<s>
The	O
cookie	B-Protocol
exchange	I-Protocol
in	O
IPsec	B-Protocol
comes	O
under	O
the	O
Oakley	B-Protocol
protocol	I-Protocol
,	O
which	O
is	O
a	O
protocol	O
of	O
key	O
management	O
.	O
</s>
<s>
The	O
cookie	B-Protocol
exchange	I-Protocol
requires	O
that	O
each	O
side	O
send	O
a	O
pseudorandom	B-Error_Name
number	I-Error_Name
,	O
the	O
cookie	B-Application
,	O
in	O
the	O
initial	O
message	O
,	O
which	O
the	O
other	O
side	O
acknowledges	O
.	O
</s>
<s>
This	O
acknowledgement	O
must	O
be	O
repeated	O
in	O
the	O
first	O
message	O
of	O
the	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
key	I-Protocol
exchange	I-Protocol
.	O
</s>
<s>
Thus	O
,	O
an	O
opponent	O
can	O
only	O
force	O
a	O
user	O
to	O
generate	O
acknowledgements	O
and	O
not	O
to	O
perform	O
the	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
calculation	O
.	O
</s>
<s>
Note	O
that	O
"	O
cookies	B-Application
"	O
in	O
the	O
sense	O
of	O
IPsec	B-Protocol
are	O
unrelated	O
to	O
HTTP	B-Application
cookies	I-Application
used	O
by	O
web	O
browsers	O
.	O
</s>
<s>
The	O
recommended	O
method	O
for	O
creating	O
the	O
cookie	B-Application
is	O
to	O
perform	O
a	O
fast	O
hash	B-Error_Name
(	O
e.g.	O
</s>
<s>
MD5	B-Algorithm
)	O
over	O
the	O
IP	B-Protocol
source	O
and	O
destination	O
addresses	O
,	O
the	O
UDP	O
source	O
and	O
destination	O
ports	O
,	O
and	O
a	O
locally	O
generated	O
secret	O
value	O
.	O
</s>
