<s>
The	O
Security	B-Protocol
Parameter	I-Protocol
Index	I-Protocol
(	O
SPI	O
)	O
is	O
an	O
identification	O
tag	O
added	O
to	O
the	O
header	O
while	O
using	O
IPsec	B-Protocol
for	O
tunneling	O
the	O
IP	O
traffic	O
.	O
</s>
<s>
The	O
SPI	O
(	O
as	O
per	O
RFC	O
2401	O
)	O
is	O
a	O
required	O
part	O
of	O
an	O
IPsec	B-Protocol
Security	B-Protocol
Association	I-Protocol
(	O
SA	O
)	O
because	O
it	O
enables	O
the	O
receiving	O
system	O
to	O
select	O
the	O
SA	O
under	O
which	O
a	O
received	O
packet	O
will	O
be	O
processed	O
.	O
</s>
