<s>
Terminal	O
Access	O
Controller	O
Access-Control	O
System	O
(	O
TACACS	B-Protocol
,	O
)	O
refers	O
to	O
a	O
family	O
of	O
related	O
protocols	B-Protocol
handling	O
remote	O
authentication	O
and	O
related	O
services	O
for	O
network	O
access	O
control	O
through	O
a	O
centralized	O
server	O
.	O
</s>
<s>
The	O
original	O
TACACS	B-Protocol
protocol	O
,	O
which	O
dates	O
back	O
to	O
1984	O
,	O
was	O
used	O
for	O
communicating	O
with	O
an	O
authentication	O
server	O
,	O
common	O
in	O
older	O
UNIX	B-Application
networks	O
including	O
but	O
not	O
limited	O
to	O
the	O
ARPANET	O
,	O
MILNET	B-General_Concept
and	O
BBNNET	O
.	O
</s>
<s>
It	O
spawned	O
related	O
protocols	B-Protocol
:	O
</s>
<s>
Extended	O
TACACS	B-Protocol
(	O
XTACACS	B-Protocol
)	O
is	O
a	O
proprietary	O
extension	O
to	O
TACACS	B-Protocol
introduced	O
by	O
Cisco	O
Systems	O
in	O
1990	O
without	O
backwards	O
compatibility	O
to	O
the	O
original	O
protocol	O
.	O
</s>
<s>
TACACS	B-Protocol
and	O
XTACACS	B-Protocol
both	O
allow	O
a	O
remote	O
access	O
server	O
to	O
communicate	O
with	O
an	O
authentication	O
server	O
in	O
order	O
to	O
determine	O
if	O
the	O
user	O
has	O
access	O
to	O
the	O
network	O
.	O
</s>
<s>
TACACS	B-Protocol
Plus	O
(	O
TACACS+	B-Protocol
)	O
is	O
a	O
protocol	O
developed	O
by	O
Cisco	O
and	O
released	O
as	O
an	O
open	O
standard	O
beginning	O
in	O
1993	O
.	O
</s>
<s>
Although	O
derived	O
from	O
TACACS	B-Protocol
,	O
TACACS+	B-Protocol
is	O
a	O
separate	O
protocol	O
that	O
handles	O
authentication	O
,	O
authorization	O
,	O
and	O
accounting	O
(	O
AAA	O
)	O
services	O
.	O
</s>
<s>
TACACS+	B-Protocol
has	O
largely	O
replaced	O
its	O
predecessors	O
.	O
</s>
<s>
TACACS	B-Protocol
was	O
originally	O
developed	O
in	O
1984	O
by	O
BBN	O
,	O
later	O
known	O
as	O
BBN	O
Technologies	O
,	O
for	O
administration	O
of	O
ARPANET	O
and	O
MILNET	B-General_Concept
,	O
which	O
ran	O
unclassified	O
network	O
traffic	O
for	O
DARPA	O
at	O
the	O
time	O
and	O
would	O
later	O
evolve	O
into	O
the	O
U.S.	O
Department	O
of	O
Defense	O
's	O
NIPRNet	B-General_Concept
.	O
</s>
<s>
Originally	O
designed	O
as	O
a	O
means	O
to	O
automate	O
authentication	O
allowing	O
someone	O
who	O
was	O
already	O
logged	O
into	O
one	O
host	O
in	O
the	O
network	O
to	O
connect	O
to	O
another	O
on	O
the	O
same	O
network	O
without	O
needing	O
to	O
re-authenticate	O
it	O
was	O
first	O
formally	O
described	O
by	O
BBN	O
's	O
Brian	O
Anderson	O
TAC	O
Access	O
Control	O
System	O
Protocols	B-Protocol
,	O
BBN	O
Tech	O
Memo	O
CC-0045	O
with	O
minor	O
TELNET	O
double	O
login	O
avoidance	O
change	O
in	O
December	O
1984	O
in	O
IETF	O
RFC	O
927	O
.	O
</s>
<s>
Cisco	O
Systems	O
began	O
supporting	O
TACACS	B-Protocol
in	O
its	O
networking	O
products	O
in	O
the	O
late	O
1980s	O
,	O
eventually	O
adding	O
several	O
extensions	O
to	O
the	O
protocol	O
.	O
</s>
<s>
In	O
1990	O
,	O
Cisco	O
's	O
extensions	O
on	O
top	O
of	O
TACACS	B-Protocol
became	O
a	O
