<s>
Alternating	B-Protocol
bit	I-Protocol
protocol	I-Protocol
(	O
ABP	O
)	O
is	O
a	O
simple	O
network	O
protocol	O
operating	O
at	O
the	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
(	O
OSI	B-Protocol
layer	I-Protocol
2	I-Protocol
)	O
that	O
retransmits	O
lost	O
or	O
corrupted	O
messages	O
using	O
FIFO	O
semantics	O
.	O
</s>
<s>
When	O
A	O
sends	O
a	O
message	O
,	O
it	O
resends	O
it	O
continuously	O
,	O
with	O
the	O
same	O
sequence	O
number	O
,	O
until	O
it	O
receives	O
an	O
acknowledgment	B-Protocol
from	O
B	O
that	O
contains	O
the	O
same	O
sequence	O
number	O
.	O
</s>
<s>
It	O
treats	O
such	O
messages	O
as	O
negative-acknowledge	B-Protocol
codes	O
(	O
NAKs	B-Protocol
)	O
.	O
</s>
<s>
The	O
protocol	O
may	O
be	O
initialized	O
by	O
sending	O
bogus	O
messages	O
and	O
acks	B-Protocol
with	O
sequence	O
number	O
1	O
.	O
</s>
<s>
Bounded	B-Protocol
Retransmission	I-Protocol
Protocol	I-Protocol
(	O
BRP	O
)	O
is	O
a	O
variant	O
of	O
the	O
alternating	B-Protocol
bit	I-Protocol
protocol	I-Protocol
introduced	O
by	O
Philips	O
.	O
</s>
<s>
Donald	O
Davies	O
 '	O
team	O
at	O
the	O
National	O
Physical	O
Laboratory	O
introduced	O
the	O
concept	O
of	O
an	O
alternating	B-Protocol
bit	I-Protocol
protocol	I-Protocol
in	O
1968	O
for	O
the	O
NPL	B-Protocol
network	I-Protocol
.	O
</s>
