<s>
TCP	B-Protocol
reset	I-Protocol
attack	I-Protocol
,	O
also	O
known	O
as	O
a	O
"	O
forged	O
TCP	B-Protocol
reset	O
"	O
or	O
"	O
spoofed	O
TCP	B-Protocol
reset	O
"	O
,	O
is	O
a	O
way	O
to	O
terminate	O
a	O
TCP	B-Protocol
connection	O
by	O
sending	O
a	O
forged	O
TCP	B-Protocol
reset	O
packet	B-Protocol
.	O
</s>
<s>
The	B-Application
Great	I-Application
Firewall	I-Application
of	I-Application
China	I-Application
,	O
and	O
Iranian	O
Internet	O
censors	O
are	O
known	O
to	O
use	O
TCP	B-Protocol
reset	I-Protocol
attacks	I-Protocol
to	O
interfere	O
with	O
and	O
block	O
connections	O
,	O
as	O
a	O
major	O
method	O
to	O
carry	O
out	O
Internet	O
censorship	O
.	O
</s>
<s>
The	O
Internet	O
is	O
a	O
system	O
for	O
individual	O
computers	O
to	O
exchange	O
electronic	O
messages	O
,	O
or	O
packets	B-Protocol
of	O
data	O
.	O
</s>
<s>
This	O
system	O
includes	O
hardware	O
to	O
carry	O
the	O
messages	O
(	O
such	O
as	O
copper	O
and	O
fiber	B-Architecture
optics	I-Architecture
cables	O
)	O
and	O
a	O
formalized	O
system	O
for	O
formatting	O
the	O
messages	O
,	O
called	O
"	O
protocols	O
"	O
.	O
</s>
<s>
The	O
basic	O
protocol	O
used	O
on	O
the	O
Internet	O
is	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
,	O
which	O
is	O
usually	O
coupled	O
with	O
additional	O
protocols	O
such	O
as	O
TCP	B-Protocol
(	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
)	O
or	O
UDP	O
(	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
)	O
.	O
</s>
<s>
TCP/IP	B-Protocol
is	O
the	O
protocol	O
set	O
used	O
for	O
email	O
and	O
web	O
browsing	O
.	O
</s>
<s>
Each	O
protocol	O
has	O
a	O
block	O
of	O
information	O
,	O
called	O
a	O
header	O
,	O
included	O
near	O
the	O
front	O
of	O
each	O
packet	B-Protocol
.	O
</s>
<s>
Headers	O
contain	O
information	O
about	O
which	O
computer	O
sent	O
the	O
packet	B-Protocol
,	O
which	O
computer	O
should	O
receive	O
it	O
,	O
the	O
packet	B-Protocol
size	O
,	O
etc	O
.	O
</s>
<s>
TCP	B-Protocol
is	O
used	O
with	O
IP	O
when	O
a	O
two-way	O
virtual	O
connection	O
is	O
required	O
between	O
two	O
computers	O
.	O
</s>
<s>
TCP	B-Protocol
is	O
a	O
connection-oriented	O
protocol	O
and	O
requires	O
a	O
logical	O
connection	O
to	O
be	O
established	O
between	O
two	O
processes	O
before	O
data	O
is	O
exchanged	O
(	O
UDP	O
on	O
the	O
other	O
hand	O
is	O
a	O
connectionless	O
IP	B-Protocol
protocol	I-Protocol
.	O
)	O
</s>
<s>
A	O
TCP/IP	B-Protocol
socket	O
is	O
used	O
when	O
communication	O
between	O
two	O
computers	O
(	O
for	O
example	O
a	O
workstation	O
with	O
a	O
browser	O
and	O
a	O
web	O
server	O
)	O
by	O
exchanging	O
a	O
stream	O
of	O
packets	B-Protocol
.	O
</s>
<s>
Using	O
a	O
TCP	B-Protocol
connection	O
gives	O
the	O
computers	O
an	O
easy	O
way	O
to	O
exchange	O
data	O
items	O
too	O
big	O
for	O
a	O
single	O
packet	B-Protocol
,	O
such	O
as	O
video	O
clips	O
,	O
email	O
attachments	O
,	O
or	O
music	O
files	O
.	O
</s>
<s>
Although	O
some	O
web	O
pages	O
are	O
small	O
enough	O
for	O
a	O
single	O
packet	B-Protocol
,	O
they	O
are	O
sent	O
over	O
TCP	B-Protocol
connections	O
for	O
convenience	O
.	O
</s>
<s>
In	O
a	O
stream	O
of	O
packets	B-Protocol
of	O
a	O
TCP	B-Protocol
connection	O
,	O
each	O
packet	B-Protocol
contains	O
a	O
TCP	B-Protocol
header	I-Protocol
.	O
</s>
<s>
In	O
most	O
packets	B-Protocol
,	O
this	O
bit	O
is	O
set	O
to	O
0	O
and	O
has	O
no	O
effect	O
;	O
however	O
,	O
if	O
this	O
bit	O
is	O
set	O
to	O
1	O
,	O
it	O
indicates	O
to	O
the	O
receiving	O
computer	O
that	O
the	O
computer	O
should	O
immediately	O
stop	O
using	O
the	O
TCP	B-Protocol
connection	O
;	O
it	O
should	O
not	O
send	O
any	O
more	O
packets	B-Protocol
using	O
the	O
connection	O
's	O
identifying	O
numbers	O
,	O
called	O
ports	O
,	O
and	O
discard	O
