<s>
The	O
Common	B-Application
Address	I-Application
Redundancy	I-Application
Protocol	I-Application
or	O
CARP	O
is	O
a	O
computer	O
networking	O
protocol	O
which	O
allows	O
multiple	O
hosts	B-Device
on	O
the	O
same	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
to	O
share	O
a	O
set	O
of	O
IP	B-Protocol
addresses	I-Protocol
.	O
</s>
<s>
Its	O
primary	O
purpose	O
is	O
to	O
provide	O
failover	B-General_Concept
redundancy	O
,	O
especially	O
when	O
used	O
with	O
firewalls	B-Application
and	O
routers	B-Protocol
.	O
</s>
<s>
In	O
some	O
configurations	O
,	O
CARP	O
can	O
also	O
provide	O
load	B-Application
balancing	I-Application
functionality	O
.	O
</s>
<s>
CARP	O
provides	O
functionality	O
similar	O
to	O
Virtual	B-Protocol
Router	I-Protocol
Redundancy	I-Protocol
Protocol	I-Protocol
(	O
VRRP	B-Protocol
)	O
and	O
to	O
Cisco	O
Systems	O
 '	O
Hot	B-Protocol
Standby	I-Protocol
Router	I-Protocol
Protocol	I-Protocol
(	O
HSRP	B-Protocol
)	O
.	O
</s>
<s>
It	O
is	O
implemented	O
in	O
several	O
BSD-based	O
operating	B-General_Concept
systems	I-General_Concept
and	O
has	O
been	O
ported	O
to	O
Linux	B-Application
(	O
ucarp	B-Application
)	O
.	O
</s>
<s>
If	O
there	O
is	O
a	O
single	O
computer	O
running	O
a	O
packet	B-Protocol
filter	I-Protocol
,	O
and	O
it	O
goes	O
down	O
,	O
the	O
networks	O
on	O
either	O
side	O
of	O
the	O
packet	B-Protocol
filter	I-Protocol
can	O
no	O
longer	O
communicate	O
with	O
each	O
other	O
,	O
or	O
they	O
communicate	O
without	O
any	O
packet	B-Application
filtering	I-Application
.	O
</s>
<s>
If	O
,	O
however	O
,	O
there	O
are	O
two	O
computers	O
running	O
a	O
packet	B-Protocol
filter	I-Protocol
,	O
running	O
CARP	O
,	O
then	O
if	O
one	O
fails	O
,	O
the	O
other	O
will	O
take	O
over	O
,	O
and	O
computers	O
on	O
either	O
side	O
of	O
the	O
packet	B-Protocol
filter	I-Protocol
will	O
not	O
be	O
aware	O
of	O
the	O
failure	O
,	O
so	O
operation	O
will	O
continue	O
as	O
normal	O
.	O
</s>
<s>
In	O
order	O
to	O
make	O
sure	O
the	O
new	O
active/primary	O
operates	O
the	O
same	O
as	O
the	O
old	O
one	O
,	O
the	O
packet	B-Protocol
filter	I-Protocol
used	O
must	O
support	O
synchronization	O
of	O
state	O
between	O
the	O
two	O
computers	O
.	O
</s>
<s>
A	O
group	O
of	O
hosts	B-Device
using	O
CARP	O
is	O
called	O
a	O
"	O
group	O
of	O
redundancy	O
"	O
.	O
</s>
<s>
The	O
group	O
of	O
redundancy	O
allocates	O
itself	O
an	O
IP	B-Protocol
address	I-Protocol
which	O
is	O
shared	O
or	O
divided	O
among	O
the	O
members	O
of	O
the	O
group	O
.	O
</s>
<s>
The	O
main	O
host	O
is	O
that	O
which	O
"	O
takes	O
"	O
the	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
Each	O
host	O
must	O
have	O
a	O
second	O
unique	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
A	O
common	O
use	O
of	O
CARP	O
is	O
the	O
creation	O
of	O
a	O
group	O
of	O
redundant	O
firewalls	B-Application
.	O
</s>
<s>
The	O
virtual	B-Operating_System
IP	I-Operating_System
address	I-Operating_System
allotted	O
to	O
the	O
group	O
of	O
redundancy	O
is	O
indicated	O
as	O
the	O
address	O
of	O
the	O
default	O
router	B-Protocol
on	O
the	O
computers	O
behind	O
this	O
group	O
of	O
firewalls	B-Application
.	O
</s>
<s>
If	O
the	O
main	O
firewall	B-Application
breaks	O
down	O
or	O
is	O
disconnected	O
from	O
