<s>
Multihoming	B-Protocol
is	O
the	O
practice	O
of	O
connecting	O
a	O
host	B-Device
or	O
a	O
computer	B-Architecture
network	I-Architecture
to	O
more	O
than	O
one	O
network	O
.	O
</s>
<s>
A	O
typical	O
host	B-Device
or	O
end-user	O
network	O
is	O
connected	O
to	O
just	O
one	O
network	O
.	O
</s>
<s>
There	O
are	O
several	O
different	O
ways	O
to	O
perform	O
multihoming	B-Protocol
.	O
</s>
<s>
A	O
single	O
host	B-Device
may	O
be	O
connected	O
to	O
multiple	O
networks	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
mobile	O
phone	O
might	O
be	O
simultaneously	O
connected	O
to	O
a	O
WiFi	O
network	O
and	O
a	O
3G	B-Architecture
network	I-Architecture
,	O
and	O
a	O
desktop	O
computer	O
might	O
be	O
connected	O
to	O
both	O
a	O
home	O
network	O
and	O
a	O
VPN	B-Application
.	O
</s>
<s>
A	O
multihomed	B-Protocol
host	B-Device
usually	O
is	O
assigned	O
multiple	O
addresses	O
,	O
one	O
per	O
connected	O
network	O
.	O
</s>
<s>
In	O
classical	O
multihoming	B-Protocol
,	O
a	O
network	O
is	O
connected	O
to	O
multiple	O
providers	O
,	O
and	O
uses	O
its	O
own	O
range	O
of	O
addresses	O
(	O
typically	O
from	O
a	O
Provider	B-Protocol
Independent	I-Protocol
(	O
PI	O
)	O
range	O
)	O
.	O
</s>
<s>
The	O
network	O
's	O
edge	B-Protocol
routers	I-Protocol
communicate	O
with	O
the	O
providers	O
using	O
a	O
dynamic	B-Protocol
routing	I-Protocol
protocol	O
,	O
typically	O
BGP	B-Protocol
,	O
which	O
announces	O
the	O
network	O
's	O
address	O
range	O
to	O
all	O
providers	O
.	O
</s>
<s>
If	O
one	O
of	O
the	O
links	O
fails	O
,	O
the	O
dynamic	B-Protocol
routing	I-Protocol
protocol	O
recognizes	O
the	O
failure	O
within	O
seconds	O
or	O
minutes	O
,	O
and	O
reconfigures	O
its	O
routing	B-Protocol
tables	I-Protocol
to	O
use	O
the	O
remaining	O
links	O
,	O
transparently	O
to	O
the	O
hosts	O
.	O
</s>
<s>
Classical	O
multihoming	B-Protocol
is	O
costly	O
,	O
since	O
it	O
requires	O
the	O
use	O
of	O
address	O
space	O
that	O
is	O
accepted	O
by	O
all	O
providers	O
,	O
a	O
public	O
Autonomous	B-Architecture
System	I-Architecture
(	O
AS	O
)	O
number	O
,	O
and	O
a	O
dynamic	B-Protocol
routing	I-Protocol
protocol	O
.	O
</s>
<s>
Since	O
multihomed	B-Protocol
address	O
space	O
cannot	O
be	O
aggregated	O
,	O
it	O
causes	O
growth	O
of	O
the	O
global	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
Multihoming	B-Protocol
with	O
multiple	O
addresses	O
is	O
cheaper	O
than	O
classical	O
multihoming	B-Protocol
,	O
and	O
can	O
be	O
used	O
without	O
any	O
cooperation	O
from	O
the	O
providers	O
(	O
e.g.	O
</s>
<s>
for	O
incoming	O
traffic	O
,	O
hosts	O
must	O
be	O
associated	O
with	O
multiple	O
