<s>
IP	B-Protocol
multicast	I-Protocol
is	O
a	O
method	O
of	O
sending	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
datagrams	B-Protocol
to	O
a	O
group	O
of	O
interested	O
receivers	O
in	O
a	O
single	O
transmission	O
.	O
</s>
<s>
It	O
uses	O
specially	O
reserved	O
multicast	B-Protocol
address	I-Protocol
blocks	O
in	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
.	O
</s>
<s>
Protocols	O
associated	O
with	O
IP	B-Protocol
multicast	I-Protocol
include	O
Internet	B-Protocol
Group	I-Protocol
Management	I-Protocol
Protocol	I-Protocol
,	O
Protocol	B-Protocol
Independent	I-Protocol
Multicast	I-Protocol
and	O
Multicast	O
VLAN	O
Registration	O
.	O
</s>
<s>
IGMP	B-Protocol
snooping	I-Protocol
is	O
used	O
to	O
manage	O
IP	B-Protocol
multicast	I-Protocol
traffic	O
on	O
layer-2	B-Protocol
networks	O
.	O
</s>
<s>
IP	B-Protocol
multicast	I-Protocol
is	O
described	O
in	O
.	O
</s>
<s>
IP	B-Protocol
multicast	I-Protocol
was	O
first	O
standardized	O
in	O
1986	O
.	O
</s>
<s>
IP	B-Protocol
multicast	I-Protocol
is	O
a	O
technique	O
for	O
one-to-many	B-General_Concept
and	O
many-to-many	B-Architecture
real-time	O
communication	O
over	O
an	O
IP	O
infrastructure	O
in	O
a	O
network	O
.	O
</s>
<s>
The	O
nodes	O
in	O
the	O
network	O
(	O
typically	O
network	B-Protocol
switches	I-Protocol
and	O
routers	B-Protocol
)	O
take	O
care	O
of	O
replicating	O
the	O
packet	O
to	O
reach	O
multiple	O
receivers	O
such	O
that	O
messages	O
are	O
sent	O
over	O
each	O
link	O
of	O
the	O
network	O
only	O
once	O
.	O
</s>
<s>
The	O
most	O
common	O
transport	B-Architecture
layer	I-Architecture
protocol	O
to	O
use	O
multicast	O
addressing	O
is	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
.	O
</s>
<s>
Reliable	B-Operating_System
multicast	I-Operating_System
protocols	O
such	O
as	O
Pragmatic	B-Protocol
General	I-Protocol
Multicast	I-Protocol
(	O
PGM	O
)	O
have	O
been	O
developed	O
to	O
add	O
loss	O
detection	O
and	O
retransmission	O
on	O
top	O
of	O
IP	B-Protocol
multicast	I-Protocol
.	O
</s>
<s>
Key	O
concepts	O
in	O
IP	B-Protocol
multicast	I-Protocol
include	O
an	O
IP	B-Protocol
multicast	I-Protocol
group	O
address	O
,	O
a	O
multicast	O
distribution	O
tree	O
and	O
receiver	O
driven	O
tree	O
creation	O
.	O
</s>
<s>
An	O
IP	B-Protocol
multicast	I-Protocol
group	O
address	O
is	O
used	O
by	O
sources	O
and	O
the	O
receivers	O
to	O
send	O
and	O
receive	O
multicast	O
messages	O
.	O
</s>
<s>
The	O
protocol	O
typically	O
used	O
by	O
receivers	O
to	O
join	O
a	O
group	O
is	O
called	O
the	O
Internet	B-Protocol
Group	I-Protocol
Management	I-Protocol
Protocol	I-Protocol
(	O
IGMP	B-Protocol
)	O
.	O
</s>
<s>
With	O
routing	O
protocols	O
based	O
on	O
shared	O
trees	O
,	O
once	O
the	O
receivers	O
join	O
a	O
particular	O
IP	B-Protocol
multicast	I-Protocol
group	O
,	O
a	O
multicast	O
distribution	O
tree	O
is	O
constructed	O
for	O
that	O
group	O
.	O
</s>
<s>
The	O
protocol	O
most	O
widely	O
used	O
for	O
this	O
is	O
