The window size field is a TCP-header field used for flow control. It tells the sender how many bytes the receiver can accept before the sender waits for an acknowledgment. This prevents a fast sender from overwhelming a receiver or its network. section 22.2.1 section 22.3
A window that grows during a transfer
PC1 advertises a 100-byte window to SRV1. SRV1 sends 100 bytes, then waits for acknowledgment. PC1 next advertises 200 bytes, allowing SRV1 to send 200 bytes in two segments before waiting. Finally, PC1 advertises 300 bytes. Because each host can adjust its own value throughout the exchange, TCP uses a sliding window. section 22.2.1
Figure 22.10 TCP flow control is achieved through the Window Size field of the TCP header. In its segments to SRV1, PC1 specifies how much data SRV1 should send before waiting for the next acknowledgment. This can be adjusted dynamically to find an appropriate rate.
Use the flow-control diagram to trace how the receiver advertises changing windows and how the sender responds before requesting another acknowledgment. section 22.2.1
