<s>
The	O
end-to-end	B-Application
principle	I-Application
is	O
a	O
design	O
framework	O
in	O
computer	B-Architecture
networking	I-Architecture
.	O
</s>
<s>
In	O
networks	B-Architecture
designed	I-Architecture
according	O
to	O
this	O
principle	O
,	O
guaranteeing	O
certain	O
application-specific	O
features	O
,	O
such	O
as	O
reliability	O
and	O
security	O
,	O
requires	O
that	O
they	O
reside	O
in	O
the	O
communicating	O
end	B-Protocol
nodes	I-Protocol
of	O
the	O
network	O
.	O
</s>
<s>
Intermediary	O
nodes	O
,	O
such	O
as	O
gateways	B-Application
and	O
routers	B-Protocol
,	O
that	O
exist	O
to	O
establish	O
the	O
network	O
,	O
may	O
implement	O
these	O
to	O
improve	O
efficiency	O
but	O
cannot	O
guarantee	O
end-to-end	O
correctness	O
.	O
</s>
<s>
The	O
essence	O
of	O
what	O
would	O
later	O
be	O
called	O
the	O
end-to-end	B-Application
principle	I-Application
was	O
contained	O
in	O
the	O
work	O
of	O
Paul	O
Baran	O
and	O
Donald	O
Davies	O
on	O
packet-switched	B-Protocol
networks	I-Protocol
in	O
the	O
1960s	O
.	O
</s>
<s>
The	O
meaning	O
of	O
the	O
end-to-end	B-Application
principle	I-Application
has	O
been	O
continuously	O
reinterpreted	O
ever	O
since	O
its	O
initial	O
articulation	O
.	O
</s>
<s>
Also	O
,	O
noteworthy	O
formulations	O
of	O
the	O
end-to-end	B-Application
principle	I-Application
can	O
be	O
found	O
before	O
the	O
seminal	O
1981	O
Saltzer	O
,	O
Reed	O
,	O
and	O
Clark	O
paper	O
.	O
</s>
<s>
The	O
end	O
hosts	B-Device
have	O
to	O
implement	O
these	O
functions	O
for	O
correctness	O
.	O
</s>
<s>
The	O
fundamental	O
notion	O
behind	O
the	O
end-to-end	B-Application
principle	I-Application
is	O
that	O
for	O
two	O
processes	B-Operating_System
communicating	O
with	O
each	O
other	O
via	O
some	O
communication	O
means	O
,	O
the	O
reliability	O
obtained	O
from	O
that	O
means	O
cannot	O
be	O
expected	O
to	O
be	O
perfectly	O
aligned	O
with	O
the	O
reliability	O
requirements	O
of	O
the	O
processes	B-Operating_System
.	O
</s>
<s>
In	O
particular	O
,	O
meeting	O
or	O
exceeding	O
very	O
high-reliability	O
requirements	O
of	O
communicating	O
processes	B-Operating_System
separated	O
by	O
networks	O
of	O
nontrivial	O
size	O
is	O
more	O
costly	O
than	O
obtaining	O
the	O
required	O
degree	O
of	O
reliability	O
by	O
positive	O
end-to-end	O
acknowledgments	B-Protocol
and	O
retransmissions	B-Protocol
(	O
referred	O
to	O
as	O
PAR	B-Protocol
or	O
ARQ	B-Error_Name
)	O
.	O
</s>
<s>
Put	O
differently	O
,	O
it	O
is	O
far	O
easier	O
to	O
obtain	O
reliability	O
beyond	O
a	O
certain	O
margin	O
by	O
mechanisms	O
in	O
the	O
end	O
hosts	B-Device
of	O
a	O
network	O
rather	O
than	O
in	O
the	O
intermediary	O
nodes	O
,	O
especially	O
when	O
the	O
latter	O
are	O
beyond	O
the	O
control	O
of	O
,	O
and	O
not	O
accountable	O
to	O
,	O
the	O
former	O
.	O
</s>
<s>
Positive	O
end-to-end	O
acknowledgments	B-Protocol
with	O
infinite	O
retries	O
can	O
obtain	O
arbitrarily	O
high	O
reliability	O
from	O
any	O
network	O
with	O
a	O
higher	O
than	O
zero	O
probability	O
of	O
successfully	O
transmitting	O
data	O
from	O
one	O
end	O
to	O
another	O
.	O
</s>
<s>
The	O
end-to-end	B-Application
