<s>
In	O
computer	B-Architecture
networking	I-Architecture
,	O
the	O
transport	B-Architecture
layer	I-Architecture
is	O
a	O
conceptual	O
division	O
of	O
methods	O
in	O
the	O
layered	B-Architecture
architecture	I-Architecture
of	O
protocols	O
in	O
the	O
network	O
stack	O
in	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
and	O
the	O
OSI	O
model	O
.	O
</s>
<s>
It	O
provides	O
services	O
such	O
as	O
connection-oriented	B-Protocol
communication	I-Protocol
,	O
reliability	B-Protocol
,	O
flow	O
control	O
,	O
and	O
multiplexing	B-Architecture
.	O
</s>
<s>
The	O
details	O
of	O
implementation	O
and	O
semantics	O
of	O
the	O
transport	B-Architecture
layer	I-Architecture
of	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
,	O
which	O
is	O
the	O
foundation	O
of	O
the	O
Internet	O
,	O
and	O
the	O
OSI	O
model	O
of	O
general	O
networking	O
are	O
different	O
.	O
</s>
<s>
The	O
protocols	O
in	O
use	O
today	O
in	O
this	O
layer	O
for	O
the	O
Internet	O
all	O
originated	O
in	O
the	O
development	O
of	O
TCP/IP	B-Protocol
.	O
</s>
<s>
In	O
the	O
OSI	O
model	O
the	O
transport	B-Architecture
layer	I-Architecture
is	O
often	O
referred	O
to	O
as	O
Layer	B-Architecture
4	I-Architecture
,	O
or	O
L4	O
,	O
while	O
numbered	O
layers	O
are	O
not	O
used	O
in	O
TCP/IP	B-Protocol
.	O
</s>
<s>
The	O
best-known	O
transport	B-Architecture
protocol	I-Architecture
of	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
is	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	B-Protocol
)	O
.	O
</s>
<s>
It	O
is	O
used	O
for	O
connection-oriented	B-Protocol
transmissions	O
,	O
whereas	O
the	O
connectionless	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	B-Protocol
)	O
is	O
used	O
for	O
simpler	O
messaging	O
transmissions	O
.	O
</s>
<s>
TCP	B-Protocol
is	O
the	O
more	O
complex	O
protocol	O
,	O
due	O
to	O
its	O
stateful	B-Application
design	I-Application
incorporating	O
reliable	O
transmission	O
and	O
data	B-General_Concept
stream	I-General_Concept
services	O
.	O
</s>
<s>
Together	O
,	O
TCP	B-Protocol
and	O
UDP	B-Protocol
comprise	O
essentially	O
all	O
traffic	O
on	O
the	O
Internet	O
and	O
are	O
the	O
only	O
protocols	O
implemented	O
in	O
every	O
major	O
operating	O
system	O
.	O
</s>
<s>
Additional	O
transport	B-Architecture
layer	I-Architecture
protocols	O
that	O
have	O
been	O
defined	O
and	O
implemented	O
include	O
the	O
Datagram	B-Protocol
Congestion	I-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
DCCP	B-Protocol
)	O
and	O
the	O
Stream	B-Protocol
Control	I-Protocol
Transmission	I-Protocol
Protocol	I-Protocol
(	O
SCTP	B-Protocol
)	O
.	O
</s>
<s>
Transport	B-Architecture
layer	I-Architecture
services	O
are	O
conveyed	O
to	O
an	O
application	B-Application
via	O
a	O
programming	O
interface	O
to	O
the	O
transport	B-Architecture
layer	I-Architecture
protocols	O
.	O
</s>
<s>
Connection-oriented	B-Protocol
communication	I-Protocol
:	O
It	O
is	O
normally	O
easier	O
for	O
an	O
application	B-Application
to	O
interpret	O
a	O
connection	O
as	O
a	O
data	B-General_Concept
stream	I-General_Concept
rather	O
than	O
having	O
to	O
deal	O
with	O
the	O
underlying	O
connection-less	O
models	O
,	O
such	O
as	O
the	O
datagram	B-Protocol
model	O
of	O
the	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	B-Protocol
)	O
and	O
of	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
.	O
</s>
<s>
Same	O
order	O
delivery	O
:	O
The	O
network	O
layer	O
does	O
n't	O
generally	O
