Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 201
TCP Flow and Window Management

Two functions are required of TCP in order for the protocol to manage the data over a connection: flow control and transmission control. Do not confuse these functions with the ICMP source quench mechanism. Source quench is for a host to inform the source of transmissions that the host is full and the host would like the sender to slow its rate of transmission.

For those readers who do not understand flow control, it is a mechanism used to control the flow of data. For example, if data is being received at a destination station faster than that station can accept it, it needs to tell the source station to slow down or to stop completely until it can clear out some space (replenish buffers).

How many segments may be outstanding at any one time? Data management using a “window” is accomplished as shown in the following slide. Data for TCP to transmit to the remote network station will be accepted by TCP from an application. This data will be placed sequentially in memory where it will wait to be sent across a connection to a remote station (for IP to send the packet). TCP places a “window” over this data in which to structure the data: data sent and acknowledged, data sent but not acknowledged, and data waiting to be sent. This is called a sliding window, for the window will slide up the data segment as each data packet is sent and acknowledged.

Refer to slide 219. Sequence numbers 100–104 have been transmitted to the destination station and the destination station has acknowledged receipt of these segments. Packets containing sequence numbers 105–108 have been transmitted by the source station, but it has not received an acknowledgment. Segments containing sequence numbers 109–114 are still in the source station and are waiting to be sent. Packets containing 115–118 are not yet in the window.

The important thing to notice is the black box covering the segments. This is the window. It will constantly move in ascending sequence order upon receipt of acknowledgments from the destination station.

This window size is variable through the use of the Window Size field in the TCP header of an acknowledgment. When the receiving station (the destination station) is running low on buffer space (an area of memory in which to store incoming data), it can inform the sender to slow its transmission rate to the amount of data it can accept. This is accomplished through the Window field in the TCP header packet. This field will contain the number of bytes (by indicating the range of sequence numbers) that the destination station is willing to accept. Slide 212 shows the TCP header, specifically, the Window field in the TCP header.

When the remote network station cannot accept any more data, it may set this Window field to a 0. It will continue to submit these 0 packets until it can again accept data (that is, the sender can send data to a host, and this host should respond with the ACK set to the previous ACK sent and a window set to 0). When buffer space is freed up, it can again submit a packet with the window size set to a number other than 0 to indicate it can again accept data. However, if the urgent bit is set, this indicates to the receiver that the sender has urgent data waiting to send.

TCP Flow and Window Management

This connection management technique allows TCP to maintain control of the data transfer by informing TCP on the sending side how much data the receiver is willing to accept. This enables both a sender and receiver of data to maintain consistent data flow over the connection.