What is Traffic Throttling in computer networks?

Traffic throttling is a congestion control mechanism used in computer networks where senders adjust their transmission rates to match the network's capacity. The goal is to operate the network just before congestion occurs, maximizing throughput while preventing packet loss and delays.

Traffic throttling approaches can be implemented in both datagram and virtual-circuit networks. These mechanisms help maintain network stability by proactively managing data flow before congestion becomes severe.

Each traffic throttling approach must solve two fundamental problems:

Problem 1: Congestion Detection

Routers must determine when congestion is approaching, ideally before it occurs. Each router continuously monitors its resource utilization using three key metrics:

• Output link utilization − Monitoring bandwidth usage on outgoing interfaces

• Buffer occupancy − Tracking queued packets inside the router

• Packet loss rate − Counting packets dropped due to insufficient buffering

Since average utilization doesn't account for traffic burstiness, routers use queueing delay as a more accurate congestion indicator. To estimate queueing delay d, routers sample queue length s periodically and update the delay using:

d_new = ? × d_old + (1-?) × s

This formula represents Exponentially Weighted Moving Average (EWMA), where constant ? determines how quickly the router forgets recent history. EWMA smooths out fluctuations like a low-pass filter. When d exceeds a threshold, the router detects congestion onset.

Problem 2: Feedback Delivery

Routers must provide timely feedback to senders causing congestion while identifying appropriate sources. The feedback mechanism must avoid sending additional packets into an already congested network.

Explicit Congestion Notification (ECN)

ECN works through the following process:

• Step 1 − Instead of generating additional packets, a congested router sets a bit in the packet header to signal congestion

• Step 2 − When the tagged packet reaches its destination, the receiver notes the congestion indication

• Step 3 − The receiver informs the sender about congestion in the next reply packet (ACK)

• Step 4 − The sender reduces its transmission rate to alleviate network congestion

Advantages of Traffic Throttling

• Proactive congestion prevention − Prevents severe congestion before it occurs

• Network stability − Maintains consistent network performance

• Fair resource sharing − Ensures equitable bandwidth distribution among users

• Reduced packet loss − Minimizes dropped packets by controlling input rates

Conclusion

Traffic throttling is essential for maintaining network stability by proactively controlling data transmission rates. Through mechanisms like ECN, networks can efficiently manage congestion without generating additional control traffic, ensuring optimal performance and fair resource utilization.