guarantee	O
that	O
packets	B-Protocol
of	O
data	O
will	O
arrive	O
in	O
the	O
same	O
order	O
that	O
they	O
were	O
sent	O
,	O
but	O
often	O
this	O
is	O
a	O
desirable	O
feature	O
.	O
</s>
<s>
This	O
is	O
usually	O
done	O
through	O
the	O
use	O
of	O
segment	O
numbering	O
,	O
with	O
the	O
receiver	O
passing	O
them	O
to	O
the	O
application	B-Application
in	O
order	O
.	O
</s>
<s>
This	O
can	O
cause	O
head-of-line	B-Protocol
blocking	I-Protocol
.	O
</s>
<s>
Reliability	B-Protocol
:	O
Packets	B-Protocol
may	O
be	O
lost	O
during	O
transport	O
due	O
to	O
network	B-Protocol
congestion	I-Protocol
and	O
errors	O
.	O
</s>
<s>
By	O
means	O
of	O
an	O
error	B-Error_Name
detection	I-Error_Name
code	I-Error_Name
,	O
such	O
as	O
a	O
checksum	B-Algorithm
,	O
the	O
transport	B-Architecture
protocol	I-Architecture
may	O
check	O
that	O
the	O
data	O
is	O
not	O
corrupted	O
,	O
and	O
verify	O
correct	O
receipt	O
by	O
sending	O
an	O
ACK	B-Protocol
or	O
NACK	B-Protocol
message	O
to	O
the	O
sender	O
.	O
</s>
<s>
Automatic	B-Error_Name
repeat	I-Error_Name
request	I-Error_Name
schemes	O
may	O
be	O
used	O
to	O
retransmit	O
lost	O
or	O
corrupted	O
data	O
.	O
</s>
<s>
Flow	O
control	O
:	O
The	O
rate	O
of	O
data	O
transmission	O
between	O
two	O
nodes	O
must	O
sometimes	O
be	O
managed	O
to	O
prevent	O
a	O
fast	O
sender	O
from	O
transmitting	O
more	O
data	O
than	O
can	O
be	O
supported	O
by	O
the	O
receiving	O
data	B-General_Concept
buffer	I-General_Concept
,	O
causing	O
a	O
buffer	B-General_Concept
overrun	O
.	O
</s>
<s>
This	O
can	O
also	O
be	O
used	O
to	O
improve	O
efficiency	O
by	O
reducing	O
buffer	B-General_Concept
underrun	O
.	O
</s>
<s>
Congestion	O
avoidance	O
:	O
Congestion	O
control	O
can	O
control	O
traffic	O
entry	O
into	O
a	O
telecommunications	O
network	O
,	O
so	O
as	O
to	O
avoid	O
congestive	O
collapse	O
by	O
attempting	O
to	O
avoid	O
oversubscription	O
of	O
any	O
of	O
the	O
processing	O
or	O
link	O
capabilities	O
of	O
the	O
intermediate	O
nodes	O
and	O
networks	O
and	O
taking	O
resource	O
reducing	O
steps	O
,	O
such	O
as	O
reducing	O
the	O
rate	O
of	O
sending	O
packets	B-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
automatic	B-Error_Name
repeat	I-Error_Name
requests	I-Error_Name
may	O
keep	O
the	O
network	O
in	O
a	O
congested	O
state	O
;	O
this	O
situation	O
can	O
be	O
avoided	O
by	O
adding	O
congestion	O
avoidance	O
to	O
the	O
flow	O
control	O
,	O
including	O
slow	O
start	O
.	O
</s>
<s>
This	O
keeps	O
the	O
bandwidth	O
consumption	O
at	O
a	O
low	O
level	O
in	O
the	O
beginning	O
of	O
the	O
transmission	O
,	O
or	O
after	O
packet	B-Protocol
retransmission	O
.	O
</s>
<s>
Multiplexing	B-Architecture
:	O
Ports	B-Protocol
can	O
provide	O
multiple	O
endpoints	O
on	O
a	O
single	O
node	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
name	O
on	O
a	O
postal	O
address	O
is	O
a	O
kind	O
of	O
multiplexing	B-Architecture
and	O
distinguishes	O
between	O
different	O
recipients	O
of	O
the	O
same	O
location	O
.	O
</s>
<s>
Computer	B-Application
applications	I-Application
will	O
each	O
listen	O
for	O
information	O
on	O
their	O
own	O
ports	B-Protocol
,	O
which	O
enables	O
the	O
use	O
of	O
more	O
than	O
one	O
network	B-General_Concept
service	I-General_Concept
at	O
the	O
same	O
time	O
.	O
</s>
<s>
It	O
is	O
part	O
of	O
the	O
transport	B-Architecture
layer	I-Architecture
in	O
the	O
TCP/IP	B-Protocol
model	I-Protocol
,	O
but	O
of	O
the	O
session	B-Application
layer	I-Application
