<s>
In	O
routing	B-Protocol
,	O
the	O
forwarding	B-Protocol
plane	I-Protocol
,	O
sometimes	O
called	O
the	O
data	B-Protocol
plane	I-Protocol
or	O
user	B-Protocol
plane	I-Protocol
,	O
defines	O
the	O
part	O
of	O
the	O
router	B-Protocol
architecture	O
that	O
decides	O
what	O
to	O
do	O
with	O
packets	O
arriving	O
on	O
an	O
inbound	O
interface	O
.	O
</s>
<s>
Most	O
commonly	O
,	O
it	O
refers	O
to	O
a	O
table	O
in	O
which	O
the	O
router	B-Protocol
looks	O
up	O
the	O
destination	O
address	O
of	O
the	O
incoming	O
packet	O
and	O
retrieves	O
the	O
information	O
necessary	O
to	O
determine	O
the	O
path	O
from	O
the	O
receiving	O
element	O
,	O
through	O
the	O
internal	O
forwarding	O
fabric	O
of	O
the	O
router	B-Protocol
,	O
and	O
to	O
the	O
proper	O
outgoing	O
interface(s )	O
.	O
</s>
<s>
In	O
such	O
cases	O
,	O
the	O
router	B-Protocol
may	O
return	O
an	O
ICMP	O
"	O
destination	O
unreachable	O
"	O
or	O
other	O
appropriate	O
code	O
.	O
</s>
<s>
Some	O
security	O
policies	O
,	O
however	O
,	O
dictate	O
that	O
the	O
router	B-Protocol
should	O
drop	O
the	O
packet	O
silently	O
,	O
in	O
order	O
that	O
a	O
potential	O
attacker	O
does	O
not	O
become	O
aware	O
that	O
a	O
target	O
is	O
being	O
protected	O
.	O
</s>
<s>
While	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
specification	O
indicates	O
that	O
an	O
Internet	B-Protocol
Control	I-Protocol
Message	I-Protocol
Protocol	I-Protocol
(	O
ICMP	O
)	O
time	O
exceeded	O
message	O
should	O
be	O
sent	O
to	O
the	O
originator	O
of	O
the	O
packet	O
(	O
i.e.	O
</s>
<s>
the	O
node	O
indicated	O
by	O
the	O
source	O
address	O
)	O
,	O
the	O
router	B-Protocol
may	O
be	O
configured	O
to	O
drop	O
the	O
packet	O
silently	O
(	O
again	O
according	O
to	O
security	O
policies	O
)	O
.	O
</s>
<s>
Depending	O
on	O
the	O
specific	O
router	B-Protocol
implementation	O
,	O
the	O
table	O
in	O
which	O
the	O
destination	O
address	O
is	O
looked	O
up	O
could	O
be	O
the	O
routing	B-Protocol
table	I-Protocol
(	O
also	O
known	O
as	O
the	O
routing	B-Protocol
information	I-Protocol
base	I-Protocol
,	O
RIB	O
)	O
,	O
or	O
a	O
separate	O
forwarding	B-Protocol
information	I-Protocol
base	I-Protocol
(	O
FIB	O
)	O
that	O
is	O
populated	O
(	O
i.e.	O
,	O
loaded	O
)	O
by	O
the	O
routing	B-Protocol
control	I-Protocol
plane	I-Protocol
,	O
but	O
used	O
by	O
the	O
forwarding	B-Protocol
plane	I-Protocol
for	O
look-ups	O
at	O
much	O
higher	O
speeds	O
.	O
</s>
<s>
Before	O
or	O
after	O
examining	O
the	O
destination	O
,	O
other	O
tables	O
may	O
be	O
consulted	O
to	O
make	O
decisions	O
to	O
drop	O
the	O
packet	B-Protocol
based	I-Protocol
on	O
other	O
characteristics	O
,	O
such	O
as	O
the	O
source	O
address	O
,	O
the	O
IP	B-Protocol
protocol	I-Protocol
identifier	O
field	O
,	O
or	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
or	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
port	O
number	O
.	O
</s>
<s>
Forwarding	B-Protocol
plane	I-Protocol
functions	O
run	O
in	O
the	O
forwarding	O
element	O
.	O
</s>
<s>
High-performance	O
