<s>
In	O
computer	O
networking	O
,	O
TCP	B-Protocol
Fast	I-Protocol
Open	I-Protocol
(	O
TFO	O
)	O
is	O
an	O
extension	O
to	O
speed	O
up	O
the	O
opening	O
of	O
successive	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
connections	O
between	O
two	O
endpoints	O
.	O
</s>
<s>
When	O
the	O
client	O
later	O
reconnects	O
,	O
it	O
sends	O
the	O
initial	O
SYN	B-Protocol
packet	I-Protocol
along	O
with	O
the	O
TFO	O
cookie	O
data	O
to	O
authenticate	O
itself	O
.	O
</s>
<s>
If	O
successful	O
,	O
the	O
server	O
may	O
start	O
sending	O
data	O
to	O
the	O
client	O
even	O
before	O
the	O
reception	O
of	O
the	O
final	O
ACK	B-Protocol
packet	I-Protocol
of	O
the	O
three-way	O
handshake	O
,	O
thus	O
skipping	O
a	O
round-trip	O
delay	O
and	O
lowering	O
the	O
latency	O
in	O
the	O
start	O
of	O
data	O
transmission	O
.	O
</s>
<s>
If	O
such	O
resistance	O
is	O
required	O
,	O
it	O
may	O
be	O
used	O
in	O
combination	O
with	O
a	O
cryptographic	O
protocol	O
such	O
as	O
TLS	B-Protocol
or	O
IPsec	B-Protocol
.	O
</s>
<s>
TFO	O
has	O
been	O
difficult	O
to	O
deploy	O
due	O
to	O
protocol	O
ossification	O
;	O
in	O
2020	O
,	O
no	O
Web	B-Application
browsers	I-Application
used	O
it	O
by	O
default	O
.	O
</s>
<s>
TCP	B-Protocol
Fast	I-Protocol
Open	I-Protocol
shares	O
the	O
goal	O
of	O
bypassing	O
the	O
three-way	O
handshake	O
of	O
TCP	O
with	O
an	O
earlier	O
proposal	O
from	O
1994	O
,	O
called	O
T/TCP	B-Protocol
(	O
RFC	O
1644	O
)	O
.	O
</s>
<s>
In	O
contrast	O
to	O
TCP	B-Protocol
Fast	I-Protocol
Open	I-Protocol
,	O
T/TCP	B-Protocol
paid	O
no	O
attention	O
to	O
security	O
,	O
opening	O
a	O
path	O
for	O
vulnerabilities	O
and	O
failing	O
to	O
gain	O
traction	O
.	O
</s>
<s>
IPv4	B-Protocol
support	O
for	O
TFO	O
was	O
merged	O
into	O
the	O
Linux	B-Application
kernel	O
mainline	O
in	O
kernel	O
versions	O
3.6	O
(	O
support	O
for	O
clients	O
)	O
and	O
3.7	O
(	O
Dec	O
2012	O
)	O
(	O
support	O
for	O
servers	O
)	O
,	O
and	O
was	O
turned	O
on	O
by	O
default	O
in	O
kernel	O
version	O
3.13	O
(	O
Jan	O
2014	O
)	O
.	O
</s>
<s>
TFO	O
support	O
for	O
IPv6	B-Protocol
servers	O
was	O
merged	O
in	O
kernel	O
version	O
3.16	O
.	O
</s>
<s>
FreeBSD	B-Operating_System
from	O
version	O
10.3	O
(	O
support	O
for	O
servers	O
)	O
and	O
12.0	O
.	O
</s>
<s>
Mozilla	B-Application
Firefox	I-Application
from	O
version	O
58	O
.	O
</s>
<s>
The	O
support	O
was	O
disabled	O
by	O
default	O
due	O
to	O
network	O
device	O
compatibility	O
issues	O
with	O
TFO	O
and	O
TLS	B-Protocol
1.3	O
and	O
eventually	O
removed	O
in	O
version	O
87	O
.	O
</s>
<s>
Google	B-Application
Chrome	I-Application
and	O
Chromium	B-Language
browsers	I-Language
have	O
support	O
for	O
TFO	O
on	O
Linux	B-Application
,	O
including	O
ChromeOS	B-Operating_System
and	O
Android	B-Application
.	O
</s>
<s>
Exim	B-Protocol
mail	B-Protocol
transfer	I-Protocol
agent	I-Protocol
(	O
MTA	O
)	O
from	O
version	O
4.88	O
.	O
</s>
<s>
Unbound	B-Protocol
DNS	I-Protocol
Resolver	I-Protocol
from	O
version	O
1.5.10	O
.	O
</s>
<s>
BIND	B-Protocol
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
from	O
version	O
9.11.0	O
.	O
</s>
<s>
Knot	B-Protocol
DNS	I-Protocol
from	O
version	O
2.6.0	O
.	O
</s>
<s>
Apple	O
's	O
iOS	B-Operating_System
9	I-Operating_System
and	O
OS	B-Operating_System
X	I-Operating_System
10.11	I-Operating_System
both	O
support	O
TCP	B-Protocol
Fast	I-Protocol
Open	I-Protocol
,	O
but	O
it	O
is	O
not	O
enabled	O
for	O
individual	O
connections	O
by	O
default	O
.	O
</s>
<s>
Microsoft	B-Application
Edge	I-Application
supports	O
TCP	B-Protocol
Fast	I-Protocol
Open	I-Protocol
since	O
Windows	B-Operating_System
10	I-Operating_System
Preview	O
build	O
14352	O
.	O
</s>
<s>
PowerDNS	O
Recursor	O
supports	O
TCP	B-Protocol
Fast	I-Protocol
Open	I-Protocol
from	O
version	O
4.1	O
.	O
</s>
<s>
dnsmasq	B-Protocol
supports	O
TCP-fastopen	O
(	O
RFC-7413	O
)	O
from	O
version	O
2.81	O
.	O
</s>
