<s>
An	O
overlay	B-Architecture
network	I-Architecture
is	O
a	O
computer	B-Architecture
network	I-Architecture
that	O
is	O
layered	B-Protocol
on	O
top	O
of	O
another	O
network	O
.	O
</s>
<s>
Nodes	B-Protocol
in	O
the	O
overlay	B-Architecture
network	I-Architecture
can	O
be	O
thought	O
of	O
as	O
being	O
connected	O
by	O
virtual	O
or	O
logical	O
links	O
,	O
each	O
of	O
which	O
corresponds	O
to	O
a	O
path	O
,	O
perhaps	O
through	O
many	O
physical	O
links	O
,	O
in	O
the	O
underlying	O
network	O
.	O
</s>
<s>
For	O
example	O
,	O
distributed	B-Architecture
systems	I-Architecture
such	O
as	O
peer-to-peer	O
networks	O
and	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
applications	O
are	O
overlay	B-Architecture
networks	I-Architecture
because	O
their	O
nodes	B-Protocol
run	O
on	O
top	O
of	O
the	O
Internet	O
.	O
</s>
<s>
The	O
Internet	O
was	O
originally	O
built	O
as	O
an	O
overlay	O
upon	O
the	O
telephone	O
network	O
,	O
while	O
today	O
(	O
through	O
the	O
advent	O
of	O
VoIP	B-Application
)	O
,	O
the	O
telephone	O
network	O
is	O
increasingly	O
turning	O
into	O
an	O
overlay	B-Architecture
network	I-Architecture
built	O
on	O
top	O
of	O
the	O
Internet	O
.	O
</s>
<s>
Enterprise	O
private	O
networks	O
were	O
first	O
overlaid	O
on	O
telecommunication	O
networks	O
such	O
as	O
Frame	B-Protocol
Relay	I-Protocol
and	O
Asynchronous	O
Transfer	O
Mode	O
packet	B-Protocol
switching	I-Protocol
infrastructures	O
but	O
migration	O
from	O
these	O
(	O
now	O
legacy	O
)	O
infrastructures	O
to	O
IP	O
based	O
MPLS	B-Protocol
networks	O
and	O
virtual	B-Application
private	I-Application
networks	I-Application
started	O
(	O
2001	O
~	O
2002	O
)	O
.	O
</s>
<s>
From	O
a	O
physical	O
standpoint	O
,	O
overlay	B-Architecture
networks	I-Architecture
are	O
quite	O
complex	O
(	O
see	O
Figure	O
1	O
)	O
as	O
they	O
combine	O
various	O
logical	O
layers	O
that	O
are	O
operated	O
and	O
built	O
by	O
various	O
entities	O
(	O
businesses	O
,	O
universities	O
,	O
government	O
etc	O
.	O
)	O
</s>
<s>
but	O
they	O
allow	O
separation	O
of	O
concerns	O
that	O
over	O
time	O
permitted	O
the	O
buildup	O
of	O
a	O
broad	O
set	O
of	O
services	O
that	O
could	O
not	O
have	O
been	O
proposed	O
by	O
a	O
single	O
telecommunication	O
operator	O
(	O
ranging	O
from	O
broadband	O
Internet	O
access	O
,	O
voice	B-Application
over	I-Application
IP	I-Application
or	O
IPTV	O
,	O
competitive	O
telecom	O
operators	O
etc	O
.	O
</s>
<s>
Telecommunication	O
transport	O
networks	O
and	O
IP	B-Protocol
networks	I-Protocol
(	O
which	O
combined	O
make	O
up	O
the	O
broader	O
Internet	O
)	O
are	O
all	O
overlaid	O
with	O
at	O
least	O
an	O
optical	O
fiber	O
layer	O
,	O
a	O
transport	O
layer	O
and	O
an	O
IP	O
or	O
circuit	O
switching	O
layers	O
(	O
in	O
the	O
case	O
of	O
the	O
PSTN	O
)	O
.	O
</s>
<s>
Nowadays	O
the	O
Internet	O
is	O
the	O
basis	O
for	O
more	O
overlaid	O
networks	O
that	O
can	O
be	O
constructed	O
in	O
order	O
to	O
permit	O
routing	B-Protocol
of	O
messages	O
to	O
destinations	O
not	O
specified	O
by	O
an	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
