qdiscs

In Linux networking, a qdisc (queuing discipline) is a component of the traffic control (tc) subsystem that governs how packets are queued and transmitted on a network device. Qdiscs operate at the egress of an interface and determine the order and rate at which packets leave the device. They are applied to a network interface using tc and form the lowest-level mechanism for traffic shaping and queue management.

A qdisc can be classless or classful. A classless qdisc makes enqueue decisions based solely on the current state of the queue, while a classful qdisc provides a hierarchy of classes and filters to direct traffic into different subqueues. Common classful qdiscs include HTB, HFSC, and CBQ, which support hierarchical scheduling and rate limiting. Classless qdiscs include pfifo_fast, fq_codel, and cake, which offer simpler queuing or congestion avoidance without user-defined classes. Each qdisc exposes parameters such as queue length, bandwidth limits, and burst settings.

Traffic management goals of qdiscs include shaping bandwidth usage, reducing latency, and preventing bufferbloat. In particular, fq_codel and cake are popular algorithms designed to provide active queue management (AQM) by distributing packets across flows and signaling congestion before queues fill. Netem is another qdisc used to emulate network conditions for testing, including delay and loss, while other qdiscs such as tbf or others focus on precise rate limiting.

Configuration and usage involve attaching a qdisc to a device, optionally with a root handle and, for classful qdiscs, creating a class and associated filters to classify traffic. The tc utility is used to add, change, or replace qdiscs and their classes, and to attach filters that steer traffic to specific classes.