Switches introduce the possibility of congestion: packets arriving faster than they can be sent out. This can happen with just two interfaces, if the inbound interface has a higher bandwidth than the outbound interface; another common source of congestion is traffic arriving on multiple inputs and all destined for the same output.
Whatever the reason, if packets are arriving for a given outbound interface faster than they can be sent, a queue will form for that interface. Once that queue is full, packets will be dropped. The most common strategy (though not the only one) is to drop any packets that arrive when the queue is full.
The term “congestion” may refer either to the point where the queue is just beginning to build up, or to the point where the queue is full and packets are lost. In their paper [CJ89], Chiu and Jain refer to the first point as the knee; this is where the slope of the load vs throughput graph flattens. They refer to the second point as the cliff; this is where packet losses may lead to a precipitous decline in throughput. Other authors use the term contention for knee-congestion.
In the Internet, most packet losses are due to congestion. This is not because congestion is especially bad (though it can be, at times), but rather that other types of losses (eg due to packet corruption) are insignificant by comparison.
We emphasize that the presence of congestion does not mean that a network has a shortage of bandwidth. Bulk-traffic senders (though not real-time senders) attempt to send as fast as possible, and congestion is simply the network’s feedback that the maximum transmission rate has been reached. For further discussion, including alternative definitions of longer-term congestion, see [BCL09].
Congestion is a sign of a problem in real-time networks, which we will consider in 20 Quality of Service. In these networks losses due to congestion must generally be kept to an absolute minimum; one way to achieve this is to limit the acceptance of new connections unless sufficient resources are available.