<s>
Link-state	B-Protocol
routing	I-Protocol
protocols	I-Protocol
are	O
one	O
of	O
the	O
two	O
main	O
classes	O
of	O
routing	B-Protocol
protocols	I-Protocol
used	O
in	O
packet	B-Protocol
switching	I-Protocol
networks	O
for	O
computer	B-Architecture
communications	I-Architecture
,	O
the	O
others	O
being	O
distance-vector	B-Protocol
routing	I-Protocol
protocols	I-Protocol
.	O
</s>
<s>
Examples	O
of	O
link-state	B-Protocol
routing	I-Protocol
protocols	I-Protocol
include	O
Open	B-Protocol
Shortest	I-Protocol
Path	I-Protocol
First	I-Protocol
(	O
OSPF	B-Protocol
)	O
and	O
Intermediate	B-Protocol
System	I-Protocol
to	I-Protocol
Intermediate	I-Protocol
System	I-Protocol
(	O
IS-IS	B-Protocol
)	O
.	O
</s>
<s>
The	O
link-state	B-Protocol
protocol	I-Protocol
is	O
performed	O
by	O
every	O
switching	O
node	O
in	O
the	O
network	O
(	O
i.e.	O
,	O
nodes	O
that	O
are	O
prepared	O
to	O
forward	O
packets	O
;	O
in	O
the	O
Internet	O
,	O
these	O
are	O
called	O
routers	B-Protocol
)	O
.	O
</s>
<s>
The	O
basic	O
concept	O
of	O
link-state	B-Protocol
routing	I-Protocol
is	O
that	O
every	O
node	O
constructs	O
a	O
map	O
of	O
the	O
connectivity	O
to	O
the	O
network	O
,	O
in	O
the	O
form	O
of	O
a	O
graph	O
,	O
showing	O
which	O
nodes	O
are	O
connected	O
to	O
which	O
other	O
nodes	O
.	O
</s>
<s>
Each	O
collection	O
of	O
best	O
paths	O
will	O
then	O
form	O
each	O
node	O
's	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
This	O
contrasts	O
with	O
distance-vector	B-Protocol
routing	I-Protocol
protocols	I-Protocol
,	O
which	O
work	O
by	O
having	O
each	O
node	O
share	O
its	O
routing	B-Protocol
table	I-Protocol
with	O
its	O
neighbours	O
,	O
in	O
a	O
link-state	B-Protocol
protocol	I-Protocol
the	O
only	O
information	O
passed	O
between	O
nodes	O
is	O
connectivity	O
related	O
.	O
</s>
<s>
Link-state	B-Protocol
algorithms	O
are	O
sometimes	O
characterized	O
informally	O
as	O
each	O
router	B-Protocol
,	O
"	O
telling	O
the	O
world	O
about	O
its	O
neighbors.	O
"	O
</s>
<s>
In	O
link-state	B-Protocol
routing	I-Protocol
protocols	I-Protocol
,	O
each	O
router	B-Protocol
possesses	O
information	O
about	O
the	O
complete	O
network	O
topology	O
.	O
</s>
<s>
Each	O
router	B-Protocol
then	O
independently	O
calculates	O
the	O
best	O
next	O
hop	O
from	O
it	O
for	O
every	O
possible	O
destination	O
in	O
the	O
network	O
using	O
local	O
information	O
of	O
the	O
topology	O
.	O
</s>
<s>
The	O
collection	O
of	O
best-next-hops	O
forms	O
the	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
This	O
contrasts	O
with	O
distance-vector	B-Protocol
routing	I-Protocol
protocols	I-Protocol
,	O
which	O
work	O
by	O
having	O
each	O
node	O
share	O
its	O
routing	B-Protocol
table	I-Protocol
with	O
its	O
neighbours	O
.	O
</s>
<s>
In	O
a	O
link-state	B-Protocol
protocol	I-Protocol
,	O
the	O
only	O
information	O
passed	O
between	O
the	O
nodes	O
is	O
the	O
information	O
used	O
to	O
construct	O
the	O
connectivity	O
