tc (Linux)
Linux traffic control — command + kernel subsystem to shape, prioritize, drop packets. Part of iproute2 package. Powerful but notoriously complex syntax. Drives everything from simple "limit bandwidth" to enterprise QoS.
Components: qdisc (queueing discipline — pfifo, fq_codel, htb, cake), class (hierarchy within htb), filter (select packets to specific class). Classic usage: tc qdisc add dev eth0 root tbf rate 1mbit burst 32kbit latency 400ms — rate-limit to 1 Mbps. Modern: CAKE as auto-configured shaper for WAN uplinks. Debug difficulty: tc -s qdisc show + tc -s class show requires expertise. iptables-mangle marks integrate with tc filter for per-app QoS.