<s>
Xerox	B-Protocol
Network	I-Protocol
Systems	I-Protocol
(	O
XNS	O
)	O
is	O
a	O
computer	B-Architecture
networking	I-Architecture
protocol	B-Protocol
suite	I-Protocol
developed	O
by	O
Xerox	O
within	O
the	O
Xerox	B-Protocol
Network	I-Protocol
Systems	I-Protocol
Architecture	I-Protocol
.	O
</s>
<s>
It	O
provided	O
general	O
purpose	O
network	O
communications	O
,	O
internetwork	B-Architecture
routing	B-Protocol
and	O
packet	O
delivery	O
,	O
and	O
higher	O
level	O
functions	O
such	O
as	O
a	O
reliable	O
stream	O
,	O
and	O
remote	B-Operating_System
procedure	I-Operating_System
calls	I-Operating_System
.	O
</s>
<s>
XNS	O
predated	O
and	O
influenced	O
the	O
development	O
of	O
the	O
Open	O
Systems	O
Interconnection	O
(	O
OSI	O
)	O
networking	O
model	O
,	O
and	O
was	O
very	O
influential	O
in	O
local	B-General_Concept
area	I-General_Concept
networking	I-General_Concept
designs	O
during	O
the	O
1980s	O
.	O
</s>
<s>
XNS	O
was	O
based	O
on	O
the	O
earlier	O
(	O
and	O
equally	O
influential	O
)	O
PARC	B-Protocol
Universal	I-Protocol
Packet	I-Protocol
(	O
PUP	O
)	O
suite	O
from	O
the	O
late	O
1970s	O
.	O
</s>
<s>
XNS	O
added	O
the	O
concept	O
of	O
a	O
network	O
number	O
,	O
allowing	O
larger	O
networks	O
to	O
be	O
constructed	O
from	O
multiple	O
smaller	O
ones	O
,	O
with	O
routers	B-Protocol
controlling	O
the	O
flow	O
of	O
information	O
between	O
the	O
networks	O
.	O
</s>
<s>
The	O
protocol	B-Protocol
suite	I-Protocol
specifications	O
for	O
XNS	O
were	O
placed	O
in	O
the	O
public	O
domain	O
in	O
1977	O
.	O
</s>
<s>
This	O
helped	O
XNS	O
become	O
the	O
canonical	O
local	B-General_Concept
area	I-General_Concept
networking	I-General_Concept
protocol	O
,	O
copied	O
to	O
various	O
degrees	O
by	O
practically	O
all	O
networking	O
systems	O
in	O
use	O
into	O
the	O
1990s	O
.	O
</s>
<s>
XNS	O
was	O
used	O
unchanged	O
by	O
3Com	O
's	O
3+Share	B-General_Concept
and	O
Ungermann-Bass	O
'	O
s	O
Net/One	O
.	O
</s>
<s>
It	O
was	O
also	O
used	O
,	O
with	O
modifications	O
,	O
as	O
the	O
basis	O
for	O
Novell	B-Operating_System
NetWare	I-Operating_System
,	O
and	O
Banyan	B-Operating_System
VINES	I-Operating_System
.	O
</s>
<s>
XNS	O
was	O
used	O
as	O
the	O
basis	O
for	O
the	O
AppleNet	B-Protocol
system	O
,	O
but	O
this	O
was	O
never	O
commercialized	O
;	O
a	O
number	O
of	O
XNS	O
's	O
solutions	O
to	O
common	O
problems	O
were	O
used	O
in	O
AppleNet	B-Protocol
's	O
replacement	O
,	O
AppleTalk	B-Application
.	O
</s>
<s>
In	O
comparison	O
to	O
the	O
OSI	O
model	O
's	O
7	O
layers	O
,	O
XNS	O
is	O
a	O
five-layer	O
system	O
,	O
like	O
the	O
later	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
.	O
</s>
<s>
Specifically	O
,	O
XNS	O
's	O
Physical	O
layer	O
is	O
really	O
the	O
Ethernet	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
system	O
,	O
also	O
being	O
developed	O
by	O
Xerox	O
at	O
the	O
same	O
time	O
,	O
and	O
a	O
number	O
of	O
its	O
design	O
decisions	O
reflect	O
that	O
fact	O
.	O
</s>
<s>
The	O
primary	O
part	O
of	O
XNS	O
is	O
its	O
definition	O
of	O
the	O
Internal	O
Transport	O
layer	O
(	O
layer	O
1	O
)	O
,	O
which	O
corresponds	O
to	O
OSI	O
's	O
Network	O
layer	O
,	O
and	O
it	O