proprietary	O
protocol	O
called	O
Extended	O
TACACS	B-Protocol
(	O
XTACACS	B-Protocol
)	O
.	O
</s>
<s>
Although	O
TACACS	B-Protocol
and	O
XTACACS	B-Protocol
are	O
not	O
open	O
standards	O
,	O
Craig	O
Finseth	O
of	O
the	O
University	O
of	O
Minnesota	O
,	O
with	O
Cisco	O
's	O
assistance	O
,	O
published	O
a	O
description	O
of	O
the	O
protocols	B-Protocol
in	O
1993	O
as	O
IETF	O
RFC	O
1492	O
for	O
informational	O
purposes	O
.	O
</s>
<s>
TACACS	B-Protocol
is	O
defined	O
in	O
RFC	O
8907	O
(	O
older	O
RFC	O
1492	O
)	O
,	O
and	O
uses	O
(	O
either	O
TCP	B-Protocol
or	O
UDP	B-Protocol
)	O
port	O
49	O
by	O
default	O
.	O
</s>
<s>
TACACS	B-Protocol
allows	O
a	O
client	O
to	O
accept	O
a	O
username	O
and	O
password	O
and	O
send	O
a	O
query	O
to	O
a	O
TACACS	B-Protocol
authentication	O
server	O
,	O
sometimes	O
called	O
a	O
TACACS	B-Protocol
daemon	O
.	O
</s>
<s>
In	O
this	O
way	O
,	O
the	O
process	O
of	O
making	O
the	O
decision	O
is	O
"	O
opened	O
up	O
"	O
and	O
the	O
algorithms	O
and	O
data	O
used	O
to	O
make	O
the	O
decision	O
are	O
under	O
the	O
complete	O
control	O
of	O
whoever	O
is	O
running	O
the	O
TACACS	B-Protocol
daemon	O
.	O
</s>
<s>
Extended	O
TACACS	B-Protocol
(	O
XTACACS	B-Protocol
)	O
extends	O
the	O
TACACS	B-Protocol
protocol	O
with	O
additional	O
functionality	O
.	O
</s>
<s>
TACACS+	B-Protocol
is	O
a	O
Cisco	O
designed	O
extension	O
to	O
TACACS	B-Protocol
that	O
encrypts	O
the	O
full	O
content	O
of	O
each	O
packet	O
.	O
</s>
<s>
TACACS+	B-Protocol
has	O
generally	O
replaced	O
TACACS	B-Protocol
and	O
XTACACS	B-Protocol
in	O
more	O
recently	O
built	O
or	O
updated	O
networks	O
.	O
</s>
<s>
TACACS+	B-Protocol
is	O
an	O
entirely	O
new	O
protocol	O
which	O
is	O
not	O
compatible	O
with	O
its	O
predecessors	O
,	O
TACACS	B-Protocol
and	O
XTACACS	B-Protocol
.	O
</s>
<s>
Because	O
TCP	B-Protocol
is	O
a	O
connection	O
oriented	O
protocol	O
,	O
TACACS+	B-Protocol
is	O
able	O
to	O
detect	O
and	O
correct	O
network	O
transmission	O
errors	O
.	O
</s>
<s>
TACACS+	B-Protocol
uses	O
TCP	B-Protocol
(	O
while	O
RADIUS	B-Protocol
operates	O
over	O
UDP	B-Protocol
)	O
.	O
</s>
<s>
Because	O
TCP	B-Protocol
is	O
a	O
connection-oriented	O
protocol	O
,	O
TACACS+	B-Protocol
has	O
to	O
implement	O
transmission	O
control	O
.	O
</s>
<s>
RADIUS	B-Protocol
,	O
however	O
,	O
is	O
not	O
required	O
to	O
detect	O
and	O
correct	O
transmission	O
errors	O
such	O
as	O
packet	B-Protocol
loss	I-Protocol
or	O
timeouts	O
,	O
etc.	O
,	O
as	O
it	O
makes	O
use	O
of	O
UDP	B-Protocol
which	O
is	O
connectionless	B-Protocol
.	O
</s>
<s>
RADIUS	B-Protocol
encrypts	O
only	O
the	O
users	O
 '	O
password	O
as	O
it	O
travels	O
from	O
the	O
RADIUS	B-Protocol
client	O
to	O
RADIUS	B-Protocol
server	I-Protocol
.	O
</s>
<s>
TACACS+	B-Protocol
encrypts	O
all	O
the	O
information	O
mentioned	O
above	O
and	O
therefore	O
does	O
not	O
have	O
the	O
vulnerabilities	O
present	O
in	O
the	O
RADIUS	B-Protocol
protocol	O
.	O
</s>
