tc (Traffic Control) is the kernel's main subsystem for shaping,
queueing, and classifying traffic. Today one part matters: netem
(network emulator), which adds artificial latency, loss, duplication,
and reordering on an interface.
Why: you check how an application behaves when the network degrades.
A common combination is netem plus tcp-handshake/tcp-states, to
watch how retransmits and timeouts catch the loss. Production CI often
runs integration tests with tc qdisc add ... loss 5%, which catches
race conditions that never surface on a clean LAN.