<s>
Anti-replay	B-Protocol
is	O
a	O
sub-protocol	O
of	O
IPsec	B-Protocol
that	O
is	O
part	O
of	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
.	O
</s>
<s>
The	O
main	O
goal	O
of	O
anti-replay	B-Protocol
is	O
to	O
avoid	O
hackers	B-Application
injecting	O
or	O
making	O
changes	O
in	O
packets	B-Protocol
that	O
travel	O
from	O
a	O
source	O
to	O
a	O
destination	O
.	O
</s>
<s>
Anti-replay	B-Protocol
protocol	O
uses	O
a	O
unidirectional	B-Device
security	B-Protocol
association	I-Protocol
in	O
order	O
to	O
establish	O
a	O
secure	B-Protocol
connection	I-Protocol
between	O
two	O
nodes	O
in	O
the	O
network	O
.	O
</s>
<s>
Once	O
a	O
secure	B-Protocol
connection	I-Protocol
is	O
established	O
,	O
the	O
anti-replay	B-Protocol
protocol	O
uses	O
packet	B-Protocol
sequence	O
numbers	O
to	O
defeat	O
replay	O
attacks	O
as	O
follows	O
:	O
When	O
the	O
source	O
sends	O
a	O
message	O
,	O
it	O
adds	O
a	O
sequence	O
number	O
to	O
its	O
packet	B-Protocol
;	O
the	O
sequence	O
number	O
starts	O
at	O
0	O
and	O
is	O
incremented	O
by	O
1	O
for	O
each	O
subsequent	O
packet	B-Protocol
.	O
</s>
<s>
The	O
destination	O
maintains	O
a	O
'	O
sliding	O
window	O
 '	O
record	O
of	O
the	O
sequence	O
numbers	O
of	O
validated	O
received	O
packets	B-Protocol
;	O
it	O
rejects	O
all	O
packets	B-Protocol
which	O
have	O
a	O
sequence	O
number	O
which	O
is	O
lower	O
than	O
the	O
lowest	O
in	O
the	O
sliding	O
window	O
(	O
i.e.	O
</s>
<s>
Accepted	O
packets	B-Protocol
,	O
once	O
validated	O
,	O
update	O
the	O
sliding	O
window	O
(	O
displacing	O
the	O
lowest	O
sequence	O
number	O
out	O
of	O
the	O
window	O
if	O
it	O
was	O
already	O
full	O
)	O
.	O
</s>
