TCP/IP Illustrated, Volume 1: The Protocols

Chapter 22. TCP Persist Timer

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 ...