<s>
Protocol	B-Protocol
spoofing	I-Protocol
is	O
used	O
in	O
data	O
communications	O
to	O
improve	O
performance	O
in	O
situations	O
where	O
an	O
existing	O
protocol	O
is	O
inadequate	O
,	O
for	O
example	O
due	O
to	O
long	O
delays	O
or	O
high	O
error	O
rates	O
.	O
</s>
<s>
In	O
most	O
applications	O
of	O
protocol	B-Protocol
spoofing	I-Protocol
,	O
a	O
communications	O
device	O
such	O
as	O
a	O
modem	B-Application
or	O
router	B-Protocol
simulates	O
(	O
"	O
spoofs	O
"	O
)	O
the	O
remote	O
endpoint	O
of	O
a	O
connection	O
to	O
a	O
locally	O
attached	O
host	O
,	O
while	O
using	O
a	O
more	O
appropriate	O
protocol	O
to	O
communicate	O
with	O
a	O
compatible	O
remote	O
device	O
that	O
performs	O
the	O
equivalent	O
spoof	O
at	O
the	O
other	O
end	O
of	O
the	O
communications	O
link	O
.	O
</s>
<s>
Error	O
correction	O
and	O
file	O
transfer	O
protocols	O
typically	O
work	O
by	O
calculating	O
a	O
checksum	B-Algorithm
or	O
CRC	O
for	O
a	O
block	O
of	O
data	O
known	O
as	O
a	O
packet	O
,	O
and	O
transmitting	O
the	O
resulting	O
number	O
at	O
the	O
end	O
of	O
the	O
packet	O
.	O
</s>
<s>
The	O
time	O
to	O
transmit	O
the	O
ACK	O
back	O
to	O
the	O
sender	O
is	O
a	O
function	O
of	O
the	O
phone	O
lines	O
,	O
as	O
opposed	O
to	O
the	O
modem	B-Application
's	O
speed	O
,	O
and	O
is	O
typically	O
about	O
of	O
a	O
second	O
on	O
short	O
links	O
and	O
may	O
be	O
much	O
longer	O
on	O
long-distance	O
links	O
or	O
data	O
networks	O
like	O
X.25	B-Protocol
.	O
</s>
<s>
For	O
instance	O
,	O
the	O
UUCP	B-Protocol
"	O
g	O
"	O
protocol	O
and	O
Kermit	B-Protocol
both	O
use	O
64-byte	O
packets	O
,	O
which	O
on	O
a	O
9600	O
bit/s	O
link	O
takes	O
about	O
of	O
a	O
second	O
to	O
send	O
.	O
</s>
<s>
XMODEM	B-Protocol
used	O
a	O
slightly	O
larger	O
128-byte	O
packet	O
,	O
which	O
takes	O
about	O
of	O
a	O
second	O
to	O
send	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
XMODEM	B-Protocol
,	O
for	O
instance	O
,	O
that	O
means	O
it	O
takes	O
a	O
minimum	O
of	O
of	O
a	O
second	O
for	O
the	O
entire	O
cycle	O
to	O
complete	O
for	O
a	O
single	O
packet	O
.	O
</s>
<s>
Protocol	B-Protocol
spoofing	I-Protocol
addresses	O
this	O
problem	O
by	O
having	O
the	O
local	O
modem	B-Application
recognize	O
that	O
a	O
data	O
transfer	O
is	O
underway	O
,	O
often	O
by	O
looking	O
for	O
packet	O
headers	O
.	O
</s>
<s>
When	O
these	O
are	O
seen	O
,	O
the	O
modem	B-Application
then	O
looks	O
for	O
the	O
end	O
of	O
the	O
packet	O
,	O
normally	O
by	O
knowing	O
the	O
number	O
of	O
bytes	O
in	O
a	O
single	O
packet	O
.	O
</s>
<s>
XMODEM	B-Protocol
,	O
for	O
instance	O
,	O
has	O
132	O
bytes	O
in	O
a	O
packet	O
due	O
to	O
the	O
header	O
and	O
checksum	B-Algorithm
being	O
added	O
to	O
the	O
128	O
bytes	O
of	O
actual	O
data	O
.	O
</s>
<s>
When	O
the	O
modem	B-Application
sees	O
the	O
packet	O
has	O
ended	O
,	O
it	O
immediately	O
sends	O
of	O
spoofed	O
ACK	O
message	O
back	O
to	O
the	O
host	O
.	O
</s>
<s>
The	O
data	O
for	O
