<s>
In	O
data	B-Architecture
networking	I-Architecture
,	O
telecommunications	O
,	O
and	O
computer	B-General_Concept
buses	I-General_Concept
,	O
an	O
acknowledgment	B-Protocol
(	O
ACK	B-Protocol
)	O
is	O
a	O
signal	O
that	O
is	O
passed	O
between	O
communicating	O
processes	B-Operating_System
,	O
computers	O
,	O
or	O
devices	O
to	O
signify	O
acknowledgment	B-Protocol
,	O
or	O
receipt	O
of	O
message	O
,	O
as	O
part	O
of	O
a	O
communications	O
protocol	O
.	O
</s>
<s>
The	O
negative-acknowledgement	O
(	O
NAK	B-Protocol
or	O
NACK	B-Protocol
)	O
is	O
a	O
signal	O
that	O
is	O
sent	O
to	O
reject	O
a	O
previously	O
received	O
message	O
or	O
to	O
indicate	O
some	O
kind	O
of	O
error	O
.	O
</s>
<s>
Acknowledgments	B-Protocol
and	O
negative	O
acknowledgments	B-Protocol
inform	O
a	O
sender	O
of	O
the	O
receiver	O
's	O
state	O
so	O
that	O
it	O
can	O
adjust	O
its	O
own	O
state	O
accordingly	O
.	O
</s>
<s>
Many	O
protocols	O
contain	O
checksums	B-Algorithm
to	O
verify	O
the	O
integrity	O
of	O
the	O
payload	O
and	O
header	B-General_Concept
.	O
</s>
<s>
Checksums	B-Algorithm
are	O
used	O
to	O
detect	O
data	O
corruption	O
.	O
</s>
<s>
If	O
a	O
message	O
is	O
received	O
with	O
an	O
invalid	O
checksum	B-Algorithm
(	O
that	O
is	O
,	O
the	O
data	O
received	O
would	O
have	O
a	O
different	O
checksum	B-Algorithm
than	O
the	O
message	O
had	O
)	O
,	O
the	O
receiver	O
can	O
know	O
that	O
some	O
information	O
was	O
corrupted	O
.	O
</s>
<s>
Most	O
often	O
,	O
when	O
checksums	B-Algorithm
are	O
employed	O
,	O
a	O
corrupted	O
message	O
received	O
will	O
either	O
not	O
be	O
served	O
an	O
ACK	B-Protocol
signal	O
,	O
or	O
will	O
be	O
served	O
a	O
NAK	B-Protocol
signal	O
.	O
</s>
<s>
ASCII	B-Protocol
code	I-Protocol
includes	O
an	O
ACK	B-Protocol
character	O
(	O
00001102	O
or	O
616	O
)	O
which	O
can	O
be	O
transmitted	O
to	O
indicate	O
successful	O
receipt	O
and	O
a	O
NAK	B-Protocol
character	O
(	O
00101012	O
or	O
1516	O
)	O
which	O
can	O
be	O
transmitted	O
to	O
indicate	O
an	O
inability	O
or	O
failure	O
to	O
receive	O
.	O
</s>
<s>
Unicode	O
provides	O
visible	O
symbols	O
for	O
these	O
characters	O
,	O
U+2406	O
( ␆	O
)	O
and	O
U+2415	O
( ␕	O
)	O
.	O
</s>
<s>
Many	O
protocols	O
are	O
acknowledgement-based	O
,	O
meaning	O
that	O
they	O
positively	O
acknowledge	O
receipt	O
of	O
messages	O
.	O
</s>
<s>
The	O
internet	O
's	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
is	O
an	O
example	O
of	O
an	O
acknowledgement-based	O
protocol	O
.	O
</s>
<s>
When	O
computers	O
communicate	O
via	O
TCP	O
,	O
received	O
packets	B-Protocol
are	O
acknowledged	O
by	O
sending	O
back	O
a	O
packet	B-Protocol
with	O
an	O
ACK	B-Protocol
bit	O
set	O
.	O
</s>
<s>
The	O
TCP	B-Protocol
protocol	I-Protocol
allows	O
these	O
acknowledgements	B-Protocol
to	O
be	O
included	O
with	O
data	O
that	O
is	O
sent	O
in	O
the	O
opposite	O
direction	O
.	O
</s>
<s>
Some	O
protocols	O
send	O
a	O
single	O
acknowledgement	B-Protocol
per	O
packet	B-Protocol
of	O
information	O
.	O
</s>
<s>
Other	O
protocols	O
such	O
as	O
TCP	O
and	O
ZMODEM	B-Device
allow	O
many	O
packets	B-Protocol
to	O
be	O
transmitted	O
before	O
receiving	O
acknowledgement	B-Protocol
