When sender transmits data much faster than the capability of the receiver, routers in between will run out of buffers and start to drop the packets. In the meantime, TCP as a reliable protocol, normally retransmit lost packets and this will ended up in a worse situation that may lead to deadlock! To prevent such a deadlock TCP uses congestion control mechanism.

* But first let’s familiarize ourselves with some handful terms.

TCP windows

A window in TCP defines the amount of data that sender can transmit before waiting for ACK.

The sender should consider two windows before data transmission; congestion window size and the receiver window size.

• The congestion window size defines the number of bytes that can be sent at any time. The sender maintains the congestion window.
• The TCP receiver window size is the maximum amount of received data, in bytes, that can be buffered at one time on.

Actual window=min (rwnd, cwnd)

Now that the sender knows the actual size of the window, it starts to send data. We will go through the whole process in part II.

References:

http://msdn.microsoft.com/en-us/library/ms819736.aspx