<s>
In	O
computer	B-Architecture
networking	I-Architecture
,	O
a	O
reliable	O
protocol	O
is	O
a	O
communication	O
protocol	O
that	O
notifies	O
the	O
sender	O
whether	O
or	O
not	O
the	O
delivery	O
of	O
data	O
to	O
intended	O
recipients	O
was	O
successful	O
.	O
</s>
<s>
Reliability	O
is	O
a	O
synonym	O
for	O
assurance	B-Protocol
,	O
which	O
is	O
the	O
term	O
used	O
by	O
the	O
ITU	O
and	O
ATM	O
Forum	O
.	O
</s>
<s>
This	O
often	O
is	O
not	O
an	O
issue	O
for	O
unicast	B-Protocol
protocols	O
,	O
but	O
it	O
may	O
become	O
a	O
problem	O
for	O
reliable	B-Operating_System
multicast	I-Operating_System
protocols	O
.	O
</s>
<s>
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
,	O
the	O
main	O
protocol	O
used	O
on	O
the	O
Internet	O
,	O
is	O
a	O
reliable	O
unicast	B-Protocol
protocol	O
;	O
it	O
provides	O
the	O
abstraction	O
of	O
a	O
reliable	O
byte	O
stream	O
to	O
applications	O
.	O
</s>
<s>
UDP	B-Protocol
is	O
an	O
unreliable	O
protocol	O
and	O
is	O
often	O
used	O
in	O
computer	O
games	O
,	O
streaming	O
media	O
or	O
in	O
other	O
situations	O
where	O
speed	O
is	O
an	O
issue	O
and	O
some	O
data	O
loss	O
may	O
be	O
tolerated	O
because	O
of	O
the	O
transitory	O
nature	O
of	O
the	O
data	O
.	O
</s>
<s>
Often	O
,	O
a	O
reliable	O
unicast	B-Protocol
protocol	O
is	O
also	O
connection	B-Protocol
oriented	I-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
TCP	O
is	O
connection	B-Protocol
oriented	I-Protocol
,	O
with	O
the	O
virtual-circuit	B-Protocol
ID	O
consisting	O
of	O
source	O
and	O
destination	O
IP	B-Protocol
addresses	I-Protocol
and	O
port	O
numbers	O
.	O
</s>
<s>
However	O
,	O
some	O
unreliable	O
protocols	O
are	O
connection	B-Protocol
oriented	I-Protocol
,	O
such	O
as	O
Asynchronous	O
Transfer	O
Mode	O
and	O
Frame	B-Protocol
Relay	I-Protocol
.	O
</s>
<s>
Building	O
on	O
the	O
packet	B-Protocol
switching	I-Protocol
concepts	O
proposed	O
by	O
Donald	O
Davies	O
,	O
the	O
first	O
communication	O
protocol	O
on	O
the	O
ARPANET	O
was	O
a	O
reliable	O
packet	O
delivery	O
procedure	O
to	O
connect	O
its	O
hosts	O
via	O
the	O
1822	B-Protocol
interface	I-Protocol
.	O
</s>
<s>
A	O
host	O
computer	O
simply	O
arranged	O
the	O
data	O
in	O
the	O
correct	O
packet	O
format	O
,	O
inserted	O
the	O
address	O
of	O
the	O
destination	O
host	O
computer	O
,	O
and	O
sent	O
the	O
message	O
across	O
the	O
interface	O
to	O
its	O
connected	O
Interface	B-Protocol
Message	I-Protocol
Processor	I-Protocol
(	O
IMP	O
)	O
.	O
</s>
<s>
Meanwhile	O
,	O
the	O
developers	O
of	O
CYCLADES	O
and	O
of	O
ALOHAnet	O
demonstrated	O
that	O
it	O
was	O
possible	O
to	O
build	O
an	O
effective	O
computer	B-Architecture
network	I-Architecture
without	O
providing	O
reliable	O
packet	O
transmission	O
.	O
</s>
<s>
This	O
led	O
to	O
the	O
development	O
of	O
the	O
end-to-end	B-Application
principle	I-Application
,	O
which	O
is	O
one	O
of	O
the	O
Internet	O
's	O
fundamental	O
design	O
principles	O
.	O
</s>
<s>
For	O
example	O