any	O
further	O
packets	B-Protocol
it	O
receives	O
with	O
headers	O
indicating	O
they	O
belong	O
to	O
that	O
connection	O
.	O
</s>
<s>
A	O
TCP	B-Protocol
reset	O
basically	O
kills	O
a	O
TCP	B-Protocol
connection	O
instantly	O
.	O
</s>
<s>
One	O
common	O
application	O
is	O
the	O
scenario	O
where	O
a	O
computer	O
(	O
computer	O
A	O
)	O
crashes	O
while	O
a	O
TCP	B-Protocol
connection	O
is	O
in	O
progress	O
.	O
</s>
<s>
The	O
computer	O
on	O
the	O
other	O
end	O
(	O
computer	O
B	O
)	O
will	O
continue	O
to	O
send	O
TCP	B-Protocol
packets	B-Protocol
since	O
it	O
does	O
not	O
know	O
that	O
computer	O
A	O
has	O
crashed	O
.	O
</s>
<s>
When	O
computer	O
A	O
reboots	O
,	O
it	O
will	O
then	O
receive	O
packets	B-Protocol
from	O
the	O
old	O
pre-crash	O
connection	O
.	O
</s>
<s>
Computer	O
A	O
has	O
no	O
context	O
for	O
these	O
packets	B-Protocol
and	O
no	O
way	O
of	O
knowing	O
what	O
to	O
do	O
with	O
them	O
,	O
so	O
it	O
might	O
send	O
a	O
TCP	B-Protocol
reset	O
to	O
computer	O
B	O
.	O
</s>
<s>
In	O
the	O
scenario	O
above	O
,	O
the	O
TCP	B-Protocol
reset	O
bit	O
was	O
sent	O
by	O
a	O
computer	O
that	O
was	O
one	O
of	O
the	O
connection	O
endpoints	O
.	O
</s>
<s>
It	O
is	O
possible	O
for	O
a	O
third	O
computer	O
to	O
monitor	O
the	O
TCP	B-Protocol
packets	B-Protocol
on	O
the	O
connection	O
and	O
then	O
send	O
a	O
"	O
forged	O
"	O
packet	B-Protocol
containing	O
a	O
TCP	B-Protocol
reset	O
to	O
one	O
or	O
both	O
endpoints	O
.	O
</s>
<s>
The	O
headers	O
in	O
the	O
forged	O
packet	B-Protocol
must	O
indicate	O
,	O
falsely	O
,	O
that	O
it	O
came	O
from	O
an	O
endpoint	O
,	O
not	O
the	O
forger	O
.	O
</s>
<s>
Every	O
field	O
in	O
the	O
IP	O
and	O
TCP	B-Protocol
headers	I-Protocol
must	O
be	O
set	O
to	O
a	O
convincing	O
forged	O
value	O
for	O
the	O
fake	O
reset	O
to	O
trick	O
the	O
endpoint	O
into	O
closing	O
the	O
TCP	B-Protocol
connection	O
.	O
</s>
<s>
Properly	O
formatted	O
forged	O
TCP	B-Protocol
resets	O
can	O
be	O
a	O
very	O
effective	O
way	O
to	O
disrupt	O
any	O
TCP	B-Protocol
connection	O
that	O
the	O
forger	O
can	O
monitor	O
.	O
</s>
<s>
One	O
application	O
of	O
a	O
forged	O
TCP	B-Protocol
reset	O
is	O
to	O
maliciously	O
disrupt	O
TCP	B-Protocol
connections	O
without	O
the	O
consent	O
of	O
the	O
two	O
parties	O
that	O
own	O
the	O
endpoints	O
.	O
</s>
<s>
However	O
,	O
network	O
security	O
systems	O
using	O
forged	O
TCP	B-Protocol
resets	O
have	O
been	O
designed	O
as	O
well	O
.	O
</s>
<s>
A	O
prototype	O
"	O
Buster	O
"	O
software	O
package	O
was	O
demonstrated	O
in	O
1995	O
that	O
would	O
send	O
forged	O
resets	O
to	O
any	O
TCP	B-Protocol
connection	O
that	O
used	O
port	O
numbers	O
in	O
a	O
short	O
list	O
.	O
</s>
<s>
Linux	O
volunteers	O
proposed	O
doing	O
something	O
similar	O
with	O
Linux	O
firewalls	O
in	O
2000	O
,	O
and	O
the	O
open	O
source	O
Snort	B-Language
used	O
TCP	B-Protocol
resets	O
to	O
disrupt	O
suspicious	O
connections	O
as	O
early	O
as	O
2003	O
.	O
</s>
<s>
By	O
late	O
2007	O
,	O
Comcast	O
began	O
using	O
forged	O
TCP	B-Protocol
resets	O
to	O
cripple	O
peer-to-peer	O
and	O
certain	O
groupware	O
applications	O
on	O
their	O
customers	O
 '	O
computers	O
.	O
</s>
<s>
In	O
2008	O
,	O
the	O
NNSquad	O
released	O
the	O
NNSquad	O
Network	O
Measurement	O
Agent	O
,	O
a	O
Windows	O
software	O
program	O
written	O
by	O
John	O
Bartas	O
,	O
which	O
could	O
detect	O
Comcast	O
's	O
forged	O
TCP	B-Protocol
resets	O
and	O
distinguish	O
them	O
from	O
real	O
endpoint-generated	O
resets	O
.	O
</s>
<s>
By	O
encrypting	O
connections	O
using	O
a	O
VPN	B-Application
,	O
the	O
attacker	O
has	O
to	O
do	O
a	O
TCP	B-Protocol
reset	I-Protocol
attack	I-Protocol
on	O
all	O
encrypted	O
connections	O
,	O
causing	O
collateral	O
damage	O
.	O
</s>
