<s>
Routing	B-Protocol
is	O
the	O
process	O
of	O
selecting	O
a	O
path	O
for	O
traffic	O
in	O
a	O
network	O
or	O
between	O
or	O
across	O
multiple	O
networks	O
.	O
</s>
<s>
Broadly	O
,	O
routing	B-Protocol
is	O
performed	O
in	O
many	O
types	O
of	O
networks	O
,	O
including	O
circuit-switched	B-Protocol
networks	I-Protocol
,	O
such	O
as	O
the	O
public	O
switched	O
telephone	O
network	O
(	O
PSTN	O
)	O
,	O
and	O
computer	B-Architecture
networks	I-Architecture
,	O
such	O
as	O
the	O
Internet	O
.	O
</s>
<s>
In	O
packet	B-Protocol
switching	I-Protocol
networks	O
,	O
routing	B-Protocol
is	O
the	O
higher-level	O
decision	O
making	O
that	O
directs	O
network	B-Protocol
packets	I-Protocol
from	O
their	O
source	O
toward	O
their	O
destination	O
through	O
intermediate	O
network	B-Protocol
nodes	I-Protocol
by	O
specific	O
packet	B-Protocol
forwarding	I-Protocol
mechanisms	O
.	O
</s>
<s>
Packet	B-Protocol
forwarding	I-Protocol
is	O
the	O
transit	O
of	O
network	B-Protocol
packets	I-Protocol
from	O
one	O
network	B-Protocol
interface	I-Protocol
to	O
another	O
.	O
</s>
<s>
Intermediate	B-Protocol
nodes	I-Protocol
are	O
typically	O
network	B-Device
hardware	I-Device
devices	O
such	O
as	O
routers	B-Protocol
,	O
gateways	B-Application
,	O
firewalls	B-Application
,	O
or	O
switches	B-Protocol
.	O
</s>
<s>
General-purpose	O
computers	O
also	O
forward	O
packets	B-Protocol
and	O
perform	O
routing	B-Protocol
,	O
although	O
they	O
have	O
no	O
specially	O
optimized	O
hardware	O
for	O
the	O
task	O
.	O
</s>
<s>
The	O
routing	B-Protocol
process	O
usually	O
directs	O
forwarding	O
on	O
the	O
basis	O
of	O
routing	B-Protocol
tables	I-Protocol
.	O
</s>
<s>
Routing	B-Protocol
tables	I-Protocol
maintain	O
a	O
record	O
of	O
the	O
routes	O
to	O
various	O
network	O
destinations	O
.	O
</s>
<s>
Routing	B-Protocol
tables	I-Protocol
may	O
be	O
specified	O
by	O
an	O
administrator	O
,	O
learned	O
by	O
observing	O
network	O
traffic	O
or	O
built	O
with	O
the	O
assistance	O
of	O
routing	B-Protocol
protocols	I-Protocol
.	O
</s>
<s>
Routing	B-Protocol
,	O
in	O
a	O
narrower	O
sense	O
of	O
the	O
term	O
,	O
often	O
refers	O
to	O
IP	B-Protocol
routing	I-Protocol
and	O
is	O
contrasted	O
with	O
bridging	B-Protocol
.	O
</s>
<s>
IP	B-Protocol
routing	I-Protocol
assumes	O
that	O
network	O
addresses	O
are	O
structured	O
and	O
that	O
similar	O
addresses	O
imply	O
proximity	O
within	O
the	O
network	O
.	O
</s>
<s>
Structured	O
addresses	O
allow	O
a	O
single	O
routing	B-Protocol
table	I-Protocol
entry	O
to	O
represent	O
the	O
route	O
to	O
a	O
group	O
of	O
devices	O
.	O
</s>
<s>
In	O
large	O
networks	O
,	O
structured	O
addressing	O
(	O
routing	B-Protocol
,	O
in	O
the	O
narrow	O
sense	O
)	O
outperforms	O
unstructured	O
addressing	O
(	O
bridging	B-Protocol
)	O
.	O
</s>
<s>
Routing	B-Protocol
has	O
become	O
the	O
dominant	O
form	O
of	O
addressing	O
on	O
the	O
Internet	O
.	O
</s>
<s>