distributed	B-Operating_System
hash	I-Operating_System
tables	I-Operating_System
can	O
be	O
used	O
to	O
route	O
messages	O
to	O
a	O
node	O
having	O
a	O
specific	O
logical	B-General_Concept
address	I-General_Concept
,	O
whose	O
IP	B-Protocol
address	I-Protocol
is	O
not	O
known	O
in	O
advance	O
.	O
</s>
<s>
Overlay	B-Architecture
networks	I-Architecture
have	O
also	O
been	O
proposed	O
as	O
a	O
way	O
to	O
improve	O
Internet	O
routing	B-Protocol
,	O
such	O
as	O
through	O
quality	O
of	O
service	O
guarantees	O
to	O
achieve	O
higher-quality	O
streaming	O
media	O
.	O
</s>
<s>
Previous	O
proposals	O
such	O
as	O
IntServ	O
,	O
DiffServ	O
,	O
and	O
IP	B-Protocol
multicast	I-Protocol
have	O
not	O
seen	O
wide	O
acceptance	O
,	O
largely	O
because	O
they	O
require	O
modification	O
of	O
all	O
routers	B-Protocol
in	O
the	O
network	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
an	O
overlay	B-Architecture
network	I-Architecture
can	O
be	O
incrementally	O
deployed	O
on	O
end-hosts	O
running	O
the	O
overlay	O
protocol	O
software	O
,	O
without	O
cooperation	O
from	O
ISPs	O
.	O
</s>
<s>
The	O
overlay	O
has	O
no	O
control	O
over	O
how	O
packets	O
are	O
routed	B-Protocol
in	O
the	O
underlying	O
network	O
between	O
two	O
overlay	O
nodes	B-Protocol
,	O
but	O
it	O
can	O
control	O
,	O
for	O
example	O
,	O
the	O
sequence	O
of	O
overlay	O
nodes	B-Protocol
a	O
message	O
traverses	O
before	O
reaching	O
its	O
destination	O
.	O
</s>
<s>
For	O
example	O
,	O
Akamai	B-Application
Technologies	I-Application
manages	O
an	O
overlay	B-Architecture
network	I-Architecture
which	O
provides	O
reliable	O
,	O
efficient	O
content	O
delivery	O
(	O
a	O
kind	O
of	O
multicast	O
)	O
.	O
</s>
<s>
Academic	O
research	O
includes	O
End	B-Protocol
System	I-Protocol
Multicast	O
and	O
Overcast	O
,	O
which	O
is	O
multicasting	O
on	O
an	O
overlay	B-Architecture
network	I-Architecture
;	O
RON	O
(	O
Resilient	O
Overlay	B-Architecture
Network	I-Architecture
)	O
for	O
resilient	O
routing	B-Protocol
;	O
and	O
OverQoS	O
for	O
quality	O
of	O
service	O
guarantees	O
,	O
among	O
others	O
.	O
</s>
<s>
The	O
dispersed	O
nature	O
of	O
the	B-Operating_System
Internet	I-Operating_System
of	I-Operating_System
things	I-Operating_System
(	O
IoT	B-Operating_System
)	O
presents	O
a	O
major	O
operational	O
challenge	O
that	O
is	O
uncommon	O
in	O
the	O
traditional	O
Internet	O
or	O
enterprise	O
networks	O
.	O
</s>
<s>
The	O
operational	O
approaches	O
for	O
management	O
and	O
security	O
used	O
in	O
enterprise	O
networks	O
,	O
where	O
most	O
hosts	O
are	O
densely	O
contained	O
in	O
buildings	O
or	O
campuses	O
,	O
do	O
not	O
translate	O
to	O
the	O
IoT	B-Operating_System
.	O
</s>
<s>
IoT	B-Operating_System
devices	O
operate	O
outside	O
of	O
the	O
enterprise	O
network	O
security	O
and	O
operational	O
perimeter	O
and	O
the	O
corporate	O
LAN	O
firewall	O
ca	O
n’t	O
protect	O
them	O
.	O
</s>
<s>
Resilient	O
Overlay	B-Architecture
Networks	I-Architecture
(	O
RON	O
)	O
are	O
architectures	O
that	O
allow	O
distributed	O
Internet	O
applications	O
to	O
detect	O
and	O
recover	O
from	O
disconnection	O