A	O
or	O
AAAA	O
DNS	B-Protocol
records	I-Protocol
so	O
that	O
they	O
are	O
reachable	O
through	O
all	O
providers	O
;	O
</s>
<s>
for	O
outgoing	O
traffic	O
,	O
a	O
technique	O
such	O
as	O
source-specific	B-Protocol
routing	I-Protocol
must	O
be	O
used	O
to	O
route	O
packets	O
through	O
the	O
correct	O
provider	O
,	O
and	O
reasonable	O
source	O
address	O
selection	O
policies	O
must	O
be	O
implemented	O
by	O
hosts	O
.	O
</s>
<s>
When	O
multihoming	B-Protocol
is	O
used	O
to	O
improve	O
reliability	O
,	O
care	O
must	O
be	O
taken	O
to	O
eliminate	O
any	O
single	B-Architecture
point	I-Architecture
of	I-Architecture
failure	I-Architecture
(	O
SPOF	B-Architecture
)	O
:	O
</s>
<s>
Routers	B-Protocol
:	O
Routers	B-Protocol
and	O
switches	B-Protocol
must	O
be	O
positioned	O
such	O
that	O
no	O
single	O
piece	O
of	O
network	O
hardware	O
controls	O
all	O
network	O
access	O
to	O
a	O
given	O
host	B-Device
.	O
</s>
<s>
In	O
particular	O
,	O
it	O
is	O
not	O
uncommon	O
to	O
see	O
multiple	O
Internet	O
uplinks	O
all	O
converge	O
on	O
a	O
single	O
edge	B-Protocol
router	I-Protocol
.	O
</s>
<s>
In	O
such	O
a	O
configuration	O
,	O
the	O
loss	O
of	O
that	O
single	O
router	B-Protocol
disconnects	O
the	O
Internet	O
uplink	O
,	O
despite	O
the	O
fact	O
that	O
multiple	O
ISPs	O
are	O
otherwise	O
in	O
use	O
.	O
</s>
<s>
Host	B-Device
connectivity	O
:	O
A	O
"	O
reliable	O
"	O
host	B-Device
must	O
be	O
connected	O
to	O
the	O
network	O
over	O
multiple	O
network	B-Protocol
interfaces	I-Protocol
,	O
each	O
connected	O
to	O
a	O
separate	O
router	B-Protocol
or	O
switch	O
.	O
</s>
<s>
Alternatively	O
,	O
and	O
preferably	O
,	O
the	O
function	O
of	O
a	O
given	O
host	B-Device
could	O
be	O
duplicated	O
across	O
multiple	O
computers	O
,	O
each	O
of	O
which	O
is	O
connected	O
to	O
a	O
different	O
router	B-Protocol
or	O
switch	O
.	O
</s>
<s>
Referencing	O
entities	O
:	O
Not	O
only	O
must	O
a	O
host	B-Device
be	O
accessible	O
,	O
but	O
in	O
many	O
cases	O
it	O
must	O
also	O
be	O
"	O
referenced	O
"	O
to	O
be	O
useful	O
.	O
</s>
<s>
For	O
most	O
servers	O
,	O
this	O
means	O
in	O
particular	O
that	O
the	O
name	B-Protocol
resolution	I-Protocol
to	O
that	O
server	O
be	O
functional	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
the	O
failure	O
of	O
a	O
single	O
element	O
blocks	O
users	O
from	O
properly	O
resolving	O
the	O
DNS	B-Protocol
name	I-Protocol
of	O
that	O
server	O
,	O
then	O
the	O
server	O
is	O
effectively	O
inaccessible	O
,	O
despite	O
its	O
otherwise	O
connected	O
state	O
.	O
</s>
<s>
By	O
increasing	O
the	O
number	O
of	O
interfaces	O
and	O
links	O
being	O
used	O
and	O
making	O
