<s>
On	O
Linux	B-Application
,	O
network	B-Application
block	I-Application
device	I-Application
(	O
NBD	O
)	O
is	O
a	O
network	O
protocol	O
that	O
can	O
be	O
used	O
to	O
forward	O
a	O
block	B-Application
device	I-Application
(	O
typically	O
a	O
hard	B-Device
disk	I-Device
or	O
partition	O
)	O
from	O
one	O
machine	O
to	O
a	O
second	O
machine	O
.	O
</s>
<s>
As	O
an	O
example	O
,	O
a	O
local	O
machine	O
can	O
access	O
a	O
hard	B-Device
disk	I-Device
drive	I-Device
that	O
is	O
attached	O
to	O
another	O
computer	O
.	O
</s>
<s>
The	O
protocol	O
was	O
originally	O
developed	O
for	O
Linux	B-Application
2.1.55	O
and	O
released	O
in	O
1997	O
.	O
</s>
<s>
There	O
are	O
Linux-compatible	O
NBD	O
implementations	O
for	O
FreeBSD	B-Operating_System
and	O
other	O
operating	O
systems	O
.	O
</s>
<s>
The	O
term	O
'	O
network	B-Application
block	I-Application
device	I-Application
 '	O
is	O
sometimes	O
also	O
used	O
generically	O
.	O
</s>
<s>
Technically	O
,	O
a	O
network	B-Application
block	I-Application
device	I-Application
is	O
realized	O
by	O
three	O
components	O
:	O
the	O
server	O
part	O
,	O
the	O
client	O
part	O
,	O
and	O
the	O
network	O
between	O
them	O
.	O
</s>
<s>
On	O
the	O
client	O
machine	O
,	O
on	O
which	O
is	O
the	O
device	B-Application
node	I-Application
,	O
a	O
kernel	O
driver	O
controls	O
the	O
device	O
.	O
</s>
<s>
Whenever	O
a	O
program	O
tries	O
to	O
access	O
the	O
device	O
,	O
the	O
kernel	O
driver	O
forwards	O
the	O
request	O
(	O
if	O
the	O
client	O
part	O
is	O
not	O
fully	O
implemented	O
in	O
the	O
kernel	O
it	O
can	O
be	O
done	O
with	O
help	O
of	O
a	O
userspace	B-Operating_System
program	O
)	O
to	O
the	O
server	O
machine	O
,	O
on	O
which	O
the	O
data	O
resides	O
physically	O
.	O
</s>
<s>
On	O
the	O
server	O
machine	O
,	O
requests	O
from	O
the	O
client	O
are	O
handled	O
by	O
a	O
userspace	B-Operating_System
program	O
.	O
</s>
<s>
Network	B-Application
block	I-Application
device	I-Application
servers	O
are	O
typically	O
implemented	O
as	O
a	O
userspace	B-Operating_System
program	O
running	O
on	O
a	O
general-purpose	O
computer	O
.	O
</s>
<s>
All	O
of	O
the	O
function	O
specific	O
to	O
network	B-Application
block	I-Application
device	I-Application
servers	O
can	O
reside	O
in	O
a	O
userspace	B-Operating_System
process	O
because	O
the	O
process	O
communicates	O
with	O
the	O
client	O
via	O
conventional	O
sockets	O
and	O
accesses	O
the	O
storage	O
via	O
a	O
conventional	O
file	B-Application
system	I-Application
interface	O
.	O
</s>
<s>
The	O
network	B-Application
block	I-Application
device	I-Application
client	O
module	O
is	O
available	O
on	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
,	O
including	O
Linux	B-Application
and	O
Bitrig	B-Application
.	O
</s>
<s>
Since	O
the	O
server	O
is	O
a	O
userspace	B-Operating_System
program	O
,	O
it	O
can	O
potentially	O
run	O
on	O
every	O
Unix-like	B-Operating_System
platform	O
;	O
for	O
example	O
,	O
NBD	O
's	O
server	O
part	O
has	O
been	O
ported	O
to	O
Solaris	B-Application
.	O
</s>
<s>
iSCSI	B-Protocol
:	O
The	O
"	O
target-utils	O
"	O
iscsi	B-Protocol
package	O
on	O
many	O
Linux	B-Application
distributions	O
.	O
</s>