Protocol	B-Protocol
Independent	I-Protocol
Multicast	I-Protocol
(	O
PIM	B-Protocol
)	O
.	O
</s>
<s>
There	O
are	O
variations	O
of	O
PIM	B-Protocol
implementations	O
:	O
Sparse	O
Mode	O
(	O
SM	O
)	O
,	O
Dense	O
Mode	O
(	O
DM	O
)	O
,	O
source-specific	B-Protocol
multicast	I-Protocol
(	O
SSM	O
)	O
and	O
Bidirectional	O
Mode	O
(	O
Bidir	O
,	O
or	O
Sparse-Dense	O
Mode	O
,	O
SDM	O
)	O
.	O
</s>
<s>
Of	O
these	O
,	O
PIM-SM	O
is	O
the	O
most	O
widely	O
deployed	O
;	O
SSM	O
and	O
Bidir	O
are	O
simpler	O
and	O
scalable	O
variations	O
developed	O
more	O
recently	O
and	O
are	O
gaining	O
in	O
popularity	O
.	O
</s>
<s>
IP	B-Protocol
multicast	I-Protocol
operation	O
does	O
not	O
require	O
an	O
active	O
source	O
to	O
know	O
about	O
the	O
receivers	O
of	O
the	O
group	O
.	O
</s>
<s>
IP	B-Protocol
multicast	I-Protocol
scales	O
to	O
a	O
large	O
receiver	O
population	O
.	O
</s>
<s>
The	O
IP	B-Protocol
multicast	I-Protocol
model	O
has	O
been	O
described	O
by	O
Internet	O
architect	O
Dave	O
Clark	O
as	O
,	O
"	O
You	O
put	O
packets	O
in	O
at	O
one	O
end	O
,	O
and	O
the	O
network	O
conspires	O
to	O
deliver	O
them	O
to	O
anyone	O
who	O
asks.	O
"	O
</s>
<s>
IP	B-Protocol
multicast	I-Protocol
creates	O
state	B-Application
information	O
per	O
multicast	O
distribution	O
tree	O
in	O
the	O
network	O
.	O
</s>
<s>
If	O
a	O
router	B-Protocol
is	O
part	O
of	O
1000	O
multicast	O
trees	O
,	O
it	O
has	O
1000	O
multicast	O
routing	O
and	O
forwarding	O
entries	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
a	O
multicast	O
router	B-Protocol
does	O
not	O
need	O
to	O
know	O
how	O
to	O
reach	O
all	O
other	O
multicast	O
trees	O
in	O
the	O
Internet	O
.	O
</s>
<s>
In	O
contrast	O
,	O
a	O
unicast	B-Protocol
router	B-Protocol
needs	O
to	O
know	O
how	O
to	O
reach	O
all	O
other	O
unicast	B-Protocol
addresses	O
in	O
the	O
Internet	O
,	O
even	O
if	O
it	O
does	O
this	O
using	O
just	O
a	O
default	O
route	O
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
aggregation	O
is	O
key	O
to	O
scaling	O
unicast	B-Protocol
routing	O
.	O
</s>
<s>
Also	O
,	O
there	O
are	O
core	O
routers	B-Protocol
that	O
carry	O
routes	O
in	O
the	O
hundreds	O
of	O
thousands	O
because	O
they	O
contain	O
the	O
Internet	O
routing	O
table	O
.	O
</s>
<s>
receive	O
data	O
corresponding	O
to	O
a	O
particular	O
multicast	B-Protocol
address	I-Protocol
)	O
must	O
use	O
IGMP	B-Protocol
to	O
join	O
.	O
</s>
<s>
Adjacent	O
routers	B-Protocol
also	O
use	O
this	O
protocol	O
to	O
communicate	O
.	O
</s>
<s>
In	O
unicast	B-Protocol
routing	O
,	O
each	O
router	B-Protocol
examines	O
the	O
destination	O
address	O
of	O
an	O
incoming	O
packet	O
and	O
looks	O
up	O
the	O
destination	O
in	O
a	O
table	O
to	O
determine	O
which	O
interface	O
to	O
use	O
in	O
order	O
for	O
that	O
packet	O
to	O
get	O
closer	O
to	O
its	O
destination	O
.	O
</s>
<s>
The	O
source	O
address	O
is	O
irrelevant	O
to	O
the	O
router	B-Protocol
.	O
</s>
<s>
However	O
,	O
in	O
multicast	O
routing	O