is	O
here	O
that	O
the	O
primary	O
internetworking	B-Architecture
protocol	O
,	O
IDP	O
,	O
is	O
defined	O
.	O
</s>
<s>
The	O
main	O
internetwork	B-Protocol
layer	I-Protocol
protocol	O
is	O
the	O
Internet	O
Datagram	O
Protocol	O
(	O
IDP	O
)	O
.	O
</s>
<s>
IDP	O
is	O
a	O
close	O
descendant	O
of	O
Pup	O
's	O
internetwork	B-Architecture
protocol	O
,	O
and	O
roughly	O
corresponds	O
to	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
layer	O
in	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
.	O
</s>
<s>
To	O
this	O
is	O
added	O
another	O
48-bit	O
address	O
section	O
provided	O
by	O
the	O
networking	O
equipment	O
;	O
32	O
bits	O
are	O
provided	O
by	O
routers	B-Protocol
to	O
identify	O
the	O
network	O
number	O
in	O
the	O
internetwork	B-Architecture
,	O
and	O
another	O
16	O
bits	O
define	O
a	O
socket	O
number	O
for	O
service	O
selection	O
within	O
a	O
single	O
host	O
.	O
</s>
<s>
Unlike	O
TCP/IP	B-Protocol
,	O
socket	O
numbers	O
are	O
part	O
of	O
the	O
full	O
network	B-Protocol
address	I-Protocol
in	O
the	O
IDP	O
header	B-General_Concept
,	O
so	O
that	O
upper-layer	O
protocols	O
do	O
not	O
need	O
to	O
implement	O
demultiplexing	O
;	O
IDP	O
also	O
supplies	O
packet	O
types	O
(	O
again	O
,	O
unlike	O
IP	O
)	O
.	O
</s>
<s>
This	O
reflects	O
the	O
fact	O
that	O
LANs	B-General_Concept
generally	O
have	O
low-error	O
rates	O
,	O
so	O
XNS	O
removed	O
error	O
correction	O
from	O
the	O
lower-level	O
protocols	O
in	O
order	O
to	O
improve	O
performance	O
.	O
</s>
<s>
Error	O
correction	O
could	O
be	O
optionally	O
added	O
at	O
higher	O
levels	O
in	O
the	O
protocol	B-Protocol
stack	I-Protocol
,	O
for	O
instance	O
,	O
in	O
XNS	O
's	O
own	O
SPP	O
protocol	O
.	O
</s>
<s>
In	O
keeping	O
with	O
the	O
low-latency	O
LAN	B-General_Concept
connections	O
it	O
runs	O
on	O
,	O
XNS	O
uses	O
a	O
short	O
packet	O
size	O
,	O
which	O
improves	O
performance	O
in	O
the	O
case	O
of	O
low	O
error	O
rates	O
and	O
short	O
turnaround	O
times	O
.	O
</s>
<s>
IDP	O
packets	O
are	O
up	O
to	O
576	O
bytes	O
long	O
,	O
including	O
the	O
30	O
byte	O
IDP	O
header	B-General_Concept
.	O
</s>
<s>
Individual	O
XNS	O
host	O
pairs	O
on	O
a	O
particular	O
network	O
might	O
use	O
larger	O
packets	O
,	O
but	O
no	O
XNS	O
router	B-Protocol
is	O
required	O
to	O
handle	O
them	O
,	O
and	O
no	O
mechanism	O
is	O
defined	O
to	O
discover	O
if	O
the	O
intervening	O
routers	B-Protocol
support	O
larger	O
packets	O
.	O
</s>
<s>
The	O
Routing	B-Protocol
Information	I-Protocol
Protocol	I-Protocol
(	O
RIP	O
)	O
,	O
a	O
descendant	O
of	O
Pup	O
's	O
Gateway	O
Information	O
Protocol	O
,	O
is	O
used	O
as	O
the	O
router	B-Protocol
information-exchange	O
system	O
,	O
and	O
(	O
slightly	O
modified	O
to	O
match	O
the	O
syntax	O
of	O
addresses	O
of	O
other	O
protocol	B-Protocol
suites	I-Protocol
)	O
,	O
remains	O
in	O
use	O
today	O
in	O
other	O
protocol	B-Protocol
suites	I-Protocol
,	O
such	O
as	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
.	O
</s>
<s>
XNS	O
also	O
implements	O
a	O
simple	O
echo	O
protocol	O
at	O
the	O
internetwork	B-Protocol
layer	I-Protocol
,	O
similar	O
to	O
IP	O
's	O
ping	B-Protocol
,	O
but	O
operating	O