maps	O
.	O
</s>
<s>
Examples	O
of	O
link-state	B-Protocol
routing	I-Protocol
protocols	I-Protocol
:	O
</s>
<s>
What	O
is	O
believed	O
to	O
be	O
the	O
first	O
adaptive	O
routing	O
network	O
of	O
computers	O
,	O
using	O
link-state	B-Protocol
routing	I-Protocol
as	O
its	O
heart	O
,	O
was	O
designed	O
and	O
implemented	O
during	O
1976-1977	O
by	O
a	O
team	O
from	O
Plessey	O
Radar	O
led	O
by	O
Bernard	O
J	O
Harris	O
;	O
the	O
project	O
was	O
for	O
"	O
Wavell	O
"	O
a	O
system	O
of	O
computer	O
command	O
and	O
control	O
for	O
the	O
British	O
Army	O
.	O
</s>
<s>
The	O
first	O
link-state	B-Protocol
routing	I-Protocol
concept	O
was	O
published	O
in	O
1979	O
by	O
John	O
M	O
.	O
McQuillan	O
(	O
then	O
at	O
Bolt	O
,	O
Beranek	O
and	O
Newman	O
)	O
as	O
a	O
mechanism	O
that	O
would	O
calculate	O
routes	O
more	O
quickly	O
when	O
network	O
conditions	O
changed	O
,	O
and	O
thus	O
lead	O
to	O
more	O
stable	O
routing	O
.	O
</s>
<s>
Later	O
work	O
at	O
BBN	O
Technologies	O
showed	O
how	O
to	O
use	O
the	O
link-state	B-Protocol
technique	O
in	O
a	O
hierarchical	O
system	O
(	O
i.e.	O
,	O
one	O
in	O
which	O
the	O
network	O
was	O
divided	O
into	O
areas	O
)	O
so	O
that	O
each	O
switching	O
node	O
does	O
not	O
need	O
a	O
map	O
of	O
the	O
entire	O
network	O
,	O
only	O
the	O
area(s )	O
in	O
which	O
it	O
is	O
included	O
.	O
</s>
<s>
The	O
technique	O
was	O
later	O
adapted	O
for	O
use	O
in	O
the	O
contemporary	O
link-state	B-Protocol
routing	I-Protocol
protocols	I-Protocol
IS-IS	B-Protocol
and	O
OSPF	B-Protocol
.	O
</s>
<s>
Cisco	O
literature	O
refers	O
to	O
Enhanced	B-Protocol
Interior	I-Protocol
Gateway	I-Protocol
Routing	I-Protocol
Protocol	I-Protocol
(	O
EIGRP	B-Protocol
)	O
as	O
a	O
"	O
hybrid	O
"	O
protocol	O
,	O
despite	O
the	O
fact	O
it	O
distributes	O
routing	B-Protocol
tables	I-Protocol
instead	O
of	O
topology	O
maps	O
.	O
</s>
<s>
However	O
,	O
it	O
does	O
synchronize	O
routing	B-Protocol
tables	I-Protocol
at	O
start	O
up	O
as	O
OSPF	B-Protocol
does	O
,	O
and	O
sends	O
specific	O
updates	O
only	O
when	O
topology	O
changes	O
occur	O
.	O
</s>
<s>
In	O
2004	O
,	O
Radia	O
Perlman	O
proposed	O
using	O
link-state	B-Protocol
routing	I-Protocol
for	O
layer	B-Protocol
2	I-Protocol
frame	O
forwarding	O
with	O
devices	O
called	O
routing	B-Protocol
bridges	I-Protocol
or	O
Rbridges	B-Protocol
.	O
</s>
<s>
The	O
Internet	O
Engineering	O
Task	O
Force	O
has	O
standardized	O
the	O
Transparent	B-Protocol
Interconnection	I-Protocol
of	I-Protocol
Lots	I-Protocol
of	I-Protocol
Links	I-Protocol
(	O
TRILL	B-Protocol
)	O
protocol	O
to	O
accomplish	O
this	O
.	O
</s>
<s>
More	O
recently	O
,	O
this	O
hierarchical	O
technique	O
was	O
applied	O
to	O
wireless	O
mesh	O
networks	O
using	O
the	O
Optimized	B-Protocol
Link	I-Protocol
State	I-Protocol
Routing	I-Protocol
Protocol	I-Protocol
(	O
OLSR	B-Protocol
)	O
.	O
</s>
<s>
This	O
is	O
used	O
in	O
some	O
ad	B-Protocol
hoc	I-Protocol
routing	I-Protocol