multiple	O
packets	O
is	O
held	O
in	O
an	O
internal	O
buffer	O
while	O
the	O
modem	B-Application
is	O
sending	O
it	O
to	O
the	O
remote	O
machine	O
.	O
</s>
<s>
However	O
,	O
this	O
also	O
requires	O
the	O
link	O
between	O
the	O
two	O
systems	O
to	O
be	O
error-free	O
,	O
as	O
the	O
modem	B-Application
has	O
already	O
ACKed	O
the	O
packets	O
even	O
before	O
they	O
have	O
been	O
sent	O
.	O
</s>
<s>
This	O
was	O
normally	O
addressed	O
by	O
using	O
a	O
modem-level	O
error	O
correction	O
protocol	O
,	O
like	O
Microcom	B-Device
Networking	I-Device
Protocols	I-Device
.	O
</s>
<s>
Protocol	B-Protocol
spoofing	I-Protocol
was	O
also	O
widely	O
used	O
with	O
another	O
feature	O
of	O
earlier	O
high-speed	O
modems	B-Application
.	O
</s>
<s>
Before	O
the	O
introduction	O
of	O
echo	O
cancellation	O
in	O
v.32	O
and	O
later	O
protocols	O
,	O
high-speed	O
modems	B-Application
typically	O
had	O
a	O
very	O
slow	O
"	O
backchannel	O
"	O
for	O
sending	O
things	O
like	O
these	O
ACKs	O
back	O
to	O
the	O
sender	O
.	O
</s>
<s>
On	O
the	O
~	O
18,500	O
bit/s	O
TrailBlazer	O
,	O
for	O
instance	O
,	O
the	O
modem	B-Application
could	O
send	O
as	O
many	O
as	O
35	O
UUCP	B-Protocol
packets	O
a	O
second	O
to	O
the	O
receiver	O
,	O
but	O
the	O
backchannel	O
offered	O
only	O
75	O
bit/s	O
,	O
not	O
nearly	O
enough	O
for	O
the	O
35	O
bytes	O
,	O
280	O
bits	O
,	O
of	O
ACK	O
messages	O
generated	O
by	O
the	O
remote	O
host	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
the	O
spoofing	O
allowed	O
the	O
sending	O
modem	B-Application
to	O
continue	O
sending	O
packets	O
as	O
fast	O
as	O
it	O
could	O
.	O
</s>
<s>
At	O
the	O
same	O
time	O
,	O
the	O
modem	B-Application
on	O
the	O
remote	O
receiving	O
end	O
dropped	O
the	O
ACK	O
packets	O
being	O
generated	O
by	O
the	O
local	O
computer	O
's	O
software	O
,	O
keeping	O
the	O
backchannel	O
clear	O
.	O
</s>
<s>
Since	O
the	O
channel	O
efficiency	O
only	O
became	O
a	O
major	O
problem	O
at	O
speeds	O
over	O
2400	O
bit/s	O
,	O
and	O
modems	B-Application
able	O
to	O
run	O
faster	O
than	O
that	O
typically	O
had	O
significant	O
processing	O
power	O
anyway	O
,	O
protocol	B-Protocol
spoofing	I-Protocol
was	O
mostly	O
associated	O
with	O
these	O
higher-speed	O
systems	O
.	O
</s>
<s>
A	O
spoofing	O
router	B-Protocol
terminates	O
the	O
TCP	O
connection	O
locally	O
and	O
translates	O
the	O
TCP	O
to	O
protocols	O
tailored	O
to	O
long	O
delays	O
over	O
the	O
satellite	O
link	O
such	O
as	O
XTP	B-Protocol
.	O
</s>
<s>
dial-on-demand	B-Protocol
WAN	O
links	O
in	O
IPX	B-Protocol
networks	O
therefore	O
never	O
become	O
idle	O
and	O
wo	O
n't	O
disconnect	O
.	O
</s>
<s>
A	O
spoofing	O
router	B-Protocol
or	O
modem	B-Application
will	O
intercept	O
the	O
SAP	O
and	O
RIP	O
broadcasts	O
,	O
and	O
re-broadcast	O
the	O
advertisements	O
from	O
its	O
own	O
routing/service	O
table	O
that	O
it	O
only	O
updates	O
when	O
the	O
link	O
is	O
active	O
for	O
other	O
reasons	O
.	O
</s>