at	O
a	O
lower	O
level	O
in	O
the	O
networking	B-Protocol
stack	I-Protocol
.	O
</s>
<s>
Instead	O
of	O
adding	O
the	O
ICMP	O
data	O
as	O
payload	O
in	O
an	O
IP	O
packet	O
,	O
as	O
in	O
ping	B-Protocol
,	O
XNS	O
's	O
echo	O
placed	O
the	O
command	O
directly	O
within	O
the	O
underlying	O
IDP	O
packet	O
.	O
</s>
<s>
The	O
same	O
might	O
be	O
achieved	O
in	O
IP	O
by	O
expanding	O
the	O
ICMP	O
Protocol	O
field	O
of	O
the	O
IP	O
header	B-General_Concept
.	O
</s>
<s>
Sequenced	O
Packet	O
Protocol	O
(	O
SPP	O
)	O
is	O
an	O
acknowledged	O
transport	O
protocol	O
,	O
analogous	O
to	O
TCP	B-Protocol
;	O
one	O
chief	O
technical	O
difference	O
is	O
that	O
the	O
sequence	B-Protocol
numbers	I-Protocol
count	O
the	O
packets	O
,	O
and	O
not	O
the	O
bytes	O
as	O
in	O
TCP	B-Protocol
and	O
PUP	O
's	O
BSP	O
;	O
it	O
is	O
the	O
direct	O
antecedent	O
to	O
Novell	B-Operating_System
's	I-Operating_System
IPX/SPX	B-Protocol
.	O
</s>
<s>
Packet	O
Exchange	O
Protocol	O
(	O
PEP	O
)	O
is	O
a	O
connectionless	O
non-reliable	O
protocol	O
similar	O
in	O
nature	O
to	O
UDP	B-Protocol
and	O
the	O
antecedent	O
to	O
Novell	B-Operating_System
's	I-Operating_System
PXP	O
.	O
</s>
<s>
In	O
the	O
original	O
Xerox	O
concept	O
,	O
application	O
protocols	O
such	O
as	O
remote	O
printing	O
,	O
filing	O
,	O
and	O
mailing	O
,	O
etc.	O
,	O
employed	O
a	O
remote	B-Operating_System
procedure	I-Operating_System
call	I-Operating_System
protocol	O
named	O
Courier	O
.	O
</s>
<s>
Courier	O
contained	O
primitives	O
to	O
implement	O
most	O
of	O
the	O
features	O
of	O
Xerox	O
's	O
Mesa	B-Language
programming	I-Language
language	I-Language
function	O
calls	O
.	O
</s>
<s>
Initially	O
,	O
XNS	O
service	O
location	O
was	O
performed	O
via	O
broadcasting	O
remote	O
procedure-calls	O
using	O
a	O
series	O
of	O
expanding	O
ring	O
broadcasts	O
(	O
in	O
consultation	O
with	O
the	O
local	O
router	B-Protocol
,	O
to	O
get	O
networks	O
at	O
increasing	O
distances	O
.	O
)	O
</s>
<s>
Due	O
to	O
its	O
tight	O
integration	O
with	O
Mesa	B-Language
as	O
an	O
underlying	O
technology	O
,	O
many	O
of	O
the	O
traditional	O
higher-level	O
protocols	O
were	O
not	O
part	O
of	O
the	O
XNS	O
system	O
itself	O
.	O
</s>
<s>
Its	O
"	O
Strong	O
credentials	O
"	O
were	O
based	O
on	O
the	O
same	O
Needham	B-Protocol
–	I-Protocol
Schroeder	I-Protocol
protocol	I-Protocol
that	O
was	O
later	O
used	O
by	O
Kerberos	B-Protocol
.	O
</s>
<s>
Xerox	O
's	O
printing	O
language	O
,	O
Interpress	B-Language
,	O
was	O
a	O
binary-formatted	O
standard	O
for	O
controlling	O
laser	O
printers	O
.	O
</s>
<s>
In	O
his	O
final	O
year	O
at	O
Harvard	O
University	O
,	O
Bob	O
Metcalfe	O
began	O
interviewing	O
at	O
a	O
number	O
of	O
companies	O
and	O
was	O
given	O
a	O
warm	O
welcome	O
by	O
Jerry	O
Elkind	O
and	O
Bob	O
Taylor	O
at	O
Xerox	O
PARC	O
,	O
who	O
were	O
beginning	O
to	O
work	O
on	O
the	O
networked	O
computer	O
workstations	O
that	O
would	O
become	O
the	O
Xerox	B-Device
Alto	I-Device
.	O
</s>
<s>
This	O
led	O
to	O
the	O
development	O
of	O
the	O
PARC	B-Protocol
Universal	I-Protocol
Packet	I-Protocol
(	O
Pup	O
)	O
system	O
,	O
and	O
by	O
late	O
1974	O
the	O
two	O
had	O
Pup	O