routers	B-Protocol
often	O
have	O
multiple	O
distributed	O
forwarding	O
elements	O
,	O
so	O
that	O
the	O
router	B-Protocol
increases	O
performance	O
with	O
parallel	O
processing	O
.	O
</s>
<s>
Depending	O
on	O
the	O
router	B-Protocol
software	O
and	O
its	O
configuration	B-General_Concept
,	O
functions	O
,	O
usually	O
implemented	O
at	O
the	O
outgoing	O
interface	O
,	O
may	O
set	O
various	O
packet	O
fields	O
,	O
such	O
as	O
the	O
DSCP	O
field	O
used	O
by	O
differentiated	O
services	O
.	O
</s>
<s>
In	O
general	O
,	O
the	O
passage	O
from	O
the	O
input	O
interface	O
directly	O
to	O
an	O
output	O
interface	O
,	O
through	O
the	O
fabric	O
with	O
minimum	O
modification	O
at	O
the	O
output	O
interface	O
,	O
is	O
called	O
the	O
fast	O
path	O
of	O
the	O
router	B-Protocol
.	O
</s>
<s>
If	O
the	O
packet	O
needs	O
significant	O
processing	O
,	O
such	O
as	O
segmentation	O
or	O
encryption	O
,	O
it	O
may	O
go	O
onto	O
a	O
slower	O
path	O
,	O
which	O
is	O
sometimes	O
called	O
the	O
services	O
plane	O
of	O
the	O
router	B-Protocol
.	O
</s>
<s>
The	O
data	B-Protocol
plane	I-Protocol
is	O
the	O
part	O
of	O
the	O
software	O
that	O
processes	O
the	O
data	O
requests	O
.	O
</s>
<s>
By	O
contrast	O
,	O
the	O
control	B-Protocol
plane	I-Protocol
is	O
the	O
part	O
of	O
the	O
software	O
that	O
configures	O
and	O
shuts	O
down	O
the	O
data	B-Protocol
plane	I-Protocol
.	O
</s>
<s>
The	O
conceptual	O
separation	O
of	O
the	O
data	B-Protocol
plane	I-Protocol
from	O
the	O
control	B-Protocol
plane	I-Protocol
has	O
been	O
done	O
for	O
years	O
.	O
</s>
<s>
An	O
early	O
example	O
is	O
Unix	B-Application
,	O
where	O
the	O
basic	O
file	O
operations	O
are	O
open	O
,	O
close	O
for	O
the	O
control	B-Protocol
plane	I-Protocol
and	O
read	O
,	O
write	O
for	O
the	O
data	B-Protocol
plane	I-Protocol
.	O
</s>
<s>
The	O
conceptual	O
separation	O
of	O
the	O
data	B-Protocol
plane	I-Protocol
from	O
the	O
control	B-Protocol
plane	I-Protocol
in	O
software	O
programming	O
has	O
proven	O
useful	O
in	O
the	O
packet	B-Protocol
switching	I-Protocol
field	O
where	O
it	O
originated	O
.	O
</s>
<s>
In	O
networking	B-Architecture
,	O
the	O
data	B-Protocol
plane	I-Protocol
is	O
sometimes	O
referred	O
to	O
as	O
the	O
forwarding	B-Protocol
plane	I-Protocol
,	O
as	O
it	O
separates	O
the	O
concerns	O
:	O
the	O
data	B-Protocol
plane	I-Protocol
is	O
optimized	O
for	O
speed	O
of	O
processing	O
,	O
and	O
for	O
simplicity	O
and	O
regularity	O
.	O
</s>
<s>
The	O
control	B-Protocol
plane	I-Protocol
is	O
optimized	O
so	O
as	O
to	O
allow	O
configuration	B-General_Concept
,	O
handling	O
policies	O
,	O
handling	O
exceptional	O
situations	O
,	O
and	O
in	O
general	O
facilitating	O
and	O
simplifying	O
the	O
data	B-Protocol
plane	I-Protocol
processing	O
.	O
</s>
<s>
Vendors	O
design	O
router	B-Protocol
products	O
for	O
specific	O
markets	O
.	O
</s>
<s>
Design	O
of	O
routers	B-Protocol
intended	O
for	O
home	O
use	O
,	O
perhaps	O
supporting	O
several	O
PCs	O
and	O
VoIP	O
telephony	O
,	O
is	O
