and system administrators; normally it is not needed for broadband connections.

Finding an RWIN Value:

On average it is best for the RWIN value to be a multiple of the MSS value, by default is program uses a multiple of 44. Ex. 44 x 1460 = 64240 If the RWIN values is greater then 65535 then scaling must be turned on (Tcp1323Opts = 1) for the value to be run by the connection.  The scaling is anther level of packets (grouping) in the connection. A scale factor of 2 on the RWIN value of 64240 gives a value of 256960. Ex.44 x 1460 = 64240 x 2^2 = 256960 This program already dose these calculations.

More Information:

TCP/IP (Transmission Control Protocol/Internet Protocol):

Transmission Control Protocol/Internet Protocol (TCP/IP) is the most popular network protocol and the basis for the Internet. Its routing capabilities provide maximum flexibility in an enterprise-wide network.

MTU (Maximum Transmission Unit):

The MTU (Maximum Transmission Unit) is the maximum amount of data (information) that can be transmitted in one packet (grouping) over a network of any type. The unit of measurement for the value of MTU is in bytes and in decimal form. The MTU value also includes the 40 byte transport header that is made up of two parts, a 20 byte TCP header and a 20 byte IP header.

MSS (Maximum Segment Size):

The MSS (Maximum Segment Size) is the maximum amount (segment) of TCP data (information) that the Winsock is equipped to accept from the network. When the computer makes a connection of any kind the two or more end must establish the rate at which connect (communicate) and at which size the packets (groupings) of data (information) well be sent. The unit of measurement for the value of MSS is in bytes and in decimal form. Knowing that the MTU value also includes the 40 byte transport header we can set the MSS value to be 40 less then MTU (MSS = MTU – 40).

RWIN (Receive Window):

The TCP RWIN (Receive Window) value is the amount of receive (obtain) data (information) that can be buffered at one time on a connection. The sending host can send only that amount of data before waiting for an acknowledgment and window update from the receiving host. The unit of measurement for the value of RWIN is in bytes and in decimal form.

How to find your MTU using a DOS or Command Shell.

ping -f -l [packetsize] [www.yourisp.com]

Sample ping: Open a DOS window in Win9x or In WinME or NT Click on “Start” then “Run” and Type “command” and Click on the “OK” button. In Windows2000 or WindowsXP use “cmd” from Run in place of Command

Sending a PING of 1472 would equal a MTU of 1500 if it comes back as a clean ping.

As you see in the first example it comes back fragmented.

At the DOS/Command Prompt type ping -f -l 1472 yourisp.com similar to the sample below.

C:\>ping -f -l 1472 broadbandnuts.com

Pinging broadbandnuts.com [] with 1472 bytes of data:

Reply from Packet needs to be fragmented but DF set.

Packet needs to be fragmented but DF set.

Packet needs to be fragmented but DF set.

Packet needs to be fragmented but DF set.

Ping statistics for

Packets: Sent = 4, Received = 1, Lost = 3 (75% loss),

Approximate round trip times in milli-seconds:

Minimum = 0ms, Maximum = 0ms, Average = 0ms

More than likely your MTU is below 1500 as in the sample below 1468 + 28 = MTU or 1496 in this case

C:\>ping -f -l 1468 broadbandnuts.com

Pinging broadbandnuts.com [] with 1468 bytes of data:

Reply from bytes=1468 time=50ms TTL=47

Reply from bytes=1468 time=40ms TTL=47

Reply from bytes=1468 time=41ms TTL=47

Reply from bytes=1468 time=40ms TTL=47

Ping statistics for

Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

Approximate round trip times in milli-seconds:

Minimum = 40ms, Maximum = 50ms, Average = 42ms

If the ping gets fragmented then it is above your MTU you are looking the highest value that does not get fragmented, then just add 28 to that value and you have your MTU. For example a MTU of 1500 would be able to have a no fragmented ping up to a value of 1472, so you add 28 to that. So you get your MTU. 1472 + 28 = 1500 (MTU).

As with all of this if you have a Question or a Suggestion see us in the Discussion forum. It's fast and FREE.


This program is intended for information and Learning only and inputs nothing into you machine. You need to use a second party program as mentioned above. We can't be held responsible for misuse.

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