<s>
The	O
RTP	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
RTCP	B-Protocol
)	O
is	O
a	O
sister	O
protocol	O
of	O
the	O
Real-time	B-Protocol
Transport	I-Protocol
Protocol	I-Protocol
(	O
RTP	O
)	O
.	O
</s>
<s>
RTCP	B-Protocol
provides	O
out-of-band	O
statistics	O
and	O
control	O
information	O
for	O
an	O
RTP	O
session	O
.	O
</s>
<s>
The	O
primary	O
function	O
of	O
RTCP	B-Protocol
is	O
to	O
provide	O
feedback	O
on	O
the	O
quality	O
of	O
service	O
(	O
QoS	O
)	O
in	O
media	O
distribution	O
by	O
periodically	O
sending	O
statistics	O
information	O
such	O
as	O
transmitted	O
octet	O
and	O
packet	O
counts	O
,	O
packet	B-Protocol
loss	I-Protocol
,	O
packet	B-Architecture
delay	I-Architecture
variation	I-Architecture
,	O
and	O
round-trip	O
delay	O
time	O
to	O
participants	O
in	O
a	O
streaming	O
multimedia	O
session	O
.	O
</s>
<s>
An	O
application	O
may	O
use	O
this	O
information	O
to	O
control	O
quality	O
of	O
service	O
parameters	O
,	O
perhaps	O
by	O
limiting	O
flow	O
,	O
or	O
using	O
a	O
different	O
codec	B-General_Concept
.	O
</s>
<s>
Typically	O
RTP	O
will	O
be	O
sent	O
on	O
an	O
even-numbered	O
UDP	B-Protocol
port	O
,	O
with	O
RTCP	B-Protocol
messages	O
being	O
sent	O
over	O
the	O
next	O
higher	O
odd-numbered	O
port	O
.	O
</s>
<s>
RTCP	B-Protocol
itself	O
does	O
not	O
provide	O
any	O
flow	O
encryption	O
or	O
authentication	O
methods	O
.	O
</s>
<s>
Such	O
mechanisms	O
may	O
be	O
implemented	O
,	O
for	O
example	O
,	O
with	O
the	O
Secure	B-Protocol
Real-time	I-Protocol
Transport	I-Protocol
Protocol	I-Protocol
(	O
SRTP	O
)	O
defined	O
in	O
RFC	O
3711	O
.	O
</s>
<s>
RTCP	B-Protocol
provides	O
basic	O
functions	O
expected	O
to	O
be	O
implemented	O
in	O
all	O
RTP	O
sessions	O
:	O
</s>
<s>
The	O
primary	O
function	O
of	O
RTCP	B-Protocol
is	O
to	O
gather	O
statistics	O
on	O
quality	O
aspects	O
of	O
the	O
media	O
distribution	O
during	O
a	O
session	O
and	O
transmit	O
this	O
data	O
to	O
the	O
session	O
media	O
source	O
and	O
other	O
session	O
participants	O
.	O
</s>
<s>
Such	O
information	O
may	O
be	O
used	O
by	O
the	O
source	O
for	O
adaptive	O
media	O
encoding	O
(	O
codec	B-General_Concept
)	O
and	O
detection	O
of	O
transmission	O
faults	O
.	O
</s>
<s>
RTCP	B-Protocol
provides	O
canonical	O
end-point	O
identifiers	O
(	O
CNAME	O
)	O
to	O
all	O
session	O
participants	O
.	O
</s>
<s>
RTCP	B-Protocol
is	O
a	O
convenient	O
means	O
to	O
reach	O
all	O
session	O
participants	O
,	O
whereas	O
RTP	O
itself	O
is	O
not	O
.	O
</s>
<s>
RTCP	B-Protocol
reports	O
are	O
expected	O
to	O
be	O
sent	O
by	O
all	O
participants	O
,	O
even	O
in	O
a	O
multicast	O
session	O
which	O
may	O
involve	O
thousands	O
of	O
recipients	O
.	O
</s>
<s>
RTCP	B-Protocol
bandwidth	O
usage	O
should	O
generally	O
not	O
exceed	O
5%	O
of	O
the	O
total	O
session	O
bandwidth	O
.	O
</s>
<s>
Furthermore	O
,	O
25%	O
of	O
the	O
RTCP	B-Protocol
bandwidth	O
should	O
be	O
reserved	O
to	O
media	O
sources	O
at	O
all	O
times	O
,	O
so	O
that	O
in	O
large	O
conferences	O
new	O
participants	O
can	O
receive	O
the	O
CNAME	O
identifiers	O
of	O
the	O
senders	O
without	O
excessive	O
delay	O
.	O
</s>
<s>
The	O
RTCP	B-Protocol
reporting	O
interval	O
is	O
randomized	O
to	O
prevent	O
unintended	O
synchronization	O
of	O
reporting	O
.	O
</s>
<s>
The	O
recommended	O
minimum	O
RTCP	B-Protocol