,	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
provides	O
an	O
unreliable	O
service	O
.	O
</s>
<s>
Together	O
,	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
and	O
IP	O
provide	O
a	O
reliable	O
service	O
,	O
whereas	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	B-Protocol
)	O
and	O
IP	O
provide	O
an	O
unreliable	O
one	O
.	O
</s>
<s>
In	O
the	O
context	O
of	O
distributed	O
protocols	O
,	O
reliability	B-Protocol
properties	I-Protocol
specify	O
the	O
guarantees	O
that	O
the	O
protocol	O
provides	O
with	O
respect	O
to	O
the	O
delivery	O
of	O
messages	O
to	O
the	O
intended	O
recipient(s )	O
.	O
</s>
<s>
An	O
example	O
of	O
a	O
reliability	O
property	O
for	O
a	O
unicast	B-Protocol
protocol	O
is	O
"	O
at	O
least	O
once	O
"	O
,	O
i.e.	O
</s>
<s>
Reliability	B-Protocol
properties	I-Protocol
for	O
multicast	O
protocols	O
can	O
be	O
expressed	O
on	O
a	O
per-recipient	O
basis	O
(	O
simple	O
reliability	B-Protocol
properties	I-Protocol
)	O
,	O
or	O
they	O
may	O
relate	O
the	O
fact	O
of	O
delivery	O
or	O
the	O
order	O
of	O
delivery	O
among	O
the	O
different	O
recipients	O
(	O
strong	B-Protocol
reliability	I-Protocol
properties	I-Protocol
)	O
.	O
</s>
<s>
In	O
the	O
context	O
of	O
multicast	O
protocols	O
,	O
strong	B-Protocol
reliability	I-Protocol
properties	I-Protocol
express	O
the	O
guarantees	O
that	O
the	O
protocol	O
provides	O
with	O
respect	O
to	O
the	O
delivery	O
of	O
messages	O
to	O
different	O
recipients	O
.	O
</s>
<s>
Strong	B-Protocol
reliability	I-Protocol
properties	I-Protocol
such	O
as	O
this	O
one	O
typically	O
require	O
that	O
messages	O
are	O
retransmitted	O
or	O
forwarded	O
among	O
the	O
recipients	O
.	O
</s>
<s>
An	O
example	O
of	O
a	O
reliability	O
property	O
stronger	O
than	O
last	O
copy	O
recall	O
is	O
atomicity	B-General_Concept
.	O
</s>
<s>
One	O
of	O
the	O
most	O
complex	O
strong	B-Protocol
reliability	I-Protocol
properties	I-Protocol
is	O
virtual	O
synchrony	O
.	O
</s>
<s>
Reliable	B-Protocol
messaging	I-Protocol
is	O
the	O
concept	O
of	O
message	B-Architecture
passing	I-Architecture
across	O
an	O
unreliable	O
infrastructure	O
whilst	O
being	O
able	O
to	O
make	O
certain	O
guarantees	O
about	O
the	O
successful	O
transmission	O
of	O
the	O
messages	O
.	O
</s>
<s>
Reliable	B-Protocol
delivery	I-Protocol
can	O
be	O
contrasted	O
with	O
best-effort	O
delivery	O
,	O
where	O
there	O
is	O
no	O
guarantee	O
that	O
messages	O
will	O
be	O
delivered	O
quickly	O
,	O
in	O
order	O
,	O
or	O
at	O
all	O
.	O
</s>
<s>
A	O
reliable	B-Protocol
delivery	I-Protocol
protocol	O
can	O
be	O
built	O
on	O
an	O
unreliable	O
protocol	O
.	O
</s>
<s>
An	O
extremely	O
common	O
example	O
is	O
the	O
layering	O
of	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
on	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
,	O
a	O
combination	O
known	O
as	O
TCP/IP	B-Protocol
.	O
</s>
<s>
Strong	B-Protocol
reliability	I-Protocol
properties	I-Protocol
are	O
offered	O
by	O
group	B-Operating_System
communication	I-Operating_System
