<s>
Source-specific	B-Protocol
routing	I-Protocol
,	O
also	O
called	O
source-address	B-Protocol
dependent	I-Protocol
routing	I-Protocol
(	O
SADR	O
)	O
,	O
is	O
a	O
routing	B-Protocol
technique	O
in	O
which	O
a	O
routing	B-Protocol
decision	O
is	O
made	O
by	O
looking	O
at	O
the	O
source	O
address	O
of	O
a	O
packet	B-Protocol
in	O
addition	O
to	O
its	O
destination	O
address	O
.	O
</s>
<s>
The	O
main	O
application	O
of	O
source-specific	B-Protocol
routing	I-Protocol
is	O
to	O
allow	O
a	O
cheap	O
form	O
of	O
multihoming	B-Protocol
without	O
the	O
need	O
for	O
provider-independent	O
addresses	O
or	O
any	O
cooperation	O
from	O
upstream	O
ISPs	O
.	O
</s>
<s>
In	O
traditional	O
next-hop	O
routing	B-Protocol
,	O
a	O
packet	B-Protocol
is	O
routed	B-Protocol
according	O
to	O
its	O
destination	O
only	O
,	O
towards	O
the	O
closest	O
router	O
that	O
announces	O
a	O
route	O
that	O
matches	O
that	O
destination	O
.	O
</s>
<s>
Consider	O
a	O
multihomed	B-Protocol
end-user	O
network	O
connected	O
to	O
two	O
ISPs	O
,	O
BT&T	O
and	O
PacketCast	O
;	O
such	O
a	O
network	O
will	O
typically	O
have	O
two	O
edge	O
routers	O
,	O
each	O
of	O
which	O
is	O
connected	O
to	O
one	O
ISP	O
.	O
</s>
<s>
Both	O
edge	O
routers	O
announce	O
a	O
default	O
route	O
,	O
meaning	O
that	O
they	O
are	O
willing	O
to	O
accept	O
packets	B-Protocol
destined	O
for	O
the	O
Internet	O
.	O
</s>
<s>
If	O
a	O
packet	B-Protocol
with	O
a	O
source	O
in	O
T	O
's	O
network	O
is	O
routed	B-Protocol
through	O
PacketCast	O
's	O
edge	O
router	O
,	O
PacketCast	O
will	O
assume	O
it	O
is	O
a	O
spoofed	O
packet	B-Protocol
,	O
and	O
drop	O
it	O
in	O
accordance	O
to	O
BCP	O
38	O
.	O
</s>
<s>
With	O
source-specific	B-Protocol
routing	I-Protocol
,	O
each	O
edge	O
router	O
announces	O
a	O
source-specific	O
default	O
route	O
:	O
a	O
route	O
that	O
applies	O
to	O
packets	B-Protocol
destined	O
to	O
the	O
Internet	O
but	O
only	O
if	O
their	O
source	O
is	O
in	O
a	O
given	O
prefix	O
.	O
</s>
<s>
The	O
effect	O
is	O
that	O
each	O
edge	O
router	O
only	O
attracts	O
packets	B-Protocol
that	O
have	O
a	O
source	O
address	O
in	O
that	O
provider	O
's	O
prefix	O
.	O
</s>
<s>
With	O
source-specific	B-Protocol
routing	I-Protocol
,	O
each	O
host	O
interface	O
has	O
multiple	O
addresses	O
,	O
one	O
per	O
provider-dependent	O
prefix	O
.	O
</s>
<s>
Various	O
techniques	O
for	O
doing	O
that	O
have	O
been	O
suggested	O
,	O
at	O
the	O
network	O
layer	O
,	O
above	O
the	O
network	O
layer	O
(	O
see	O
Shim6	B-Protocol
)	O
,	O
or	O
by	O
using	O
multipath	O
techniques	O
at	O
the	O
higher	O
layers	O
(	O
see	O
Multipath	B-Protocol
TCP	I-Protocol
and	O
Multipath	O
Mosh	B-Protocol
)	O
.	O
</s>
<s>
On	O
a	O
network	O
with	O
a	O
single	O
edge	O
router	O
,	O
it	O
is	O
possible	O
to	O
implement	O
source-specific	B-Protocol
routing	I-Protocol
by	O
manual	O
manipulation	O
of	O
routing	B-Protocol
tables	O
.	O
</s>
<s>
With	O
multiple	O
routers	O
,	O
explicit	O
support	O
for	O
source-specific	B-Protocol
routing	I-Protocol
is	O
required	O
in	O
the	O
routing	B-Protocol
protocol	O
.	O
</s>
<s>
As	O
of	O
early	O
2016	O
,	O
there	O
are	O
two	O
routing	B-Protocol
protocols	O
that	O
implement	O
support	O
for	O
source-specific	B-Protocol
routing	I-Protocol
:	O
</s>
<s>
The	O
Babel	B-Protocol
routing	I-Protocol
protocol	I-Protocol
has	O
support	O
for	O
source-specific	B-Protocol
routing	I-Protocol
for	O
both	O
IPv4	O
and	O
IPv6	O
;	O
this	O
is	O
implemented	O
for	O
IPv6	O
in	O
babeld	O
and	O
in	O
BIRD	O
(	O
earlier	O
versions	O
of	O
babeld	O
supported	O
source-specific	B-Protocol
routing	I-Protocol
for	O
IPv4	O
)	O
;	O
</s>
<s>
There	O
exists	O
an	O
implementation	O
of	O
IS-IS	B-Protocol
with	O
support	O
for	O
source-specific	B-Protocol
routing	I-Protocol
for	O
IPv6	O
only	O
.	O
</s>
<s>
The	O
IETF	O
Homenet	O
protocol	O
suite	O
requires	O
support	O
for	O
source-specific	B-Protocol
routing	I-Protocol
in	O
its	O
routing	B-Protocol
protocol	O
.	O
</s>
