<s>
Connection-oriented	B-Protocol
communication	I-Protocol
is	O
a	O
network	O
communication	O
mode	O
in	O
telecommunications	O
and	O
computer	O
networking	O
,	O
where	O
a	O
communication	B-Protocol
session	I-Protocol
or	O
a	O
semi-permanent	O
connection	O
is	O
established	O
before	O
any	O
useful	O
data	O
can	O
be	O
transferred	O
.	O
</s>
<s>
The	O
alternative	O
to	O
connection-oriented	B-Protocol
transmission	O
is	O
connectionless	B-Protocol
communication	I-Protocol
,	O
for	O
example	O
the	O
datagram	B-Protocol
mode	O
communication	O
used	O
by	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
and	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
,	O
where	O
data	O
may	O
be	O
delivered	O
out	O
of	O
order	O
,	O
since	O
different	O
network	B-Protocol
packets	I-Protocol
are	O
routed	O
independently	O
and	O
may	O
be	O
delivered	O
over	O
different	O
paths	O
.	O
</s>
<s>
Connection-oriented	B-Protocol
communication	I-Protocol
may	O
be	O
implemented	O
with	O
a	O
circuit	B-Protocol
switched	I-Protocol
connection	O
,	O
or	O
a	O
packet-mode	O
virtual	B-Protocol
circuit	I-Protocol
connection	O
.	O
</s>
<s>
In	O
the	O
latter	O
case	O
,	O
it	O
may	O
use	O
either	O
a	O
transport	B-Architecture
layer	I-Architecture
virtual	B-Protocol
circuit	I-Protocol
protocol	O
such	O
as	O
the	O
TCP	B-Protocol
protocol	I-Protocol
,	O
allowing	O
data	O
to	O
be	O
delivered	O
in	O
order	O
.	O
</s>
<s>
Although	O
the	O
lower-layer	O
switching	O
is	O
connectionless	B-Protocol
,	O
or	O
it	O
may	O
be	O
a	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
or	O
network	B-Application
layer	I-Application
switching	O
mode	O
,	O
where	O
all	O
data	B-Protocol
packets	I-Protocol
belonging	O
to	O
the	O
same	O
traffic	O
stream	O
are	O
delivered	O
over	O
the	O
same	O
path	O
,	O
and	O
traffic	O
flows	O
are	O
identified	O
by	O
some	O
connection	O
identifier	O
reducing	O
the	O
overhead	O
of	O
routing	O
decisions	O
on	O
a	O
packet-by-packet	O
basis	O
for	O
the	O
network	O
.	O
</s>
<s>
Connection-oriented	B-Protocol
protocol	I-Protocol
services	O
are	O
often	O
,	O
but	O
not	O
always	O
,	O
reliable	B-Protocol
network	O
services	O
that	O
provide	O
acknowledgment	O
after	O
successful	O
delivery	O
and	O
automatic	B-Error_Name
repeat	I-Error_Name
request	I-Error_Name
functions	O
in	O
case	O
of	O
missing	O
or	O
corrupted	O
data	O
.	O
</s>
<s>
Asynchronous	O
Transfer	O
Mode	O
,	O
Frame	B-Protocol
Relay	I-Protocol
and	O
MPLS	B-Protocol
are	O
examples	O
of	O
a	O
connection-oriented	B-Protocol
,	O
unreliable	B-Protocol
protocol	O
.	O
</s>
<s>
SMTP	B-Protocol
is	O
an	O
example	O
of	O
a	O
connection-oriented	B-Protocol
protocol	I-Protocol
in	O
which	O
if	O
a	O
message	O
is	O
not	O
delivered	O
,	O
an	O
error	O
report	O
is	O
sent	O
to	O
the	O
sender	O
which	O
makes	O
SMTP	B-Protocol
a	O
reliable	B-Protocol
protocol	O
.	O
</s>
<s>
Because	O
they	O
can	O
keep	O
track	O
of	O
a	O
conversation	O
,	O
connection-oriented	B-Protocol
protocols	I-Protocol
are	O
sometimes	O
described	O
as	O
stateful	O
.	O
</s>
<s>
Circuit	B-Protocol
switched	I-Protocol
communication	O
,	O
for	O
example	O
the	O
public	O
switched	O
telephone	O
network	O
,	O
ISDN	B-Protocol
,	O
SONET/SDH	B-Protocol
and	O
optical	B-Protocol
mesh	I-Protocol
networks	I-Protocol
,	O
are	O
intrinsically	O
connection-oriented	B-Protocol
communications	I-Protocol
systems	O
.	O
</s>
<s>
Packet	B-Protocol
switched	I-Protocol
communication	O
may	O
also	O
be	O
