3.2.5.5 Receiving a FLOOD Message
Section 3.1.5.4 specifies the rules for handling received FLOOD messages. In addition to those rules, when sending an ACK message in response to a FLOOD message with a nonzero Validate Key field, a Publisher MUST check its Locally Registered Key List. If the Validate Key is not in the list, the Publisher MUST set the N flag in the ACK message.