the	O
network	O
,	O
the	O
virtual	B-Operating_System
IP	I-Operating_System
address	I-Operating_System
will	O
be	O
taken	O
by	O
one	O
of	O
the	O
firewall	B-Application
slaves	O
and	O
the	O
service	O
availability	O
will	O
not	O
be	O
interrupted	O
.	O
</s>
<s>
In	O
the	O
late	O
1990s	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
began	O
work	O
on	O
a	O
protocol	O
for	O
router	B-Protocol
redundancy	O
.	O
</s>
<s>
In	O
1997	O
,	O
Cisco	O
informed	O
the	O
IETF	O
that	O
it	O
had	O
patents	O
in	O
this	O
area	O
and	O
,	O
in	O
1998	O
,	O
pointed	O
out	O
its	O
patent	O
on	O
HSRP	B-Protocol
.	O
</s>
<s>
Nonetheless	O
,	O
IETF	O
continued	O
work	O
on	O
VRRP	B-Protocol
.	O
</s>
<s>
After	O
some	O
debate	O
,	O
the	O
IETF	O
VRRP	B-Protocol
working	O
group	O
decided	O
to	O
approve	O
the	O
standard	O
,	O
despite	O
its	O
reliance	O
on	O
patented	O
techniques	O
,	O
as	O
long	O
as	O
Cisco	O
made	O
the	O
patent	O
available	O
to	O
third	O
parties	O
under	O
reasonable	O
and	O
non-discriminatory	O
licensing	O
terms	O
.	O
</s>
<s>
Because	O
VRRP	B-Protocol
fixed	O
problems	O
with	O
the	O
HSRP	B-Protocol
protocol	O
,	O
Cisco	O
began	O
using	O
VRRP	B-Protocol
instead	O
,	O
while	O
still	O
claiming	O
it	O
as	O
its	O
own	O
.	O
</s>
<s>
Cisco	O
informed	O
the	O
OpenBSD	B-Operating_System
developers	O
that	O
it	O
would	O
enforce	O
its	O
patent	O
on	O
HSRP	B-Protocol
.	O
</s>
<s>
As	O
Cisco	O
's	O
licensing	O
terms	O
prevented	O
an	O
open-source	O
VRRP	B-Protocol
implementation	O
,	O
the	O
OpenBSD	B-Operating_System
developers	O
begun	O
developing	O
CARP	O
instead	O
.	O
</s>
<s>
OpenBSD	B-Operating_System
focuses	O
on	O
security	O
.	O
</s>
<s>
This	O
made	O
CARP	O
fundamentally	O
different	O
from	O
VRRP	B-Protocol
and	O
ensured	O
that	O
CARP	O
did	O
not	O
infringe	O
on	O
Cisco	O
's	O
patent	O
.	O
</s>
<s>
Later	O
,	O
it	O
was	O
integrated	O
into	O
FreeBSD	B-Operating_System
(	O
first	O
released	O
in	O
May	O
2005	O
with	O
FreeBSD	B-Operating_System
5.4	O
)	O
,	O
NetBSD	B-Device
and	O
Linux	B-Application
(	O
ucarp	B-Application
)	O
.	O
</s>
<s>
OpenBSD	B-Operating_System
uses	O
VRRP	B-Protocol
's	O
protocol	O
number	O
and	O
MAC	O
addresses	O
.	O
</s>
<s>
The	O
OpenBSD	B-Operating_System
project	I-Operating_System
requested	O
unique	O
numbers	O
from	O
the	O
Internet	O
Assigned	O
Numbers	O
Authority	O
(	O
IANA	O
)	O
but	O
was	O
denied	O
.	O
</s>
<s>
The	O
OpenBSD	B-Operating_System
developers	O
met	O
neither	O
requirement	O
.	O
</s>
<s>
OpenBSD	B-Operating_System
's	O
website	O
states	O
the	O
following	O
:	O
</s>
<s>
IANA	O
had	O
assigned	O
protocol	O
number	O
112	O
to	O
VRRP	B-Protocol
(	O
in	O
1998	O
,	O
via	O
RFC	O
2338	O
)	O
.	O
</s>
<s>
Protocol	O
number	O
112	O
remains	O
in	O
use	O
by	O
VRRP	B-Protocol
.	O
</s>
<s>
CARP	O
also	O
uses	O
a	O
range	O
of	O
Ethernet	O
MAC	O
addresses	O
which	O
IEEE	O
had	O
assigned	O
to	O
IANA/IETF	O
for	O
the	O
VRRP	B-Protocol
protocol	O
.	O
</s>
<s>
In	O
spite	O
of	O
the	O
overlap	O
,	O
it	O
is	O
still	O
possible	O
to	O
use	O
VRRP	B-Protocol
and	O
CARP	O
in	O
the	O
same	O
broadcast	B-Architecture
domain	I-Architecture
,	O
as	O
long	O
as	O
the	O
VRRP	B-Protocol
group	O
ID	O
and	O
the	O
CARP	O
virtual	O
host	O
ID	O
are	O
different	O
.	O
</s>