connection-oriented	B-Protocol
,	O
which	O
is	O
called	O
virtual	B-Protocol
circuit	I-Protocol
mode	O
communication	O
.	O
</s>
<s>
Due	O
to	O
the	O
packet	B-Protocol
switching	I-Protocol
,	O
the	O
communication	O
may	O
suffer	O
from	O
variable	O
bit	O
rate	O
and	O
delay	O
,	O
due	O
to	O
varying	O
traffic	O
load	O
and	O
packet	B-Protocol
queue	O
lengths	O
.	O
</s>
<s>
Connection-oriented	B-Protocol
communication	I-Protocol
are	O
not	O
necessarily	O
reliable	B-Protocol
protocols	O
.	O
</s>
<s>
Connection-oriented	B-Protocol
transport	B-Architecture
layer	I-Architecture
protocols	O
provide	O
connection-oriented	B-Protocol
communications	I-Protocol
over	O
connectionless	B-Protocol
communications	I-Protocol
systems	O
.	O
</s>
<s>
A	O
connection-oriented	B-Protocol
transport	B-Architecture
layer	I-Architecture
protocol	O
,	O
such	O
as	O
TCP	B-Protocol
,	O
may	O
be	O
based	O
on	O
a	O
connectionless	B-Protocol
network	B-Application
layer	I-Application
protocol	O
(	O
such	O
as	O
IP	O
)	O
,	O
but	O
still	O
achieves	O
in-order	O
delivery	O
of	O
a	O
byte-stream	O
,	O
by	O
means	O
of	O
segment	O
sequence	O
numbering	O
on	O
the	O
sender	O
side	O
,	O
packet	B-Protocol
buffering	O
and	O
data	B-Protocol
packet	I-Protocol
reordering	O
on	O
the	O
receiver	O
side	O
.	O
</s>
<s>
In	O
a	O
connection-oriented	B-Protocol
packet-switched	B-Protocol
data-link	B-Protocol
or	O
network-layer	B-Application
protocol	O
,	O
all	O
data	O
is	O
sent	O
over	O
the	O
same	O
path	O
during	O
a	O
communication	B-Protocol
session	I-Protocol
.	O
</s>
<s>
Rather	O
than	O
using	O
complete	O
routing	O
information	O
for	O
each	O
packet	B-Protocol
(	O
source	O
and	O
destination	O
addresses	O
)	O
as	O
in	O
connectionless	B-Protocol
datagram	B-Protocol
switching	O
such	O
as	O
conventional	O
IP	O
routers	O
,	O
a	O
connection-oriented	B-Protocol
protocol	I-Protocol
identifies	O
traffic	O
flows	O
only	O
by	O
a	O
channel	O
or	O
data	O
stream	O
number	O
,	O
often	O
denoted	O
virtual	B-Protocol
circuit	I-Protocol
identifier	O
(	O
VCI	O
)	O
.	O
</s>
<s>
Thus	O
,	O
the	O
actual	O
packet	B-Protocol
switching	I-Protocol
and	O
data	O
transfer	O
can	O
be	O
taken	O
care	O
of	O
by	O
fast	O
hardware	O
,	O
as	O
opposed	O
to	O
slower	O
software-based	O
routing	O
.	O
</s>
<s>
Typically	O
,	O
this	O
connection	O
identifier	O
is	O
a	O
small	O
integer	O
(	O
for	O
example	O
,	O
10	O
bits	O
for	O
Frame	B-Protocol
Relay	I-Protocol
and	O
24	O
bits	O
for	O
ATM	O
)	O
.	O
</s>
<s>
ATM	O
and	O
Frame	B-Protocol
Relay	I-Protocol
,	O
for	O
example	O
,	O
are	O
both	O
examples	O
of	O
connection-oriented	B-Protocol
,	O
unreliable	B-Protocol
data	B-Protocol
link	I-Protocol
layer	I-Protocol
protocols	O
.	O
</s>
<s>
Reliable	B-Protocol
connectionless	B-Protocol
protocols	I-Protocol
exist	O
as	O
well	O
,	O
for	O
example	O
AX.25	B-Protocol
network	B-Application
layer	I-Application
protocol	O
when	O
it	O
passes	O
data	O
in	O
I-frames	O
,	O
but	O
this	O
combination	O
is	O
rare	O
,	O
and	O
reliable-connectionless	O
is	O
uncommon	O
in	O
modern	O
networks	O
.	O
</s>
<s>
Some	O
connection-oriented	B-Protocol
protocols	I-Protocol
have	O
been	O
designed	O
or	O
altered	O
to	O
accommodate	O
both	O
connection-oriented	B-Protocol
and	O
connectionless	B-Protocol
data	O
.	O
</s>
<s>
Examples	O
of	O
connection-oriented	B-Protocol
packet-mode	O
communication	O
,	O
i.e.	O
</s>
<s>
virtual	B-Protocol
circuit	I-Protocol
mode	O
communication	O
:	O
</s>