in	O
the	O
OSI	O
model	O
.	O
</s>
<s>
The	O
transport	B-Architecture
layer	I-Architecture
is	O
responsible	O
for	O
delivering	O
data	O
to	O
the	O
appropriate	O
application	B-Application
process	O
on	O
the	O
host	O
computers	O
.	O
</s>
<s>
This	O
involves	O
statistical	B-Algorithm
multiplexing	I-Algorithm
of	O
data	O
from	O
different	O
application	B-Application
processes	O
,	O
i.e.	O
</s>
<s>
forming	O
data	O
segments	O
,	O
and	O
adding	O
source	O
and	O
destination	O
port	B-Protocol
numbers	I-Protocol
in	O
the	O
header	O
of	O
each	O
transport	B-Architecture
layer	I-Architecture
data	O
segment	O
.	O
</s>
<s>
Together	O
with	O
the	O
source	O
and	O
destination	O
IP	O
address	O
,	O
the	O
port	B-Protocol
numbers	I-Protocol
constitute	O
a	O
network	B-Protocol
socket	I-Protocol
,	O
i.e.	O
</s>
<s>
In	O
the	O
OSI	O
model	O
,	O
this	O
function	O
is	O
supported	O
by	O
the	O
session	B-Application
layer	I-Application
.	O
</s>
<s>
Some	O
transport	B-Architecture
layer	I-Architecture
protocols	O
,	O
for	O
example	O
TCP	B-Protocol
,	O
but	O
not	O
UDP	B-Protocol
,	O
support	O
virtual	B-Protocol
circuits	I-Protocol
,	O
i.e.	O
</s>
<s>
provide	O
connection-oriented	B-Protocol
communication	I-Protocol
over	O
an	O
underlying	O
packet-oriented	O
datagram	B-Protocol
network	O
.	O
</s>
<s>
A	O
byte-stream	O
is	O
delivered	O
while	O
hiding	O
the	O
packet	B-Algorithm
mode	I-Algorithm
communication	O
for	O
the	O
application	B-Application
processes	O
.	O
</s>
<s>
This	O
involves	O
connection	O
establishment	O
,	O
dividing	O
of	O
the	O
data	B-General_Concept
stream	I-General_Concept
into	O
packets	B-Protocol
called	O
segments	O
,	O
segment	O
numbering	O
and	O
reordering	O
of	O
out-of-order	O
data	O
.	O
</s>
<s>
Finally	O
,	O
some	O
transport	B-Architecture
layer	I-Architecture
protocols	O
,	O
for	O
example	O
TCP	B-Protocol
,	O
but	O
not	O
UDP	B-Protocol
,	O
provide	O
end-to-end	O
reliable	O
communication	O
,	O
i.e.	O
</s>
<s>
error	B-Error_Name
recovery	I-Error_Name
by	O
means	O
of	O
error	B-Error_Name
detecting	I-Error_Name
code	I-Error_Name
and	O
automatic	B-Error_Name
repeat	I-Error_Name
request	I-Error_Name
(	O
ARQ	O
)	O
protocol	O
.	O
</s>
<s>
The	O
ARQ	B-Error_Name
protocol	I-Error_Name
also	O
provides	O
flow	O
control	O
,	O
which	O
may	O
be	O
combined	O
with	O
congestion	O
avoidance	O
.	O
</s>
<s>
UDP	B-Protocol
is	O
a	O
very	O
simple	O
protocol	O
and	O
does	O
not	O
provide	O
virtual	B-Protocol
circuits	I-Protocol
,	O
nor	O
reliable	O
communication	O
,	O
delegating	O
these	O
functions	O
to	O
the	O
application	B-Application
program	I-Application
.	O
</s>
<s>
UDP	B-Protocol
packets	I-Protocol
are	O
called	O
datagrams	B-Protocol
,	O
rather	O
than	O
segments	O
.	O
</s>
<s>
TCP	B-Protocol
is	O
used	O
for	O
many	O
protocols	O
,	O
including	O
HTTP	B-Protocol
web	O
browsing	O
and	O
email	O
transfer	O
.	O
</s>
<s>
UDP	B-Protocol
may	O
be	O
used	O
for	O
multicasting	O
and	O
broadcasting	B-Operating_System
,	O
since	O
retransmissions	O
are	O
not	O
possible	O
to	O
a	O
large	O
amount	O
of	O
hosts	O
.	O
</s>
<s>
UDP	B-Protocol
typically	O
gives	O
higher	O
throughput	O
and	O
shorter	O
latency	O
and	O
is	O
therefore	O
often	O
used	O
for	O
real-time	O
multimedia	O
communication	O
where	O
packet	B-Protocol
loss	O
occasionally	O
can	O
be	O
accepted	O
,	O
for	O
example	O
IP-TV	O
and	O
IP-telephony	O
,	O
and	O
for	O
online	O
computer	O
games	O
.	O
</s>
<s>
Many	O
non-IP-based	O
