Packet Lab
Follow one packet hop by hop and inspect its state, then inject failures — loss, latency, DNS outage, expired certificate, blocked port — and predict the behaviour.
Pick a source, a destination, a protocol and a port, and follow a single packet from `send()` through the socket, transport, IP and link layers, out of the NIC, through a NAT router and every hop after it, watching which headers exist, which fields change and which stay fixed — and what a router does in the microseconds it holds the packet.
Eight buttons — drop a packet, add 200 ms, add 5% loss, kill DNS, kill a router, expire the certificate, block the port, reset from the server — each produce a distinct symptom at the application, a distinct chain of events through the layers, and a distinct tool that confirms it; the skill is predicting all three before pressing the button.