<s>
The	O
explicit	O
multi-unicast	O
(	O
Xcast	B-Protocol
)	O
is	O
a	O
variation	O
of	O
multicast	O
that	O
supports	O
a	O
great	O
number	O
of	O
multicast	O
sessions	O
with	O
a	O
small	O
number	O
of	O
recipients	O
in	O
each	O
.	O
</s>
<s>
It	O
adds	O
all	O
the	O
destination	O
IP	B-Protocol
addresses	I-Protocol
in	O
the	O
IP	B-Protocol
header	O
,	O
instead	O
of	O
using	O
a	O
multicast	O
address	O
.	O
</s>
<s>
The	O
traditional	O
multicast	O
schemes	O
over	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	B-Protocol
)	O
scale	O
to	O
multicast	O
groups	O
with	O
many	O
members	O
,	O
but	O
they	O
have	O
scalability	O
problems	O
for	O
a	O
great	O
number	O
of	O
groups	O
.	O
</s>
<s>
Xcast	B-Protocol
minimizes	O
bandwidth	O
consumption	O
for	O
small	O
groups	O
,	O
by	O
eliminating	O
the	O
signaling	O
protocols	O
and	O
state	O
information	O
for	O
every	O
session	O
of	O
the	O
standard	O
IP	B-Protocol
multicast	I-Protocol
scheme	O
.	O
</s>
<s>
In	O
Xcast	B-Protocol
,	O
the	O
source	O
node	O
keeps	O
all	O
destinations	O
of	O
the	O
multicast	O
channel	O
through	O
which	O
packets	O
will	O
be	O
sent	O
.	O
</s>
<s>
The	O
source	O
encodes	O
the	O
destinations	O
list	O
in	O
the	O
Xcast	B-Protocol
header	O
and	O
sends	O
the	O
packet	O
to	O
a	O
router	B-Protocol
.	O
</s>
<s>
Each	O
router	B-Protocol
looks	O
in	O
a	O
routing	B-Protocol
table	I-Protocol
to	O
determine	O
the	O
next	O
hop	O
of	O
each	O
packet	O
,	O
analyzes	O
its	O
header	O
,	O
parses	O
the	O
destination	O
field	O
basing	O
on	O
the	O
following	O
jump	O
of	O
every	O
destination	O
and	O
copies	O
the	O
packets	O
as	O
many	O
different	O
paths	O
as	O
they	O
need	O
to	O
follow	O
.	O
</s>
<s>
After	O
that	O
,	O
the	O
router	B-Protocol
copies	O
the	O
packet	O
with	O
its	O
correct	O
Xcast	B-Protocol
header	O
to	O
every	O
following	O
jump	O
.	O
</s>
<s>
The	O
packet	O
is	O
treated	O
just	O
like	O
a	O
unicast	B-Protocol
packet	O
,	O
which	O
is	O
called	O
Xcast	B-Protocol
to	O
Unicast	B-Protocol
(	O
X2U	O
)	O
.	O
</s>
<s>
The	O
IP	B-Protocol
multicast	I-Protocol
standard	O
was	O
designed	O
to	O
scale	O
to	O
multicast	O
groups	O
with	O
many	O
members	O
.	O
</s>
<s>
Multicast	O
routing	O
protocols	O
keep	O
routing	B-Protocol
tables	I-Protocol
that	O
record	O
multicast	O
group	O
addresses	O
with	O
members	O
.	O
</s>
<s>
IP	B-Protocol
Multicast	I-Protocol
protocols	O
announce	O
a	O
source	O
or	O
maintain	O
routes	O
between	O
routers	B-Protocol
.	O
</s>
<s>
Xcast	B-Protocol
follows	O
philosophy	O
that	O
worked	O
well	O
to	O
grow	O
the	O
Internet	O
:	O
keep	O
the	O
center	O
of	O
the	O
network	B-Architecture
simple	O
,	O
and	O
do	O
the	O
complicated	O
operations	O
on	O
the	O
sides	O
.	O
</s>
<s>
An	O
open	B-License
source	I-License
implementation	O
was	O
available	O
from	O
IBM	O
starting	O
in	O
2001	O
.	O
</s>
<s>
A	O
MediaWiki-based	O
web	O
site	O
(	O
English	O
language	O
,	O
but	O
registered	O
