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Background: Calculate the optimal TCP windowscale value
Posted by Admin Kayako, Last modified by Wouter Debie on 28 November 2017 05:33 PM

The window size in a TCP connection is a very important parameter. It has a huge influence on the performance, because the receive is the amount of data a host can accept without acknowledging the sender. If the sender has not received acknowledgement for the first packet it sent, it will stop and wait and if this wait exceeds a certain limit, it may even retransmit. If the receive window is too small, a server will wait too often. If a window is too big, chances are higher that a server will send too fast and packet loss will happen.

The optimal value for the receive window is often called the bandwidth-delay product. It is the amount of data which can be sent over a link during the time the server sends the first packet and the time the server receives an acknowledgement for this packet.

So:

Bandwidth-delay product = RTT x BandWidth

Now, to optimize the TCP performance, the calculated optimal receive window should be at least this size and a multiple of the MTU of the link. But, there is also a third important parameter: or TCP, the receive window is announced by two parameters: the windows size in the TCP header, and if supported, the windows scale factor.  This means the ideal receive window must be a value which can be expressed by a window size and scale value.

To calculate the optimized value for a TCP connection, we have created an spreadsheet where you can just enter the different parameters, and you will get the optimal values for both the announced windowsize and windows scale.

Download Spreadsheet.

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