<s>
NACK-Oriented	B-Protocol
Reliable	I-Protocol
Multicast	I-Protocol
(	O
NORM	O
)	O
is	O
a	O
transport	B-Architecture
layer	I-Architecture
Internet	B-Protocol
protocol	I-Protocol
designed	O
to	O
provide	O
reliable	O
transport	O
in	O
multicast	O
groups	O
in	O
data	O
networks	O
.	O
</s>
<s>
NORM	O
operates	O
on	O
top	O
of	O
the	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
,	O
and	O
ensures	O
reliable	O
communication	O
based	O
upon	O
a	O
negative	O
acknowledgement	O
(	O
NACK	O
)	O
,	O
selective	O
Automatic	O
Repeat	O
Request	O
(	O
ARQ	O
)	O
mechanism	O
,	O
as	O
opposed	O
to	O
the	O
positive	O
acknowledgement	O
(	O
ACK	O
)	O
approach	O
that	O
the	O
standard	O
Transmission	O
Control	O
Protocol	O
(	O
TCP	O
)	O
uses	O
.	O
</s>
<s>
To	O
support	O
further	O
scalability	O
,	O
NORM	O
also	O
employs	O
packet	O
erasure	O
coding	O
using	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
(	O
FEC	O
)	O
codes	O
coupled	O
with	O
suppression	O
of	O
redundant	O
NACK	O
feedback	O
from	O
the	O
receiver	O
group	O
.	O
</s>
<s>
Along	O
with	O
supporting	O
reliable	O
transport	O
,	O
NORM	O
also	O
provides	O
TCP-compatible	O
congestion	B-Protocol
control	I-Protocol
as	O
well	O
as	O
end-to-end	O
flow	O
control	O
.	O
</s>
<s>
Unlike	O
TCP	O
,	O
which	O
uses	O
the	O
ACK	O
mechanism	O
for	O
congestion	B-Protocol
control	I-Protocol
and	O
flow	O
control	O
,	O
NORM	O
uses	O
separate	O
mechanisms	O
for	O
each	O
.	O
</s>
<s>
Although	O
NORM	O
was	O
developed	O
primarily	O
to	O
support	O
multicast	O
group	O
communication	O
,	O
it	O
also	O
supports	O
unicast	B-Protocol
(	O
point-to-point	O
)	O
data	O
transfers	O
.	O
</s>
<s>
In	O
the	O
TCP/IP	B-Protocol
network	I-Protocol
model	O
,	O
the	O
transport	B-Architecture
layer	I-Architecture
of	O
the	O
network	O
(	O
Layer	B-Architecture
4	I-Architecture
)	O
is	O
responsible	O
for	O
the	O
reliable	O
transport	O
of	O
data	O
.	O
</s>
<s>
The	O
TCP	O
protocol	O
is	O
the	O
principal	O
means	O
of	O
ensuring	O
reliable	O
unicast	B-Protocol
(	O
point-to-point	O
)	O
transport	O
.	O
</s>
<s>
MDP	O
also	O
used	O
packet-level	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
coding	O
concepts	O
as	O
a	O
repair	O
mechanism	O
.	O
</s>
<s>
In	O
July	O
2005	O
the	O
NACK-based	O
protocol	O
building	O
blocks	O
and	O
protocol	O
instantiation	O
were	O
submitted	O
as	O
“	O
Experimental	O
”	O
in	O
RFC	O
3940	O
,	O
and	O
in	O
November	O
2009	O
“	O
NACK-Oriented	B-Protocol
Reliable	I-Protocol
Multicast	I-Protocol
(	O
NORM	O
)	O
Transport	B-Architecture
Protocol	I-Architecture
”	O
was	O
approved	O
in	O
RFC	O
5740	O
.	O
</s>
<s>
Typically	O
a	O
NORM	O
session	O
takes	O
place	O
using	O
pre-determined	O
IP	B-Protocol
addresses	I-Protocol
and	O
port	B-Protocol
numbers	I-Protocol
,	O
which	O
are	O
usually	O
associated	O
with	O
IP	B-Protocol
multicast	I-Protocol
group	I-Protocol
addresses	I-Protocol
that	O
are	O
determined	O
prior	O
to	O
the	O
session	O
.	O
</s>
<s>
These	O
addresses	O
may	O
be	O
determined	O
using	O
other	O
protocols	O
,	O
such	O
as	O
the	O
Session	B-Language
Description	I-Language
Protocol	I-Language
(	O
SDP	O
)	O
[	O
RFC	O
4566 ]	O
or	O
the	O
Session	B-Protocol
Announcement	I-Protocol
Protocol	I-Protocol
(	O
SAP	O
)	O
[	O
RFC	O
2974 ]	O
.	O
</s>
<s>
Although	O
NORM	O
was	O
specifically	O
designed	O
for	O
multicast	O
group	O
communication	O
,	O
it	O
supports	O
unicast	B-Protocol
communication	O
,	O
also	O
.	O
</s>
<s>
An	O
optional	O
extension	O
field	O
that	O
specifies	O
the	O
error	O
correction	O
encoding	O
being	O
used	O
,	O
the	O
congestion	B-Protocol
control	I-Protocol
algorithm	O
,	O
or	O
other	O
session	O
management	O
information	O
,	O
can	O
be	O
inserted	O
between	O
the	O
header	O
