<s>
Routing	B-Protocol
in	I-Protocol
delay-tolerant	I-Protocol
networking	I-Protocol
concerns	O
itself	O
with	O
the	O
ability	O
to	O
transport	B-Architecture
,	O
or	O
route	O
,	O
data	O
from	O
a	O
source	O
to	O
a	O
destination	O
,	O
which	O
is	O
a	O
fundamental	O
ability	O
all	O
communication	O
networks	O
must	O
have	O
.	O
</s>
<s>
In	O
these	O
challenging	O
environments	O
,	O
popular	O
ad	O
hoc	O
routing	B-Protocol
protocols	I-Protocol
such	O
as	O
AODV	B-Device
and	O
DSR	B-Device
fail	O
to	O
establish	O
routes	O
.	O
</s>
<s>
However	O
,	O
when	O
instantaneous	O
end-to-end	O
paths	O
are	O
difficult	O
or	O
impossible	O
to	O
establish	O
,	O
routing	B-Protocol
protocols	I-Protocol
must	O
take	O
to	O
a	O
"	O
store	O
and	O
forward	O
"	O
approach	O
,	O
where	O
data	O
is	O
incrementally	O
moved	O
and	O
stored	O
throughout	O
the	O
network	O
in	O
hopes	O
that	O
it	O
will	O
eventually	O
reach	O
its	O
destination	O
.	O
</s>
<s>
There	O
are	O
many	O
characteristics	O
DTN	O
protocols	O
,	O
including	O
routing	B-Protocol
,	O
must	O
take	O
into	O
consideration	O
.	O
</s>
<s>
However	O
,	O
due	O
to	O
the	O
laws	B-Application
of	I-Application
physics	I-Application
,	O
it	O
is	O
possible	O
to	O
predict	O
the	O
future	O
in	O
terms	O
of	O
the	O
times	O
contacts	O
will	O
be	O
available	O
,	O
and	O
how	O
long	O
they	O
will	O
last	O
.	O
</s>
<s>
These	O
nodes	O
,	O
sometimes	O
referred	O
to	O
as	O
Data	B-Architecture
Mules	I-Architecture
,	O
are	O
exploited	O
for	O
their	O
mobility	O
.	O
</s>
<s>
Since	O
they	O
are	O
the	O
primary	O
source	O
of	O
transitive	O
communication	O
between	O
two	O
non-neighboring	O
nodes	O
in	O
the	O
network	O
,	O
an	O
important	O
routing	B-Protocol
question	O
is	O
how	O
to	O
properly	O
distribute	O
data	O
among	O
these	O
nodes	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
a	O
routing	B-Protocol
protocol	I-Protocol
will	O
most	O
likely	O
have	O
more	O
options	O
available	O
during	O
contact	O
opportunities	O
,	O
and	O
may	O
not	O
have	O
to	O
utilize	O
each	O
one	O
.	O
</s>
<s>
An	O
example	O
of	O
this	O
type	O
of	O
network	O
is	O
a	O
disaster	O
recovery	O
network	O
where	O
all	O
nodes	O
(	O
generally	O
people	O
and	O
vehicles	B-Device
)	O
are	O
mobile	O
.	O
</s>
<s>
Routing	B-Protocol
protocols	I-Protocol
can	O
utilize	O
this	O
information	O
to	O
best	O
determine	O
how	O
messages	O
should	O
be	O
transmitted	O
and	O
stored	O
to	O
not	O
over-burden	O
limited	O
resources	O
.	O
</s>
<s>
While	O
there	O
are	O
many	O
characteristics	O
of	O
routing	B-Protocol
protocols	I-Protocol
,	O
one	O
of	O
the	O
most	O
immediate	O
ways	O
to	O
create	O
a	O
taxonomy	O
is	O
based	O
on	O
whether	O
or	O
not	O
the	O
protocol	O
creates	O
replicas	B-Architecture
of	O
messages	O
.	O
</s>
<s>
Routing	B-Protocol
protocols	I-Protocol
that	O
never	O
replicate	O
a	O
message	O
are	O
considered	O
forwarding-based	O
,	O
whereas	O
protocols	O
that	O
do	O
replicate	O
messages	O
are	O
considered	O
replication-based	O
.	O
</s>
<s>
Forwarding-based	O
approaches	O
are	O
generally	O
much	O
