tcp provides mechanisms for flow control. flow control helps maintain the reliability of tcp transmission by adjusting the rate of data flow between source and destination for a given session. flow control is accomplished by limiting the amount of data segments forwarded at one time and by requiring acknowledgments of receipt prior to sending more. the first thing that tcp determines (to accomplish flow control) is the amount of data segments that the destination device can accept. the tcp header includes a 16-bit field called the window size. this is the number of bytes that the destination device of a tcp session is able to accept and process at one time. at first ,during the session startup ,the initial window size is agreed upon the 3-way handshake between source and destination. once agreed upon, the source device must limit the amount of data segments sent to the destination device based on the window size. only after the source device receives an acknowledgement that the data segments have been received, can it continue sending more data for the session.