driven	O
by	O
keeping	O
the	O
cost	O
as	O
low	O
as	O
possible	O
.	O
</s>
<s>
In	O
such	O
a	O
router	B-Protocol
,	O
there	O
is	O
no	O
separate	O
forwarding	O
fabric	O
,	O
and	O
there	O
is	O
only	O
one	O
active	O
forwarding	O
path	O
:	O
into	O
the	O
main	O
processor	O
and	O
out	O
of	O
the	O
main	O
processor	O
.	O
</s>
<s>
Routers	B-Protocol
for	O
more	O
demanding	O
applications	O
accept	O
greater	O
cost	O
and	O
complexity	O
to	O
get	O
higher	O
throughput	O
in	O
their	O
forwarding	B-Protocol
planes	I-Protocol
.	O
</s>
<s>
Several	O
design	O
factors	O
affect	O
router	B-Protocol
forwarding	O
performance	O
:	O
</s>
<s>
Routers	B-Protocol
may	O
have	O
one	O
or	O
more	O
processors	O
.	O
</s>
<s>
Higher-performance	O
routers	B-Protocol
invariably	O
have	O
multiple	O
processing	O
elements	O
,	O
which	O
may	O
be	O
general-purpose	O
processor	O
chips	O
or	O
specialized	O
application-specific	O
integrated	O
circuits	O
(	O
ASIC	O
)	O
.	O
</s>
<s>
In	O
such	O
designs	O
,	O
the	O
main	O
processor	O
does	O
not	O
participate	O
in	O
forwarding	O
,	O
but	O
only	O
in	O
control	B-Protocol
plane	I-Protocol
and	O
management	O
processing	O
.	O
</s>
<s>
Performance	O
measurements	O
on	O
single	O
routers	B-Protocol
,	O
or	O
narrowly	O
defined	O
systems	O
of	O
routers	B-Protocol
,	O
are	O
the	O
province	O
of	O
the	O
Benchmarking	O
Working	O
Group	O
(	O
BMWG	O
)	O
.	O
</s>
<s>
A	O
classic	O
RFC	O
2544	O
benchmark	O
uses	O
half	O
the	O
router	B-Protocol
's	O
(	O
i.e.	O
,	O
the	O
device	O
under	O
test	O
(	O
DUT	O
)	O
)	O
ports	O
for	O
input	O
of	O
a	O
defined	O
load	O
,	O
and	O
measures	O
the	O
time	O
at	O
which	O
the	O
outputs	O
appear	O
at	O
the	O
output	O
ports	O
.	O
</s>
<s>
Perhaps	O
the	O
first	O
step	O
in	O
speeding	O
routers	B-Protocol
was	O
to	O
have	O
a	O
separate	O
RIB	O
and	O
FIB	O
in	O
main	O
memory	O
,	O
with	O
the	O
FIB	O
,	O
typically	O
with	O
fewer	O
entries	O
than	O
the	O
RIB	O
,	O
being	O
organized	O
for	O
fast	O
destination	O
lookup	O
.	O
</s>
<s>
In	O
contrast	O
,	O
the	O
RIB	O
was	O
optimized	O
for	O
efficient	O
updating	O
by	O
routing	B-Protocol
protocols	O
.	O
</s>
<s>
Early	O
uniprocessing	O
routers	B-Protocol
usually	O
organized	O
the	O
FIB	O
as	O
a	O
hash	B-Algorithm
table	I-Algorithm
,	O
while	O
the	O
RIB	O
might	O
be	O
a	O
linked	B-Data_Structure
list	I-Data_Structure
.	O
</s>
<s>
When	O
routers	B-Protocol
started	O
to	O
have	O
separate	O
forwarding	O
processors	O
,	O
these	O
processors	O
usually	O
had	O
far	O
less	O
memory	O
than	O
the	O
main	O
processor	O
,	O
such	O
that	O
the	O
forwarding	O
processor	O
could	O
hold	O
only	O
the	O
most	O
frequently	O
used	O
routes	O
.	O
</s>
<s>
Such	O
a	O
cache	O
,	O
however	O
,	O
was	O
far	O
too	O
small	O
for	O
general	O
Internet	O
routing	B-Protocol
.	O
</s>
<s>
Different	O
router	B-Protocol
designs	O
behaved	O
in	O
different	O
ways	O
when	O
a	O
destination	O
was	O
not	O
in	O
the	O
cache	O
.	O
</s>
<s>
A	O
cache	O
miss	O
condition	O
might	O
result	O
in	O
the	O
packet	O