,	O
the	O
source	O
address	O
(	O
which	O
is	O
a	O
simple	O
unicast	B-Protocol
address	I-Protocol
)	O
is	O
used	O
to	O
determine	O
data	O
stream	O
direction	O
.	O
</s>
<s>
The	O
router	B-Protocol
determines	O
which	O
downstream	O
interfaces	O
are	O
destinations	O
for	O
this	O
multicast	O
group	O
(	O
the	O
destination	O
address	O
)	O
,	O
and	O
sends	O
the	O
packet	O
out	O
through	O
the	O
appropriate	O
interfaces	O
.	O
</s>
<s>
The	O
term	O
reverse-path	B-Protocol
forwarding	I-Protocol
is	O
used	O
to	O
describe	O
this	O
concept	O
of	O
routing	O
packets	O
away	O
from	O
the	O
source	O
,	O
rather	O
than	O
towards	O
the	O
destination	O
.	O
</s>
<s>
A	O
number	O
of	O
errors	O
can	O
happen	O
if	O
packets	O
intended	O
for	O
unicast	B-Protocol
are	O
accidentally	O
sent	O
to	O
a	O
multicast	B-Protocol
address	I-Protocol
;	O
in	O
particular	O
,	O
sending	O
ICMP	O
packets	O
to	O
a	O
multicast	B-Protocol
address	I-Protocol
has	O
been	O
used	O
in	O
the	O
context	O
of	O
DoS	O
attacks	O
as	O
a	O
way	O
of	O
achieving	O
packet	O
amplification	O
.	O
</s>
<s>
On	O
the	O
local	O
network	O
,	O
multicast	O
delivery	O
is	O
controlled	O
by	O
IGMP	B-Protocol
(	O
on	O
IPv4	B-Protocol
network	I-Protocol
)	O
and	O
MLD	B-Protocol
(	O
on	O
IPv6	B-Protocol
network	O
)	O
;	O
inside	O
a	O
routing	B-Protocol
domain	I-Protocol
,	O
PIM	B-Protocol
or	O
MOSPF	B-Protocol
are	O
used	O
;	O
between	O
routing	B-Protocol
domains	I-Protocol
,	O
one	O
uses	O
inter-domain	O
multicast	O
routing	O
protocols	O
,	O
such	O
as	O
MBGP	B-Protocol
.	O
</s>
<s>
Unicast	B-Protocol
packets	O
are	O
delivered	O
to	O
a	O
specific	O
recipient	O
on	O
an	O
Ethernet	O
or	O
IEEE	O
802.3	O
subnet	O
by	O
setting	O
a	O
specific	O
layer	B-Protocol
2	I-Protocol
MAC	O
address	O
on	O
the	O
Ethernet	O
packet	O
address	O
.	O
</s>
<s>
IPv4	B-Protocol
multicast	O
packets	O
are	O
delivered	O
using	O
the	O
Ethernet	O
MAC	O
address	O
range	O
through	O
(	O
with	O
an	O
OUI	O
owned	O
by	O
the	O
IANA	O
)	O
.	O
</s>
<s>
The	O
lower	O
23	O
bits	O
of	O
the	O
28-bit	O
multicast	B-Protocol
IP	I-Protocol
address	O
are	O
mapped	O
into	O
the	O
23	O
bits	O
of	O
available	O
Ethernet	O
address	O
space	O
.	O
</s>
<s>
For	O
IPv6	B-Protocol
multicast	I-Protocol
addresses	O
,	O
the	O
Ethernet	O
MAC	O
is	O
derived	O
by	O
the	O
four	O
low-order	O
octets	O
OR'ed	O
with	O
the	O
MAC	O
,	O
so	O
for	O
example	O
the	O
IPv6	B-Protocol
address	O
would	O
map	O
to	O
the	O
Ethernet	O
MAC	O
address	O
.	O
</s>
<s>
There	O
are	O
switches	O
that	O
listen	O
to	O
IGMP	B-Protocol
traffic	O
and	O
maintain	O
a	O
state	B-Application
table	O
of	O
which	O
network	O
systems	O
are	O
subscribed	O
to	O
a	O
given	O
multicast	O
group	O
.	O
</s>
<s>
This	O
process	O
of	O
listening	O
to	O
the	O
IGMP	B-Protocol
traffic	O
is	O
called	O
IGMP	B-Protocol
snooping	I-Protocol
.	O
</s>
<s>
Additionally	O
,	O
some	O
switches	O
with	O
layer	O
3	O
capabilities	O
can	O
act	O
as	O
an	O
IGMP	B-Protocol
