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The cookie exchange in IPsec comes under the Oakley protocol, which is a protocol of key management. The cookie exchange requires that each side send a pseudorandom number, the cookie, in the initial message, which the other side acknowledges. This acknowledgement must be repeated in the first message of the Diffie-Hellman key exchange. If the source address was forged, the opponent gets no answer. Thus, an opponent can only force a user to generate acknowledgements and not to perform the Diffie-Hellman calculation. Note that "cookies" in the sense of IPsec are unrelated to HTTP cookies used by web browsers.
The recommended method for creating the cookie is to perform a fast hash (e.g. MD5) over the IP source and destination addresses, the UDP source and destination ports, and a locally generated secret value.