or	O
interference	O
.	O
</s>
<s>
Current	O
wide	O
area	O
routing	B-Protocol
protocols	O
that	O
take	O
at	O
least	O
several	O
minutes	O
to	O
recover	O
from	O
are	O
improved	O
upon	O
with	O
this	O
application	O
layer	O
overlay	O
.	O
</s>
<s>
The	O
RON	O
nodes	B-Protocol
monitor	O
the	O
Internet	O
paths	O
among	O
themselves	O
and	O
will	O
determine	O
whether	O
or	O
not	O
to	O
reroute	O
packets	O
directly	O
over	O
the	O
internet	O
or	O
over	O
other	O
RON	O
nodes	B-Protocol
thus	O
optimizing	O
application	O
specific	O
metrics	O
.	O
</s>
<s>
The	O
Resilient	O
Overlay	B-Architecture
Network	I-Architecture
has	O
a	O
relatively	O
simple	O
conceptual	O
design	O
.	O
</s>
<s>
RON	O
nodes	B-Protocol
are	O
deployed	O
at	O
various	O
locations	O
on	O
the	O
Internet	O
.	O
</s>
<s>
These	O
nodes	B-Protocol
form	O
an	O
application	O
layer	O
overlay	O
that	O
cooperate	O
in	O
routing	B-Protocol
packets	O
.	O
</s>
<s>
Each	O
of	O
the	O
RON	O
nodes	B-Protocol
monitor	O
the	O
quality	O
of	O
the	O
Internet	O
paths	O
between	O
each	O
other	O
and	O
uses	O
this	O
information	O
to	O
accurately	O
and	O
automatically	O
select	O
paths	O
from	O
each	O
packet	O
,	O
thus	O
reducing	O
the	O
amount	O
of	O
time	O
required	O
to	O
recover	O
from	O
poor	O
quality	O
of	O
service	O
.	O
</s>
<s>
Overlay	O
multicast	O
is	O
also	O
known	O
as	O
End	B-Protocol
System	I-Protocol
or	O
Peer-to-Peer	O
Multicast	O
.	O
</s>
<s>
High	O
bandwidth	O
multi-source	O
multicast	O
among	O
widely	O
distributed	O
nodes	B-Protocol
is	O
a	O
critical	O
capability	O
for	O
a	O
wide	O
range	O
of	O
applications	O
,	O
including	O
audio	O
and	O
video	O
conferencing	O
,	O
multi-party	O
games	O
and	O
content	O
distribution	O
.	O
</s>
<s>
The	O
limited	O
deployment	O
of	O
IP	B-Protocol
Multicast	I-Protocol
,	O
a	O
best	O
effort	O
network	O
layer	O
multicast	O
protocol	O
,	O
has	O
led	O
to	O
considerable	O
interest	O
in	O
alternate	O
approaches	O
that	O
are	O
implemented	O
at	O
the	O
application	O
layer	O
,	O
using	O
only	O
end-systems	B-Protocol
.	O
</s>
<s>
Each	O
edge	O
in	O
this	O
topology	O
corresponds	O
to	O
a	O
unicast	O
path	O
between	O
two	O
end-systems	B-Protocol
or	O
peers	O
in	O
the	O
underlying	O
Internet	O
.	O
</s>
<s>
All	O
multicast-related	O
functionality	O
is	O
implemented	O
at	O
the	O
peers	O
instead	O
of	O
at	O
routers	B-Protocol
,	O
and	O
the	O
goal	O
of	O
the	O
multicast	O
protocol	O
is	O
to	O
construct	O
and	O
maintain	O
an	O
efficient	O
overlay	O
for	O
data	O
transmission	O
.	O
</s>
<s>
Overlay	B-Architecture
network	I-Architecture
protocols	O
based	O
on	O
TCP/IP	B-Protocol
include	O
:	O
</s>
<s>
Many	O
peer-to-peer	O
protocols	O
including	O
Gnutella	B-Protocol
,	O
Gnutella2	O
,	O
Freenet	B-Protocol
,	O
I2P	B-Protocol
and	O
Tor	B-Application
.	O
</s>
<s>
Overlay	B-Architecture
network	I-Architecture
protocols	O
based	O
on	O
UDP/IP	O
include	O
:	O
</s>
<s>
Distributed	B-Operating_System
hash	I-Operating_System
tables	I-Operating_System
(	O
DHTs	O
)	O
based	O
on	O
Kademlia	B-Operating_System
algorithm	O
,	O
such	O
as	O
KAD	O
,	O
etc	O
.	O
</s>