less	O
wasteful	O
of	O
network	O
resources	O
,	O
as	O
only	O
a	O
single	O
copy	O
of	O
a	O
message	O
exists	O
in	O
storage	O
in	O
the	O
network	O
at	O
any	O
given	O
time	O
.	O
</s>
<s>
This	O
eliminates	O
the	O
need	O
for	O
the	O
destination	O
to	O
provide	O
feedback	O
to	O
the	O
network	O
(	O
except	O
for	O
,	O
perhaps	O
,	O
an	O
acknowledgments	B-Protocol
sent	O
to	O
the	O
sender	O
)	O
,	O
to	O
indicate	O
outstanding	O
copies	O
can	O
be	O
deleted	O
.	O
</s>
<s>
Unfortunately	O
,	O
forwarding-based	O
approaches	O
do	O
not	O
allow	O
for	O
sufficient	O
message	O
delivery	O
rates	O
in	O
many	O
DTNs	O
.	O
</s>
<s>
Replication-based	O
protocols	O
,	O
on	O
the	O
other	O
hand	O
,	O
allow	O
for	O
greater	O
message	O
delivery	O
rates	O
,	O
since	O
multiple	O
copies	O
exist	O
in	O
the	O
network	O
,	O
and	O
only	O
one	O
(	O
or	O
in	O
some	O
cases	O
,	O
as	O
with	O
erasure	O
coding	O
,	O
a	O
few	O
)	O
must	O
reach	O
the	O
destination	O
.	O
</s>
<s>
Furthermore	O
,	O
many	O
flooding-based	O
protocols	O
are	O
inherently	O
not	O
scalable	O
.	O
</s>
<s>
name	O
=	O
"	O
spyropoulos2005	O
"	O
/	O
>	O
attempt	O
to	O
compromise	O
by	O
limiting	O
the	O
number	O
of	O
possible	O
replicas	B-Architecture
of	O
a	O
given	O
message	O
.	O
</s>
<s>
It	O
is	O
important	O
to	O
note	O
that	O
the	O
vast	O
majority	O
of	O
DTN	O
routing	B-Protocol
protocols	I-Protocol
are	O
heuristic-based	O
,	O
and	O
non-optimal	O
.	O
</s>
<s>
name	O
=	O
"	O
balasubramanian2007	O
"	O
/	O
>	O
More	O
specifically	O
"	O
online	B-Algorithm
algorithms	I-Algorithm
without	O
complete	O
future	O
knowledge	O
and	O
with	O
unlimited	O
computational	O
power	O
,	O
or	O
computationally	O
limited	O
algorithms	O
with	O
complete	O
future	O
knowledge	O
,	O
can	O
be	O
arbitrarily	O
far	O
from	O
optimal	O
"	O
.	O
</s>
<s>
Replication-based	O
protocols	O
have	O
recently	O
obtained	O
much	O
attention	O
in	O
the	O
scientific	O
community	O
,	O
as	O
they	O
can	O
allow	O
for	O
substantially	O
better	O
message	O
delivery	O
ratios	O
than	O
in	O
forwarding-based	O
protocols	O
.	O
</s>
<s>
These	O
types	O
of	O
routing	B-Protocol
protocols	I-Protocol
allow	O
for	O
a	O
message	O
to	O
be	O
replicated	O
;	O
each	O
of	O
the	O
replicas	B-Architecture
,	O
as	O
well	O
as	O
the	O
original	O
message	O
itself	O
,	O
are	O
generally	O
referred	O
to	O
as	O
message	O
copies	O
or	O
message	O
replicas	B-Architecture
.	O
</s>
<s>
Possible	O
issues	O
with	O
replication-based	O
routing	B-Protocol
include	O
:	O
</s>
<s>
network	B-Protocol
congestion	I-Protocol
in	O
clustered	O
areas	O
,	O
</s>
<s>
Since	O
network	O
resources	O
may	O
quickly	O
become	O
constrained	O
,	O
deciding	O
which	O
messages	O
to	O
transmit	O
first	O
and	O
which	O
messages	O
to	O
drop	O
first	O
play	O
critical	O
roles	O
in	O
many	O
routing	B-Protocol
protocols	I-Protocol
.	O
</s>
<s>
Epidemic	O
routing	B-Protocol
is	O
flooding-based	O
in	O
nature	O
,	O
as	O
nodes	O
continuously	O
replicate	O
and	O
transmit	O
messages	O
to	O
newly	O
discovered	O
contacts	O
that	O
do	O
not	O
already	O
