<s>
The	O
Interface	B-Protocol
Message	I-Protocol
Processor	I-Protocol
(	O
IMP	O
)	O
was	O
the	O
packet	B-Protocol
switching	I-Protocol
node	B-Protocol
used	O
to	O
interconnect	O
participant	O
networks	O
to	O
the	O
ARPANET	O
from	O
the	O
late	O
1960s	O
to	O
1989	O
.	O
</s>
<s>
It	O
was	O
the	O
first	O
generation	O
of	O
gateways	B-Application
,	O
which	O
are	O
known	O
today	O
as	O
routers	B-Protocol
.	O
</s>
<s>
An	O
IMP	O
was	O
a	O
ruggedized	B-Device
Honeywell	O
DDP-516	B-Device
minicomputer	B-Architecture
with	O
special-purpose	O
interfaces	O
and	O
software	O
.	O
</s>
<s>
In	O
later	O
years	O
the	O
IMPs	O
were	O
made	O
from	O
the	O
non-ruggedized	O
Honeywell	B-Device
316	I-Device
which	O
could	O
handle	O
two-thirds	O
of	O
the	O
communication	O
traffic	O
at	O
approximately	O
one-half	O
the	O
cost	O
.	O
</s>
<s>
An	O
IMP	O
requires	O
the	O
connection	O
to	O
a	O
host	O
computer	O
via	O
a	O
special	O
bit-serial	B-Protocol
interface	O
,	O
defined	O
in	O
BBN	B-Protocol
Report	I-Protocol
1822	I-Protocol
.	O
</s>
<s>
The	O
concept	O
of	O
an	O
"	O
Interface	O
computer	O
"	O
was	O
first	O
proposed	O
in	O
1966	O
by	O
Donald	O
Davies	O
for	O
the	O
NPL	B-Protocol
network	I-Protocol
in	O
England	O
.	O
</s>
<s>
BBN	O
began	O
programming	O
work	O
in	O
February	O
1969	O
on	O
modified	O
Honeywell	O
DDP-516s	B-Device
.	O
</s>
<s>
The	O
IMP	O
had	O
an	O
error-control	O
mechanism	O
that	O
discarded	O
packets	B-Protocol
with	O
errors	O
without	O
acknowledging	O
receipt	O
;	O
the	O
source	O
IMP	O
,	O
upon	O
not	O
receiving	O
an	O
acknowledging	O
receipt	O
,	O
would	O
subsequently	O
re-send	O
a	O
duplicate	O
packet	B-Protocol
.	O
</s>
<s>
Based	O
on	O
the	O
requirements	O
of	O
ARPA	O
's	O
request	O
for	O
proposal	O
,	O
the	O
IMP	O
used	O
a	O
24-bit	O
checksum	B-Algorithm
for	O
error	O
correction	O
.	O
</s>
<s>
BBN	O
chose	O
to	O
make	O
the	O
IMP	O
hardware	O
calculate	O
the	O
checksum	B-Algorithm
,	O
because	O
it	O
was	O
a	O
faster	O
option	O
than	O
using	O
a	O
software	O
calculation	O
.	O
</s>
<s>
It	O
used	O
an	O
SDS	B-Device
Sigma	I-Device
7	I-Device
host	O
computer	O
.	O
</s>
<s>
It	O
was	O
attached	O
to	O
an	O
SDS	B-Device
940	I-Device
host	O
.	O
</s>
<s>
According	O
to	O
a	O
report	O
filed	O
by	O
Heart	O
,	O
a	O
preliminary	O
test	O
in	O
late	O
1969	O
based	O
on	O
a	O
27-hour	O
period	O
of	O
activity	O
on	O
the	O
UCSB-SRI	O
line	O
found	O
"	O
approximately	O
one	O
packet	B-Protocol
per	O
20,000	O
in	O
error	O
;	O
"	O
subsequent	O
tests	O
"	O
uncovered	O
a	O
100%	O
variation	O
in	O
this	O
number	O
-	O
apparently	O
due	O
to	O
many	O
unusually	O
long	O
periods	O
of	O
time	O
(	O
on	O
the	O
order	O
of	O
hours	O
)	O
with	O
no	O
detected	O
errors.	O
"	O
</s>
<s>
A	O
variant	O
of	O
the	O
IMP	O
existed	O
,	O
called	O
the	O
TIP	O
,	O
which	O
connected	O
terminals	O
as	O
well	O
as	O
computers	O
to	O
the	O
network	O
;	O
it	O
was	O
based	O
on	O
the	O
Honeywell	B-Device
316	I-Device
,	O
a	O
later	O
version	O
of	O
the	O
516	O
.	O
</s>
<s>
Later	O
,	O
some	O
Honeywell-based	O
IMPs	O
were	O
replaced	O
with	O
multiprocessing	O
BBN	O
Pluribus	B-Device
IMPs	O
,	O
but	O
ultimately	O
BBN	O
developed	O
a	O
microprogrammed	B-Device
clone	O
of	O
the	O
Honeywell	O
machine	O
.	O
</s>
<s>
Most	O
IMPs	O
were	O
either	O
taken	O
apart	O
,	O
junked	O
or	O
transferred	O
to	O
MILNET	B-General_Concept
.	O
</s>
<s>
BBN	B-Protocol
Report	I-Protocol
1822	I-Protocol
specifies	O
the	O
method	O
for	O
connecting	O
a	O
host	O
computer	O
to	O
an	O
IMP	O
.	O
</s>
<s>
However	O
,	O
it	O
is	O
accurate	O
to	O
say	O
that	O
the	O
1822	O
protocol	O
incorporates	O
the	O
physical	B-Application
layer	I-Application
,	O
the	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
,	O
and	O
the	O
network	B-Application
layer	I-Application
.	O
</s>
<s>
The	O
interface	O
visible	O
to	O
the	O
host	O
system	O
passes	O
network	B-Application
layer	I-Application
addresses	O
directly	O
to	O
a	O
physical	B-Application
layer	I-Application
device	O
.	O
</s>
<s>
To	O
transmit	O
data	O
,	O
the	O
host	O
constructs	O
a	O
message	O
containing	O
the	O
numeric	O
address	O
of	O
another	O
host	O
on	O
the	O
network	O
(	O
similar	O
to	O
an	O
IP	B-Protocol
address	I-Protocol
on	O
the	O
Internet	O
)	O
and	O
a	O
data	O
field	O
,	O
and	O
transmits	O
the	O
message	O
across	O
the	O
1822	O
interface	O
to	O
the	O
IMP	O
.	O
</s>
<s>
The	O
IMP	O
routes	O
the	O
message	O
to	O
the	O
destination	O
host	O
using	O
protocols	O
that	O
were	O
eventually	O
adopted	O
by	O
Internet	B-Protocol
routers	I-Protocol
.	O
</s>
<s>
While	O
packets	B-Protocol
transmitted	O
across	O
the	O
Internet	O
are	O
assumed	O
to	O
be	O
unreliable	O
,	O
1822	O
messages	O
were	O
guaranteed	O
to	O
be	O
transmitted	O
reliably	O
to	O
the	O
addressed	O
destination	O
.	O
</s>
<s>
The	O
specification	O
incorporated	O
an	O
alternating	B-Protocol
bit	I-Protocol
protocol	I-Protocol
,	O
of	O
the	O
type	O
proposed	O
by	O
Donald	O
Davies	O
 '	O
team	O
for	O
the	O
NPL	B-Protocol
network	I-Protocol
in	O
1968	O
.	O
</s>