Bridging	B-Protocol
is	O
still	O
widely	O
used	O
within	O
local	B-General_Concept
area	I-General_Concept
networks	I-General_Concept
.	O
</s>
<s>
Routing	B-Protocol
schemes	O
differ	O
in	O
how	O
they	O
deliver	O
messages	O
:	O
</s>
<s>
This	O
article	O
focuses	O
on	O
unicast	O
routing	B-Protocol
algorithms	I-Protocol
.	O
</s>
<s>
With	O
static	B-Protocol
routing	I-Protocol
,	O
small	O
networks	O
may	O
use	O
manually	O
configured	O
routing	B-Protocol
tables	I-Protocol
.	O
</s>
<s>
Larger	O
networks	O
have	O
complex	O
topologies	B-Architecture
that	O
can	O
change	O
rapidly	O
,	O
making	O
the	O
manual	O
construction	O
of	O
routing	B-Protocol
tables	I-Protocol
unfeasible	O
.	O
</s>
<s>
Nevertheless	O
,	O
most	O
of	O
the	O
public	O
switched	O
telephone	O
network	O
(	O
PSTN	O
)	O
uses	O
pre-computed	O
routing	B-Protocol
tables	I-Protocol
,	O
with	O
fallback	O
routes	O
if	O
the	O
most	O
direct	O
route	O
becomes	O
blocked	O
(	O
see	O
routing	B-Protocol
in	O
the	O
PSTN	O
)	O
.	O
</s>
<s>
Dynamic	B-Protocol
routing	I-Protocol
attempts	O
to	O
solve	O
this	O
problem	O
by	O
constructing	O
routing	B-Protocol
tables	I-Protocol
automatically	O
,	O
based	O
on	O
information	O
carried	O
by	O
routing	B-Protocol
protocols	I-Protocol
,	O
allowing	O
the	O
network	O
to	O
act	O
nearly	O
autonomously	O
in	O
avoiding	O
network	O
failures	O
and	O
blockages	O
.	O
</s>
<s>
Dynamic	B-Protocol
routing	I-Protocol
dominates	O
the	O
Internet	O
.	O
</s>
<s>
Examples	O
of	O
dynamic-routing	O
protocols	O
and	O
algorithms	O
include	O
Routing	B-Protocol
Information	I-Protocol
Protocol	I-Protocol
(	O
RIP	O
)	O
,	O
Open	B-Protocol
Shortest	I-Protocol
Path	I-Protocol
First	I-Protocol
(	O
OSPF	B-Protocol
)	O
and	O
Enhanced	B-Protocol
Interior	I-Protocol
Gateway	I-Protocol
Routing	I-Protocol
Protocol	I-Protocol
(	O
EIGRP	B-Protocol
)	O
.	O
</s>
<s>
Distance	O
vector	O
algorithms	O
use	O
the	O
Bellman	B-Algorithm
–	I-Algorithm
Ford	I-Algorithm
algorithm	I-Algorithm
.	O
</s>
<s>
(	O
This	O
information	O
—	O
the	O
list	O
of	O
destinations	O
,	O
the	O
total	O
cost	O
to	O
each	O
,	O
and	O
the	O
next	O
hop	B-Protocol
to	O
send	O
data	O
to	O
get	O
there	O
—	O
makes	O
up	O
the	O
routing	B-Protocol
table	I-Protocol
,	O
or	O
distance	O
table	O
.	O
)	O
</s>
<s>
Over	O
time	O
,	O
all	O
the	O
nodes	O
in	O
the	O
network	O
discover	O
the	O
best	O
next	O
hop	B-Protocol
and	O
total	O
cost	O
for	O
all	O
destinations	O
.	O
</s>
<s>
When	O
a	O
network	B-Protocol
node	I-Protocol
goes	O
down	O
,	O
any	O
nodes	O
that	O
used	O
it	O
as	O
their	O
next	O
hop	B-Protocol
discard	O
the	O
entry	O
and	O
convey	O
the	O
updated	O
routing	B-Protocol
information	I-Protocol
to	O
all	O
adjacent	O
nodes	O
,	O
which	O
in	O
turn	O
repeat	O
the	O
process	O
.	O
</s>
<s>
When	O
applying	O
link-state	B-Protocol
algorithms	O
,	O
a	O
graphical	O
map	O
of	O
the	O
network	O
is	O
the	O
fundamental	O
data	O
used	O
for	O
