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 window size when they receive positive or negative acknowledgments, 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. For a broad class of window based protocols, the induced stochastic process can be analyzed using the theory of semi-Markov processes