report	O
interval	O
per	O
station	O
is	O
5	O
seconds	O
.	O
</s>
<s>
Stations	O
should	O
not	O
transmit	O
RTCP	B-Protocol
reports	O
more	O
often	O
than	O
once	O
every	O
5	O
seconds	O
.	O
</s>
<s>
Version	O
:	O
(	O
2	O
bits	O
)	O
Identifies	O
the	O
version	O
of	O
RTP	O
,	O
which	O
is	O
the	O
same	O
in	O
RTCP	B-Protocol
packets	O
as	O
in	O
RTP	O
data	O
packets	O
.	O
</s>
<s>
PT	O
(	O
Packet	O
type	O
)	O
:	O
(	O
8	O
bits	O
)	O
Contains	O
a	O
constant	O
to	O
identify	O
RTCP	B-Protocol
packet	O
type	O
.	O
</s>
<s>
Length	O
:	O
(	O
16	O
bits	O
)	O
Indicates	O
the	O
length	O
of	O
this	O
RTCP	B-Protocol
packet	O
(	O
including	O
the	O
header	O
itself	O
)	O
in	O
32-bit	O
units	O
minus	O
one	O
.	O
</s>
<s>
Note	O
that	O
multiple	O
reports	O
can	O
be	O
concatenated	O
into	O
a	O
single	O
compound	O
RTCP	B-Protocol
packet	O
,	O
each	O
with	O
its	O
own	O
packet	O
header	O
.	O
</s>
<s>
RTCP	B-Protocol
distinguishes	O
several	O
types	O
of	O
packets	O
:	O
sender	O
report	O
,	O
receiver	O
report	O
,	O
source	O
description	O
,	O
and	O
goodbye	O
.	O
</s>
<s>
In	O
addition	O
,	O
the	O
protocol	O
is	O
extensible	O
and	O
allows	O
application-specific	O
RTCP	B-Protocol
packets	O
.	O
</s>
<s>
A	O
standards-based	O
extension	O
of	O
RTCP	B-Protocol
is	O
the	O
extended	O
report	O
packet	O
type	O
introduced	O
by	O
RFC	O
3611	O
.	O
</s>
<s>
Application-specific	O
message	O
(	O
APP	O
)	O
The	O
application-specific	O
message	O
provides	O
a	O
mechanism	O
to	O
design	O
application-specific	O
extensions	O
to	O
the	O
RTCP	B-Protocol
protocol	O
.	O
</s>
<s>
In	O
large-scale	O
applications	O
,	O
such	O
as	O
in	O
Internet	O
Protocol	O
television	O
(	O
IPTV	O
)	O
,	O
very	O
long	O
delays	O
(	O
minutes	O
to	O
hours	O
)	O
between	O
RTCP	B-Protocol
reports	O
may	O
occur	O
,	O
because	O
of	O
the	O
RTCP	B-Protocol
bandwidth	O
control	O
mechanism	O
required	O
to	O
control	O
congestion	O
(	O
see	O
Protocol	O
functions	O
)	O
.	O
</s>
<s>
Methods	O
have	O
been	O
introduced	O
to	O
alleviate	O
the	O
problems	O
:	O
RTCP	B-Protocol
filtering	O
,	O
RTCP	B-Protocol
biasing	O
and	O
hierarchical	O
aggregation	O
.	O
</s>
<s>
The	O
Hierarchical	O
Aggregation	O
(	O
or	O
also	O
known	O
as	O
RTCP	B-Protocol
feedback	O
hierarchy	O
)	O
is	O
an	O
optimization	O
of	O
the	O
RTCP	B-Protocol
feedback	O
model	O
and	O
its	O
aim	O
is	O
to	O
shift	O
the	O
maximum	O
number	O
of	O
users	O
limit	O
further	O
together	O
with	O
quality	O
of	O
service	O
(	O
QoS	O
)	O
measurement	O
.	O
</s>
<s>
The	O
RTCP	B-Protocol
bandwidth	O
is	O
constant	O
and	O
takes	O
just	O
5%	O
of	O
session	O
bandwidth	O
.	O
</s>
<s>
The	O
Hierarchical	O
Aggregation	O
is	O
used	O
with	O
Source-Specific	B-Protocol
Multicast	I-Protocol
where	O
only	O
a	O
single	O
source	O
is	O
allowed	O
,	O
i.e.	O
</s>
<s>
Another	O
type	O
of	O
multicast	O
could	O
be	O
Any-Source	B-Protocol
Multicast	I-Protocol
but	O
it	O
is	O
not	O
so	O
suitable	O
for	O
large-scale	O
applications	O
with	O
huge	O
number	O
of	O
users	O
.	O
</s>
<s>
The	O
function	O
of	O
this	O
member	O
is	O
to	O
receive	O
Receiver	O
Reports	O
(	O
RR	O
)	O
(	O
see	O
RTCP	B-Protocol
)	O
and	O
retransmit	O
summarized	O
RR	O
packets	O
,	O
so-called	O
Receiver	O
Summary	O
Information	O
(	O
RSI	O
)	O
to	O
a	O
sender	O
(	O
in	O
case	O
of	O
single-level	O
hierarchy	O
)	O
.	O
</s>