networks	O
,	O
such	O
as	O
X.25	B-Protocol
,	O
Frame	B-Protocol
Relay	I-Protocol
and	O
ATM	O
,	O
implement	O
the	O
connection-oriented	B-Protocol
communication	I-Protocol
at	O
the	O
network	O
or	O
data	O
link	O
layer	O
rather	O
than	O
the	O
transport	B-Architecture
layer	I-Architecture
.	O
</s>
<s>
In	O
X.25	B-Protocol
,	O
in	O
telephone	O
network	O
modems	O
and	O
in	O
wireless	O
communication	O
systems	O
,	O
reliable	O
node-to-node	O
communication	O
is	O
implemented	O
at	O
lower	O
protocol	O
layers	O
.	O
</s>
<s>
The	O
OSI	O
connection-mode	O
transport	B-Architecture
layer	I-Architecture
protocol	O
specification	O
defines	O
five	O
classes	O
of	O
transport	B-Architecture
protocols	I-Architecture
:	O
TP0	O
,	O
providing	O
the	O
least	O
error	B-Error_Name
recovery	I-Error_Name
,	O
to	O
TP4	O
,	O
which	O
is	O
designed	O
for	O
less	O
reliable	O
networks	O
.	O
</s>
<s>
Due	O
to	O
protocol	B-Protocol
ossification	I-Protocol
,	O
TCP	B-Protocol
and	O
UDP	B-Protocol
are	O
the	O
only	O
widely-used	O
transport	B-Architecture
protocols	I-Architecture
on	O
the	O
Internet	O
.	O
</s>
<s>
To	O
avoid	O
middlebox	O
intolerance	O
,	O
new	O
transport	B-Architecture
protocols	I-Architecture
may	O
mimic	O
the	O
wire	O
image	O
of	O
a	O
tolerated	O
protocol	O
,	O
or	O
be	B-Protocol
encapsulated	I-Protocol
in	O
UDP	B-Protocol
,	O
accepting	O
some	O
overhead	O
(	O
e.g.	O
,	O
due	O
to	O
outer	O
checksums	B-Algorithm
made	O
redundant	O
by	O
inner	O
integrity	O
checks	O
)	O
.	O
</s>
<s>
QUIC	B-Protocol
takes	O
the	O
latter	O
approach	O
,	O
rebuilding	O
reliable	O
stream	O
transport	O
on	O
top	O
of	O
UDP	B-Protocol
.	O
</s>
<s>
This	O
list	O
shows	O
some	O
protocols	O
that	O
are	O
commonly	O
placed	O
in	O
the	O
transport	B-Architecture
layers	I-Architecture
of	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
,	O
the	O
OSI	B-Protocol
protocol	I-Protocol
suite	I-Protocol
,	O
NetWare	B-Operating_System
's	O
IPX/SPX	B-Protocol
,	O
AppleTalk	B-Application
,	O
and	O
Fibre	B-Architecture
Channel	I-Architecture
.	O
</s>
<s>
Feature	O
UDP	B-Protocol
UDP-Lite	B-Protocol
TCP	B-Protocol
Multipath	B-Protocol
TCP	I-Protocol
SCTP	B-Protocol
DCCP	B-Protocol
RUDP	B-Protocol
Packet	B-Protocol
header	O
size	O
8	O
bytes	O
8	O
bytes	O
20	O
–	O
60	O
bytes	O
50	O
–	O
90	O
bytes	O
12	O
bytes	O
12	O
or	O
16	O
bytes	O
14+	O
bytes	O
Typical	O
data-packet	O
overhead	O
8	O
bytes	O
8	O
bytes	O
20	O
bytes	O
?	O
</s>
<s>
ISO/IEC	O
8073/ITU	O
-T	O
Recommendation	O
X.224	O
,	O
"	O
Information	O
Technology	O
-	O
Open	O
Systems	O
Interconnection	O
-	O
Protocol	O
for	O
providing	O
the	O
connection-mode	O
transport	O
service	O
"	O
,	O
defines	O
five	O
classes	O
of	O
connection-mode	O
transport	B-Architecture
protocols	I-Architecture
designated	O
class	O
0	O
(	O
TP0	O
)	O
to	O
class	O
4	O
(	O
TP4	O
)	O
.	O
</s>
<s>
Class	O
0	O
contains	O
no	O
error	B-Error_Name
recovery	I-Error_Name
and	O
was	O
designed	O
for	O
use	O
on	O
network	O
layers	O
that	O
provide	O
error-free	O
connections	O
.	O
</s>
<s>
Class	O
4	O
is	O
closest	O
to	O
TCP	B-Protocol
,	O
although	O
TCP	B-Protocol
contains	O
functions	O
,	O
such	O
as	O
the	O
graceful	O
close	O
,	O
which	O
OSI	O
assigns	O
to	O
the	O
session	B-Application
layer	I-Application
.	O
</s>
<s>
There	O
is	O
also	O
a	O
connectionless	O
transport	B-Architecture
protocol	I-Architecture
,	O
specified	O
by	O
ISO/IEC	O
8602/ITU	O
-T	O
Recommendation	O
X.234	O
.	O
</s>