for	O
any	O
of	O
them	O
,	O
a	O
procedure	O
necessary	O
to	O
fill	O
high	O
bandwidth-delay	O
product	O
links	O
with	O
a	O
large	O
number	O
of	O
bytes	O
in	O
flight	O
.	O
</s>
<s>
Other	O
protocols	O
are	O
NAK-based	O
,	O
meaning	O
that	O
they	O
only	O
respond	O
to	O
messages	O
if	O
there	O
is	O
a	O
problem	O
.	O
</s>
<s>
Examples	O
include	O
most	O
reliable	B-Operating_System
multicast	I-Operating_System
protocols	O
which	O
send	O
a	O
NAK	B-Protocol
when	O
the	O
receiver	O
detects	O
missing	O
packets	B-Protocol
.	O
</s>
<s>
Still	O
other	O
protocols	O
make	O
use	O
of	O
both	O
NAKs	B-Protocol
and	O
ACKs	B-Protocol
.	O
</s>
<s>
Binary	B-Protocol
Synchronous	I-Protocol
Communications	I-Protocol
(	O
Bisync	B-Protocol
)	O
and	O
Adaptive	O
Link	O
Rate	O
(	O
for	O
Energy-Efficient	O
Ethernet	O
)	O
are	O
examples	O
.	O
</s>
<s>
Still	O
other	O
protocols	O
such	O
as	O
the	O
RC-5	O
,	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
,	O
and	O
X10	O
protocols	O
perform	O
blind	O
transmission	O
with	O
no	O
acknowledgement	B-Protocol
,	O
often	O
transmitting	O
the	O
same	O
message	O
multiple	O
times	O
in	O
hopes	O
that	O
at	O
least	O
one	O
copy	O
of	O
the	O
message	O
gets	O
through	O
.	O
</s>
<s>
The	O
acknowledgement	B-Protocol
function	O
is	O
used	O
in	O
the	O
automatic	B-Error_Name
repeat	I-Error_Name
request	I-Error_Name
(	O
ARQ	O
)	O
function	O
.	O
</s>
<s>
Acknowledgement	B-Protocol
frames	O
are	O
numbered	O
in	O
coordination	O
with	O
the	O
frames	O
that	O
have	O
been	O
received	O
and	O
then	O
sent	O
to	O
the	O
transmitter	O
.	O
</s>
<s>
This	O
allows	O
the	O
transmitter	O
to	O
avoid	O
overflow	B-General_Concept
or	O
underrun	O
at	O
the	O
receiver	O
,	O
and	O
to	O
become	O
aware	O
of	O
any	O
missed	O
frames	O
.	O
</s>
<s>
In	O
Binary	B-Protocol
Synchronous	I-Protocol
Communications	I-Protocol
,	O
the	O
NAK	B-Protocol
is	O
used	O
to	O
indicate	O
that	O
a	O
transmission	O
error	O
was	O
detected	O
in	O
the	O
previously	O
received	O
block	O
and	O
that	O
the	O
receiver	O
is	O
ready	O
to	O
accept	O
retransmission	B-Protocol
of	O
that	O
block	O
.	O
</s>
<s>
Bisync	B-Protocol
does	O
not	O
use	O
a	O
single	O
ACK	B-Protocol
character	O
but	O
has	O
two	O
control	O
sequences	O
for	O
alternate	O
even/odd	O
block	O
acknowledgement	B-Protocol
.	O
</s>
<s>
Some	O
computer	B-General_Concept
buses	I-General_Concept
have	O
a	O
dedicated	O
acknowledge	O
wire	O
in	O
the	O
control	B-Architecture
bus	I-Architecture
used	O
to	O
acknowledge	O
bus	O
operations	O
:	O
DACK	O
used	O
for	O
ISA	O
DMA	O
;	O
DATACK	O
used	O
in	O
the	O
STEbus	B-Architecture
,	O
the	O
data	O
transfer	O
acknowledge	O
pin	O
of	O
the	O
Motorola	B-Device
68000	I-Device
that	O
inspired	O
the	O
title	O
of	O
DTACK	B-Architecture
Grounded	I-Architecture
,	O
etc	O
.	O
</s>
<s>
Some	O
computer	B-General_Concept
buses	I-General_Concept
do	O
not	O
always	O
acknowledge	O
every	O
write	O
;	O
some	O
or	O
all	O
of	O
the	O
writes	O
use	O
a	O
posted	B-Architecture
write	I-Architecture
.	O
</s>
<s>
The	O
I²C	O
serial	O
bus	O
has	O
a	O
time	O
slot	O
for	O
an	O
acknowledgment	B-Protocol
bit	O
after	O
each	O
byte	O
.	O
</s>
