I never said "an application can set the window size"

But the OP asked: "how can i change the tcp parameters (i.e. tcp window size, scaling"; and the answer is: he cannot.

The application can influence this

Great. You've outlined a mechanism for potentially effecting some indeterminate change in the window size/scale factor product. Now show how to do something useful with it.

The primary benefit of adjusting those (that) parameter is to improve throughput by tailoring the effective buffer size to the bandwidth and latency of the link.

To make the adjustment, you need to know:

• The effective bandwidth;
• The effective latency;
• The application retrieve rate.

But, none of these factors are available to application code. Not only is that information not available from the stack; even if it was, it would do you no good because the only parameter you can tune is the retrieve buffer size; and that has to be done before the connection is made; so you cannot even try to instrument these from within the application and adjust in light of your findings.