routing	O
less	O
deterministic	O
,	O
multihoming	B-Protocol
complicates	O
network	O
administration	O
.	O
</s>
<s>
Classical	O
multihoming	B-Protocol
is	O
the	O
dominant	O
technique	O
for	O
IPv4	O
.	O
</s>
<s>
This	O
requires	O
that	O
a	O
network	O
have	O
its	O
own	O
public	O
IP	O
address	O
range	O
and	O
a	O
public	O
AS	B-Architecture
number	I-Architecture
.	O
</s>
<s>
While	O
multihoming	B-Protocol
with	O
multiple	O
addresses	O
has	O
been	O
implemented	O
for	O
IPv4	O
,	O
it	O
is	O
not	O
generally	O
used	O
,	O
as	O
host	B-Device
implementations	O
do	O
not	O
deal	O
well	O
with	O
multiple	O
addresses	O
per	O
interface	O
which	O
requires	O
the	O
use	O
of	O
"	O
virtual	O
interfaces	O
"	O
.	O
</s>
<s>
It	O
is	O
also	O
possible	O
to	O
implement	O
multihoming	B-Protocol
for	O
IPv4	O
using	O
multiple	O
NAT	B-Protocol
gateways	O
.	O
</s>
<s>
Both	O
classical	O
multihoming	B-Protocol
and	O
multihoming	B-Protocol
with	O
multiple	O
addresses	O
may	O
be	O
used	O
in	O
IPv6	O
.	O
</s>
<s>
Provider	B-Protocol
Independent	I-Protocol
Address	I-Protocol
Space	I-Protocol
(	O
PI	O
)	O
is	O
available	O
in	O
IPv6	O
.	O
</s>
<s>
Critics	O
say	O
that	O
the	O
increased	O
size	O
of	O
routing	B-Protocol
tables	I-Protocol
needed	O
to	O
handle	O
multi-homing	B-Protocol
in	O
this	O
way	O
will	O
overwhelm	O
current	O
router	B-Protocol
hardware	O
.	O
</s>
<s>
However	O
,	O
from	O
a	O
pragmatic	O
perspective	O
,	O
allocating	O
a	O
/32	O
is	O
equivalent	O
in	O
global	O
address	O
space	O
cost	O
to	O
allocating	O
a	O
single	O
IPv4	O
address	O
,	O
and	O
this	O
may	O
be	O
acceptable	O
if	O
,	O
as	O
seems	O
to	O
be	O
likely	O
for	O
the	O
foreseeable	O
future	O
,	O
the	O
number	O
of	O
multihomed	B-Protocol
sites	O
can	O
be	O
numbered	O
only	O
in	O
the	O
millions	O
,	O
as	O
opposed	O
to	O
the	O
many	O
billions	O
of	O
non-multihomed	O
endpoints	O
which	O
are	O
anticipated	O
to	O
comprise	O
the	O
vast	O
majority	O
of	O
IPv6	O
endpoints	O
.	O
</s>
<s>
RIPE	O
allocates	O
IPv6	O
provider-independent	B-Protocol
address	I-Protocol
spaces	I-Protocol
/48	O
or	O
shorter	O
from	O
2001:0678::/29	O
.	O
</s>
<s>
Multihoming	B-Protocol
with	O
multiple	O
addresses	O
has	O
been	O
implemented	O
for	O
IPv6	O
.	O
</s>
<s>
For	O
outgoing	O
traffic	O
,	O
this	O
requires	O
support	O
on	O
the	O
host	B-Device
,	O
either	O
protocol	O
agnostic	O
(	O
Multipath	B-Protocol
TCP	I-Protocol
,	O
SCTP	B-Protocol
,	O
QUIC	B-Protocol
,	O
etc	O
.	O
)	O
</s>
<s>
SHIM6	B-Protocol
)	O
.	O
</s>
<s>
DNS	B-Protocol
and	O
firewall	O
records	O
must	O
be	O
updated	O
to	O
redirect	O
traffic	O
to	O
a	O
different	O
/48	O
subnet	O
.	O
</s>