systems	I-Operating_System
(	O
GCSs	O
)	O
such	O
as	O
IS-IS	B-Protocol
,	O
Appia	B-Operating_System
framework	I-Operating_System
,	O
Spread	O
,	O
JGroups	B-Language
or	O
QuickSilver	B-General_Concept
Scalable	I-General_Concept
Multicast	I-General_Concept
.	O
</s>
<s>
The	O
QuickSilver	B-General_Concept
Properties	I-General_Concept
Framework	I-General_Concept
is	O
a	O
flexible	O
platform	O
that	O
allows	O
strong	B-Protocol
reliability	I-Protocol
properties	I-Protocol
to	O
be	O
expressed	O
in	O
a	O
purely	O
declarative	O
manner	O
,	O
using	O
a	O
simple	O
rule-based	O
language	O
,	O
and	O
automatically	O
translated	O
into	O
a	O
hierarchical	O
protocol	O
.	O
</s>
<s>
One	O
protocol	O
that	O
implements	O
reliable	B-Protocol
messaging	I-Protocol
is	O
WS-ReliableMessaging	O
,	O
which	O
handles	O
reliable	B-Protocol
delivery	I-Protocol
of	O
SOAP	B-Application
messages	O
.	O
</s>
<s>
The	O
ATM	O
Service-Specific	O
Coordination	O
Function	O
provides	O
for	O
transparent	O
assured	O
delivery	O
with	O
AAL5	B-Protocol
.	O
</s>
<s>
There	O
is	O
,	O
however	O
,	O
a	O
problem	O
with	O
the	O
definition	O
of	O
reliability	O
as	O
"	O
delivery	O
or	O
notification	O
of	O
failure	O
"	O
in	O
real-time	B-General_Concept
computing	I-General_Concept
.	O
</s>
<s>
safety-critical	B-Application
,	O
safety-involved	B-Application
,	O
and	O
some	O
secure	O
mission-critical	O
systems	O
,	O
must	O
be	O
proved	B-Architecture
to	O
perform	O
at	O
some	O
specified	O
minimum	O
level	O
.	O
</s>
<s>
In	O
hard	O
real-time	B-General_Concept
systems	I-General_Concept
,	O
all	O
data	O
must	O
be	O
delivered	O
by	O
the	O
deadline	O
or	O
it	O
is	O
considered	O
a	O
system	O
failure	O
.	O
</s>
<s>
In	O
firm	O
real-time	B-General_Concept
systems	I-General_Concept
,	O
late	O
data	O
is	O
still	O
valueless	O
but	O
the	O
system	O
can	O
tolerate	O
some	O
amount	O
of	O
late	O
or	O
missing	O
data	O
.	O
</s>
<s>
There	O
are	O
a	O
number	O
of	O
protocols	O
that	O
are	O
capable	O
of	O
addressing	O
real-time	O
requirements	O
for	O
reliable	B-Protocol
delivery	I-Protocol
and	O
timeliness	O
:	O
</s>
<s>
STANAG	O
3910	O
,	O
also	O
known	O
as	O
EFABus	O
in	O
its	O
use	O
on	O
the	O
Eurofighter	B-Application
Typhoon	I-Application
,	O
is	O
,	O
in	O
effect	O
,	O
a	O
version	O
of	O
MIL-1553	O
augmented	O
with	O
a	O
20Mbit/s	O
shared	O
media	O
bus	O
for	O
data	O
transfers	O
,	O
retaining	O
the	O
1Mbit/s	O
shared	O
media	O
bus	O
for	O
control	O
purposes	O
.	O
</s>
<s>
The	O
Asynchronous	O
Transfer	O
Mode	O
(	O
ATM	O
)	O
,	O
the	O
Avionics	B-Protocol
Full-Duplex	I-Protocol
Switched	I-Protocol
Ethernet	I-Protocol
(	O
AFDX	B-Protocol
)	O
,	O
and	O
Time	B-Protocol
Triggered	I-Protocol
Ethernet	I-Protocol
(	O
TTEthernet	B-Protocol
)	O
are	O
examples	O
of	O
packet-switched	B-Protocol
networks	I-Protocol
protocols	O
where	O
the	O
timeliness	O
and	O
reliability	O
of	O
data	O
transfers	O
can	O
be	O
assured	O
by	O
the	O
network	O
.	O
</s>
<s>
AFDX	B-Protocol