principle	I-Application
does	O
not	O
extend	O
to	O
functions	O
beyond	O
end-to-end	O
error	O
control	O
and	O
correction	O
,	O
and	O
security	O
.	O
</s>
<s>
In	O
a	O
2001	O
paper	O
,	O
Blumenthal	O
and	O
Clark	O
note	O
:	O
"[F]rom	O
the	O
beginning	O
,	O
the	O
end-to-end	O
arguments	O
revolved	O
around	O
requirements	O
that	O
could	O
be	O
implemented	O
correctly	O
at	O
the	O
endpoints	O
;	O
if	O
implementation	O
inside	O
the	O
network	O
is	O
the	O
only	O
way	O
to	O
accomplish	O
the	O
requirement	O
,	O
then	O
an	O
end-to-end	B-Application
argument	I-Application
is	O
n't	O
appropriate	O
in	O
the	O
first	O
place.	O
"	O
</s>
<s>
The	O
end-to-end	B-Application
principle	I-Application
is	O
closely	O
related	O
,	O
and	O
sometimes	O
seen	O
as	O
a	O
direct	O
precursor	O
,	O
to	O
the	O
principle	O
of	O
net	O
neutrality	O
.	O
</s>
<s>
In	O
the	O
1960s	O
,	O
Paul	O
Baran	O
and	O
Donald	O
Davies	O
,	O
in	O
their	O
pre-ARPANET	O
elaborations	O
of	O
networking	O
,	O
made	O
brief	O
comments	O
about	O
reliability	O
that	O
capture	O
the	O
essence	O
of	O
the	O
later	O
end-to-end	B-Application
principle	I-Application
.	O
</s>
<s>
The	O
ARPANET	O
was	O
the	O
first	O
large-scale	O
general-purpose	O
packet	B-Protocol
switching	I-Protocol
network	O
implementing	O
several	O
of	O
the	O
basic	O
notions	O
previously	O
touched	O
on	O
by	O
Baran	O
and	O
Davies	O
.	O
</s>
<s>
Davies	O
had	O
worked	O
on	O
simulation	O
of	O
datagram	B-Protocol
networks	O
.	O
</s>
<s>
Building	O
on	O
this	O
idea	O
,	O
Louis	O
Pouzin	O
's	O
CYCLADES	O
network	O
was	O
the	O
first	O
to	O
make	O
the	O
hosts	B-Device
responsible	O
for	O
the	O
reliable	O
delivery	O
of	O
data	O
,	O
rather	O
than	O
this	O
being	O
a	O
centralized	O
service	O
of	O
the	O
network	O
itself	O
.	O
</s>
<s>
Concepts	O
implemented	O
in	O
this	O
network	O
influenced	O
TCP/IP	B-Protocol
architecture	O
.	O
</s>
<s>
The	O
ARPANET	O
demonstrated	O
several	O
important	O
aspects	O
of	O
the	O
end-to-end	B-Application
principle	I-Application
.	O
</s>
<s>
If	O
the	O
basic	O
premise	O
of	O
a	O
distributed	O
network	O
is	O
packet	B-Protocol
switching	I-Protocol
,	O
then	O
functions	O
such	O
as	O
reordering	O
and	O
duplicate	O
detection	O
inevitably	O
have	O
to	O
be	O
implemented	O
at	O
the	O
logical	O
endpoints	O
of	O
such	O
a	O
network	O
.	O
</s>
<s>
Dave	O
Clark	O
,	O
one	O
of	O
the	O
authors	O
of	O
the	O
end-to-end	B-Application
principle	I-Application
paper	O
,	O
concludes	O
:	O
"	O
The	O
discovery	O
of	O
packets	O
is	O
not	O
a	O
consequence	O
of	O
the	O
end-to-end	B-Application
argument	I-Application
.	O
</s>
<s>
It	O
is	O
the	O
success	O
of	O
packets	O
that	O
make	O
the	O
end-to-end	B-Application
argument	I-Application
relevant.	O
"	O
</s>
<s>
In	O
order	O
to	O
remedy	O
any	O
potential	O
failures	O
of	O
packet	O
transmission	O
normal	O
ARPANET	O
messages	O
were	O
handed	O
from	O
one	O
node	O
to	O
the	O
next	O
node	O
with	O
a	O
positive	O
acknowledgment	B-Protocol
and	O
retransmission	B-Protocol
scheme	O
;	O
after	O
a	O
successful	O
handover	O
they	O
were	O
then	O
discarded	O
,	O
no	O
source-to-destination	O