and	O
payload	O
sections	O
.	O
</s>
<s>
NORM_INFO	O
messages	O
,	O
defined	O
in	O
RFC	O
5740	O
Section	O
4.2.2	O
,	O
allow	O
optional	O
,	O
out-of-band	B-Protocol
data	I-Protocol
to	O
be	O
sent	O
in	O
association	O
with	O
the	O
data	O
content	O
objects	O
.	O
</s>
<s>
These	O
messages	O
serve	O
to	O
collect	O
round-trip	O
timing	O
,	O
gather	O
and	O
send	O
data	O
related	O
to	O
congestion	B-Protocol
control	I-Protocol
,	O
synchronize	O
repair	O
windows	O
,	O
and	O
make	O
notifications	O
of	O
a	O
sender	O
's	O
status	O
.	O
</s>
<s>
REPAIR_ADV	O
:	O
Advertises	O
sender	O
's	O
aggregated	O
repair/feedback	O
state	O
for	O
suppression	O
of	O
unicast	B-Protocol
feedback	O
from	O
receivers	O
.	O
</s>
<s>
Additionally	O
,	O
these	O
messages	O
contain	O
fields	O
to	O
provide	O
information	O
to	O
the	O
sender(s )	O
related	O
to	O
round-trip	O
timing	O
collection	O
and	O
congestion	B-Protocol
control	I-Protocol
.	O
</s>
<s>
NORM_ACK	O
messages	O
,	O
defined	O
in	O
RFC	O
5740	O
4.3.2	O
,	O
are	O
used	O
primarily	O
to	O
support	O
congestion	B-Protocol
control	I-Protocol
operations	O
and	O
round-trip	O
timing	O
measurements	O
.	O
</s>
<s>
Header	O
extensions	O
typically	O
carry	O
information	O
related	O
to	O
baseline	O
FEC	O
,	O
congestion	B-Protocol
control	I-Protocol
operations	O
,	O
and	O
other	O
session	O
management	O
information	O
.	O
</s>
<s>
At	O
the	O
same	O
time	O
,	O
the	O
sender	O
periodically	O
transmits	O
NORM_CMD(CC )	O
messages	O
as	O
needed	O
to	O
collect	O
feedback	O
information	O
necessary	O
for	O
congestion	B-Protocol
control	I-Protocol
and	O
other	O
session	O
management	O
activities	O
.	O
</s>
<s>
Within	O
the	O
context	O
of	O
general	O
operations	O
,	O
NORM	O
has	O
specific	O
mechanisms	O
to	O
address	O
session	O
control	O
,	O
congestion	B-Protocol
control	I-Protocol
,	O
flow	O
control	O
,	O
reliability	O
,	O
and	O
NACK	O
management	O
.	O
</s>
<s>
Although	O
NORM	O
is	O
not	O
a	O
connection-oriented	B-Protocol
protocol	I-Protocol
,	O
like	O
TCP	O
,	O
it	O
does	O
have	O
both	O
connection-oriented	B-Protocol
and	O
connection-less	O
characteristics	O
.	O
</s>
<s>
Unlike	O
the	O
connection-oriented	B-Protocol
TCP	O
protocol	O
,	O
NORM	O
does	O
not	O
establish	O
a	O
session	O
with	O
a	O
setup	O
mechanism	O
(	O
i.e.	O
,	O
a	O
three-way	O
handshake	O
)	O
.	O
</s>
<s>
Rather	O
,	O
NORM	O
uses	O
predesignated	O
port	B-Protocol
numbers	I-Protocol
and	O
addresses	O
that	O
senders	O
and	O
receivers	O
must	O
know	O
.	O
</s>
<s>
NORM	O
uses	O
a	O
TCP-Friendly	O
congestion	B-Protocol
control	I-Protocol
scheme	O
that	O
enables	O
it	O
to	O
fairly	O
share	O
available	O
bandwidth	O
with	O
TCP	O
and	O
other	O
transport	B-Architecture
protocols	I-Architecture
.	O
</s>
<s>
Specifically	O
,	O
NORM	O
's	O
congestion	B-Protocol
control	I-Protocol
scheme	O
uses	O
a	O
rate	O
control	O
procedure	O
,	O
and	O
it	O
is	O
an	O
adaptation	O
of	O
the	O
TCP-Friendly	O
Multicast	O
Congestion	B-Protocol
Control	I-Protocol
 [ TFMCC ] 	O
Protocol	O
Specification	O
approach	O
described	O
in	O
RFC4654	O
.	O
</s>
<s>
The	O
TFMCC	O
congestion	B-Protocol
control	I-Protocol
approach	O
is	O
an	O
equation-based	O
approach	O
.	O
</s>
<s>
As	O
with	O
TCP	O
,	O
the	O
NORM	O
congestion	B-Protocol
control	I-Protocol
scheme	O
assumes	O
packet	O
loss	O
is	O
due	O
to	O
buffer	O
overflows	O
,	O
and	O
the	O
sender	O
consequently	O
backs	O
off	O
it	O
transmission	O
rate	O
when	O
packet	O
loss	O
reaches	O
a	O
specified	O
level	O
.	O
</s>
<s>
NORM	O
ensures	O
reliable	O
transport	O
through	O
the	O
use	O
of	O
two	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
coding	O
schemes	O
:	O
systematic	O
codes	O
and	O
non-systematic	O
codes	O
.	O
</s>
