<s>
Real	B-Protocol
Data	I-Protocol
Transport	I-Protocol
(	O
RDT	O
)	O
is	O
a	O
proprietary	O
transport	B-Architecture
protocol	I-Architecture
for	O
the	O
actual	O
audio-video	O
data	O
,	O
developed	O
by	O
RealNetworks	O
in	O
the	O
1990s	O
.	O
</s>
<s>
It	O
is	O
commonly	O
used	O
in	O
companion	O
with	O
a	O
control	O
protocol	O
for	O
streaming	O
media	O
like	O
the	O
IETF	O
's	O
Real	B-Protocol
Time	I-Protocol
Streaming	I-Protocol
Protocol	I-Protocol
(	O
RTSP	B-Protocol
)	O
.	O
</s>
<s>
A	O
non-proprietary	O
alternative	O
for	O
RDT	O
is	O
IETF	O
's	O
Real-time	B-Protocol
Transport	I-Protocol
Protocol	I-Protocol
(	O
RTP	O
)	O
,	O
which	O
is	O
also	O
implemented	O
in	O
RealNetworks	O
players	O
.	O
</s>
<s>
As	O
reported	O
in	O
a	O
2002	O
book	O
about	O
firewalls	O
,	O
RDT	O
used	O
two	O
unidirectional	O
UDP	B-Protocol
connections	O
,	O
one	O
for	O
the	O
data	O
sent	O
from	O
the	O
server	O
to	O
the	O
client	O
and	O
another	O
in	O
the	O
opposite	O
direction	O
for	O
retransmission	O
requests	O
.	O
</s>
<s>
Another	O
2003	O
book	O
reported	O
that	O
RDT	O
was	O
also	O
seen	O
carried	O
over	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
,	O
as	O
a	O
fall-back	O
mechanism	O
.	O
</s>
<s>
RDT	O
is	O
now	O
included	O
as	O
part	O
of	O
the	O
Helix	B-Application
Community	I-Application
project	O
.	O
</s>