re-transmission	B-Protocol
in	O
case	O
of	O
packet	O
loss	O
was	O
catered	O
for	O
.	O
</s>
<s>
This	O
use	O
case	O
was	O
catered	O
for	O
in	O
the	O
ARPANET	O
by	O
providing	O
a	O
raw	O
message	O
service	O
that	O
dispensed	O
with	O
various	O
reliability	O
measures	O
so	O
as	O
to	O
provide	O
faster	O
and	O
lower	O
latency	O
data	O
transmission	O
service	O
to	O
the	O
end	O
hosts	B-Device
.	O
</s>
<s>
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
is	O
a	O
connectionless	B-Protocol
datagram	B-Protocol
service	I-Protocol
with	O
no	O
delivery	O
guarantees	O
.	O
</s>
<s>
End-to-end	O
acknowledgment	B-Protocol
and	O
retransmission	B-Protocol
is	O
the	O
responsibility	O
of	O
the	O
connection-oriented	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
which	O
sits	O
on	O
top	O
of	O
IP	O
.	O
</s>
<s>
The	O
functional	O
split	O
between	O
IP	O
and	O
TCP	O
exemplifies	O
the	O
proper	O
application	O
of	O
the	O
end-to-end	B-Application
principle	I-Application
to	O
transport	O
protocol	O
design	O
.	O
</s>
<s>
An	O
example	O
of	O
the	O
end-to-end	B-Application
principle	I-Application
is	O
that	O
of	O
an	O
arbitrarily	O
reliable	O
file	O
transfer	O
between	O
two	O
endpoints	O
in	O
a	O
distributed	O
network	O
of	O
a	O
varying	O
,	O
nontrivial	O
size	O
:	O
The	O
only	O
way	O
two	O
endpoints	O
can	O
obtain	O
a	O
completely	O
reliable	O
transfer	O
is	O
by	O
transmitting	O
and	O
acknowledging	O
a	O
checksum	B-Algorithm
for	O
the	O
entire	O
data	O
stream	O
;	O
in	O
such	O
a	O
setting	O
,	O
lesser	O
checksum	B-Algorithm
and	O
acknowledgment	B-Protocol
(	O
ACK/NACK	O
)	O
protocols	O
are	O
justified	O
only	O
for	O
the	O
purpose	O
of	O
optimizing	O
performancethey	O
are	O
useful	O
to	O
the	O
vast	O
majority	O
of	O
clients	O
,	O
but	O
are	O
not	O
enough	O
to	O
fulfill	O
the	O
reliability	O
requirement	O
of	O
this	O
particular	O
application	O
.	O
</s>
<s>
A	O
thorough	O
checksum	B-Algorithm
is	O
hence	O
best	O
done	O
at	O
the	O
endpoints	O
,	O
and	O
the	O
network	O
maintains	O
a	O
relatively	O
low	O
level	O
of	O
complexity	O
and	O
reasonable	O
performance	O
for	O
all	O
clients	O
.	O
</s>
<s>
The	O
most	O
important	O
limitation	O
of	O
the	O
end-to-end	B-Application
principle	I-Application
is	O
that	O
its	O
basic	O
premise	O
,	O
placing	O
functions	O
in	O
the	O
application	O
endpoints	O
rather	O
than	O
in	O
the	O
intermediary	O
nodes	O
,	O
is	O
not	O
trivial	O
to	O
implement	O
.	O
</s>
<s>
An	O
example	O
of	O
the	O
limitations	O
of	O
the	O
end-to-end	B-Application
principle	I-Application
exists	O
in	O
mobile	O
devices	O
,	O
for	O
instance	O
with	O
mobile	B-Protocol
IPv6	I-Protocol
.	O
</s>
<s>
Further	O
problems	O
can	O
be	O
seen	O
with	O
a	O
decrease	O
in	O
network	O
transparency	O
from	O
the	O
addition	O
of	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
,	O
which	O
IPv4	B-Protocol
relies	O
on	O
to	O
combat	O
address	B-Protocol
exhaustion	I-Protocol
.	O
</s>
<s>
With	O
the	O
introduction	O
of	O
IPv6	B-Protocol
,	O
users	O
once	O
again	O
have	O
unique	O
identifiers	O
,	O
allowing	O
for	O
true	O
end-to-end	B-Application
connectivity	I-Application
.	O
</s>
