sands-lab/nika/tc-fault-skill

Traffic Control Faults

What These Faults Mean

• link_bandwidth_throttling: a device interface owns a tbf shaping rule that caps throughput.
• link_high_packet_corruption: a netem corrupt rule randomly damages packets, so TCP retransmits and throughput collapses.
• incast_traffic_network_limitation: the qdisc stack combines netem and tbf, creating fan-in or queueing behavior that looks worse than a simple bandwidth cap.

Leading Signals

• tc qdisc show reveals a non-default shaping stack on the implicated interface.
• The qdisc layout itself is the decisive proof; RTT or throughput symptoms only tell you where to look.

When To Use

• Quiet throughput degradation
• Extreme jitter with little or no packet loss
• Suspected qdisc or shaping on a server or router interface
• If ownership is still unclear, run tc_snapshot.py from diagnosis-methodology-skill/scripts to find non-default qdisc stacks before targeted tc checks.

Exact Calls

• exec_shell(host_name="<host>", command="tc qdisc show dev <interface>")
• get_tc_statistics(host_name="<host>", intf_name="<interface>")
• exec_shell(host_name="<host>", command="tc -s qdisc show dev <interface>")

Required Tool Usage

• get_tc_statistics always needs both host_name and intf_name.
• Check the specific interface under test, not a default placeholder. If the bottleneck may be on a different interface, use that one.

Guardrails

• The decisive signal is the qdisc stack itself, not just high RTT.
• Report only the device that actually owns the shaping rule.
• Sending clients are victims, not faulty devices, unless a different skill proves otherwise.