<s>
File	B-Protocol
eXchange	I-Protocol
Protocol	I-Protocol
(	O
FXP	O
or	O
FXSP	O
)	O
is	O
a	O
method	O
of	O
data	O
transfer	O
which	O
uses	O
FTP	B-Protocol
to	O
transfer	O
data	O
from	O
one	O
remote	O
server	B-Application
to	O
another	O
(	O
inter-server	B-Protocol
)	O
without	O
routing	O
this	O
data	O
through	O
the	O
client	B-Protocol
's	O
connection	O
.	O
</s>
<s>
Conventional	O
FTP	B-Protocol
involves	O
a	O
single	O
server	B-Application
and	O
a	O
single	O
client	B-Protocol
;	O
all	O
data	O
transmission	O
is	O
done	O
between	O
these	O
two	O
.	O
</s>
<s>
In	O
the	O
FXP	O
session	O
,	O
a	O
client	B-Protocol
maintains	O
a	O
standard	O
FTP	B-Protocol
connection	O
to	O
two	O
servers	O
,	O
and	O
can	O
direct	O
either	O
server	B-Application
to	O
connect	O
to	O
the	O
other	O
to	O
initiate	O
a	O
data	O
transfer	O
.	O
</s>
<s>
The	O
advantage	O
of	O
using	O
FXP	O
over	O
FTP	B-Protocol
is	O
evident	O
when	O
a	O
high-bandwidth	O
server	B-Application
demands	O
resources	O
from	O
another	O
high-bandwidth	O
server	B-Application
,	O
but	O
only	O
a	O
low-bandwidth	O
client	B-Protocol
,	O
such	O
as	O
a	O
network	O
administrator	O
working	O
away	O
from	O
location	O
,	O
has	O
the	O
authority	O
to	O
access	O
the	O
resources	O
on	O
both	O
servers	O
.	O
</s>
<s>
Enabling	O
FXP	O
support	O
can	O
make	O
a	O
server	B-Application
vulnerable	O
to	O
an	O
exploit	O
known	O
as	O
FTP	B-Protocol
bounce	I-Protocol
.	O
</s>
<s>
As	O
a	O
result	O
of	O
this	O
,	O
FTP	B-Protocol
server	I-Protocol
software	O
often	O
has	O
FXP	O
disabled	O
by	O
default	O
.	O
</s>
<s>
Some	O
FTP	B-Protocol
Servers	I-Protocol
such	O
as	O
glFTPd	B-Protocol
,	O
cuftpd	O
,	O
RaidenFTPD	O
,	O
drftpd	O
,	O
and	O
wzdftpd	O
support	O
negotiation	O
of	O
a	O
secure	O
data	O
channel	O
between	O
two	O
servers	O
using	O
either	O
of	O
the	O
FTP	B-Protocol
protocol	I-Protocol
extension	O
commands	O
;	O
CPSV	O
or	O
SSCN	O
.	O
</s>
<s>
This	O
normally	O
works	O
by	O
the	O
client	B-Protocol
issuing	O
CPSV	O
in	O
lieu	O
of	O
the	O
PASV	O
command	O
—	O
or	O
by	O
sending	O
SSCN	O
prior	O
to	O
PASV	O
transfers	O
—	O
which	O
instructs	O
the	O
server	B-Application
to	O
create	O
either	O
a	O
SSL	B-Protocol
or	O
TLS	B-Protocol
connection	O
.	O
</s>
<s>
However	O
,	O
both	O
methods	O
—	O
CPSV	O
and	O
SSCN	O
—	O
may	O
be	O
susceptible	O
to	O
man-in-the-middle	O
attacks	O
,	O
if	O
the	O
two	O
FTP	B-Protocol
servers	I-Protocol
do	O
not	O
verify	O
each	O
other	O
's	O
SSL	B-Protocol
certificates	O
.	O
</s>
<s>
SSCN	O
was	O
first	O
introduced	O
by	O
RaidenFTPD	O
and	O
SmartFTP	B-Application
in	O
2003	O
and	O
has	O
been	O
widely	O
adopted	O
.	O
</s>
<s>
Although	O
FXP	O
is	O
often	O
considered	O
a	O
distinct	O
protocol	O
,	O
it	O
is	O
in	O
fact	O
merely	O
an	O
extension	O
of	O
the	O
FTP	B-Protocol
protocol	I-Protocol
and	O
is	O
specified	O
in	O
:	O
</s>
