The reduction in the quality of service that occurs when a network node or a link carries more data than it can handle is called “Network Congestion”. The congestion in the network can lead to effects like packet loss or even blocking of new connections.
Therefore, Congestion in networks can be defined as, “A state that occurs in network layers because of the heavy message traffic that results in slowing down the network response time is called congestion”.
Following are the effects of Congestion −
Because of the increase in the response time, the overall performance is reduced.
Also, in worst situations, because of the delay that takes place, re-transmission can also occur which worsens the scenario.
The broader ways to classify the congestions are as follows −
To control the congestion in networks, the control techniques are broadly classified under two categories, which are as follows −
Now let us discuss about closed loop congestion control in detail −
The basic mechanisms of closed loop congestion control are as follows −
When a router is congested, it informs the previous upstream router to reduce the rate of outgoing packets.
If a node becomes congested it can slow down or halt flow of packets from other nodes and halt flow of packets from other nodes.
It means that other nodes have to apply control on incoming packet rates control on incoming packet rates.
Propagates back to source and it is not used in ATM or frame relay.
It is diagrammatically represented as follows −
It is sent by router to source, similar to ICMP’s source quench packet.
It is diagrammatically represented as follows −
It looks for delay in some other action.
Transmission delay may increase with Transmission delay may increase with congestion.
Packets may be discarded and the source can detect these as implicit indications of congestion.
Useful on connectionless (datagram) networks and it is used in frame relay LAPF.
The router sends an explicit signal. The network alerts end systems of increasing congestion and is used on connection-oriented networks. The end systems take steps to reduce offered load.
Backwards − Congestion avoidance info sent in opposite direction of packet travel.
Forwards − Congestion avoidance info sent in the same direction as packet travel.