What is Network Congestion? Common Causes and How to Fix Them?

Network Congestion occurs when the traffic flowing through a network exceeds its maximum capacity. In most cases, congestion is a temporary issue with the network caused due to a sudden upsurge of traffic, however, sometimes, a network is continually congested, indicating a deeper problem. End-users perceive network congestion as Network Slowdown or a very large delay in processing requests.

Network congestion is also a contributing factor in the following underlying issues:

• High Latency – 
  In a congested network, the time taken by a packet to reach its destination increases significantly, hence a higher latency rate is observed.
• Connection timeouts –
  Ideally, the service should wait for the arrival of packets but in several cases, the connection terminates due to timeout.
• Packet loss –
  Many packets cannot reach their destination if the network is congested, and will be dropped eventually due to timeout.

Causes of network congestion :

1. Excessive bandwidth consumption –
   Certain users or devices on the network may occasionally utilize more bandwidth than the average user or device. This can put a strain on the network and its routing equipment (routers, switches, and cables), causing network congestion.
    
2. Poor subnet management –
   For better resource management, a big network is divided into subnets. However, network congestion could arise if the subnets are not scaled according to usage patterns and resource requirements.
    
3. Broadcast Storms –
   A broadcast storm occurs when there is a sudden upsurge in the number of requests to a network. As a result, a network may be unable to handle all of the requests at the same time.
    
4. Multicasting –
   Multicasting occurs when a network allows multiple computers to communicate with each other at the same time. In multicasting, a collision can occur when two packets are sent at the same time. Such frequent collisions may cause a network to be congested.
    
5. Border Gateway Protocol –
   All traffic is routed by BGP via the shortest possible path. However, while routing a packet, it doesn’t consider the amount of traffic present in the route. In such scenarios, there is a possibility all the packets are being routed via the same route which may lead to network congestion.
    
6. Too many devices –
   Every network has a limit on the amount of data it can manage. This capacity establishes a limit on how much bandwidth and traffic your network can handle before performance degrades. If the network has too many devices linked to it, the network may become burdened with data requests.
    
7. Outdated Hardware –
   When data is transmitted over old switches, routers, servers, and Internet exchanges, bottlenecks can emerge. Data transmission can get hampered or slowed down due to outdated hardware. As a result, network congestion occurs.
    
8. Over-subscription –
   A cost-cutting tactic that can result in the network being compelled to accommodate far more traffic than it was designed to handle (at the same time).

Effects of network congestion :

1. Queueing delay
2. Packet Loss
3. Slow Network
4. Blocking of new connections
5. Low throughput

Test for network congestion :

• Run Command Prompt as administrator.
• Type tracert google.com in the CMD window.

Tracert

• Take note of how many hops it takes to get to the final server.
• For every hop, check out the value of ping.

Congestion at the network layer is related to two issues, throughput and delay.

1. Based on delay

When the load is much less than the capacity of the network, the delay is at a minimum .
This minimum delay is composed of propagation delay and processing delay, both of which are negligible.
However, when the load reaches the network capacity ,the delay increases sharply because we now need to add the queuing delay to the total delay.
The delay becomes infinite when the load is greater than the capacity.

delay as a function load

2. Based on Throughout

When the load is below the capacity of the network, the throughput increases proportionally with the load.
We expect the throughput to remain constant after the load reaches the capacity, but instead the throughput declines sharply.
The reason is the discarding of packets by the routers.
When the load exceeds the capacity, the queues become full and the routers have to discard some packets.
Discarding packets does not reduce the number of packets in the network because the sources retransmit the packets, using time-out mechanisms, when the packets do not reach the destinations.

throughput as a function of delay

How to fix network congestion?

1. Divide your network into subnets that can be resized to meet traffic.
2. TCP/IP settings should be adjusted to balance packet send/request speeds.
3. Use a CDN (Content Delivery Network) to save time by directing more requests to edge servers.
4. Choke packets are used to reduce the output of sender devices, which helps to avoid network congestion.
5. In case the default route becomes congested, you can employ multi-hop routing so that traffic can be managed.
6. Upgrade your Internet plan to allow for more devices and increased bandwidth. Check to see if your devices are up to date and not outdated (even the cables).

A good practice is to monitor your network for any abnormal changes in the traffic. This helps in identifying the issue in advance and planning out improvements.