in	O
Japan	O
)	O
indicates	O
activity	O
from	O
2004	O
through	O
2007	O
.	O
</s>
<s>
Routers	B-Protocol
do	O
not	O
need	O
to	O
keep	O
information	O
for	O
every	O
session	O
or	O
channel	O
.	O
</s>
<s>
This	O
makes	O
Xcast	B-Protocol
very	O
scalable	O
about	O
the	O
number	O
of	O
sessions	O
it	O
can	O
support	O
.	O
</s>
<s>
They	O
are	O
routed	O
correctly	O
thanks	O
to	O
the	O
common	O
unicast	B-Protocol
protocols	O
.	O
</s>
<s>
Xcast	B-Protocol
minimizes	O
the	O
network	B-Architecture
latencies	O
and	O
maximizes	O
efficiency	O
.	O
</s>
<s>
With	O
traditional	O
IP	B-Protocol
multicast	I-Protocol
routing	O
protocols	O
it	O
is	O
necessary	O
to	O
establish	O
a	O
communication	O
between	O
unicast	B-Protocol
and	O
multicast	O
routing	O
protocols	O
.	O
</s>
<s>
Xcast	B-Protocol
reacts	O
immediately	O
to	O
unicast	B-Protocol
routing	O
changes	O
.	O
</s>
<s>
Easier	O
security	O
and	O
register	B-General_Concept
.	O
</s>
<s>
With	O
Xcast	B-Protocol
all	O
sources	O
know	O
the	O
channel	O
members	O
and	O
all	O
routers	B-Protocol
are	O
able	O
to	O
know	O
the	O
number	O
of	O
times	O
each	O
packet	O
has	O
been	O
duplicated	O
in	O
its	O
domain	O
.	O
</s>
<s>
The	O
receptors	O
can	O
be	O
heterogeneous	O
since	O
Xcast	B-Protocol
allows	O
that	O
every	O
receptor	O
is	O
able	O
to	O
have	O
its	O
own	O
requirements	O
of	O
service	O
in	O
each	O
multicast	O
channel	O
.	O
</s>
<s>
Simplicity	O
when	O
implementing	O
reliable	O
protocols	O
over	O
Xcast	B-Protocol
.	O
</s>
<s>
Flexibility	O
:	O
unicast	B-Protocol
,	O
multicast	O
and	O
Xcast	B-Protocol
represent	O
costs	O
of	O
bandwidth	O
,	O
signalization	O
and	O
processing	O
respectively	O
.	O
</s>
<s>
Depending	O
on	O
how	O
the	O
network	B-Architecture
is	O
built	O
or	O
its	O
load	O
at	O
certain	O
moment	O
,	O
it	O
may	O
be	O
better	O
to	O
use	O
one	O
system	O
or	O
another	O
.	O
</s>
<s>
Xcast	B-Protocol
is	O
just	O
another	O
alternative	O
.	O
</s>
<s>
Every	O
processing	O
step	O
looks	O
into	O
the	O
routing	B-Protocol
table	I-Protocol
,	O
so	O
it	O
is	O
consulted	O
the	O
same	O
number	O
times	O
as	O
a	O
unicast	B-Protocol
to	O
each	O
destination	O
.	O
</s>
<s>
Xcast	B-Protocol
is	O
designed	O
for	O
sessions	O
with	O
few	O
users	O
in	O
each	O
,	O
so	O
in	O
many	O
routers	B-Protocol
the	O
headers	O
will	O
only	O
have	O
just	O
one	O
address	O
.	O
</s>
<s>
Xcast	B-Protocol
allows	O
efficient	O
applications	O
such	O
as	O
VoIP	O
,	O
video	O
conferencing	O
,	O
or	O
collaborative	O
meetings	O
.	O
</s>
<s>
These	O
applications	O
could	O
be	O
done	O
using	O
just	O
unicast	B-Protocol
,	O
but	O
in	O
cases	O
with	O
limited	O
bandwidth	O
,	O
the	O
Xcast	B-Protocol
efficiency	O
might	O
be	O
useful	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
since	O
Xcast	B-Protocol
does	O
not	O
scale	O
to	O
groups	O
with	O
many	O
members	O
,	O
it	O
can	O
not	O
substitute	O
for	O
all	O
other	O
multicast	O
models	O
.	O
</s>