being	O
sent	O
back	O
to	O
the	O
main	O
processor	O
,	O
to	O
be	O
looked	O
up	O
in	O
a	O
slow	O
path	O
that	O
had	O
access	O
to	O
the	O
full	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
Depending	O
on	O
the	O
router	B-Protocol
design	O
,	O
a	O
cache	O
miss	O
might	O
cause	O
an	O
update	O
to	O
the	O
fast	O
hardware	O
cache	O
or	O
the	O
fast	O
cache	O
in	O
main	O
memory	O
.	O
</s>
<s>
Whenever	O
a	O
RIB	O
entry	O
changed	O
,	O
the	O
router	B-Protocol
changed	O
the	O
corresponding	O
FIB	O
entry	O
.	O
</s>
<s>
Various	O
search	B-Application
algorithms	I-Application
have	O
been	O
used	O
for	O
FIB	O
lookup	O
.	O
</s>
<s>
While	O
well-known	O
general-purpose	O
data	O
structures	O
were	O
first	O
used	O
,	O
such	O
as	O
hash	B-Algorithm
tables	I-Algorithm
,	O
specialized	O
algorithms	O
,	O
optimized	O
for	O
IP	O
addresses	O
,	O
emerged	O
.	O
</s>
<s>
A	O
multicore	B-Architecture
CPU	I-Architecture
architecture	O
is	O
commonly	O
used	O
to	O
implement	O
high-performance	O
networking	B-Architecture
systems	O
.	O
</s>
<s>
Various	O
forms	O
of	O
fast	O
RAM	O
and	O
,	O
eventually	O
,	O
basic	O
content-addressable	B-Data_Structure
memory	I-Data_Structure
(	O
CAM	B-Data_Structure
)	O
were	O
used	O
to	O
speed	O
lookup	O
.	O
</s>
<s>
CAM	B-Data_Structure
,	O
while	O
useful	O
in	O
layer	O
2	O
switches	O
that	O
needed	O
to	O
look	O
up	O
a	O
relatively	O
small	O
number	O
of	O
fixed-length	O
MAC	O
addresses	O
,	O
had	O
limited	O
utility	O
with	O
IP	O
addresses	O
having	O
variable-length	O
routing	B-Protocol
prefixes	O
(	O
see	O
Classless	B-Protocol
Inter-Domain	I-Protocol
Routing	I-Protocol
)	O
.	O
</s>
<s>
Ternary	O
CAM	B-Data_Structure
(	O
CAM	B-Data_Structure
)	O
,	O
while	O
expensive	O
,	O
lends	O
itself	O
to	O
variable-length	O
prefix	O
lookups	O
.	O
</s>
<s>
A	O
next	O
step	O
in	O
speeding	O
routers	B-Protocol
was	O
to	O
have	O
a	O
specialized	O
forwarding	O
processor	O
separate	O
from	O
the	O
main	O
processor	O
.	O
</s>
<s>
The	O
fast	O
routing	B-Protocol
processor	O
typically	O
had	O
a	O
small	O
FIB	O
,	O
with	O
hardware	O
memory	O
(	O
e.g.	O
,	O
static	B-Architecture
random-access	I-Architecture
memory	I-Architecture
(	O
SRAM	O
)	O
)	O
faster	O
and	O
more	O
expensive	O
than	O
the	O
FIB	O
in	O
main	O
memory	O
.	O
</s>
<s>
Next	O
,	O
routers	B-Protocol
began	O
to	O
have	O
multiple	O
forwarding	O
elements	O
,	O
that	O
communicated	O
through	O
a	O
high-speed	O
shared	O
bus	O
or	O
through	O
a	O
shared	O
memory	O
.	O
</s>
<s>
While	O
a	O
router	B-Protocol
might	O
have	O
16	O
forwarding	O
engines	O
,	O
if	O
there	O
was	O
a	O
single	O
bus	O
,	O
only	O
one	O
packet	O
transfer	O
at	O
a	O
time	O
was	O
possible	O
.	O
</s>
<s>
With	O
a	O
small	O
number	O
of	O
forwarding	O
engines	O
,	O
crossbar	O
forwarding	O
fabrics	O
are	O
practical	O
and	O
efficient	O
for	O
high-performance	O
routing	B-Protocol
.	O
</s>
<s>
There	O
are	O
multistage	O
designs	O
for	O
crossbar	O
systems	O
,	O
such	O
as	O
Clos	B-Device
networks	I-Device
.	O
</s>
