<s>
In	O
computer	B-Architecture
networking	I-Architecture
,	O
out-of-band	B-Protocol
data	I-Protocol
is	O
the	O
data	O
transferred	O
through	O
a	O
stream	O
that	O
is	O
independent	O
from	O
the	O
main	O
in-band	O
data	O
stream	O
.	O
</s>
<s>
An	O
out-of-band	B-Protocol
data	I-Protocol
mechanism	O
provides	O
a	O
conceptually	O
independent	O
channel	O
,	O
which	O
allows	O
any	O
data	O
sent	O
via	O
that	O
mechanism	O
to	O
be	O
kept	O
separate	O
from	O
in-band	O
data	O
.	O
</s>
<s>
The	O
out-of-band	B-Protocol
data	I-Protocol
mechanism	O
should	O
be	O
provided	O
as	O
an	O
inherent	O
characteristic	O
of	O
the	O
data	O
channel	O
and	O
transmission	O
protocol	O
,	O
rather	O
than	O
requiring	O
a	O
separate	O
channel	O
and	O
endpoints	O
to	O
be	O
established	O
.	O
</s>
<s>
The	O
term	O
"	O
out-of-band	B-Protocol
data	I-Protocol
"	O
probably	O
derives	O
from	O
out-of-band	O
signaling	O
,	O
as	O
used	O
in	O
the	O
telecommunications	O
industry	O
.	O
</s>
<s>
Consider	O
a	O
networking	O
application	O
that	O
tunnels	B-Protocol
data	O
from	O
a	O
remote	O
data	O
source	O
to	O
a	O
remote	O
destination	O
.	O
</s>
<s>
The	O
receiving	O
end	O
will	O
be	O
notified	O
in	O
some	O
fashion	O
of	O
the	O
arrival	O
of	O
out-of-band	B-Protocol
data	I-Protocol
,	O
and	O
it	O
can	O
read	O
the	O
out	O
of	O
band	O
data	O
and	O
know	O
that	O
this	O
is	O
a	O
message	O
intended	O
for	O
it	O
from	O
the	O
sending	O
end	O
,	O
independent	O
of	O
the	O
data	O
from	O
the	O
data	O
source	O
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
implement	O
out-of-band	B-Protocol
data	I-Protocol
transmission	O
using	O
a	O
physically	O
separate	O
channel	O
,	O
but	O
most	O
commonly	O
out-of-band	B-Protocol
data	I-Protocol
is	O
a	O
feature	O
provided	O
by	O
a	O
transmission	O
protocol	O
using	O
the	O
same	O
channel	O
as	O
normal	O
data	O
.	O
</s>
<s>
Depending	O
on	O
the	O
implementation	O
,	O
there	O
may	O
be	O
some	O
mechanism	O
for	O
notifying	O
or	O
interrupting	O
the	O
receiving	O
application	O
when	O
out-of-band	B-Protocol
data	I-Protocol
has	O
arrived	O
.	O
</s>
<s>
The	O
most	O
commonly	O
used	O
protocol	O
containing	O
an	O
out-of-band	B-Protocol
data	I-Protocol
mechanism	O
is	O
the	O
Internet	O
's	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
It	O
implements	O
out-of-band	B-Protocol
data	I-Protocol
using	O
an	O
"	O
urgent	O
pointer	O
,	O
"	O
which	O
marks	O
certain	O
data	O
in	O
the	O
transmitted	O
data	O
stream	O
as	O
out-of-band	O
.	O
</s>
<s>
Unfortunately	O
,	O
a	O
long-existing	O
discrepancy	O
between	O
and	O
limits	O
the	O
usability	O
of	O
this	O
feature	O
of	O
TCP	O
;	O
nonetheless	O
,	O
it	O
is	O
heavily	O
used	O
by	O
certain	O
standard	O
application	O
protocols	O
,	O
notably	O
the	O
Telnet	B-Protocol
protocol	O
.	O
</s>
<s>
On	O
Unix-like	O
computers	O
,	O
out-of-band	B-Protocol
data	I-Protocol
can	O
be	O
read	O
with	O
the	O
recv( )	O
system	O
call	O
.	O
</s>
<s>
A	O
process	O
or	O
process	O
group	O
can	O
be	O
configured	O
to	O
receive	O
SIGURG	B-Protocol
signals	O
when	O
out-of-band	B-Protocol
data	I-Protocol
is	O
available	O
for	O
reading	O
on	O
a	O
socket	B-Protocol
,	O
by	O
using	O
the	O
F_SETOWN	O
command	O
of	O
the	O
fcntl( )	O
system	O
call	O
.	O
</s>
<s>
This	O
is	O
a	O
form	O
of	O
asynchronous	B-Architecture
I/O	I-Architecture
.	O
</s>
<s>
The	O
Serial	O
Advanced	O
Technology	O
Attachment	O
(	O
SATA	O
)	O
protocol	O
commonly	O
used	O
to	O
connect	O
disk	B-Device
drives	I-Device
to	O
computers	O
implements	O
an	O
out-of-band	B-Protocol
data	I-Protocol
protocol	O
.	O
</s>
<s>
Some	O
sources	O
define	O
"	O
out-of-band	B-Protocol
data	I-Protocol
"	O
as	O
including	O
the	O
characteristic	O
that	O
out-of-band	B-Protocol
data	I-Protocol
should	O
be	O
prioritized	O
ahead	O
of	O
queued	O
in-band	O
data	O
.	O
</s>
<s>
This	O
is	O
reinforced	O
by	O
TCP	O
's	O
(	O
RFC	O
793	O
)	O
referring	O
to	O
the	O
out-of-band	B-Protocol
data	I-Protocol
mechanism	O
as	O
"	O
urgent	O
data.	O
"	O
</s>
<s>
However	O
,	O
prioritization	O
is	O
neither	O
an	O
essential	O
nor	O
a	O
necessarily	O
desirable	O
characteristic	O
of	O
out-of-band	B-Protocol
data	I-Protocol
;	O
moreover	O
,	O
TCP	O
implementations	O
vary	O
greatly	O
on	O
how	O
they	O
treat	O
the	O
urgency	O
of	O
out-of-band	B-Protocol
data	I-Protocol
.	O
</s>
