What is $R_\min$ in this formulat for optimal buffer size?
$R_{min}$ is the "Writing Rate in Application Buffer" (from router in your example). 

This formula makes sense for different conditions like $R_{app}=R_{min}$. In this case the buffer size should be infinite (very large buffer) and it is intuitively correct. $R_{app} &gt;R_{min}$ makes the size negative which shows that buffering does not help while the reading data is very faster than buffering data in user side.