<s>
A	O
load-balanced	B-Protocol
switch	I-Protocol
is	O
a	O
switch	O
architecture	O
which	O
guarantees	O
100%	O
throughput	O
with	O
no	O
central	O
arbitration	O
at	O
all	O
,	O
at	O
the	O
cost	O
of	O
sending	O
each	O
packet	O
across	O
the	O
crossbar	O
twice	O
.	O
</s>
<s>
Load-balanced	B-Protocol
switches	I-Protocol
are	O
a	O
subject	O
of	O
research	O
for	O
large	O
routers	B-Protocol
scaled	O
past	O
the	O
point	O
of	O
practical	O
central	O
arbitration	O
.	O
</s>
<s>
Internet	B-Protocol
routers	I-Protocol
are	O
typically	O
built	O
using	O
line	O
cards	O
connected	O
with	O
a	O
switch	O
.	O
</s>
<s>
Routers	B-Protocol
supporting	O
moderate	O
total	O
bandwidth	O
may	O
use	O
a	O
bus	B-General_Concept
as	O
their	O
switch	O
,	O
but	O
high	O
bandwidth	O
routers	B-Protocol
typically	O
use	O
some	O
sort	O
of	O
crossbar	O
interconnection	O
.	O
</s>
<s>
A	O
load-balanced	B-Protocol
switch	I-Protocol
is	O
not	O
related	O
to	O
a	O
load	O
balancing	O
switch	O
,	O
which	O
refers	O
to	O
a	O
kind	O
of	O
router	B-Protocol
used	O
as	O
a	O
front	O
end	O
to	O
a	O
farm	O
of	O
web	O
servers	O
to	O
spread	O
requests	O
to	O
a	O
single	O
website	O
across	O
many	O
servers	O
.	O
</s>
<s>
As	O
shown	O
in	O
the	O
figure	O
to	O
the	O
right	O
,	O
a	O
load-balanced	B-Protocol
switch	I-Protocol
has	O
N	O
input	O
line	O
cards	O
,	O
each	O
of	O
rate	O
R	O
,	O
each	O
connected	O
to	O
N	O
buffers	O
by	O
a	O
link	O
of	O
rate	O
R/N	O
.	O
</s>
<s>
Each	O
buffer	O
in	O
a	O
load-balanced	B-Protocol
switch	I-Protocol
acts	O
as	O
a	O
shared-memory	O
switch	O
,	O
and	O
a	O
load-balanced	B-Protocol
switch	I-Protocol
is	O
essentially	O
a	O
way	O
to	O
scale	O
up	O
a	O
shared-memory	O
switch	O
,	O
at	O
the	O
cost	O
of	O
additional	O
latency	O
associated	O
with	O
forwarding	O
packets	O
at	O
rate	O
R/N	O
twice	O
.	O
</s>
<s>
The	O
Stanford	O
group	O
investigating	O
load-balanced	B-Protocol
switches	I-Protocol
is	O
concentrating	O
on	O
implementations	O
where	O
the	O
number	O
of	O
buffers	O
is	O
equal	O
to	O
the	O
number	O
of	O
line	O
cards	O
.	O
</s>
<s>
But	O
the	O
basic	O
load-balanced	B-Protocol
switch	I-Protocol
architecture	O
does	O
not	O
require	O
that	O
the	O
buffers	O
be	O
placed	O
on	O
the	O
line	O
cards	O
,	O
or	O
that	O
there	O
be	O
the	O
same	O
number	O
of	O
buffers	O
and	O
line	O
cards	O
.	O
</s>
<s>
One	O
interesting	O
property	O
of	O
a	O
load-balanced	B-Protocol
switch	I-Protocol
is	O
that	O
,	O
although	O
the	O
mesh	O
connecting	O
line	O
cards	O
to	O
buffers	O
is	O
required	O
to	O
connect	O
every	O
line	O
card	O
to	O
every	O
buffer	O
,	O
there	O
is	O
no	O
requirement	O
that	O
the	O
mesh	O
act	O
as	O
a	O
non-blocking	O
crossbar	O
,	O
nor	O
that	O
the	O
connections	O
be	O
responsive	O
to	O
any	O
traffic	O
pattern	O
.	O
</s>
<s>
Although	O
reordering	O
is	O
legal	O
,	O
it	O
is	O
typically	O
undesirable	O
because	O
TCP	B-Protocol
does	O
not	O
perform	O
well	O
with	O
reordered	O
packets	O
.	O
</s>
<s>
By	O
adding	O
yet	O
more	O
latency	O
and	O
buffering	O
,	O
the	O
load-balanced	B-Protocol
switch	I-Protocol
can	O
maintain	O
packet	O
order	O
within	O
flows	O
using	O
only	O
local	O
information	O
.	O
</s>
<s>
Since	O
the	O
line	O
cards	O
in	O
a	O
load-balanced	B-Protocol
switch	I-Protocol
do	O
not	O
need	O
to	O
be	O
physically	O
near	O
one	O
another	O
,	O
one	O
possible	O
implementation	O
is	O
to	O
use	O
an	O
entire	O
continent	O
-	O
or	O
global-sized	O
backbone	O
network	O
as	O
the	O
interconnection	O
mesh	O
,	O
and	O
core	O
routers	B-Protocol
as	O
the	O
"	O
line	O
cards	O
"	O
.	O
</s>
<s>
A	O
load-balanced	B-Protocol
switch	I-Protocol
backbone	O
can	O
deliver	O
100%	O
throughput	O
with	O
an	O
overcapacity	O
of	O
just	O
2x	O
,	O
as	O
measured	O
across	O
the	O
whole	O
system	O
.	O
</s>
<s>
These	O
map	O
well	O
to	O
the	O
constant-rate	O
2R/N	O
channels	O
of	O
the	O
load-balanced	B-Protocol
switch	I-Protocol
's	O
mesh	O
.	O
</s>
