fq_codel
Linux qdisc: Fair Queueing + Controlled Delay. Default in Linux 3.6+ (2012). Combines per-flow fair queueing (mice protected from elephants) with the CoDel algorithm for buffer-bloat prevention.
CoDel: track packet time in queue, if over 5ms target start dropping head-of-queue packets → TCP lowers rate. fq: hash-based per-flow queue so a heavy flow doesn't monopolize buffer. Result: dramatic reduction in tail latency on loaded links without tuning. Modern WiFi router: fq_codel default for excellent gaming + video-call experience under load. Competitor: CAKE (all-in-one shaper for WAN uplinks).