<s>
Extremely	O
Opportunistic	O
Routing	O
(	O
ExOR	B-Protocol
)	O
is	O
a	O
combination	O
of	O
routing	B-Protocol
protocol	I-Protocol
and	O
media	B-Protocol
access	I-Protocol
control	I-Protocol
for	O
a	O
wireless	B-General_Concept
ad	I-General_Concept
hoc	I-General_Concept
network	I-General_Concept
,	O
invented	O
by	O
Sanjit	O
Biswas	O
and	O
Robert	O
Morris	O
of	O
the	O
MIT	O
Artificial	O
Intelligence	O
Laboratory	O
,	O
and	O
described	O
in	O
a	O
2005	O
paper	O
.	O
</s>
<s>
Previously	O
open	O
source	O
,	O
ExOR	B-Protocol
was	O
available	O
in	O
2005	O
but	O
is	O
no	O
longer	O
obtainable	O
.	O
</s>
<s>
ExOR	B-Protocol
is	O
valuable	O
because	O
it	O
can	O
operate	O
available	O
digital	O
radios	O
to	O
use	O
some	O
previously	O
impractical	O
algorithmic	O
optimizations	O
.	O
</s>
<s>
The	O
algorithm	O
is	O
designed	O
to	O
convey	O
packets	B-Protocol
of	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
,	O
so	O
that	O
it	O
enables	O
the	O
maximum	O
number	O
of	O
other	O
services	O
.	O
</s>
<s>
One	O
of	O
the	O
crucial	O
features	O
of	O
the	O
laptop	O
was	O
to	O
be	O
a	O
wireless	B-General_Concept
ad	I-General_Concept
hoc	I-General_Concept
network	I-General_Concept
that	O
would	O
permit	O
the	O
laptops	O
to	O
cooperate	O
to	O
provide	O
more	O
resources	O
than	O
an	O
individual	O
computer	O
could	O
afford	O
.	O
</s>
<s>
A	O
wireless	B-General_Concept
ad	I-General_Concept
hoc	I-General_Concept
network	I-General_Concept
would	O
cost	O
less	O
and	O
use	O
less	O
power	O
if	O
it	O
used	O
standard	O
radios	O
(	O
i.e.	O
</s>
<s>
The	O
starting	O
radio	O
,	O
the	O
source	O
,	O
broadcasts	O
a	O
batch	O
of	O
packets	B-Protocol
.	O
</s>
<s>
As	O
timers	O
in	O
intermediate	O
radios	O
expire	O
,	O
radios	O
further	O
from	O
the	O
destination	O
retransmit	O
the	O
packets	B-Protocol
that	O
no	O
closer	O
radio	O
has	O
yet	O
retransmitted	O
.	O
</s>
<s>
The	O
timers	O
in	O
intermediate	O
radios	O
are	O
set	O
to	O
an	O
estimate	O
of	O
the	O
transmission	O
time	O
that	O
closer	O
radios	O
will	O
need	O
in	O
order	O
to	O
transmit	O
packets	B-Protocol
.	O
</s>
<s>
The	O
estimate	O
is	O
calculated	O
based	O
on	O
the	O
number	O
of	O
packets	B-Protocol
in	O
the	O
batch	O
,	O
and	O
the	O
probabilities	O
of	O
a	O
correct	O
transmission	O
from	O
each	O
intermediate	O
radio	O
.	O
</s>
<s>
ExOR	B-Protocol
uses	O
a	O
conventional	O
routing	B-Protocol
protocol	I-Protocol
"	O
RRTc	O
"	O
to	O
collect	O
information	O
about	O
the	O
probability	O
of	O
a	O
successful	O
transmission	O
between	O
each	O
pair	O
of	O
digital	O
radios	O
in	O
the	O
network	O
.	O
</s>
<s>
The	O
authors	O
were	O
concerned	O
that	O
retransmitting	O
packets	B-Protocol
could	O
use	O
up	O
too	O
much	O
of	O
the	O
available	O
radio	O
time	O
.	O
</s>
<s>
ExOR	B-Protocol
therefore	O
tries	O
to	O
reduce	O
retransmissions	O
of	O
packets	B-Protocol
to	O
the	O
minimum	O
possible	O
.	O
</s>
<s>
This	O
accounts	O
for	O
ExOR	B-Protocol
's	O
high	O
efficiency	O
.	O
</s>
<s>
The	O
radios	O
 '	O
numbers	O
are	O
placed	O
in	O
a	O
list	O
sorted	B-Algorithm
by	O
distance	O
to	O
the	O
destination	O
,	O
from	O
closest	O
to	O
furthest	O
.	O
</s>
<s>
Also	O
,	O
the	O
source	O
radio	O
starts	O
a	O
list	O
of	O
the	O
packets	B-Protocol
in	O
the	O
batch	O
in	O
order	O
to	O
measure	O
packets	B-Protocol
 '	O
progress	O
.	O
</s>
<s>
This	O
"	O
batch	O
map	O
"	O
is	O
an	O
array	O
of	O
radio	O
numbers	O
,	O
one	O
per	O
packet	B-Protocol
.	O
</s>
<s>
Each	O
radio	O
number	O
is	O
the	O
radio	O
that	O
transmitted	O
that	O
packet	B-Protocol
,	O
and	O
was	O
closest	O
to	O
the	O
destination	O
radio	O
.	O
</s>
<s>
Each	O
data	B-Protocol
packet	I-Protocol
has	O
the	O
list	O
of	O
radios	O
,	O
and	O
packets	B-Protocol
placed	O
in	O
the	O
front	O
.	O
</s>
<s>
The	O
list	O
saves	O
space	O
in	O
