There are several reasons why congestion is caused. For example, if a stream of packets begin arriving on some of the input lines and all need to be sent on the same output line a queue will build up. If there is not enough memory to store all of them, packets will start to get lost.
Adding more memory may be of some help up to a limit, but it has been discovered that if routers have an infinite amount of memory, it only makes the congestion worse, because by the time the packages get to the front of the queue, they have been timed out and duplicates have been despatched.
All these packages will be forwarded to the next router increasing the load on the entire system, right up to the destination. Slow processors can be another cause for congestion.
ADVERTISEMENTS:
If the routers are slow at performing their housekeeping like queuing buffers, updating tables, etc. then queues can be built up even though there may be excess line capacity. Low bandwidth lines may be another reason for causing congestion.
Therefore, solving any one reason may not solve the problem of congestion totally. The actual problem is, frequently, a mismatch of capacities between the capacity of the subnet and the volume of traffic. The problem can only be eliminated if all the components are carefully balanced.
Congestion tends to make things worse for itself. For example, if a router has no free buffers, it must ignore newly arrived packets. When a packet is discarded, the despatching router may time out that packet and send another copy, perhaps many times.
Since it cannot discard the packet until it has been acknowledged, congestion at the receiver forces the sender to refrain from releasing a buffer it would have normally freed. This way the congestion continues to spiral up until it causes the entire system to collapse.
ADVERTISEMENTS:
Congestion control is concerned with making sure that the subnet is able to cope with the offered traffic. It is concerned with the behaviour of all hosts, routers, the store and forward facility in the routers and all the other factors that tend to reduce the carrying capacity of the subnet.