possess	O
a	O
copy	O
of	O
the	O
message	O
.	O
</s>
<s>
In	O
the	O
most	O
simple	O
case	O
,	O
epidemic	O
routing	B-Protocol
is	O
flooding	B-Protocol
;	O
however	O
,	O
more	O
sophisticated	O
techniques	O
can	O
be	O
used	O
to	O
limit	O
the	O
number	O
of	O
message	O
transfers	O
.	O
</s>
<s>
Epidemic	O
routing	B-Protocol
has	O
its	O
roots	O
in	O
ensuring	O
distributed	O
databases	O
remain	O
synchronized	O
,	O
and	O
many	O
of	O
these	O
techniques	O
,	O
such	O
as	O
rumor	O
mongering	O
,	O
can	O
be	O
directly	O
applied	O
to	O
routing	B-Protocol
.	O
</s>
<s>
Epidemic	O
routing	B-Protocol
is	O
particularly	O
resource	O
hungry	O
because	O
it	O
deliberately	O
makes	O
no	O
attempt	O
to	O
eliminate	O
replications	B-Architecture
that	O
would	O
be	O
unlikely	O
to	O
improve	O
the	O
delivery	O
probability	O
of	O
messages	O
.	O
</s>
<s>
Data	B-Architecture
Mules	I-Architecture
(	O
mostly	O
associated	O
with	O
a	O
human	O
)	O
move	O
in	O
a	O
society	O
and	O
accordingly	O
tend	O
to	O
have	O
greater	O
probabilities	O
of	O
meeting	O
certain	O
Mules	O
than	O
others	O
.	O
</s>
<s>
The	O
Probabilistic	O
Routing	B-Protocol
Protocol	I-Protocol
using	O
History	O
of	O
Encounters	O
and	O
Transitivity	O
(	O
PRoPHET	O
)	O
protocol	O
uses	O
an	O
algorithm	O
that	O
attempts	O
to	O
exploit	O
the	O
non-randomness	O
of	O
real-world	O
encounters	O
by	O
maintaining	O
a	O
set	O
of	O
probabilities	O
for	O
successful	O
delivery	O
to	O
known	O
destinations	O
in	O
the	O
DTN	O
(	O
delivery	O
predictabilities	O
)	O
and	O
replicating	O
messages	O
during	O
opportunistic	O
encounters	O
only	O
if	O
the	O
Mule	O
that	O
does	O
not	O
have	O
the	O
message	O
appears	O
to	O
have	O
a	O
better	O
chance	O
of	O
delivering	O
it	O
.	O
</s>
<s>
MaxProp	O
is	O
flooding-based	O
in	O
nature	O
,	O
in	O
that	O
if	O
a	O
contact	O
is	O
discovered	O
,	O
all	O
messages	O
not	O
held	O
by	O
the	O
contact	O
will	O
attempt	O
to	O
be	O
replicated	O
and	O
transferred	O
.	O
</s>
<s>
In	O
essence	O
,	O
MaxProp	O
maintains	O
an	O
ordered-queue	B-Application
based	O
on	O
the	O
destination	O
of	O
each	O
message	O
,	O
ordered	O
by	O
the	O
estimated	O
likelihood	O
of	O
a	O
future	O
transitive	O
path	O
to	O
that	O
destination	O
.	O
</s>
<s>
Note	O
that	O
this	O
phase	O
is	O
completely	O
local	O
and	O
does	O
not	O
require	O
transmitting	O
routing	B-Protocol
information	I-Protocol
between	O
nodes	O
.	O
</s>
<s>
In	O
conjunction	O
with	O
the	O
core	O
routing	B-Protocol
described	O
above	O
,	O
MaxProp	O
allows	O
for	O
many	O
complementary	O
mechanisms	O
,	O
each	O
helping	O
the	O
message	O
delivery	O
ratio	O
in	O
general	O
.	O
</s>
<s>
First	O
,	O
acknowledgements	B-Protocol
are	O
injected	O
into	O
the	O
network	O
by	O
nodes	O
that	O
successfully	O
receive	O
a	O
message	O
(	O
and	O
are	O
the	O
final	O
destination	O
of	O
that	O
message	O
)	O
.	O
</s>
<s>
These	O
acknowledgements	B-Protocol
are	O
128-bit	B-Algorithm
hashes	I-Algorithm
of	O
the	O
message	O
that	O
are	O
flooded	O
into	O
the	O
network	O
,	O
and	O
instruct	O
nodes	O
to	O
delete	O