and	O
TTEthernet	B-Protocol
are	O
also	O
based	O
on	O
IEEE	O
802.3	O
Ethernet	O
,	O
though	O
not	O
entirely	O
compatible	O
with	O
it	O
.	O
</s>
<s>
ATM	O
uses	O
connection-oriented	B-Protocol
virtual	B-Protocol
channels	I-Protocol
(	O
VCs	O
)	O
which	O
have	O
fully	O
deterministic	O
paths	O
through	O
the	O
network	O
,	O
and	O
usage	B-Protocol
and	I-Protocol
network	I-Protocol
parameter	I-Protocol
control	I-Protocol
(	O
UPC/NPC	O
)	O
,	O
which	O
are	O
implemented	O
within	O
the	O
network	O
,	O
to	O
limit	O
the	O
traffic	O
on	O
each	O
VC	O
separately	O
.	O
</s>
<s>
However	O
,	O
for	O
the	O
reliability	O
and	O
timeliness	O
to	O
be	O
proved	B-Architecture
,	O
and	O
for	O
the	O
proofs	O
to	O
be	O
tolerant	O
of	O
faults	O
in	O
and	O
malicious	O
actions	O
by	O
the	O
equipment	O
connected	O
to	O
the	O
network	O
,	O
the	O
calculations	O
of	O
these	O
resource	O
usages	O
cannot	O
be	O
based	O
on	O
any	O
parameters	O
that	O
are	O
not	O
actively	O
enforced	O
by	O
the	O
network	O
,	O
i.e.	O
</s>
<s>
AFDX	B-Protocol
uses	O
frequency	O
domain	O
bandwidth	O
allocation	O
and	O
traffic	O
policing	O
,	O
that	O
allows	O
the	O
traffic	O
on	O
each	O
virtual	O
link	O
(	O
VL	O
)	O
to	O
be	O
limited	O
so	O
that	O
the	O
requirements	O
for	O
shared	O
resources	O
can	O
be	O
predicted	O
and	O
congestion	O
prevented	O
so	O
it	O
can	O
be	O
proved	B-Architecture
not	O
to	O
affect	O
the	O
critical	O
data	O
.	O
</s>
<s>
However	O
,	O
the	O
techniques	O
for	O
predicting	O
the	O
resource	O
requirements	O
and	O
proving	O
that	O
congestion	O
is	O
prevented	O
are	O
not	O
part	O
of	O
the	O
AFDX	B-Protocol
standard	O
.	O
</s>
<s>
TTEthernet	B-Protocol
provides	O
the	O
lowest	O
possible	O
latency	O
in	O
transferring	O
data	O
across	O
the	O
network	O
by	O
using	O
time-domain	O
control	O
methods	O
–	O
each	O
time	O
triggered	O
transfer	O
is	O
scheduled	O
at	O
a	O
specific	O
time	O
so	O
that	O
contention	O
for	O
shared	O
resources	O
is	O
controlled	O
and	O
thus	O
the	O
possibility	O
of	O
congestion	O
is	O
eliminated	O
.	O
</s>
<s>
the	O
cyclic	B-Operating_System
executive	I-Operating_System
.	O
</s>
<s>
This	O
is	O
especially	O
true	O
where	O
the	O
transfers	O
over	O
the	O
bus	O
or	O
network	O
are	O
cyclically	O
scheduled	O
(	O
as	O
is	O
commonly	O
the	O
case	O
with	O
MIL-STD-1553B	O
and	O
STANAG	O
3910	O
,	O
and	O
necessarily	O
so	O
with	O
AFDX	B-Protocol
and	O
TTEthernet	B-Protocol
)	O
but	O
the	O
application	O
processes	O
are	O
not	O
synchronized	O
with	O
this	O
schedule	O
.	O
</s>
<s>
With	O
both	O
AFDX	B-Protocol
and	O
TTEthernet	B-Protocol
,	O
there	O
are	O
additional	O
functions	O
required	O
of	O
the	O
interfaces	O
,	O
e.g.	O
</s>
<s>
AFDX	B-Protocol
's	O
Bandwidth	O
Allocation	O
Gap	O
control	O
,	O
and	O
TTEthernet	B-Protocol
's	O
requirement	O
for	O
very	O
close	O
synchronization	O
of	O
the	O
sources	O
of	O
time-triggered	O
data	O
,	O
that	O
make	O
it	O
difficult	O
to	O
use	O
standard	O
Ethernet	O
interfaces	O
.	O
</s>
