<s>
In	O
computer	B-Architecture
networking	I-Architecture
,	O
cut-through	B-Protocol
switching	I-Protocol
,	O
also	O
called	O
cut-through	B-Protocol
forwarding	I-Protocol
is	O
a	O
method	O
for	O
packet	B-Protocol
switching	I-Protocol
systems	O
,	O
wherein	O
the	O
switch	O
starts	O
forwarding	O
a	O
frame	B-Protocol
(	O
or	O
packet	B-Protocol
)	O
before	O
the	O
whole	O
frame	B-Protocol
has	O
been	O
received	O
,	O
normally	O
as	O
soon	O
as	O
the	O
destination	O
address	O
and	O
outgoing	O
interface	O
is	O
determined	O
.	O
</s>
<s>
Compared	O
to	O
store	B-Protocol
and	I-Protocol
forward	I-Protocol
,	O
this	O
technique	O
reduces	O
latency	O
through	O
the	O
switch	O
and	O
relies	O
on	O
the	O
destination	O
devices	O
for	O
error	O
handling	O
.	O
</s>
<s>
Pure	O
cut-through	B-Protocol
switching	I-Protocol
is	O
only	O
possible	O
when	O
the	O
speed	O
of	O
the	O
outgoing	O
interface	O
is	O
at	O
least	O
equal	O
or	O
higher	O
than	O
the	O
incoming	O
interface	O
speed	O
.	O
</s>
<s>
Adaptive	B-Device
switching	I-Device
dynamically	O
selects	O
between	O
cut-through	O
and	O
store	B-Protocol
and	I-Protocol
forward	I-Protocol
behaviors	O
based	O
on	O
current	O
network	O
conditions	O
.	O
</s>
<s>
Cut-through	B-Protocol
switching	I-Protocol
is	O
closely	O
associated	O
with	O
wormhole	B-Protocol
switching	I-Protocol
.	O
</s>
<s>
When	O
cut-through	B-Protocol
switching	I-Protocol
is	O
used	O
in	O
Ethernet	O
the	O
switch	O
is	O
not	O
able	O
to	O
verify	O
the	O
integrity	O
of	O
an	O
incoming	O
frame	B-Protocol
before	O
forwarding	O
it	O
.	O
</s>
<s>
A	O
cut-through	O
switch	O
will	O
forward	O
corrupted	O
frames	O
,	O
whereas	O
a	O
store	B-Protocol
and	I-Protocol
forward	I-Protocol
switch	O
will	O
drop	O
them	O
.	O
</s>
<s>
The	O
technology	O
was	O
developed	O
by	O
Kalpana	O
,	O
the	O
company	O
that	O
introduced	O
the	O
first	O
Ethernet	B-Protocol
switch	I-Protocol
.	O
</s>
<s>
The	O
primary	O
advantage	O
of	O
cut-through	O
Ethernet	B-Protocol
switches	I-Protocol
,	O
compared	O
to	O
store-and-forward	B-Protocol
Ethernet	B-Protocol
switches	I-Protocol
,	O
is	O
lower	O
latency	O
.	O
</s>
<s>
Cut-through	O
Ethernet	B-Protocol
switches	I-Protocol
can	O
support	O
an	O
end-to-end	O
network	B-Protocol
delay	I-Protocol
latency	O
of	O
about	O
10	O
microseconds	O
.	O
</s>
<s>
End-to-end	O
application	O
latencies	O
below	O
3	O
microseconds	O
require	O
specialized	O
hardware	O
such	O
as	O
InfiniBand	B-Architecture
.	O
</s>
<s>
Fragment	O
free	O
is	O
a	O
variation	O
on	O
cut-through	B-Protocol
switching	I-Protocol
that	O
partially	O
addresses	O
this	O
problem	O
by	O
assuring	O
that	O
