<s>
Pragmatic	B-Protocol
General	I-Protocol
Multicast	I-Protocol
(	O
PGM	O
)	O
is	O
a	O
reliable	B-Operating_System
multicast	I-Operating_System
computer	B-Architecture
network	I-Architecture
transport	O
protocol	O
.	O
</s>
<s>
PGM	O
provides	O
a	O
reliable	O
sequence	O
of	O
packets	O
to	O
multiple	O
recipients	O
simultaneously	O
,	O
making	O
it	O
suitable	O
for	O
applications	O
like	O
multi-receiver	O
file-transfer	B-Protocol
.	O
</s>
<s>
Multicast	O
is	O
a	O
network	B-Architecture
addressing	O
method	O
for	O
the	O
delivery	O
of	O
information	O
to	O
a	O
group	O
of	O
destinations	O
simultaneously	O
using	O
the	O
most	O
efficient	O
strategy	O
to	O
deliver	O
the	O
messages	O
over	O
each	O
link	O
of	O
the	O
network	B-Architecture
only	O
once	O
,	O
creating	O
copies	O
only	O
when	O
the	O
links	O
to	O
the	O
multiple	O
destinations	O
split	O
(	O
typically	O
network	B-Protocol
switches	I-Protocol
and	O
routers	B-Protocol
)	O
.	O
</s>
<s>
However	O
,	O
like	O
the	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
,	O
multicast	O
does	O
not	O
guarantee	O
the	O
delivery	O
of	O
a	O
message	O
stream	O
.	O
</s>
<s>
A	O
reliable	B-Operating_System
multicast	I-Operating_System
protocol	O
,	O
like	O
PGM	O
,	O
adds	O
the	O
ability	O
for	O
receivers	O
to	O
detect	O
lost	O
and/or	O
out-of-order	O
messages	O
and	O
take	O
corrective	O
action	O
(	O
similar	O
in	O
principle	O
to	O
TCP	B-Protocol
)	O
,	O
resulting	O
in	O
a	O
gap-free	O
,	O
in-order	O
message	O
stream	O
.	O
</s>
<s>
While	O
TCP	B-Protocol
uses	O
ACKs	O
to	O
acknowledge	O
groups	O
of	O
packets	O
sent	O
(	O
something	O
that	O
would	O
be	O
uneconomical	O
over	O
multicast	O
)	O
,	O
PGM	O
uses	O
the	O
concept	O
of	O
negative	O
acknowledgements	O
(	O
NAKs	O
)	O
.	O
</s>
<s>
A	O
NAK	O
is	O
sent	O
unicast	B-Protocol
back	O
to	O
the	O
host	O
via	O
a	O
defined	O
network-layer	O
hop-by-hop	B-Protocol
procedure	O
whenever	O
there	O
is	O
a	O
detection	O
of	O
data	O
loss	O
of	O
a	O
specific	O
sequence	O
.	O
</s>
<s>
It	O
is	O
not	O
yet	O
a	O
standard	O
,	O
but	O
has	O
been	O
implemented	O
in	O
some	O
networking	O
devices	O
and	O
operating	B-General_Concept
systems	I-General_Concept
,	O
including	O
Windows	B-Application
XP	I-Application
and	O
later	O
versions	O
of	O
Microsoft	B-Application
Windows	I-Application
,	O
as	O
well	O
as	O
in	O
third-party	O
libraries	O
for	O
Linux	B-Application
,	O
Windows	O
and	O
Solaris	B-Application
.	O
</s>
