Flow Control

Flow control prevents network congestion by ensuring that sending network devices do not overwhelm the receiving devices with data. A high-speed computer, for example, could generate traffic faster than the network can transfer it or faster than the destination network device can receive and process it.

The three methods used for handling network congestion are as follows:

• Buffering Used by network devices, such as routers, to temporarily store bursts of excess data in memory until the data can be processed.

  - Occasional data bursts are often handled by buffering.

  - Excess data bursts can exhaust memory forcing the network device to discard any additional datagrams that arrive for transmission. In this case, it is up to the upper-layer protocols to detect the loss of data and request retransmission of the missing data.

• Source-quench messages Used by receiving network devices (routers) to prevent their buffers from overflowing. The receiving device sends source-quench messages to request that the sender reduce its current rate of data transmission.

  - The receiver begins discarding received data due to overflowing buffers.

  - The receiver then begins sending source-quench messages to the sending device at the rate of one message for each packet dropped. The source device receives these source-quench messages and slows down the sending data rate until source-quench messages are no longer received by the sender.

  - The sender gradually increases the sending data rate for as long as no further source-quench requests are received (by the distant-end).

• Windowing A flow-control scheme in which the source requires an acknowledgment from the destination after a certain number of packets have been transmitted. For example, with a window size of three, the source requires an acknowledgment after sending three packets, as the following details:

  - The source device sends three packets to the destination device.

  - After receiving these three packets, the destination device sends an acknowledgment to the source.

  - The source receives the acknowledgment and sends three more packets.

  - If the destination does not receive one or more of these packets for some reason, such as overflowing buffers, the receiver does not receive enough packets to send an acknowledgment. The source then retransmits the packets at a slower transmission rate.