220.127.116.11 Receiving Message #1
On receipt of message #1, the host MUST validate the message, as specified in [RFC2408] section 5. If message #1 is correctly validated, the host MUST construct message #2 in response, as follows:
The message MUST consist of only an ISAKMP header and a Notify payload structure, as specified in [RFC2408] section 3.14.
The notify message type MUST be set to NOTIFY_DOS_COOKIE, and the notification data MUST contain an 8-byte cookie value. The cookie generation mechanism is implementation-dependent but SHOULD be stateless to provide good DoS protection.<29>
The host MUST then silently discard message #1, even if the message is correctly validated.