Flow control

The purpose of TCP's flow control mechanisms is to allow communication between hosts with wildly varying memory dimensions. In each TCP segment, the sender of the segment indicates its available buffer space. A TCP sender must not send more data than the buffer space indicated by the receiver.

In our implementations, the application cannot send more data than the receiving host can buffer. Before sending data, the application checks how many bytes it is allowed to send and does not send more data than the other host can accept. If the remote host cannot accept any data at all, the stack initiates the zero window probing mechanism.

The application is responsible for controlling the size of the window size indicated in sent segments. If the application must wait or buffer data, it can explicitly close the window so that the sender will not send data until the application is able to handle it.