extra	O
copies	O
of	O
the	O
message	O
from	O
their	O
buffers	O
.	O
</s>
<s>
This	O
helps	O
promote	O
initial	O
rapid	O
message	O
replication	B-Architecture
to	O
give	O
new	O
messages	O
a	O
"	O
head	O
start	O
"	O
.	O
</s>
<s>
RAPID	O
,	O
which	O
is	O
an	O
acronym	O
for	O
Resource	B-Algorithm
Allocation	I-Algorithm
Protocol	O
for	O
Intentional	O
DTN	O
routing	B-Protocol
,	O
was	O
developed	O
at	O
the	O
University	O
of	O
Massachusetts	O
,	O
Amherst	O
.	O
</s>
<s>
It	O
was	O
first	O
introduced	O
in	O
the	O
SIGCOMM	O
2007	O
publication	O
,	O
DTN	O
Routing	B-Protocol
as	O
a	O
Resource	B-Algorithm
Allocation	I-Algorithm
Problem	I-Algorithm
.	O
</s>
<s>
The	O
authors	O
of	O
RAPID	O
argue	O
as	O
a	O
base	O
premise	O
that	O
prior	O
DTN	O
routing	B-Protocol
algorithms	I-Protocol
incidentally	O
effect	O
performance	O
metrics	O
,	O
such	O
as	O
average	O
delay	O
and	O
message	O
delivery	O
ratio	O
.	O
</s>
<s>
The	O
goal	O
of	O
RAPID	O
is	O
to	O
intentionally	O
effect	O
a	O
single	O
routing	B-Protocol
metric	O
.	O
</s>
<s>
RAPID	O
,	O
like	O
MaxProp	O
,	O
is	O
flooding-based	O
,	O
and	O
will	O
therefore	O
attempt	O
to	O
replicate	O
all	O
packets	O
if	O
network	O
resources	O
allow	O
.	O
</s>
<s>
Replication	B-Architecture
:	O
Packets	O
are	O
replicated	O
based	O
on	O
marginal	O
utility	O
(	O
the	O
change	O
is	O
utility	O
over	O
the	O
size	O
of	O
the	O
packet	O
)	O
.	O
</s>
<s>
Spray	O
and	O
Wait	O
is	O
a	O
routing	B-Protocol
protocol	I-Protocol
that	O
attempts	O
to	O
gain	O
the	O
delivery	O
ratio	O
benefits	O
of	O
replication-based	O
routing	B-Protocol
as	O
well	O
as	O
the	O
low	O
resource	O
utilization	O
benefits	O
of	O
forwarding-based	O
routing	B-Protocol
.	O
</s>
<s>
It	O
was	O
first	O
presented	O
at	O
the	O
2005	O
ACM	O
SIGCOMM	O
conference	O
,	O
under	O
the	O
publication	O
"	O
Spray	O
and	O
Wait	O
:	O
An	O
Efficient	O
Routing	B-Protocol
Scheme	O
for	O
Intermittently	O
Connected	O
Mobile	O
Networks	O
"	O
.	O
</s>
<s>
There	O
are	O
two	O
main	O
versions	O
of	O
Spray	O
and	O
Wait	O
:	O
vanilla	B-General_Concept
and	O
binary	O
.	O
</s>
<s>
The	O
simplest	O
way	O
to	O
achieve	O
this	O
,	O
known	O
as	O
the	O
vanilla	B-General_Concept
version	O
,	O
is	O
for	O
the	O
source	O
to	O
transmit	O
a	O
single	O
copy	O
of	O
the	O
message	O
to	O
the	O
first	O
distinct	O
nodes	O
it	O
encounters	O
after	O
the	O
message	O
is	O
created	O
.	O
</s>
<s>
The	O
benefit	O
of	O
Binary	O
Spray	O
and	O
Wait	O
is	O
that	O
messages	O
are	O
disseminated	O
faster	O
than	O
the	O
vanilla	B-General_Concept
version	O
.	O
</s>
<s>
They	O
study	O
the	O
social	O
structures	O
of	O
the	O
between	O
devices	O
and	O
leverage	O
them	O
in	O
the	O
design	O
of	O
forwarding	B-Protocol
algorithms	O
for	O
Pocket	O
Switched	O
Networks(PSNs )	O
.	O
</s>
<s>
According	O
to	O
this	O
finding	O
,	O
they	O
propose	O
Bubble	O
Rap	O
,	O
a	O
social-based	O
forwarding	B-Protocol
algorithm	O
,	O
to	O
improve	O
the	O
forwarding	B-Protocol
efficiency	O
significantly	O
compared	O
to	O
history-based	O
PROPHET	O
and	O
social-based	O
SimBet	O