collision	O
fragments	O
are	O
not	O
forwarded	O
.	O
</s>
<s>
Fragment	O
free	O
will	O
hold	O
the	O
frame	B-Protocol
until	O
the	O
first	O
64	O
bytes	O
are	O
read	O
from	O
the	O
source	O
to	O
detect	O
a	O
collision	O
before	O
forwarding	O
.	O
</s>
<s>
The	O
theory	O
here	O
is	O
that	O
frames	O
that	O
are	O
damaged	O
(	O
usually	O
by	O
collisions	O
)	O
are	O
often	O
shorter	O
than	O
the	O
minimum	O
valid	O
ethernet	O
frame	B-Protocol
size	O
of	O
64	O
bytes	O
.	O
</s>
<s>
With	O
a	O
fragment-free	O
buffer	O
the	O
first	O
64	O
bytes	O
of	O
each	O
frame	B-Protocol
,	O
updates	O
the	O
source	O
MAC	O
and	O
port	O
if	O
necessary	O
,	O
reads	O
the	O
destination	O
MAC	O
,	O
and	O
forwards	O
the	O
frame	B-Protocol
.	O
</s>
<s>
If	O
the	O
frame	B-Protocol
is	O
less	O
than	O
64	O
bytes	O
,	O
it	O
is	O
discarded	O
.	O
</s>
<s>
Because	O
the	O
switch	O
only	O
ever	O
buffers	O
64	O
bytes	O
of	O
each	O
frame	B-Protocol
,	O
Fragment	O
Free	O
is	O
a	O
faster	O
mode	O
than	O
Store	B-Protocol
and	I-Protocol
Forward	I-Protocol
,	O
but	O
there	O
still	O
exists	O
a	O
risk	O
of	O
forwarding	O
bad	O
frames	O
.	O
</s>
<s>
There	O
are	O
certain	O
scenarios	O
that	O
force	O
a	O
cut-through	O
Ethernet	B-Protocol
switch	I-Protocol
to	O
buffer	O
the	O
entire	O
frame	B-Protocol
,	O
acting	O
like	O
a	O
store-and-forward	B-Protocol
Ethernet	B-Protocol
switch	I-Protocol
for	O
that	O
frame	B-Protocol
:	O
</s>
<s>
Speed	O
:	O
When	O
the	O
outgoing	O
port	O
is	O
faster	O
than	O
the	O
incoming	O
port	O
,	O
the	O
switch	O
must	O
buffer	O
the	O
entire	O
frame	B-Protocol
received	O
from	O
the	O
lower-speed	O
port	O
before	O
the	O
switch	O
can	O
start	O
transmitting	O
that	O
frame	B-Protocol
out	O
the	O
high-speed	O
port	O
,	O
to	O
prevent	O
underrun	O
.	O
</s>
<s>
(	O
When	O
the	O
outgoing	O
port	O
is	O
slower	O
than	O
the	O
incoming	O
port	O
,	O
the	O
switch	O
can	O
perform	O
cut-through	B-Protocol
switching	I-Protocol
and	O
start	O
transmitting	O
that	O
frame	B-Protocol
before	O
it	O
is	O
entirely	O
received	O
,	O
although	O
it	O
must	O
still	O
buffer	O
some	O
of	O
the	O
frame	B-Protocol
)	O
.	O
</s>
<s>
Congestion	O
:	O
When	O
a	O
cut-through	O
switch	O
decides	O
a	O
frame	B-Protocol
from	O
one	O
incoming	O
port	O
needs	O
to	O
go	O
out	O
through	O
an	O
outgoing	O
port	O
,	O
but	O
that	O
outgoing	O
port	O
is	O
already	O
busy	O
sending	O
a	O
frame	B-Protocol
from	O
a	O
second	O
incoming	O
port	O
,	O
the	O
switch	O
must	O
buffer	O
some	O
or	O
all	O
of	O
the	O
frame	B-Protocol
from	O
the	O
first	O
incoming	O
port	O
.	O
</s>
<s>
Cut-through	B-Protocol
switching	I-Protocol
is	O
the	O
dominant	O
