Active queue management

Intentional dropping of network packets during congestion

In routers and switches, active queue management (AQM) is the policy of dropping packets inside a buffer associated with a network interface controller (NIC) before that buffer becomes full, often with the goal of reducing network congestion or improving end to end latency. This task is performed by the network scheduler, which for this purpose uses various algorithms such as random early detection (RED), Explicit Congestion Notification (ECN), or controlled delay (CoDel). RFC 7567 recommends active queue management as a best practice.

An Internet router typically maintains a set of queues, one or more per interface, that hold packets scheduled to go out on that interface. Historically, such queues use a drop tail discipline: a packet is put onto the queue if the queue is shorter than its maximum size (measured in packets or in bytes), and dropped otherwise. Active queue disciplines drop or mark packets before the queue is full. Typically, they operate by maintaining one or more drop/mark probabilities, and occasionally dropping or marking packets according to the probabilities before the queue is full.

Drop tail queues have a tendency to penalise bursty flows, and to cause global synchronisation between flows. By dropping packets probabilistically, AQM disciplines typically avoid both of these issues. By providing endpoints with congestion indication before the queue is full, AQM disciplines are able to maintain a shorter queue length than drop tail queues, which combats bufferbloat and reduces network latency.

An active queue management and denial of Service (AQM&DoS) simulation platform is established based on the NS 2 simulation code of the RRED algorithm. The AQM&DoS simulation platform can simulate a variety of DoS attacks (Distributed DoS, Spoofing DoS, Low rate DoS, etc.) and AQM algorithms (RED, RRED, SFB, etc.). It automatically calculates and records the average throughput of normal TCP flows before and after DoS attacks to facilitate the analysis of the impact of DoS attacks on normal TCP flows and AQM algorithms.