<s>
Retransmission	B-Protocol
,	O
essentially	O
identical	O
with	O
automatic	B-Error_Name
repeat	I-Error_Name
request	I-Error_Name
(	O
ARQ	O
)	O
,	O
is	O
the	O
resending	O
of	O
packets	B-Protocol
which	O
have	O
been	O
either	O
damaged	O
or	O
lost	O
.	O
</s>
<s>
Retransmission	B-Protocol
is	O
one	O
of	O
the	O
basic	O
mechanisms	O
used	O
by	O
protocols	O
operating	O
over	O
a	O
packet	B-Protocol
switched	I-Protocol
computer	B-Architecture
network	I-Architecture
to	O
provide	O
reliable	O
communication	O
(	O
such	O
as	O
that	O
provided	O
by	O
a	O
reliable	O
byte	O
stream	O
,	O
for	O
example	O
TCP	B-Protocol
)	O
.	O
</s>
<s>
Such	O
networks	O
are	O
usually	O
"	O
unreliable	O
"	O
,	O
meaning	O
they	O
offer	O
no	O
guarantees	O
that	O
they	O
will	O
not	O
delay	O
,	O
damage	O
,	O
or	O
lose	O
packets	B-Protocol
,	O
or	O
deliver	O
them	O
out	O
of	O
order	O
.	O
</s>
<s>
Protocols	O
which	O
provide	O
reliable	O
communication	O
over	O
such	O
networks	O
use	O
a	O
combination	O
of	O
acknowledgments	B-Protocol
(	O
i.e.	O
</s>
<s>
an	O
explicit	O
receipt	O
from	O
the	O
destination	O
of	O
the	O
data	O
)	O
,	O
retransmission	B-Protocol
of	O
missing	O
or	O
damaged	O
packets	B-Protocol
(	O
usually	O
initiated	O
by	O
a	O
time-out	O
)	O
,	O
and	O
checksums	B-Algorithm
to	O
provide	O
that	O
reliability	O
.	O
</s>
<s>
There	O
are	O
several	O
forms	O
of	O
acknowledgement	B-Protocol
which	O
can	O
be	O
used	O
alone	O
or	O
together	O
in	O
networking	O
protocols	O
:	O
</s>
<s>
Positive	O
Acknowledgement	B-Protocol
:	O
the	O
receiver	O
explicitly	O
notifies	O
the	O
sender	O
which	O
packets	B-Protocol
,	O
messages	O
,	O
or	O
segments	O
were	O
received	O
correctly	O
.	O
</s>
<s>
Positive	O
Acknowledgement	B-Protocol
therefore	O
also	O
implicitly	O
informs	O
the	O
sender	O
which	O
packets	B-Protocol
were	O
not	O
received	O
and	O
provides	O
detail	O
on	O
packets	B-Protocol
which	O
need	O
to	O
be	O
retransmitted	O
.	O
</s>
<s>
Negative	O
Acknowledgment	B-Protocol
(	O
NACK	B-Protocol
)	O
:	O
the	O
receiver	O
explicitly	O
notifies	O
the	O
sender	O
which	O
packets	B-Protocol
,	O
messages	O
,	O
or	O
segments	O
were	O
received	O
incorrectly	O
and	O
thus	O
may	O
need	O
to	O
be	O
retransmitted	O
(	O
RFC	O
4077	O
)	O
.	O
</s>
<s>
Selective	O
Acknowledgment	B-Protocol
(	O
SACK	O
)	O
:	O
the	O
receiver	O
explicitly	O
lists	O
which	O
packets	B-Protocol
,	O
messages	O
,	O
or	O
segments	O
in	O
a	O
stream	O
are	O
acknowledged	O
(	O
either	O
negatively	O
or	O
positively	O
)	O
.	O
</s>
<s>
Positive	O
selective	O
acknowledgment	B-Protocol
is	O
an	O
option	O
in	O
TCP	B-Protocol
(	O
RFC	O
2018	O
)	O
that	O
is	O
useful	O
in	O
Satellite	O
Internet	O
access	O
(	O
RFC	O
2488	O
)	O
.	O
</s>
<s>
Cumulative	O
Acknowledgment	B-Protocol
:	O
the	O
receiver	O
acknowledges	O
that	O
it	O
correctly	O
received	O
a	O
packet	B-Protocol
,	O
message	O
,	O
or	O
segment	O
in	O
a	O
stream	O
which	O
implicitly	O
informs	O
the	O
sender	O
that	O
the	O
previous	O
packets	B-Protocol
were	O
received	O
correctly	O
.	O
</s>
<s>
TCP	B-Protocol
uses	O
cumulative	O
acknowledgment	B-Protocol
with	O
its	O
TCP	B-Protocol
sliding	O
window	O
.	O
</s>
<s>
Retransmission	B-Protocol
is	O
a	O
very	O
simple	O
concept	O
.	O
</s>
