“Loop DoS relies on IP spoofing and can be triggered from a single host that sends one message to start the communication.”
One spoofed UDP packet gets two servers yelling error messages at each other until both fall over. CISPA counts 300,000 vulnerable hosts across DNS, NTP, TFTP and relics like Chargen and Echo. Broadcom, Cisco, Honeywell, Microsoft and MikroTik confirmed they ship it. BCP38 anti-spoofing filtering would have killed this class of attack years ago, and ISPs still treat it as optional.