fq_codel
also: Fair Queuing Controlled Delay, fq-codel
fq_codel is a network packet scheduling algorithm that combines fair queuing with CoDel (Controlled Delay) to reduce latency and buffer bloat in network transmission.
fq_codel (Fair Queuing Controlled Delay) is a qdisc (queue discipline) in Linux that manages how packets are sent over network interfaces. It divides traffic into separate flows and applies CoDel's intelligent dropping mechanism to each, ensuring low latency while maintaining fairness between connections.
CoDel works by monitoring packet sojourn time—how long packets sit in the queue—and drops packets when delay exceeds a target threshold. Fair queuing ensures that no single flow monopolizes bandwidth, so a large download won't starve interactive traffic like video calls.
Example: Instead of letting a bulk file transfer fill the queue and cause 100ms lag in your SSH session, fq_codel keeps both flows responsive by limiting queue buildup and allocating bandwidth fairly.
This is the default qdisc on most modern Linux systems and is particularly effective for reducing buffer bloat on home routers and edge networks.