<s>
TCP	B-Protocol
Cookie	I-Protocol
Transactions	I-Protocol
(	O
TCPCT	B-Protocol
)	O
is	O
specified	O
in	O
RFC	O
6013	O
(	O
historic	O
status	O
,	O
formerly	O
experimental	O
)	O
as	O
an	O
extension	O
of	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
intended	O
to	O
secure	O
it	O
against	O
denial-of-service	O
attacks	O
,	O
such	O
as	O
resource	O
exhaustion	O
by	O
SYN	O
flooding	O
and	O
malicious	O
connection	O
termination	O
by	O
third	O
parties	O
.	O
</s>
<s>
Unlike	O
the	O
original	O
SYN	B-Protocol
cookies	I-Protocol
approach	O
,	O
TCPCT	B-Protocol
does	O
not	O
conflict	O
with	O
other	O
TCP	O
extensions	O
,	O
but	O
requires	O
TCPCT	B-Protocol
support	O
in	O
the	O
client	O
(	O
initiator	O
)	O
as	O
well	O
as	O
the	O
server	O
(	O
responder	O
)	O
TCP	O
stack	O
.	O
</s>
<s>
The	O
immediate	O
reason	O
for	O
the	O
TCPCT	B-Protocol
extension	O
is	O
deployment	O
of	O
the	O
DNSSEC	B-Protocol
protocol	O
.	O
</s>
<s>
Prior	O
to	O
DNSSEC	B-Protocol
,	O
DNS	B-Protocol
requests	O
primarily	O
used	O
short	O
UDP	B-Protocol
packets	I-Protocol
,	O
but	O
due	O
to	O
the	O
size	O
of	O
DNSSEC	B-Protocol
exchanges	O
,	O
and	O
shortcomings	O
of	O
IP	B-Protocol
fragmentation	I-Protocol
,	O
UDP	B-Protocol
is	O
less	O
practical	O
for	O
DNSSEC	B-Protocol
.	O
</s>
<s>
Thus	O
DNSSEC-enabled	O
requests	O
create	O
a	O
large	O
number	O
of	O
short-lived	O
TCP	O
connections	O
.	O
</s>
<s>
TCPCT	B-Protocol
avoids	O
resource	O
exhaustion	O
on	O
server-side	O
by	O
not	O
allocating	O
any	O
resources	O
until	O
the	O
completion	O
of	O
the	O
three-way	O
handshake	O
.	O
</s>
<s>
Additionally	O
,	O
TCPCT	B-Protocol
allows	O
the	O
server	O
to	O
release	O
memory	O
immediately	O
after	O
the	O
connection	O
closes	O
,	O
while	O
it	O
persists	O
in	O
the	O
TIME-WAIT	O
state	O
.	O
</s>
<s>
TCPCT	B-Protocol
support	O
was	O
partly	O
merged	O
into	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
in	O
December	O
2009	O
,	O
but	O
was	O
removed	O
in	O
May	O
2013	O
because	O
it	O
was	O
never	O
fully	O
implemented	O
and	O
had	O
a	O
performance	O
cost	O
.	O
</s>
<s>
TCPCT	B-Protocol
was	O
deprecated	O
in	O
2016	O
in	O
favor	O
of	O
TCP	B-Protocol
Fast	I-Protocol
Open	I-Protocol
.	O
</s>