each	O
node	O
.	O
</s>
<s>
Using	O
this	O
map	O
,	O
each	O
router	B-Protocol
independently	O
determines	O
the	O
least-cost	O
path	O
from	O
itself	O
to	O
every	O
other	O
node	O
using	O
a	O
standard	O
shortest	O
paths	O
algorithm	O
such	O
as	O
Dijkstra	B-Algorithm
's	I-Algorithm
algorithm	I-Algorithm
.	O
</s>
<s>
This	O
tree	O
then	O
serves	O
to	O
construct	O
the	O
routing	B-Protocol
table	I-Protocol
,	O
which	O
specifies	O
the	O
best	O
next	O
hop	B-Protocol
to	O
get	O
from	O
the	O
current	O
node	O
to	O
any	O
other	O
node	O
.	O
</s>
<s>
A	O
link-state	B-Protocol
routing	I-Protocol
algorithm	O
optimized	O
for	O
mobile	B-Protocol
ad	I-Protocol
hoc	I-Protocol
networks	I-Protocol
is	O
the	O
optimized	O
Link	B-Protocol
State	I-Protocol
Routing	I-Protocol
Protocol	I-Protocol
(	O
OLSR	O
)	O
.	O
</s>
<s>
OLSR	O
is	O
proactive	O
;	O
it	O
uses	O
Hello	O
and	O
Topology	B-Architecture
Control	O
(	O
TC	O
)	O
messages	O
to	O
discover	O
and	O
disseminate	O
link-state	B-Protocol
information	O
through	O
the	O
mobile	B-Protocol
ad	I-Protocol
hoc	I-Protocol
network	I-Protocol
.	O
</s>
<s>
Using	O
Hello	O
messages	O
,	O
each	O
node	O
discovers	O
2-hop	O
neighbor	O
information	O
and	O
elects	O
a	O
set	O
of	O
multipoint	O
relays	O
(	O
MPRs	O
)	O
.	O
</s>
<s>
MPRs	O
distinguish	O
OLSR	O
from	O
other	O
link-state	B-Protocol
routing	I-Protocol
protocols	I-Protocol
.	O
</s>
<s>
Distance	O
vector	O
and	O
link-state	B-Protocol
routing	I-Protocol
are	O
both	O
intra-domain	B-Architecture
routing	I-Architecture
protocols	O
.	O
</s>
<s>
They	O
are	O
used	O
inside	O
an	O
autonomous	B-Architecture
system	I-Architecture
,	O
but	O
not	O
between	O
autonomous	B-Architecture
systems	I-Architecture
.	O
</s>
<s>
Both	O
of	O
these	O
routing	B-Protocol
protocols	I-Protocol
become	O
intractable	O
in	O
large	O
networks	O
and	O
cannot	O
be	O
used	O
in	O
inter-domain	B-Architecture
routing	I-Architecture
.	O
</s>
<s>
Distance	O
vector	O
routing	B-Protocol
is	O
subject	O
to	O
instability	O
if	O
there	O
are	O
more	O
than	O
a	O
few	O
hops	B-Protocol
in	O
the	O
domain	O
.	O
</s>
<s>
Link	B-Protocol
state	I-Protocol
routing	I-Protocol
needs	O
significant	O
resources	O
to	O
calculate	O
routing	B-Protocol
tables	I-Protocol
.	O
</s>
<s>
Path-vector	O
routing	B-Protocol
is	O
used	O
for	O
inter-domain	B-Architecture
routing	I-Architecture
.	O
</s>
<s>
It	O
is	O
similar	O
to	O
distance	O
vector	O
routing	B-Protocol
.	O
</s>
<s>
Path-vector	O
routing	B-Protocol
assumes	O
that	O
one	O
node	O
(	O
there	O
can	O
be	O
many	O
)	O
in	O
each	O
autonomous	B-Architecture
system	I-Architecture
acts	O
on	O
behalf	O
of	O
the	O
entire	O
autonomous	B-Architecture
system	I-Architecture
.	O
</s>
<s>
The	O
speaker	O
node	O
creates	O
a	O
routing	B-Protocol
table	I-Protocol
and	O
advertises	O
it	O
to	O
neighboring	O
speaker	O
nodes	O
in	O
neighboring	O
autonomous	B-Architecture
systems	I-Architecture
