A network is congested when one or more network components must discard packets due to lack of buffer space. Given the above architecture, it is possible to see how network congestion can occur. A source of data flow on the network cannot reserve bandwidth across the network to its data’s destination. It, therefore, is unable to determine what rate of data flow can be sustained between it and the destination.
If a source transmits data at a rate too high to be sustained between it and the destination, one or more routers will begin to queue the packets in their buffers. If the queueing continues, the buffers will become full and packets from the source will be discarded, causing data loss. If the source is attempting to guarantee transmission reliability, retransmission of data and increased transmission time between the source and the destination is the result. Figure 2 from [Jain & Ramakrishnan 88] demonstrates the problem of congestion.
As the load (rate of data transmitted) through the network increases, the throughput (rate of data reaching the destination) increases linearly. However, as the load reaches the network’s capacity, the buffers in the routers begin to fill. This increases the response time (time for data to traverse the network between source and destination) and lowers the throughput.
Once the routers’ buffers begin to overflow, packet loss occurs. Increases in load beyond this point increase the probability of packet loss. Under extreme load, response time approaches infinity and the throughput approaches zero; this is the point of congestion collapse. This point is known as the cliff due to the extreme drop in throughput. Figure 2 also shows a plot of power, defined as the ratio of throughput to response time. The power peaks at the knee of the figure.
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