TCP Features : Flow Control

Computers that send and receive TCP data segments can operate at different data rates because of differences in CPU and network bandwidth. As a result, it is quite possible for a sender to send data at a much faster rate than the receiver can handle. TCP implements a flow control mechanism that controls the amount of data sent by the sender. TCP uses a sliding window mechanism for implementing flow control.
TCP flow control mechanism exhibits the following properties:
• Octets that are to the left of the window range have already been sent and acknowledged.
• Octets in the window range can be sent without any delay. Some of the octets in
the window range may already have been sent, but they have not been acknowledged. Other octets may be waiting to be sent.
• Octets that are to the right of the window range have not been sent. These octets can be sent only when they fall in the window range.

The left edge of the window is the lowest numbered octet that has not been acknowledged. The window can advance; that is, the left edge of the window can move to the right when an acknowledgment is received for data that has been sent. The TCP packet containing the acknowledgment contains information about the window size that the sender should use.
The window size reflects the amount of buffer space available for new data at the receiver. If this buffer space size shrinks because the receiver is being overrun, the receiver will send back a smaller window size. In the extreme case, it is possible for the receiver to send a window size of only one octet, which means that only one octet can be sent. This situation is referred to as the silly window syndrome, and most TCP implementations take special measures to avoid it.
A TCP module sending back a window size of zero indicates to the sender that its buffers are full and no additional data should be sent. TCP includes mechanisms to shrink window size when the receiver experiences congestion of data and to expand window size as the congestion problem clears.
The goal of the sliding window mechanism is to keep the channel full of data and to reduce to a minimum the delays experienced by waiting for acknowledgments.