This is the device-wide subset of received chunks that are control chunks rather than data. A high ratio of control to data chunks received can mean peers are heartbeating aggressively or generating more SACKs than necessary, adding CPU load to this device's stack without carrying real signaling payload. If this number tracks close to what the number of active associations times an expected heartbeat interval would predict, the control traffic is just normal keepalive load rather than anything abnormal.