.	O
</s>
<s>
The	O
idea	O
is	O
the	O
same	O
as	O
distance	O
vector	O
routing	B-Protocol
except	O
that	O
only	O
speaker	O
nodes	O
in	O
each	O
autonomous	B-Architecture
system	I-Architecture
can	O
communicate	O
with	O
each	O
other	O
.	O
</s>
<s>
The	O
speaker	O
node	O
advertises	O
the	O
path	O
,	O
not	O
the	O
metric	O
,	O
of	O
the	O
nodes	O
in	O
its	O
autonomous	B-Architecture
system	I-Architecture
or	O
other	O
autonomous	B-Architecture
systems	I-Architecture
.	O
</s>
<s>
The	O
path-vector	O
routing	B-Protocol
algorithm	I-Protocol
is	O
similar	O
to	O
the	O
distance	O
vector	O
algorithm	O
in	O
the	O
sense	O
that	O
each	O
border	O
router	B-Protocol
advertises	O
the	O
destinations	O
it	O
can	O
reach	O
to	O
its	O
neighboring	O
router	B-Protocol
.	O
</s>
<s>
The	O
path	O
,	O
expressed	O
in	O
terms	O
of	O
the	O
domains	O
(	O
or	O
confederations	O
)	O
traversed	O
so	O
far	O
,	O
is	O
carried	O
in	O
a	O
special	O
path	O
attribute	O
that	O
records	O
the	O
sequence	O
of	O
routing	B-Protocol
domains	O
through	O
which	O
the	O
reachability	O
information	O
has	O
passed	O
.	O
</s>
<s>
A	O
route	O
is	O
defined	O
as	O
a	O
pairing	O
between	O
a	O
destination	O
and	O
the	O
attributes	O
of	O
the	O
path	O
to	O
that	O
destination	O
,	O
thus	O
the	O
name	O
,	O
path-vector	O
routing	B-Protocol
;	O
The	O
routers	B-Protocol
receive	O
a	O
vector	O
that	O
contains	O
paths	O
to	O
a	O
set	O
of	O
destinations	O
.	O
</s>
<s>
Path	O
selection	O
involves	O
applying	O
a	O
routing	B-Protocol
metric	O
to	O
multiple	O
routes	O
to	O
select	O
(	O
or	O
predict	O
)	O
the	O
best	O
route	O
.	O
</s>
<s>
Most	O
routing	B-Protocol
algorithms	I-Protocol
use	O
only	O
one	O
network	O
path	O
at	O
a	O
time	O
.	O
</s>
<s>
Multipath	B-Protocol
routing	I-Protocol
and	O
specifically	O
equal-cost	B-Protocol
multi-path	I-Protocol
routing	I-Protocol
techniques	O
enable	O
the	O
use	O
of	O
multiple	O
alternative	O
paths	O
.	O
</s>
<s>
In	O
computer	B-Architecture
networking	I-Architecture
,	O
the	O
metric	O
is	O
computed	O
by	O
a	O
routing	B-Protocol
algorithm	I-Protocol
,	O
and	O
can	O
cover	O
information	O
such	O
as	O
bandwidth	O
,	O
network	B-Protocol
delay	I-Protocol
,	O
hop	B-Protocol
count	I-Protocol
,	O
path	O
cost	O
,	O
load	O
,	O
maximum	B-Protocol
transmission	I-Protocol
unit	I-Protocol
,	O
reliability	O
,	O
and	O
communication	O
cost	O
.	O
</s>
<s>
The	O
routing	B-Protocol
table	I-Protocol
stores	O
only	O
the	O
best	O
possible	O
routes	O
,	O
while	O
link-state	B-Protocol
or	O
topological	O
databases	O
may	O
store	O
all	O
other	O
information	O
as	O
well	O
.	O
</s>
<s>
In	O
case	O
of	O
overlapping	O
or	O
equal	O
routes	O
,	O
algorithms	O
consider	O
the	O
following	O
elements	O
in	O
priority	O
order	O
to	O
decide	O
which	O
routes	O
to	O
install	O
into	O
the	O
routing	B-Protocol
table	I-Protocol
:	O
</s>
<s>
Prefix	O
length	O
:	O
A	O
matching	O
route	B-Protocol