successfully	O
running	O
on	O
Ethernet	O
.	O
</s>
<s>
They	O
filed	O
a	O
patent	O
on	O
the	O
concepts	O
,	O
with	O
Metcalfe	O
adding	O
several	O
other	O
names	O
because	O
he	O
believed	O
they	O
deserved	O
mention	O
,	O
and	O
then	O
submitted	O
a	O
paper	O
on	O
the	O
concept	O
to	O
Communications	O
of	O
the	O
ACM	O
on	O
"	O
Ethernet	O
:	O
Distributed	O
Packet	O
Switching	O
for	O
Local	O
Computer	B-Architecture
Networks	I-Architecture
"	O
,	O
published	O
in	O
July	O
1976	O
.	O
</s>
<s>
The	O
AI	O
Lab	O
would	O
then	O
go	O
on	O
to	O
make	O
their	O
own	O
version	O
of	O
Ethernet	O
,	O
Chaosnet	B-General_Concept
.	O
</s>
<s>
Dalal	O
was	O
also	O
being	O
heavily	O
recruited	O
by	O
Bob	O
Kahn	O
's	O
ARPANET	O
team	O
(	O
working	O
on	O
TCP/IP	B-Protocol
)	O
,	O
but	O
when	O
Cerf	O
left	O
to	O
join	O
DARPA	O
,	O
Dalal	O
agreed	O
to	O
move	O
to	O
PARC	O
and	O
started	O
there	O
in	O
1977	O
.	O
</s>
<s>
Dalal	O
also	O
attempted	O
to	O
remain	O
involved	O
in	O
the	O
TCP	B-Protocol
efforts	O
underway	O
at	O
DARPA	O
,	O
but	O
eventually	O
gave	O
up	O
and	O
focussed	O
fully	O
on	O
Pup	O
.	O
</s>
<s>
This	O
was	O
essentially	O
a	O
version	O
of	O
Pup	O
with	O
the	O
added	O
concept	O
of	O
sockets	O
and	O
an	O
internetwork	B-Architecture
,	O
which	O
allowed	O
routers	B-Protocol
to	O
forward	O
packets	O
across	O
connected	O
networks	O
.	O
</s>
<s>
However	O
,	O
it	O
played	O
an	O
important	O
role	O
in	O
the	O
development	O
of	O
networking	O
technology	O
in	O
the	O
1980s	O
,	O
by	O
influencing	O
software	O
and	O
hardware	O
vendors	O
to	O
seriously	O
consider	O
the	O
need	O
for	O
computing	O
platforms	O
to	O
support	O
more	O
than	O
one	O
network	B-Protocol
protocol	I-Protocol
stack	I-Protocol
simultaneously	O
.	O
</s>
<s>
Among	O
these	O
were	O
Net/One	O
,	O
3+	O
,	O
Banyan	B-Operating_System
VINES	I-Operating_System
and	O
Novell	B-Operating_System
's	I-Operating_System
IPX/SPX	B-Protocol
.	O
</s>
<s>
These	O
systems	O
added	O
their	O
own	O
concepts	O
on	O
top	O
of	O
the	O
XNS	O
addressing	O
and	O
routing	B-Protocol
system	O
;	O
VINES	B-Operating_System
added	O
a	O
directory	O
service	O
among	O
other	O
services	O
,	O
while	O
Novell	B-Operating_System
NetWare	I-Operating_System
added	O
a	O
number	O
of	O
user-facing	O
services	O
like	O
printing	O
and	O
file	O
sharing	O
.	O
</s>
<s>
AppleTalk	B-Application
used	O
XNS-like	O
routing	B-Protocol
,	O
but	O
had	O
incompatible	O
addresses	O
using	O
shorter	O
numbers	O
.	O
</s>
<s>
XNS	O
also	O
helped	O
to	O
validate	O
the	O
design	O
of	O
the	O
4.2BSD	B-Operating_System
network	O
subsystem	O
by	O
providing	O
a	O
second	O
protocol	B-Protocol
suite	I-Protocol
,	O
one	O
which	O
was	O
significantly	O
different	O
from	O
the	O
Internet	B-Protocol
protocols	I-Protocol
;	O
by	O
implementing	O
both	O
stacks	O
in	O
the	O
same	O
kernel	O
,	O
Berkeley	O
researchers	O
demonstrated	O
that	O
the	O
design	O
was	O
suitable	O
for	O
more	O
than	O
just	O
IP	O
.	O
</s>
<s>
Additional	O
BSD	B-Operating_System
modifications	O
were	O
eventually	O
necessary	O
to	O
support	O
the	O
full	O
range	O
of	O
Open	O
Systems	O
Interconnection	O
(	O
OSI	O
)	O
protocols	O
.	O
</s>