each	O
packet	B-Protocol
by	O
using	O
radio	O
numbers	O
rather	O
than	O
IP	O
addresses	O
.	O
</s>
<s>
Then	O
,	O
the	O
sending	O
radio	O
broadcasts	O
the	O
first	O
batch	O
of	O
data	B-Protocol
packets	I-Protocol
.	O
</s>
<s>
Radios	O
that	O
receive	O
a	O
packet	B-Protocol
but	O
are	O
not	O
in	O
the	O
list	O
in	O
the	O
packet	B-Protocol
ignore	O
the	O
data	B-Protocol
packets	I-Protocol
.	O
</s>
<s>
These	O
radios	O
throw	O
away	O
the	O
packets	B-Protocol
as	O
soon	O
as	O
the	O
packets	B-Protocol
are	O
received	O
.	O
</s>
<s>
Radios	O
that	O
are	O
in	O
the	O
packet	B-Protocol
's	O
list	O
of	O
radios	O
save	O
the	O
data	B-Protocol
packets	I-Protocol
that	O
they	O
receive	O
.	O
</s>
<s>
When	O
a	O
radio	O
times	O
out	O
,	O
it	O
transmits	O
the	O
packets	B-Protocol
that	O
no	O
radio	O
closer	O
to	O
the	O
destination	O
has	O
retransmitted	O
.	O
</s>
<s>
These	O
packets	B-Protocol
include	O
the	O
radio	O
's	O
best	O
available	O
information	O
about	O
the	O
progress	O
of	O
the	O
packets	B-Protocol
in	O
the	O
batch	O
(	O
i.e.	O
</s>
<s>
In	O
particular	O
,	O
each	O
packet	B-Protocol
's	O
batch	O
map	O
contains	O
the	O
retransmitter	O
's	O
radio	O
number	O
for	O
each	O
packet	B-Protocol
that	O
it	O
retransmits	O
.	O
</s>
<s>
When	O
a	O
radio	O
receives	O
a	O
packet	B-Protocol
sent	O
from	O
a	O
radio	O
that	O
is	O
closer	O
to	O
the	O
destination	O
,	O
it	O
erases	O
its	O
own	O
copy	O
of	O
that	O
packet	B-Protocol
.	O
</s>
<s>
There	O
's	O
no	O
need	O
for	O
it	O
to	O
retransmit	O
that	O
packet	B-Protocol
.	O
</s>
<s>
However	O
,	O
it	O
also	O
updates	O
its	O
batch	O
map	O
about	O
the	O
progress	O
of	O
the	O
packets	B-Protocol
in	O
the	O
batch	O
.	O
</s>
<s>
In	O
this	O
way	O
,	O
the	O
information	O
about	O
the	O
progress	O
of	O
the	O
packets	B-Protocol
flows	O
backward	O
toward	O
the	O
source	O
as	O
radios	O
farther	O
from	O
the	O
destination	O
update	O
their	O
batch	O
maps	O
by	O
eavesdropping	O
on	O
retransmissions	O
.	O
</s>
<s>
Since	O
the	O
retransmissions	O
closer	O
to	O
the	O
source	O
radio	O
occur	O
later	O
,	O
the	O
packet	B-Protocol
progress	O
information	O
flows	O
back	O
to	O
the	O
source	O
radio	O
,	O
even	O
though	O
no	O
acknowledge	O
packets	B-Protocol
are	O
ever	O
transmitted	O
.	O
</s>
<s>
At	O
the	O
end	O
,	O
there	O
are	O
usually	O
a	O
few	O
packets	B-Protocol
that	O
did	O
n't	O
go	O
anywhere	O
.	O
</s>
<s>
ExOR	B-Protocol
is	O
more	O
efficient	O
with	O
large	O
blocks	O
of	O
data	O
.	O
</s>
<s>
So	O
,	O
blocks	O
of	O
data	O
more	O
than	O
100,000	O
bytes	B-Application
are	O
broken	O
into	O
groups	O
of	O
data	B-Protocol
packets	I-Protocol
called	O
batches	O
.	O
</s>
<s>
Since	O
the	O
major	O
internet	B-Protocol
protocol	I-Protocol
TCP	O
sends	O
a	O
stream	O
of	O
data	O
,	O
ExOR	B-Protocol
uses	O
local	O
proxy	O
data	O
servers	O
to	O
accumulate	O
blocks	O
of	O
data	O
.	O
</s>
<s>
Each	O
packet	B-Protocol
is	O
retransmitted	O
a	O
minimal	O
number	O
of	O
times	O
,	O
and	O
covers	O
the	O
longest	O
possible	O
distance	O
on	O
each	O
transmission	O
.	O
</s>
<s>
Some	O
time	O
is	O
wasted	O
by	O
having	O
the	O
receiver	O
broadcast	O
packet	B-Protocol
information	O
,	O
but	O
this	O
is	O
far	O
less	O
than	O
the	O
normal	O
routing	O
schemes	O
,	O
which	O
can	O
retransmit	O
when	O
an	O
acknowledge	O
message	O
is	O
lost	O
.	O
</s>
<s>
There	O
are	O
no	O
acknowledge	O
packets	B-Protocol
,	O
and	O
no	O
collisions	O
with	O
them	O
.	O
</s>
<s>
The	O
authors	O
say	O
that	O
the	O
protocol	O
is	O
roughly	O
twice	O
as	O
efficient	O
as	O
normal	O
routing	B-Protocol
protocols	I-Protocol
with	O
fixed	O
"	O
optimal	O
"	O
routing	O
.	O
</s>
<s>
ExOR	B-Protocol
's	O
efficiency	O
estimates	O
are	O
based	O
on	O
a	O
real	O
implementation	O
with	O
a	O
Linux	B-Application
routing	O
toolkit	O
called	O
click	O
.	O
</s>