querier	O
.	O
</s>
<s>
In	O
networks	O
where	O
there	O
is	O
no	O
router	B-Protocol
present	O
to	O
act	O
as	O
a	O
multicast	O
router	B-Protocol
,	O
a	O
switch	O
with	O
IGMP	B-Protocol
snooping	I-Protocol
querier	O
enabled	O
can	O
be	O
used	O
to	O
generate	O
the	O
needed	O
IGMP	B-Protocol
messages	O
to	O
get	O
users	O
to	O
subscribe	O
to	O
multicast	O
traffic	O
.	O
</s>
<s>
802.11	O
wireless	O
networking	O
uses	O
the	O
same	O
range	O
of	O
MAC	O
addresses	O
as	O
wired	O
Ethernet	O
to	O
map	O
IP	B-Protocol
multicast	I-Protocol
addresses	O
.	O
</s>
<s>
However	O
,	O
an	O
802.11	O
wireless	O
network	O
handles	O
multicast	O
traffic	O
differently	O
,	O
depending	O
on	O
the	O
configuration	O
of	O
delivery	B-General_Concept
traffic	I-General_Concept
indication	I-General_Concept
message	I-General_Concept
(	O
DTIM	O
)	O
,	O
and	O
beacon	O
interval	O
settings	O
.	O
</s>
<s>
If	O
no	O
stations	O
within	O
the	O
basic	B-General_Concept
service	I-General_Concept
set	I-General_Concept
are	O
in	O
power	O
save	O
mode	O
,	O
multicast	O
packets	O
are	O
sent	O
immediately	O
when	O
they	O
arrive	O
.	O
</s>
<s>
There	O
are	O
various	O
methods	O
for	O
coping	O
with	O
this	O
,	O
such	O
as	O
choosing	O
to	O
unicast	B-Protocol
multicast	O
data	O
repeatedly	O
to	O
each	O
client	O
,	O
or	O
requesting	O
ACKs	O
from	O
each	O
client	O
.	O
</s>
<s>
IP	B-Protocol
multicast	I-Protocol
is	O
an	O
internet	O
communication	O
method	O
where	O
a	O
single	O
data	O
packet	O
can	O
be	O
transmitted	O
from	O
a	O
sender	O
and	O
replicated	O
to	O
a	O
set	O
of	O
receivers	O
.	O
</s>
<s>
In	O
contrast	O
,	O
transmission	O
of	O
multicast	O
on	O
media	O
that	O
is	O
point-to-point	O
or	O
point-to-multipoint	B-General_Concept
requires	O
the	O
packet	O
to	O
be	O
replicated	O
for	O
each	O
link	O
.	O
</s>
<s>
The	O
use	O
of	O
IPsec	B-Protocol
as	O
a	O
communication	O
link	O
requires	O
a	O
point-to-point	O
connection	O
establishment	O
.	O
</s>
<s>
A	O
new	O
method	O
was	O
required	O
to	O
securely	O
transmit	O
multicast	O
and	O
this	O
was	O
referred	O
to	O
as	O
Secure	B-Protocol
Multicast	I-Protocol
or	O
Multicast	O
Security	O
.	O
</s>
<s>
The	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
created	O
a	O
new	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
to	O
securely	O
transmit	O
multicast	O
traffic	O
across	O
a	O
packet	O
network	O
.	O
</s>
<s>
The	O
protocol	O
definition	O
was	O
developed	O
in	O
the	O
Multicast	O
Security	O
Workgroup	O
and	O
led	O
to	O
several	O
Request	O
for	O
Comments	O
(	O
RFC	O
)	O
that	O
are	O
now	O
used	O
as	O
standards	O
for	O
securing	O
IP	B-Protocol
multicast	I-Protocol
traffic	O
.	O
</s>
<s>
For	O
this	O
,	O
the	O
IETF	O
created	O
the	O
Group	B-Protocol
Domain	I-Protocol
of	I-Protocol
Interpretation	I-Protocol
(	O
GDOI	B-Protocol
)	O
protocol	O
defined	O
in	O
RFC	O
6407	O
.	O
</s>
<s>
The	O
protocol	O
allows	O
the	O
sender	O
and	O
receiver	O
to	O
join	O
a	O
key	O
server	O
where	O
