What is Weighted Random Early Detection

Weighted Random Early Detection (WRED): An Overview and Implementation


Weighted Random Early Detection (WRED) is a congestion avoidance mechanism used in network routers to manage the flow of packets and prevent network congestion. It is a technique that allows routers to selectively discard packets based on their importance, helping to maintain a stable and efficient network performance. In this article, we will delve into the details of WRED, how it works, and its implementation in modern networks.

The Need for Congestion Avoidance

In computer networks, congestion occurs when the network traffic exceeds its capacity, leading to a degradation in network performance and delay in packet delivery. Without a mechanism to handle congestion, a network can become overwhelmed, resulting in dropped packets, increased latency, and reduced overall efficiency. Congestion avoidance mechanisms like WRED are designed to address these issues by dynamically managing network traffic. By selectively discarding packets before a network becomes congested, WRED aims to prevent congestion from occurring in the first place, resulting in improved network performance and user experience.

How WRED Works

At its core, WRED is an algorithm that determines which packets to drop based on their importance. WRED accomplishes this by assigning weights to different packets, enabling the router to prioritize traffic flows and make informed decisions about packet discards. Here is a step-by-step breakdown of how WRED works:
  • 1. Traffic Measurement: WRED continuously monitors the average queue length or packet arrival rate to assess the levels of congestion in a network.
  • 2. Packet Drop Probability Calculation: Based on the measured congestion levels, WRED calculates a drop probability for incoming packets. The drop probability is determined by the weight assigned to each flow and the level of congestion.
  • 3. Packet Discard Decision: When a packet arrives, WRED compares its drop probability with a randomly generated value. If the drop probability is higher than the generated value, the packet is discarded. Otherwise, it is forwarded to the destination.
By implementing this decision-making process, WRED ensures that packets with lower drop probabilities are given priority while maintaining fairness among different traffic flows.

Benefits of WRED

WRED offers several benefits that make it an effective congestion avoidance mechanism:
  • Improved QoS: By selectively dropping packets, WRED allows routers to prioritize traffic and provide better Quality of Service (QoS) for critical applications.
  • Prevents Tail Drops: WRED prevents tail drop scenarios where a sudden increase in traffic causes the router buffer to become full, resulting in dropped packets. By discarding packets before the buffer reaches full capacity, WRED helps prevent congestion from occurring.
  • Enhanced Fairness: WRED ensures fairness among different traffic flows by assigning weights to packets. This prevents one flow from dominating network resources, leading to a more equitable distribution of bandwidth.
  • Scalability: WRED is a scalable solution that can handle varying levels of congestion and adapt to changing network conditions.

Implementing WRED

To implement WRED in a network, routers need to be configured with WRED parameters that define the behavior of the algorithm. These parameters include:
  • Min Threshold: The minimum threshold at which WRED starts to consider dropping packets.
  • Max Threshold: The maximum threshold at which WRED aggressively drops packets to alleviate congestion.
  • Drop Probability: The probability with which packets are dropped based on the congestion levels and assigned weights.
During configuration, network administrators define these parameters to ensure WRED operates optimally for their network environment. Fine-tuning these parameters is essential to strike a balance between congestion avoidance and maintaining a high-quality user experience.


Weighted Random Early Detection (WRED) is a crucial tool for managing network congestion and ensuring optimal performance. By selectively discarding packets based on their importance, WRED facilitates congestion avoidance and prevents network overload. Its benefits, including improved Quality of Service, prevention of tail drops, enhanced fairness, and scalability, make it a valuable component in modern network routers. Understanding WRED's workflow and properly configuring its parameters allow network administrators to maintain stable and efficient network operations in the face of increasing traffic demands.

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