AQM (active queue management)

Active Queue Management (AQM) is a technique used in network protocols to regulate network congestion. Congestion occurs when the amount of data transmitted exceeds the capacity of the network to handle it. AQM manages congestion by selectively dropping or marking packets at the network routers. AQM algorithms aim to reduce packet loss, improve network utilization and fairness, and minimize the round-trip delay of packets.

In this article, we will discuss AQM in detail, including its working, benefits, and challenges.

Background

TCP (Transmission Control Protocol) is a widely used transport protocol that controls congestion in the network. When a TCP sender detects packet loss, it assumes that the network is congested and reduces the transmission rate. This results in reduced congestion in the network. However, if the packet loss rate is high, TCP sends packets at a low rate, resulting in underutilization of the network.

In the past, the network was designed to handle best-effort traffic, where packets were dropped when congestion occurred. However, with the increase in real-time applications, such as video conferencing and online gaming, packet loss has become unacceptable. AQM algorithms were developed to reduce packet loss and improve network utilization.

AQM Techniques

AQM algorithms aim to provide congestion control by selectively dropping or marking packets. AQM techniques can be classified into two categories: drop-based and mark-based.

Drop-based AQM

Drop-based AQM algorithms selectively drop packets when congestion occurs. The simplest form of drop-based AQM is tail drop. In tail drop, packets are dropped when the buffer at the router is full. This results in a sudden increase in packet loss when the buffer is full, leading to congestion collapse.

To avoid congestion collapse, random early detection (RED) was developed. In RED, packets are randomly dropped before the buffer is full. The probability of dropping a packet is proportional to the average queue length. This results in a smooth increase in packet loss when the buffer approaches its capacity, providing an early warning of congestion.

Other drop-based AQM algorithms include BLUE (Binary Exponential Backoff), FRED (Flow Random Early Drop), and DCTCP (Data Center TCP).

Mark-based AQM

Mark-based AQM algorithms selectively mark packets when congestion occurs. The most commonly used mark-based AQM algorithm is ECN (Explicit Congestion Notification). In ECN, packets are marked with an ECN bit in the header when congestion occurs. The ECN bit indicates that congestion has been detected and allows the TCP sender to reduce the transmission rate. ECN provides an early warning of congestion without dropping packets.

AQM Benefits

AQM algorithms provide several benefits, including:

Reduced Packet Loss

AQM algorithms reduce packet loss by providing an early warning of congestion. This allows TCP senders to reduce the transmission rate, avoiding congestion collapse.

Improved Network Utilization

AQM algorithms improve network utilization by preventing underutilization of the network due to low transmission rates caused by high packet loss.

Fairness

AQM algorithms provide fairness by treating all flows equally. When congestion occurs, AQM algorithms drop or mark packets from all flows, preventing one flow from dominating the network.

Improved QoS

AQM algorithms improve QoS (Quality of Service) by reducing delay and jitter. By providing an early warning of congestion, AQM algorithms prevent long queuing delays, reducing the round-trip delay of packets.

AQM Challenges

Despite the benefits of AQM algorithms, there are several challenges, including:

Complexity

AQM algorithms are complex and require careful tuning. The parameters of AQM algorithms, such as the drop probability or marking probability, need to be carefully chosen to provide optimal performance.

Compatibility

AQM algorithms need to be compatible with existing protocols and applications. For example, some applications may not be designed to handle marked packets, leading to reduced performance.

Deployment

AQM algorithms need to be widely deployed to be effective. However, deployment can be challenging due to the wide variety of devices and protocols used in the network.

Security

AQM algorithms can be vulnerable to attacks, such as denial of service (DoS) attacks, where an attacker floods the network with packets. AQM algorithms need to be designed to prevent or mitigate such attacks.

Conclusion

Active Queue Management (AQM) is a technique used to regulate network congestion by selectively dropping or marking packets at network routers. AQM algorithms aim to reduce packet loss, improve network utilization and fairness, and minimize the round-trip delay of packets. AQM techniques can be classified into drop-based and mark-based. Drop-based AQM algorithms selectively drop packets, while mark-based AQM algorithms selectively mark packets. AQM algorithms provide several benefits, including reduced packet loss, improved network utilization, fairness, and improved QoS. However, there are several challenges, including complexity, compatibility, deployment, and security. Despite the challenges, AQM algorithms are an important tool for managing congestion in the network and improving network performance.