How to check the speed of transmission / reception of data?

Question

C++ / C#

I want to make a program (on Windows) similar to NetWorx. It measures the speed of data transmission and reception and displays them as on the screen.

I will deal with the conclusion, but I do not understand how to measure this speed. How, with what and what should be measured to obtain information about the speed? (even xs what other question to ask, I don’t understand what to do at all)