CoDel (Controlled delay)

Introduction

CoDel (Controlled Delay) is a congestion control algorithm designed to manage queue delays in networking. CoDel aims to reduce bufferbloat, a condition where an excessively large buffer can cause increased delay and packet loss. This algorithm was developed by Van Jacobson and Kathleen Nichols in 2012 as a response to the problem of bufferbloat.

In this article, we will explain how CoDel works, its benefits, and its implementation in networking.

Understanding Bufferbloat

Before we dive into CoDel, it's important to understand the concept of bufferbloat. Bufferbloat is a condition that occurs when a network device has a large buffer, and it fills up with packets. The excess packets in the buffer can cause delays and increased latency, leading to dropped packets and decreased network performance.

This condition can occur at any point in the network, from a user's computer to the router or switch. It can also occur in both wired and wireless networks.

CoDel Overview

CoDel is designed to manage bufferbloat by controlling the delay in the buffer. The algorithm works by monitoring the average delay of the packets in the queue and making decisions based on that delay.

When a packet enters the queue, CoDel measures its delay. If the delay is below a certain threshold, CoDel lets the packet through without delay. However, if the delay is above the threshold, CoDel adds a small delay to the packet to prevent further congestion.

This delay is known as a "droptail," and it is added to the packet's existing delay to create a total delay. The droptail is calculated based on the time the packet has spent in the queue and the average delay of the packets in the queue.

If a packet's total delay exceeds a maximum threshold, CoDel drops the packet. Dropping the packet signals to the sender that the network is congested and that the sender should slow down its transmission rate.

CoDel's Benefits

CoDel has several benefits over traditional queuing algorithms, such as First In First Out (FIFO) and Random Early Detection (RED).

1. Reduced Latency: CoDel's primary benefit is its ability to reduce latency. By controlling the delay in the buffer, CoDel prevents packets from sitting in the buffer for an extended period of time, reducing the overall delay in the network.
2. Improved Throughput: CoDel also improves network throughput by preventing bufferbloat. By dropping packets when the buffer becomes congested, CoDel prevents the buffer from becoming full, which can cause packet loss and decreased throughput.
3. Fairness: CoDel also provides fairness in network traffic by preventing one flow from dominating the buffer. By dropping packets from all flows when the buffer is congested, CoDel ensures that all flows have an equal opportunity to send packets.

CoDel Implementation

CoDel can be implemented in a variety of networking devices, including routers, switches, and end-hosts. In this section, we will discuss how CoDel can be implemented in a router.

CoDel is typically implemented in the kernel of the router's operating system. When a packet enters the router, it is placed in a queue in the kernel, and CoDel monitors the queue for congestion.

CoDel uses two parameters to control the delay in the buffer: the target delay and the interval. The target delay is the desired delay for the packets in the queue, and the interval is the time between updates to the delay measurements.

CoDel uses a sliding window to measure the average delay of the packets in the queue. The size of the window is determined by the interval parameter, and it is used to calculate the average delay of the packets in the queue.

When a packet enters the queue, CoDel measures its delay and adds it to the sliding window. If the delay is below the target delay, CoDel lets the packet through without delay. However, if the delay is above the target delay, CoDel calculates the droptail delay and adds it to the packet's existing delay.

The droptail delay is calculated based on the time the packet has spent in the queue and the average delay of the packets in the queue. CoDel uses a variant of the binary search algorithm to determine the droptail delay.

If the packet's total delay exceeds a maximum threshold, CoDel drops the packet. CoDel uses a variable threshold for dropping packets, known as the "drop probability," which is based on the delay measurements.

CoDel also uses a feature called "time-based dropping" to prevent long-standing flows from dominating the buffer. Time-based dropping drops packets from flows that have been active for a certain period of time, regardless of their delay.

CoDel can be implemented in combination with other queuing algorithms, such as Hierarchical Fair Service Curve (HFSC) and Class-Based Queueing (CBQ). These algorithms can be used to prioritize certain types of traffic, such as voice or video, while still using CoDel to manage buffer delays.

Conclusion

CoDel is a congestion control algorithm designed to manage bufferbloat in networking. By controlling the delay in the buffer, CoDel reduces latency, improves throughput, and provides fairness in network traffic.

CoDel can be implemented in a variety of networking devices, including routers, switches, and end-hosts. It is typically implemented in the kernel of the device's operating system and uses two parameters, the target delay and the interval, to control the delay in the buffer.

CoDel is a significant improvement over traditional queuing algorithms, such as FIFO and RED, and it is widely used in networking today. As the demand for faster and more reliable networks continues to grow, CoDel and other congestion control algorithms will play an increasingly important role in managing network traffic.