policies	O
and	O
keys	O
are	O
encrypted	O
and	O
distributed	O
to	O
the	O
members	O
of	O
the	O
secure	B-Protocol
multicast	I-Protocol
group	O
.	O
</s>
<s>
Multicast	O
,	O
by	O
its	O
very	O
nature	O
,	O
is	O
not	O
a	O
connection-oriented	O
mechanism	O
,	O
so	O
protocols	O
such	O
as	O
TCP	B-Protocol
,	O
which	O
allows	O
for	O
retransmission	O
of	O
missing	O
packets	O
,	O
are	O
not	O
appropriate	O
.	O
</s>
<s>
One	O
such	O
scheme	O
,	O
proposed	O
by	O
Cisco	O
,	O
is	O
PGM	O
(	O
originally	O
Pretty	O
Good	O
Multicasting	O
,	O
but	O
changed	O
for	O
trademark	O
reasons	O
to	O
Pragmatic	B-Protocol
General	I-Protocol
Multicast	I-Protocol
)	O
,	O
documented	O
in	O
RFC	O
3208	O
.	O
</s>
<s>
In	O
this	O
scheme	O
,	O
multicast	O
packets	O
have	O
sequence	B-Protocol
numbers	I-Protocol
and	O
when	O
a	O
packet	O
is	O
missed	O
a	O
recipient	O
can	O
request	O
that	O
the	O
packet	O
be	O
re-multicast	O
with	O
other	O
members	O
of	O
the	O
Multicast	O
group	O
ignoring	O
the	O
replacement	O
data	O
if	O
not	O
needed	O
.	O
</s>
<s>
An	O
expanded	O
version	O
,	O
PGM-CC	O
,	O
has	O
attempted	O
to	O
make	O
IP	B-Protocol
Multicasting	I-Protocol
more	O
"	O
TCP	B-Protocol
friendly	O
"	O
by	O
stepping	O
the	O
entire	O
group	O
down	O
to	O
the	O
bandwidth	O
available	O
by	O
the	O
worst	O
receiver	O
.	O
</s>
<s>
Two	O
other	O
schemes	O
documented	O
by	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
are	O
:	O
the	O
standards-track	O
protocol	O
NACK-Oriented	O
Reliable	B-Operating_System
Multicast	I-Operating_System
(	O
NORM	O
)	O
,	O
documented	O
in	O
RFC	O
5740	O
and	O
RFC	O
5401	O
,	O
and	O
the	O
protocol	O
File	O
Delivery	O
over	O
Unidirectional	O
Transport	O
(	O
FLUTE	O
)	O
,	O
documented	O
in	O
RFC	O
6726	O
.	O
</s>
<s>
Other	O
such	O
protocols	O
exist	O
,	O
such	O
as	O
Scalable	O
Reliable	B-Operating_System
Multicast	I-Operating_System
,	O
and	O
are	O
defined	O
by	O
a	O
variety	O
of	O
sources	O
.	O
</s>
<s>
A	O
list	O
of	O
reliable	B-Operating_System
multicast	I-Operating_System
protocols	O
from	O
the	O
ACM	O
SIGCOMM	O
Multicast	O
Workshop	O
,	O
August	O
27	O
,	O
1996	O
,	O
documents	O
a	O
number	O
of	O
approaches	O
to	O
the	O
problem	O
.	O
</s>
<s>
Independent	O
groups	O
like	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
Multicast	I-Protocol
Standards	O
Initiative	O
(	O
IPMSI	O
)	O
have	O
claimed	O
that	O
the	O
lack	O
of	O
a	O
truly	O
scalable	O
Secure	O
Reliable	O
IP	B-Protocol
Multicast	I-Protocol
protocol	O
like	O
the	O
proposed	O
Secure	B-Protocol
Multicast	I-Protocol
for	O
Advanced	O
Repeating	O
of	O
Television	O
(	O
SMART	O
)	O
have	O
hampered	O
the	O
adoption	O
of	O
IP	B-Protocol
Multicast	I-Protocol
in	O
inter-domain	O
routing	O
.	O
</s>
<s>
Reliable	O
IP	B-Protocol
Multicasting	I-Protocol
protocols	O
,	O
such	O
as	O
PGM	O
and	O
SMART	O
,	O
are	O
experimental	O
;	O
the	O
only	O
standards-track	O
protocol	O
is	O
NORM	O
(	O
the	O
standards-track	O
revision	O
of	O
RFC	O
