What is Traffic Throttling in computer networks?

Traffic throttling is one of the approaches for congestion control. In the internet and other computer networks, senders trying to adjust the transmission need to send as much traffic as the network can readily deliver. In this setting the network aim is to operate just before the onset of congestion.

There are some approaches to throttling traffic that can be used in both datagram and virtual-circuit networks.

Each approach has to solve two problems −

Firs

Routers have to determine when congestion is approaching ideally before it has arrived. Each router can continuously monitor the resources it is using.

There are three possibilities, which are as follows −

• Utilisation of output links.

• Buffering of queued packets inside the router.

• Numbers of packets are lost due to insufficient buffering.

Second

Average of utilization does not directly account for burstiness of most traffic and queueing delay inside routers directly captures any congestion experienced by packets.

To manage the good estimation of queueing delay d, a sample of queue length s, can be made periodically and d updated according to,

$$\mathrm{d_{new}=\alpha d_{old} +(1-\alpha)s}$$

Where the constant α determines how fast the router forgets recent history. This is called EWMA (Exponentially Weighted Moving Average)

It smoothest out fluctuations and is equivalent to allow-pass filter. Whenever d moves above the threshold, the router notes the onset of congestion.

Routers must deliver timely feedback to the senders that are causing the congestion. Routers must also identify the appropriate senders. It must then warn carefully, without sending many more packets into an already congested network.

There are many feedback mechanisms one of them is as follows −

Explicit Congestion Notification (ECN)

The Explicit Congestion Notification (ECN) is diagrammatically represented as follows −

Explanation of ECN

Step 1 − Instead of generating additional packets to warn of congestion, a router can tag any packet it forwards by setting a bit in the packet header to signal that it is experiencing congestion.

Step 2 − When the network delivers the packet, the destination can note that there is congestion and inform the sender when it sends a reply packet.

Step 3 − The sender can then throttle its transmissions as before.

Step 4 − This design is called explicit congestion notification and is mostly used on the Internet.