protocols	I-Protocol
that	O
use	O
radio	O
frequency	O
transmission	O
.	O
</s>
<s>
In	O
2012	O
,	O
the	O
IEEE	O
completed	O
and	O
approved	O
the	O
standardization	O
of	O
the	O
use	O
of	O
IS-IS	B-Protocol
to	O
control	O
Ethernet	O
forwarding	O
with	O
IEEE	B-Protocol
802.1aq	I-Protocol
shortest	B-Protocol
path	I-Protocol
bridging	I-Protocol
(	O
SPB	O
)	O
.	O
</s>
<s>
As	O
previously	O
mentioned	O
,	O
the	O
first	O
main	O
stage	O
in	O
the	O
link-state	B-Protocol
algorithm	O
is	O
to	O
give	O
a	O
map	O
of	O
the	O
network	O
to	O
every	O
node	O
.	O
</s>
<s>
Next	O
,	O
each	O
node	O
periodically	O
(	O
and	O
in	O
case	O
of	O
connectivity	O
changes	O
)	O
sends	O
a	O
short	O
message	O
,	O
the	O
link-state	B-Protocol
advertisement	I-Protocol
,	O
which	O
:	O
</s>
<s>
Identifies	O
all	O
the	O
other	O
nodes	O
(	O
either	O
routers	B-Protocol
or	O
networks	O
)	O
to	O
which	O
it	O
is	O
directly	O
connected	O
.	O
</s>
<s>
As	O
a	O
necessary	O
precursor	O
,	O
each	O
node	O
in	O
the	O
network	O
remembers	O
,	O
for	O
every	O
one	O
of	O
its	O
neighbors	O
,	O
the	O
sequence	O
number	O
of	O
the	O
last	O
link-state	B-Protocol
message	O
which	O
it	O
received	O
from	O
that	O
node	O
.	O
</s>
<s>
When	O
a	O
link-state	B-Protocol
advertisement	I-Protocol
is	O
received	O
at	O
a	O
node	O
,	O
the	O
node	O
looks	O
up	O
the	O
sequence	O
number	O
it	O
has	O
stored	O
for	O
the	O
source	O
of	O
that	O
link-state	B-Protocol
message	O
:	O
if	O
this	O
message	O
is	O
newer	O
(	O
i.e.	O
,	O
has	O
a	O
higher	O
sequence	O
number	O
)	O
,	O
it	O
is	O
saved	O
,	O
the	O
sequence	O
number	O
is	O
updated	O
,	O
and	O
a	O
copy	O
is	O
sent	O
in	O
turn	O
to	O
each	O
of	O
that	O
node	O
's	O
neighbors	O
.	O
</s>
<s>
This	O
procedure	O
rapidly	O
gets	O
a	O
copy	O
of	O
the	O
latest	O
version	O
of	O
each	O
node	O
's	O
link-state	B-Protocol
advertisement	I-Protocol
to	O
every	O
node	O
in	O
the	O
network	O
.	O
</s>
<s>
Networks	O
running	O
link	B-Protocol
state	I-Protocol
algorithms	O
can	O
also	O
be	O
segmented	O
into	O
hierarchies	O
which	O
limit	O
the	O
scope	O
of	O
route	O
changes	O
.	O
</s>
<s>
These	O
features	O
mean	O
that	O
link	B-Protocol
state	I-Protocol
algorithms	O
scale	O
better	O
to	O
larger	O
networks	O
.	O
</s>
<s>
Finally	O
,	O
with	O
the	O
complete	O
set	O
of	O
link-state	B-Protocol
advertisements	I-Protocol
(	O
one	O
from	O
each	O
node	O
in	O
the	O
network	O
)	O
in	O
hand	O
,	O
each	O
node	O
produces	O
the	O
graph	O
for	O
the	O
map	O
of	O
the	O
network	O
.	O
</s>
<s>
The	O
algorithm	O
iterates	O
over	O
the	O
collection	O
of	O
link-state	B-Protocol
advertisements	I-Protocol
;	O
for	O
each	O
one	O
,	O
it	O
makes	O
links	O
on	O
the	O
map	O
of	O
the	O
network	O
,	O
from	O
the	O
node	O
which	O
sent	O
that	O
message	O
,	O
to	O
all	O
the	O
nodes	O
which	O
that	O
message	O
indicates	O
are	O
neighbors	O
of	O
the	O
sending	O
node	O
.	O
</s>
<s>
The	O
link-state	B-Protocol
message	O
giving	O
