22.1 Introduction

We've seen that TCP has the receiver perform flow control by specifying the amount of data it is willing to accept from the sender: the window size. What happens when the window size goes to 0? This effectively stops the sender from transmitting data, until the window becomes nonzero.

We saw this scenario in Figure 20.3. When the sender received segment 9, opening the window that was shut down by segment 8, it immediately started sending data. TCP must handle the case of this acknowledgment that opens the window (segment 9) being lost. Acknowledgments are not reliably transmitted ”that is, TCP does not ACK acknowledgments, it only ACKs segments containing data.

If an acknowledgment is lost, we could end up with both sides waiting for the other: the receiver waiting to receive data (since it provided the sender with a nonzero window) and the sender waiting to receive the window update allowing it to send. To prevent this form of deadlock from occurring the sender uses a persist timer that causes it to query the receiver periodically, to find out if the window has been increased. These segments from the sender are called window probes. In this chapter we'll examine window probes and the persist timer. We'll also examine the silly window syndrome, which is tied to the persist timer.