TCP Flow Control

A powerful server can send data much faster than a small client can process it. Eventually the client's receive buffer fills. How does the sender know when to slow down?

The receiver tells the sender how much buffer space it has available: its receive window (rwnd). It advertises this capacity in the TCP header's Window field, sent with acknowledgments (ACKs). A zero window pauses ordinary data transmission; as the application reads data and frees space, the receiver advertises an updated window. Receiver: "I can accept another 32 KB." The sender limits outstanding data accordingly.