information	O
about	O
the	O
neighbors	O
is	O
recomputed	O
,	O
and	O
then	O
flooded	O
throughout	O
the	O
network	O
,	O
whenever	O
there	O
is	O
a	O
change	O
in	O
the	O
connectivity	O
between	O
the	O
node	O
and	O
its	O
neighbors	O
;	O
e.g.	O
,	O
when	O
a	O
link	O
fails	O
.	O
</s>
<s>
As	O
initially	O
mentioned	O
,	O
the	O
second	O
main	O
stage	O
in	O
the	O
link-state	B-Protocol
algorithm	O
is	O
to	O
produce	O
routing	B-Protocol
tables	I-Protocol
,	O
by	O
inspecting	O
the	O
maps	O
.	O
</s>
<s>
Each	O
node	O
independently	O
runs	O
an	O
algorithm	O
over	O
the	O
map	O
to	O
determine	O
the	O
shortest	O
path	O
from	O
itself	O
to	O
every	O
other	O
node	O
in	O
the	O
network	O
;	O
generally	O
some	O
variant	O
of	O
Dijkstra	B-Algorithm
's	I-Algorithm
algorithm	I-Algorithm
is	O
used	O
.	O
</s>
<s>
A	O
node	O
maintains	O
two	O
data	O
structures	O
:	O
a	O
tree	B-Application
containing	O
nodes	O
which	O
are	O
"	O
done	O
"	O
,	O
and	O
a	O
list	O
of	O
candidates	O
.	O
</s>
<s>
The	O
variant	O
of	O
a	O
greedy	B-Algorithm
algorithm	I-Algorithm
then	O
repetitively	O
does	O
the	O
following	O
:	O
</s>
<s>
All	O
neighbour	O
nodes	O
which	O
are	O
directly	O
connected	O
to	O
the	O
node	O
are	O
just	O
added	O
to	O
the	O
tree	B-Application
(	O
excepting	O
any	O
nodes	O
which	O
are	O
already	O
in	O
either	O
the	O
tree	B-Application
or	O
the	O
candidate	O
list	O
)	O
.	O
</s>
<s>
Each	O
node	O
in	O
the	O
candidate	O
list	O
is	O
compared	O
to	O
each	O
of	O
the	O
nodes	O
already	O
in	O
the	O
tree	B-Application
.	O
</s>
<s>
The	O
candidate	O
node	O
which	O
is	O
closest	O
to	O
any	O
of	O
the	O
nodes	O
already	O
in	O
the	O
tree	B-Application
is	O
itself	O
moved	O
into	O
the	O
tree	B-Application
and	O
attached	O
to	O
the	O
appropriate	O
neighbor	O
node	O
.	O
</s>
<s>
When	O
a	O
node	O
is	O
moved	O
from	O
the	O
candidate	O
list	O
into	O
the	O
tree	B-Application
,	O
it	O
is	O
removed	O
from	O
the	O
candidate	O
list	O
and	O
is	O
not	O
considered	O
in	O
subsequent	O
iterations	O
of	O
the	O
algorithm	O
.	O
</s>
<s>
(	O
When	O
there	O
are	O
none	O
,	O
all	O
the	O
nodes	O
in	O
the	O
network	O
will	O
have	O
been	O
added	O
to	O
the	O
tree	B-Application
.	O
)	O
</s>
<s>
This	O
procedure	O
ends	O
with	O
the	O
tree	B-Application
containing	O
all	O
the	O
nodes	O
in	O
the	O
network	O
,	O
with	O
the	O
node	O
on	O
which	O
the	O
algorithm	O
is	O
running	O
as	O
the	O
root	O
of	O
the	O
tree	B-Application
.	O
</s>
<s>
The	O
shortest	O
path	O
from	O
that	O
node	O
to	O
any	O
other	O
node	O
is	O
indicated	O
by	O
the	O
list	O
of	O
nodes	O
one	O
traverses	O
to	O
get	O
from	O
the	O
root	O
of	O
the	O
tree	B-Application
,	O
to	O
the	O
desired	O
node	O
in	O
the	O
tree	B-Application
.	O
</s>
<s>
With	O
the	O
shortest	O
paths	O
in	O
hand	O
,	O
the	O
next	O
step	O
is	O
to	O
fill	O
in	O
the	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
For	O
any	O
given	O
destination	O
node	O
,	O
the	O
best	O
path	O
