Window based network flow control protocols, such as TCP, modulate the number of unacknowledged packets the protocol is allowed to have outstanding. Such protocols change the size when they receive positive or negative acknowledgements, where the latter kind may be inferred from timeouts. Together with a communications channel that loses packets at random, such a protocol induces a stochastic process on the window size. We consider a broad class of window based protocols, and analyze various statistics of the induced stochastic process. We demonstrate that all these protocols can be treated analytically using the theory of semi-Markov processes