table	I-Protocol
entry	O
with	O
a	O
longer	O
subnet	O
mask	O
is	O
always	O
preferred	O
as	O
it	O
specifies	O
the	O
destination	O
more	O
exactly	O
.	O
</s>
<s>
Metric	O
:	O
When	O
comparing	O
routes	O
learned	O
via	O
the	O
same	O
routing	B-Protocol
protocol	I-Protocol
,	O
a	O
lower	O
metric	O
is	O
preferred	O
.	O
</s>
<s>
Metrics	O
cannot	O
be	O
compared	O
between	O
routes	O
learned	O
from	O
different	O
routing	B-Protocol
protocols	I-Protocol
.	O
</s>
<s>
Administrative	B-Protocol
distance	I-Protocol
:	O
When	O
comparing	O
route	B-Protocol
table	I-Protocol
entries	O
from	O
different	O
sources	O
such	O
as	O
different	O
routing	B-Protocol
protocols	I-Protocol
and	O
static	O
configuration	O
,	O
a	O
lower	O
administrative	B-Protocol
distance	I-Protocol
indicates	O
a	O
more	O
reliable	O
source	O
and	O
thus	O
a	O
preferred	O
route	O
.	O
</s>
<s>
Because	O
a	O
routing	B-Protocol
metric	O
is	O
specific	O
to	O
a	O
given	O
routing	B-Protocol
protocol	I-Protocol
,	O
multi-protocol	O
routers	B-Protocol
must	O
use	O
some	O
external	O
heuristic	B-Algorithm
to	O
select	O
between	O
routes	O
learned	O
from	O
different	O
routing	B-Protocol
protocols	I-Protocol
.	O
</s>
<s>
Cisco	O
routers	B-Protocol
,	O
for	O
example	O
,	O
attribute	O
a	O
value	O
known	O
as	O
the	O
administrative	B-Protocol
distance	I-Protocol
to	O
each	O
route	O
,	O
where	O
smaller	O
administrative	B-Protocol
distances	I-Protocol
indicate	O
routes	O
learned	O
from	O
a	O
protocol	O
assumed	O
to	O
be	O
more	O
reliable	O
.	O
</s>
<s>
This	O
is	O
useful	O
for	O
debugging	O
network	O
connections	O
or	O
routing	B-Protocol
tables	I-Protocol
.	O
</s>
<s>
In	O
some	O
small	O
systems	O
,	O
a	O
single	O
central	O
device	O
decides	O
ahead	O
of	O
time	O
the	O
complete	O
path	O
of	O
every	O
packet	B-Protocol
.	O
</s>
<s>
In	O
some	O
other	O
small	O
systems	O
,	O
whichever	O
edge	O
device	O
injects	O
a	O
packet	B-Protocol
into	O
the	O
network	O
decides	O
ahead	O
of	O
time	O
the	O
complete	O
path	O
of	O
that	O
particular	O
packet	B-Protocol
.	O
</s>
<s>
Once	O
it	O
has	O
this	O
information	O
,	O
it	O
can	O
use	O
an	O
algorithm	O
such	O
as	O
A*	B-Protocol
search	I-Protocol
algorithm	I-Protocol
to	O
find	O
the	O
best	O
path	O
.	O
</s>
<s>
In	O
high-speed	O
systems	O
,	O
there	O
are	O
so	O
many	O
packets	B-Protocol
transmitted	O
every	O
second	O
that	O
it	O
is	O
infeasible	O
for	O
a	O
single	O
device	O
to	O
calculate	O
the	O
complete	O
path	O
for	O
each	O
and	O
every	O
packet	B-Protocol
.	O
</s>
<s>
Early	O
high-speed	O
systems	O
dealt	O
with	O
this	O
with	O
circuit	B-Protocol
switching	I-Protocol
by	O
setting	O
up	O
a	O
path	O
once	O
for	O
the	O
first	O
packet	B-Protocol
between	O
some	O
source	O
and	O
some	O
destination	O
;	O
later	O
packets	B-Protocol
between	O
that	O
same	O
source	O
and	O
that	O
same	O
destination	O
continue	O
to	O
follow	O
the	O
same	O
path	O
without	O
recalculating	O