algorithms	O
.	O
</s>
<s>
CafRep	O
is	O
a	O
fully	O
localised	O
adaptive	O
forwarding	B-Protocol
&	O
replication	B-Architecture
protocol	O
with	O
congestion	O
control	O
and	O
avoidance	O
to	O
enable	O
congestion-aware	O
mobile	O
social	O
framework	O
in	O
heterogeneous	O
DTNs	O
.	O
</s>
<s>
CafRep	O
uses	O
a	O
combined	O
social	O
,	O
buffer	O
and	O
delay	O
metrics	O
for	O
congestion-aware	O
message	O
forwarding	B-Protocol
and	O
replication	B-Architecture
that	O
maximises	O
message	O
delivery	O
ratio	O
and	O
availability	O
of	O
nodes	O
while	O
minimising	O
latency	O
and	O
packet	O
loss	O
rates	O
at	O
times	O
of	O
increasing	O
congestion	O
levels	O
.	O
</s>
<s>
At	O
the	O
core	O
of	O
CafRep	O
is	O
a	O
combined	O
relative	O
utility	O
driven	O
heuristics	B-Algorithm
that	O
allow	O
highly	O
adaptive	O
forwarding	B-Protocol
and	O
replication	B-Architecture
policies	O
by	O
managing	O
to	O
detect	O
and	O
offload	O
congested	O
parts	O
of	O
the	O
network	O
and	O
adapting	O
the	O
sending/forwarding	O
rates	O
based	O
on	O
resource	O
and	O
contact	O
predictions	O
.	O
</s>
<s>
RACOD	O
:	O
Routing	B-Protocol
Using	O
Ant	O
Colony	O
Optimization	O
in	O
DTN	O
introduces	O
learning	O
of	O
paths	O
using	O
ACO	O
and	O
also	O
intelligently	O
decides	O
which	O
message	O
to	O
drop	O
and	O
which	O
message	O
to	O
transfer	O
.	O
</s>
<s>
DTLSR	O
is	O
implemented	O
in	O
the	O
DTN2	O
BP	O
implementation	O
and	O
aims	O
to	O
provide	O
a	O
straightforward	O
extension	O
of	O
link-state	B-Protocol
routing	I-Protocol
.	O
</s>
<s>
With	O
DTLSR	O
,	O
link	B-Protocol
state	I-Protocol
announcements	O
are	O
sent	O
as	O
in	O
OLSR	B-Protocol
,	O
but	O
links	O
that	O
are	O
deemed	O
'	O
down	O
 '	O
are	O
not	O
immediately	O
removed	O
from	O
the	O
graph	O
.	O
</s>
<s>
The	O
SABR	O
protocol	O
is	O
an	O
extension	O
of	O
Contact	O
Graph	O
Routing	B-Protocol
that	O
seeks	O
to	O
provide	O
a	O
routing	B-Protocol
solution	O
for	O
a	O
wide	O
range	O
of	O
scenarios	O
that	O
include	O
both	O
scheduled	O
and	O
discovered	O
connectivity	O
.	O
</s>
<s>
SABR	O
then	O
makes	O
forwarding	B-Protocol
decisions	O
based	O
on	O
an	O
earliest-arrival-time	O
metric	O
where	O
bundles	O
are	O
routed	B-Protocol
over	O
the	O
time-varying	O
connectivity	O
graph	O
.	O
</s>
<s>
SABR	O
uses	O
historical	O
contact	O
information	O
and	O
neighbor	O
discovery	O
to	O
address	O
routing	B-Protocol
over	O
non-scheduled	O
links	O
.	O
</s>
<s>
The	O
majority	O
of	O
existing	O
routing	B-Protocol
and	O
data	O
delivery	O
protocols	O
for	O
DTNs	O
assume	O
that	O
mobile	O
nodes	O
willingly	O
participate	O
in	O
data	O
delivery	O
,	O
share	O
their	O
resources	O
with	O
each	O
other	O
,	O
and	O
follow	O
the	O
rules	O
of	O
underlying	O
networking	O
protocols	O
.	O
</s>
<s>
An	O
adversary	O
,	O
for	O
example	O
,	O
may	O
drop	O
received	O
messages	O
but	O
produce	O
forged	O
routing	B-Protocol
metrics	O
or	O
false	O
information	O
with	O
the	O
aim	O
of	O
either	O
attracting	O
more	O
messages	O
or	O
decreasing	O
its	O
detection	O
probability	O
.	O
</s>