3941	O
is	O
specified	O
in	O
RFC	O
5401	O
,	O
the	O
standards-track	O
revision	O
of	O
RFC	O
3940	O
is	O
specified	O
in	O
RFC	O
5740	O
)	O
.	O
</s>
<s>
Since	O
multicast	O
is	O
a	O
different	O
transmission	O
mode	O
from	O
unicast	B-Protocol
,	O
only	O
protocols	O
designed	O
for	O
multicast	O
can	O
be	O
sensibly	O
used	O
with	O
multicast	O
.	O
</s>
<s>
Most	O
of	O
the	O
existing	O
application	B-Application
protocols	O
that	O
use	O
multicast	O
run	O
on	O
top	O
of	O
the	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
.	O
</s>
<s>
In	O
many	O
applications	O
,	O
the	O
Real-time	B-Protocol
Transport	I-Protocol
Protocol	I-Protocol
(	O
RTP	O
)	O
is	O
used	O
for	O
framing	O
of	O
multimedia	O
content	O
over	O
multicast	O
;	O
the	O
Resource	B-Protocol
Reservation	I-Protocol
Protocol	I-Protocol
(	O
RSVP	B-Protocol
)	O
may	O
be	O
used	O
for	O
bandwidth	O
reservation	O
in	O
a	O
network	O
supporting	O
multicast	O
distribution	O
.	O
</s>
<s>
Multicast	B-Protocol
DNS	I-Protocol
(	O
mDNS	B-Protocol
)	O
can	O
be	O
used	O
to	O
resolve	O
domain	O
or	O
host	O
names	O
without	O
a	O
dedicated	O
DNS	O
server	O
by	O
using	O
multicast	O
.	O
</s>
<s>
IP	B-Protocol
multicast	I-Protocol
is	O
widely	O
deployed	O
in	O
enterprises	O
,	O
commercial	O
stock	O
exchanges	O
,	O
and	O
multimedia	O
content	O
delivery	O
networks	O
.	O
</s>
<s>
A	O
common	O
enterprise	O
use	O
of	O
IP	B-Protocol
multicast	I-Protocol
is	O
for	O
IPTV	O
applications	O
such	O
as	O
live	O
television	O
distribution	O
and	O
televised	O
company	O
meetings	O
.	O
</s>
<s>
In	O
the	O
hospitality	O
industry	O
IP	B-Protocol
multicast	I-Protocol
has	O
become	O
common	O
for	O
IPTV	O
distribution	O
in	O
hotels	O
,	O
and	O
in	O
the	O
retail	O
sector	O
IP	B-Protocol
multicast	I-Protocol
is	O
now	O
widely	O
used	O
for	O
TV	O
distribution	O
and	O
video	O
advertising	O
applications	O
.	O
</s>
<s>
Pay-TV	O
operators	O
and	O
some	O
educational	O
institutions	O
with	O
significant	O
on-campus	O
student	O
housing	O
have	O
deployed	O
IP	B-Protocol
multicast	I-Protocol
to	O
deliver	O
one-way	O
streaming	O
media	O
such	O
as	O
high-speed	O
video	O
to	O
large	O
groups	O
of	O
receivers	O
.	O
</s>
<s>
Some	O
technical	O
conferences	O
and	O
meetings	O
are	O
transmitted	O
using	O
IP	B-Protocol
multicast	I-Protocol
.	O
</s>
<s>
The	O
key	O
advantage	O
of	O
multicast	O
boot	O
images	O
over	O
unicasting	B-Protocol
boot	O
images	O
is	O
significantly	O
lower	O
network	O
bandwidth	O
usage	O
.	O
</s>
<s>
IP	B-Protocol
multicast	I-Protocol
has	O
also	O
seen	O
deployment	O
within	O
the	O
financial	O
sector	O
for	O
applications	O
such	O
as	O
stock	O
tickers	O
and	O
hoot-n-holler	O
systems	O
.	O
</s>
<s>
The	O
large	O
state	B-Application
requirements	O
in	O
routers	B-Protocol
make	O
applications	O
using	O
a	O
large	O
number	O
of	O
trees	O
unable	O
to	O
work	O
while	O
using	O
IP	B-Protocol
multicast	I-Protocol
.	O
</s>
<s>
Take	O
presence	B-Protocol
information	I-Protocol