until	O
the	O
circuit	O
teardown	O
.	O
</s>
<s>
Later	O
high-speed	O
systems	O
inject	O
packets	B-Protocol
into	O
the	O
network	O
without	O
any	O
one	O
device	O
ever	O
calculating	O
a	O
complete	O
path	O
for	O
packets	B-Protocol
.	O
</s>
<s>
Such	O
systems	O
generally	O
use	O
next-hop	O
routing	B-Protocol
.	O
</s>
<s>
Most	O
systems	O
use	O
a	O
deterministic	O
dynamic	B-Protocol
routing	I-Protocol
algorithm	O
.	O
</s>
<s>
A	O
few	O
routing	B-Protocol
algorithms	I-Protocol
do	O
not	O
use	O
a	O
deterministic	O
algorithm	O
to	O
find	O
the	O
best	O
link	O
for	O
a	O
packet	B-Protocol
to	O
get	O
from	O
its	O
original	O
source	O
to	O
its	O
final	O
destination	O
.	O
</s>
<s>
Instead	O
,	O
to	O
avoid	O
congestion	O
hot	O
spots	O
in	O
packet	B-Protocol
systems	O
,	O
a	O
few	O
algorithms	O
use	O
a	O
randomized	O
Valiant	O
's	O
paradigm	O
—	O
that	O
routes	O
a	O
path	O
to	O
a	O
randomly	O
picked	O
intermediate	O
destination	O
,	O
and	O
from	O
there	O
to	O
its	O
true	O
final	O
destination	O
.	O
</s>
<s>
In	O
many	O
early	O
telephone	O
switches	B-Protocol
,	O
a	O
randomizer	O
was	O
often	O
used	O
to	O
select	O
the	O
start	O
of	O
a	O
path	O
through	O
a	O
multistage	O
switching	O
fabric	O
.	O
</s>
<s>
In	O
some	O
networks	O
,	O
routing	B-Protocol
is	O
complicated	O
by	O
the	O
fact	O
that	O
no	O
single	O
entity	O
is	O
responsible	O
for	O
selecting	O
paths	O
;	O
instead	O
,	O
multiple	O
entities	O
are	O
involved	O
in	O
selecting	O
paths	O
or	O
even	O
parts	O
of	O
a	O
single	O
path	O
.	O
</s>
<s>
With	O
such	O
routing	B-Protocol
,	O
the	O
equilibrium	O
routes	O
can	O
be	O
longer	O
than	O
optimal	O
for	O
all	O
drivers	O
.	O
</s>
<s>
In	O
a	O
single-agent	O
model	O
used	O
,	O
for	O
example	O
,	O
for	O
routing	B-Protocol
automated	O
guided	O
vehicles	O
(	O
AGVs	O
)	O
on	O
a	O
terminal	O
,	O
reservations	O
are	O
made	O
for	O
each	O
vehicle	O
to	O
prevent	O
simultaneous	O
use	O
of	O
the	O
same	O
part	O
of	O
an	O
infrastructure	O
.	O
</s>
<s>
This	O
approach	O
is	O
also	O
referred	O
to	O
as	O
context-aware	O
routing	B-Protocol
.	O
</s>
<s>
The	O
Internet	O
is	O
partitioned	O
into	O
autonomous	B-Architecture
systems	I-Architecture
(	O
ASs	O
)	O
such	O
as	O
internet	O
service	O
providers	O
(	O
ISPs	O
)	O
,	O
each	O
of	O
which	O
controls	O
routes	O
involving	O
its	O
network	O
.	O
</s>
<s>
Routing	B-Protocol
occurs	O
at	O
multiple	O
levels	O
.	O
</s>
<s>
First	O
,	O
AS-level	O
paths	O
are	O
selected	O
via	O
the	O
BGP	B-Protocol
protocol	O
that	O
produces	O
a	O
sequence	O
of	O
ASs	O
through	O
which	O
packets	B-Protocol
flow	O
.	O
</s>
<s>
These	O
routing	B-Protocol
decisions	O
often	O
correlate	O
with	O
business	O
relationships	O
with	O
these	O
neighboring	O
ASs	O
,	O
which	O
may	O
be	O
unrelated	O
to	O
path	O
quality	O
or	O
latency	O
.	O
</s>
<s>
Second	O
,	O
once	O
an	O
AS-level	O
path	O
has	O
been	O
selected	O
,	O
there	O
are	O
often	O
multiple	O