for	O
that	O
destination	O
is	O
the	O
node	O
which	O
is	O
the	O
first	O
step	O
from	O
the	O
root	B-Application
node	I-Application
,	O
down	O
the	O
branch	O
in	O
the	O
shortest-path	O
tree	B-Application
which	O
leads	O
toward	O
the	O
desired	O
destination	O
node	O
.	O
</s>
<s>
To	O
create	O
the	O
routing	B-Protocol
table	I-Protocol
,	O
it	O
is	O
only	O
necessary	O
to	O
walk	O
the	O
tree	B-Application
,	O
remembering	O
the	O
identity	O
of	O
the	O
node	O
at	O
the	O
head	O
of	O
each	O
branch	O
,	O
and	O
filling	O
in	O
the	O
routing	B-Protocol
table	I-Protocol
entry	O
for	O
each	O
node	O
one	O
comes	O
across	O
with	O
that	O
identity	O
.	O
</s>
<s>
Whenever	O
a	O
change	O
in	O
the	O
connectivity	O
map	O
happens	O
,	O
it	O
is	O
necessary	O
to	O
recompute	O
the	O
shortest-path	O
tree	B-Application
,	O
and	O
then	O
recreate	O
the	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
Work	O
by	O
BBN	O
Technologies	O
discovered	O
how	O
to	O
recompute	O
only	O
that	O
part	O
of	O
the	O
tree	B-Application
which	O
could	O
have	O
been	O
affected	O
by	O
a	O
given	O
change	O
in	O
the	O
map	O
.	O
</s>
<s>
Also	O
,	O
the	O
routing	B-Protocol
table	I-Protocol
would	O
normally	O
be	O
filled	O
in	O
as	O
the	O
shortest-path	O
tree	B-Application
is	O
computed	O
,	O
instead	O
of	O
making	O
it	O
a	O
separate	O
operation	O
.	O
</s>
<s>
With	O
Fisheye	B-Protocol
State	I-Protocol
Routing	I-Protocol
(	O
FSR	O
)	O
the	O
LSA	O
are	O
sent	O
with	O
different	O
time-to-live	O
values	O
to	O
restrict	O
their	O
diffusion	O
and	O
limit	O
the	O
overhead	O
due	O
to	O
control	O
messages	O
.	O
</s>
<s>
The	O
same	O
concept	O
is	O
used	O
also	O
in	O
the	O
Hazy	B-Protocol
Sighted	I-Protocol
Link	I-Protocol
State	I-Protocol
Routing	I-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
This	O
can	O
occur	O
since	O
each	O
node	O
computes	O
its	O
shortest-path	O
tree	B-Application
and	O
its	O
routing	B-Protocol
table	I-Protocol
without	O
interacting	O
in	O
any	O
way	O
with	O
any	O
other	O
nodes	O
.	O
</s>
<s>
The	O
Optimized	B-Protocol
Link	I-Protocol
State	I-Protocol
Routing	I-Protocol
Protocol	I-Protocol
(	O
OLSR	B-Protocol
)	O
is	O
a	O
link-state	B-Protocol
routing	I-Protocol
protocol	I-Protocol
optimized	O
for	O
mobile	B-General_Concept
ad	I-General_Concept
hoc	I-General_Concept
networks	I-General_Concept
(	O
which	O
can	O
also	O
be	O
used	O
on	O
other	O
wireless	B-General_Concept
ad	I-General_Concept
hoc	I-General_Concept
networks	I-General_Concept
)	O
.	O
</s>
<s>
OLSR	B-Protocol
is	O
proactive	O
and	O
uses	O
hello	O
and	O
topology	O
control	O
(	O
TC	O
)	O
messages	O
to	O
discover	O
and	O
disseminate	O
link-state	B-Protocol
information	O
into	O
the	O
mobile	B-General_Concept
ad	I-General_Concept
hoc	I-General_Concept
network	I-General_Concept
.	O
</s>
<s>
MPRs	O
make	O
OLSR	B-Protocol
distinct	O
from	O
other	O
link-state	B-Protocol
routing	I-Protocol
protocols	I-Protocol
.	O
</s>