switching	O
architecture	O
in	O
Fibre	O
Channel	O
due	O
to	O
the	O
low-latency	O
performance	O
required	O
for	O
SCSI	O
traffic	O
.	O
</s>
<s>
Brocade	O
has	O
implemented	O
cut-through	B-Protocol
switching	I-Protocol
in	O
its	O
Fibre	O
Channel	O
ASICs	O
since	O
the	O
1990s	O
and	O
has	O
been	O
implemented	O
in	O
tens	O
of	O
millions	O
of	O
ports	O
in	O
production	O
SANs	O
worldwide	O
.	O
</s>
<s>
CRC	O
errors	O
are	O
detected	O
in	O
a	O
cut-through	O
switch	O
and	O
indicated	O
by	O
marking	O
the	O
corrupted	O
frame	B-Protocol
EOF	O
field	O
as	O
"	O
invalid	O
"	O
.	O
</s>
<s>
The	O
destination	O
devices	O
(	O
host	O
or	O
storage	O
)	O
sees	O
the	O
invalid	O
EOF	O
and	O
discards	O
the	O
frame	B-Protocol
prior	O
to	O
sending	O
it	O
to	O
the	O
application	O
or	O
LUN	O
.	O
</s>
<s>
As	O
soon	O
as	O
the	O
destination	O
device	O
receives	O
the	O
EOF	O
marker	O
as	O
"	O
invalid	O
"	O
,	O
recovery	O
of	O
the	O
corrupted	O
frame	B-Protocol
can	O
begin	O
.	O
</s>
<s>
With	O
store	B-Protocol
and	I-Protocol
forward	I-Protocol
,	O
the	O
corrupted	O
frame	B-Protocol
is	O
discarded	O
at	O
the	O
switch	O
forcing	O
a	O
SCSI	O
timeout	O
and	O
a	O
SCSI	O
retry	O
for	O
recovery	O
that	O
can	O
result	O
in	O
delays	O
of	O
tens	O
of	O
seconds	O
.	O
</s>
<s>
Cut-through	B-Protocol
switching	I-Protocol
was	O
one	O
of	O
the	O
important	O
features	O
of	O
IP	O
networks	O
using	O
ATM	O
networks	O
since	O
the	O
edge	O
routers	O
of	O
the	O
ATM	O
network	O
were	O
able	O
to	O
use	O
cell	B-Protocol
switching	I-Protocol
through	O
the	O
core	O
of	O
the	O
network	O
with	O
low	O
latency	O
at	O
all	O
points	O
.	O
</s>
<s>
With	O
higher	O
speed	O
links	O
,	O
this	O
has	O
become	O
less	O
of	O
a	O
problem	O
since	O
packet	B-Protocol
latency	O
has	O
become	O
much	O
smaller	O
.	O
</s>
<s>
Cut-through	B-Protocol
switching	I-Protocol
is	O
very	O
popular	O
in	O
InfiniBand	B-Architecture
networks	O
,	O
since	O
these	O
are	O
often	O
deployed	O
in	O
environments	O
where	O
latency	O
is	O
a	O
prime	O
concern	O
,	O
such	O
as	O
supercomputer	B-Architecture
clusters	O
.	O
</s>
<s>
by	O
the	O
Exim	B-Protocol
mail	B-Protocol
transfer	I-Protocol
agent	I-Protocol
.	O
</s>
<s>
rather	O
than	O
the	O
traditional	O
bounce	O
message	O
necessitated	O
by	O
the	O
more	O
usual	O
store-and-forward	B-Protocol
operation	O
.	O
</s>
<s>
Cut-through	B-Protocol
switching	I-Protocol
has	O
been	O
applied	O
to	O
make	O
block-relay	O
lower	O
latency	O
in	O
Bitcoin	B-Protocol
.	O
</s>
<s>
Low	O
latency	O
is	O
critical	O
for	O
Bitcoin	B-Protocol
miners	O
to	O
reduce	O
the	O
rate	O
at	O
which	O
their	O
blocks	O
are	O
orphaned	O
.	O
</s>