corresponding	O
router-level	O
paths	O
to	O
choose	O
from	O
.	O
</s>
<s>
In	O
choosing	O
the	O
single	O
router-level	O
path	O
,	O
it	O
is	O
common	O
practice	O
for	O
each	O
ISP	O
to	O
employ	O
hot-potato	B-Protocol
routing	I-Protocol
:	O
sending	O
traffic	O
along	O
the	O
path	O
that	O
minimizes	O
the	O
distance	O
through	O
the	O
ISP	O
's	O
own	O
network	O
—	O
even	O
if	O
that	O
path	O
lengthens	O
the	O
total	O
distance	O
to	O
the	O
destination	O
.	O
</s>
<s>
When	O
routing	B-Protocol
a	O
message	O
from	O
a	O
source	O
in	O
A	O
's	O
London	O
network	O
to	O
a	O
destination	O
in	O
B	O
's	O
New	O
York	O
network	O
,	O
A	O
may	O
choose	O
to	O
immediately	O
send	O
the	O
message	O
to	O
B	O
in	O
London	O
.	O
</s>
<s>
A	O
2003	O
measurement	O
study	O
of	O
Internet	O
routes	O
found	O
that	O
,	O
between	O
pairs	O
of	O
neighboring	O
ISPs	O
,	O
more	O
than	O
30%	O
of	O
paths	O
have	O
inflated	O
latency	O
due	O
to	O
hot-potato	B-Protocol
routing	I-Protocol
,	O
with	O
5%	O
of	O
paths	O
being	O
delayed	O
by	O
at	O
least	O
12ms	O
.	O
</s>
<s>
Inflation	O
due	O
to	O
AS-level	O
path	O
selection	O
,	O
while	O
substantial	O
,	O
was	O
attributed	O
primarily	O
to	O
BGP	B-Protocol
's	O
lack	O
of	O
a	O
mechanism	O
to	O
directly	O
optimize	O
for	O
latency	O
,	O
rather	O
than	O
to	O
selfish	O
routing	B-Protocol
policies	I-Protocol
.	O
</s>
<s>
It	O
was	O
also	O
suggested	O
that	O
,	O
were	O
an	O
appropriate	O
mechanism	O
in	O
place	O
,	O
ISPs	O
would	O
be	O
willing	O
to	O
cooperate	O
to	O
reduce	O
latency	O
rather	O
than	O
use	O
hot-potato	B-Protocol
routing	I-Protocol
.	O
</s>
<s>
As	O
the	O
Internet	O
and	O
IP	O
networks	O
have	O
become	O
mission	B-Application
critical	I-Application
business	O
tools	O
,	O
there	O
has	O
been	O
increased	O
interest	O
in	O
techniques	O
and	O
methods	O
to	O
monitor	O
the	O
routing	B-Protocol
posture	O
of	O
networks	O
.	O
</s>
<s>
Incorrect	O
routing	B-Protocol
or	O
routing	B-Protocol
issues	O
cause	O
undesirable	O
performance	O
degradation	O
,	O
flapping	B-Protocol
or	O
downtime	O
.	O
</s>
<s>
Monitoring	O
routing	B-Protocol
in	O
a	O
network	O
is	O
achieved	O
using	O
route	O
analytics	O
tools	O
and	O
techniques	O
.	O
</s>
<s>
In	O
networks	O
where	O
a	O
logically	O
centralized	O
control	O
is	O
available	O
over	O
the	O
forwarding	O
state	O
,	O
for	O
example	O
,	O
using	O
software-defined	O
networking	O
,	O
routing	B-Protocol
techniques	O
can	O
be	O
used	O
that	O
aim	O
to	O
optimize	O
global	O
and	O
network-wide	O
performance	O
metrics	O
.	O
</s>
<s>
Work	O
on	O
the	O
later	O
over	O
private	O
WAN	O
discusses	O
modeling	O
routing	B-Protocol
as	O
a	O
graph	O
optimization	O
problem	O
by	O
pushing	O
all	O
the	O
queuing	O
to	O
the	O
end-points	O
.	O
</s>
<s>
The	O
authors	O
also	O
propose	O
a	O
heuristic	B-Algorithm
to	O
solve	O
the	O
problem	O
efficiently	O
while	O
sacrificing	O
negligible	O
performance	O
.	O
</s>