as	O
an	O
example	O
where	O
each	O
person	O
needs	O
to	O
keep	O
at	O
least	O
one	O
tree	O
of	O
its	O
subscribers	O
,	O
if	O
not	O
several	O
.	O
</s>
<s>
No	O
mechanism	O
has	O
yet	O
been	O
demonstrated	O
that	O
would	O
allow	O
the	O
IP	B-Protocol
multicast	I-Protocol
model	O
to	O
scale	O
to	O
millions	O
of	O
senders	O
and	O
millions	O
of	O
multicast	O
groups	O
and	O
,	O
thus	O
,	O
it	O
is	O
not	O
yet	O
possible	O
to	O
make	O
fully	O
general	O
multicast	O
applications	O
practical	O
.	O
</s>
<s>
RFC	O
3170	O
(	O
IP	B-Protocol
Multicast	I-Protocol
Applications	O
:	O
Challenges	O
&	O
Solutions	O
)	O
provides	O
an	O
overview	O
of	O
deployment	O
issues	O
.	O
</s>
<s>
IP	B-Protocol
multicasting	I-Protocol
was	O
first	O
developed	O
by	O
Steve	O
Deering	O
while	O
at	O
Stanford	O
University	O
for	O
which	O
he	O
received	O
the	O
IEEE	O
Internet	O
Award	O
.	O
</s>
<s>
The	O
MBONE	B-Protocol
was	O
a	O
long-running	O
experimental	O
approach	O
to	O
enabling	O
multicast	O
between	O
sites	O
through	O
the	O
use	O
of	O
tunnels	O
.	O
</s>
<s>
While	O
the	O
MBONE	B-Protocol
is	O
no	O
longer	O
operational	O
,	O
there	O
is	O
renewed	O
interest	O
in	O
tunneling	O
multicast	O
traffic	O
once	O
again	O
in	O
order	O
to	O
make	O
the	O
service	O
available	O
to	O
a	O
wide	O
array	O
of	O
end	O
users	O
.	O
</s>
<s>
CastGate	O
was	O
an	O
attempt	O
from	O
the	O
ETRO-TELE	O
research	O
group	O
at	O
the	O
Vrije	O
Universiteit	O
Brussel	O
to	O
adopt	O
IP	B-Protocol
multicast	I-Protocol
on	O
the	O
Internet	O
.	O
</s>
<s>
The	O
CastGate	O
project	O
tried	O
to	O
fix	O
this	O
by	O
allowing	O
end	O
users	O
to	O
connect	O
through	O
an	O
automatically	O
configured	O
IP	B-Protocol
tunnel	I-Protocol
over	O
networks	O
which	O
did	O
not	O
natively	O
support	O
IP	B-Protocol
multicast	I-Protocol
.	O
</s>
<s>
The	O
hope	O
was	O
if	O
enough	O
content	O
providers	O
and	O
users	O
used	O
this	O
service	O
,	O
then	O
more	O
Internet	O
service	O
providers	O
would	O
enable	O
IP	B-Protocol
multicast	I-Protocol
natively	O
to	O
their	O
customers	O
.	O
</s>
<s>
CastGate	O
supplied	O
a	O
software	O
client	O
for	O
both	O
Microsoft	B-Application
Windows	I-Application
and	O
Linux	B-Application
to	O
connect	O
to	O
the	O
CastGate	O
tunnel	O
network	O
.	O
</s>
<s>
It	O
also	O
supplied	O
tools	O
to	O
add	O
tunnel	O
servers	O
and	O
tools	O
to	O
receive	O
Session	B-Protocol
Announcement	I-Protocol
Protocol	I-Protocol
announcements	O
from	O
the	O
multicast	O
network	O
with	O
video	O
and	O
audio	O
streams	O
.	O
</s>
<s>
Starting	O
in	O
2005	O
,	O
the	O
BBC	O
began	O
encouraging	O
UK-based	O
Internet	O
service	O
providers	O
to	O
adopt	O
multicast-addressable	O
services	O
in	O
their	O
networks	O
by	O
providing	O
BBC	O
Radio	O
at	O
higher	O
quality	O
than	O
is	O
available	O
via	O
their	O
unicast-addressed	O
services	O
.	O
</s>
<s>
–	O
Nack-Oriented	O
Reliable	B-Operating_System
Multicast	I-Operating_System
from	O
the	O
U.S.	O
</s>
