<s>
A	O
network	O
socket	O
is	O
a	O
software	O
structure	O
within	O
a	O
network	B-Protocol
node	I-Protocol
of	O
a	O
computer	B-Architecture
network	I-Architecture
that	O
serves	O
as	O
an	O
endpoint	O
for	O
sending	O
and	O
receiving	O
data	O
across	O
the	O
network	O
.	O
</s>
<s>
The	O
structure	O
and	O
properties	O
of	O
a	O
socket	O
are	O
defined	O
by	O
an	O
application	B-Application
programming	I-Application
interface	I-Application
(	O
API	B-Application
)	O
for	O
the	O
networking	O
architecture	O
.	O
</s>
<s>
Sockets	O
are	O
created	O
only	O
during	O
the	O
lifetime	O
of	O
a	O
process	B-Operating_System
of	O
an	O
application	O
running	O
in	O
the	O
node	O
.	O
</s>
<s>
Because	O
of	O
the	O
standardization	O
of	O
the	O
TCP/IP	B-Protocol
protocols	I-Protocol
in	O
the	O
development	O
of	O
the	O
Internet	O
,	O
the	O
term	O
network	O
socket	O
is	O
most	O
commonly	O
used	O
in	O
the	O
context	O
of	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
,	O
and	O
is	O
therefore	O
often	O
also	O
referred	O
to	O
as	O
Internet	O
socket	O
.	O
</s>
<s>
In	O
this	O
context	O
,	O
a	O
socket	O
is	O
externally	O
identified	O
to	O
other	O
hosts	O
by	O
its	O
socket	O
address	O
,	O
which	O
is	O
the	O
triad	O
of	O
transport	B-Architecture
protocol	I-Architecture
,	O
IP	B-Protocol
address	I-Protocol
,	O
and	O
port	B-Protocol
number	I-Protocol
.	O
</s>
<s>
The	O
term	O
socket	O
is	O
also	O
used	O
for	O
the	O
software	O
endpoint	O
of	O
node-internal	O
inter-process	B-Operating_System
communication	I-Operating_System
(	O
IPC	O
)	O
,	O
which	O
often	O
uses	O
the	O
same	O
API	B-Application
as	O
a	O
network	O
socket	O
.	O
</s>
<s>
Similarly	O
,	O
the	O
term	O
port	B-Protocol
is	O
used	O
for	O
external	O
physical	O
endpoints	O
at	O
a	O
node	O
or	O
device	O
.	O
</s>
<s>
The	O
application	B-Application
programming	I-Application
interface	I-Application
(	O
API	B-Application
)	O
for	O
the	O
network	B-Protocol
protocol	I-Protocol
stack	I-Protocol
creates	O
a	O
handle	O
for	O
each	O
socket	O
created	O
by	O
an	O
application	O
,	O
commonly	O
referred	O
to	O
as	O
a	O
socket	O
descriptor	O
.	O
</s>
<s>
In	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
,	O
this	O
descriptor	O
is	O
a	O
type	O
of	O
file	B-Application
descriptor	I-Application
.	O
</s>
<s>
It	O
is	O
stored	O
by	O
the	O
application	O
process	B-Operating_System
for	O
use	O
with	O
every	O
read	O
and	O
write	O
operation	O
on	O
the	O
communication	O
channel	O
.	O
</s>
<s>
At	O
the	O
time	O
of	O
creation	O
with	O
the	O
API	B-Application
,	O
a	O
network	O
socket	O
is	O
bound	O
to	O
the	O
combination	O
of	O
a	O
type	O
of	O
network	O
protocol	O
to	O
be	O
used	O
for	O
transmissions	O
,	O
a	O
network	O
address	O
of	O
the	O
host	O
,	O
and	O
a	O
port	B-Protocol
number	I-Protocol
.	O
</s>
<s>
They	O
are	O
used	O
as	O
service	O
types	O
,	O
and	O
,	O
once	O
created	O
by	O
a	O
process	B-Operating_System
,	O
serve	O
as	O
an	O
externally	O
(	O
from	O
the	O
network	O
)	O
addressable	O
location	O
component	O
,	O
so	O
that	O
other	O
hosts	O
may	O
establish	O
connections	O
.	O
</s>
<s>
Network	O
sockets	O
may	O
be	O
dedicated	O
for	O
persistent	O
connections	O
for	O
communication	O
between	O
two	O
nodes	O
,	O
or	O
they	O
may	O
participate	O
in	O
connectionless	B-Protocol
and	O
multicast	O
communications	O
.	O
</s>
<s>
In	O
practice	O
,	O
due	O
to	O
the	O
proliferation	O
of	O
the	O
TCP/IP	B-Protocol
protocols	I-Protocol
in	O
use	O
on	O
the	O
Internet	O
,	O
the	O
term	O
network	O
socket	O
usually	O
refers	O
to	O
use	O
with	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	B-Protocol
)	O
.	O
</s>
<s>
An	O
application	O
can	O
communicate	O
with	O
a	O
remote	O
process	B-Operating_System
by	O
exchanging	O
data	O
with	O
TCP/IP	B-Protocol
by	O
knowing	O
the	O
combination	O
of	O
protocol	O
type	O
,	O
IP	B-Protocol
address	I-Protocol
,	O
and	O
port	B-Protocol
number	I-Protocol
.	O
</s>
<s>
The	O
remote	O
process	B-Operating_System
establishes	O
a	O
network	O
socket	O
in	O
its	O
own	O
instance	O
of	O
the	O
protocol	B-Protocol
stack	I-Protocol
,	O
and	O
uses	O
the	O
networking	O
API	B-Application
to	O
connect	O
to	O
the	O
application	O
,	O
presenting	O
its	O
own	O
socket	O
address	O
for	O
use	O
by	O
the	O
application	O
.	O
</s>
<s>
A	O
protocol	B-Protocol
stack	I-Protocol
,	O
usually	O
provided	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
(	O
rather	O
than	O
as	O
a	O
separate	O
library	O
,	O
for	O
instance	O
)	O
,	O
is	O
a	O
set	O
of	O
services	O
that	O
allow	O
processes	O
to	O
communicate	O
over	O
a	O
network	O
using	O
the	O
protocols	O
that	O
the	O
stack	O
implements	O
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
forwards	O
the	O
payload	O
of	O
incoming	O
IP	B-Protocol
packets	B-Protocol
to	O
the	O
corresponding	O
application	O
by	O
extracting	O
the	O
socket	O
address	O
information	O
from	O
the	O
IP	B-Protocol
and	O
transport	B-Architecture
protocol	I-Architecture
headers	B-General_Concept
and	O
stripping	O
the	O
headers	B-General_Concept
from	O
the	O
application	O
data	O
.	O
</s>
<s>
The	O
application	B-Application
programming	I-Application
interface	I-Application
(	O
API	B-Application
)	O
that	O
programs	O
use	O
to	O
communicate	O
with	O
the	O
protocol	B-Protocol
stack	I-Protocol
,	O
using	O
network	O
sockets	O
,	O
is	O
called	O
a	O
socket	O
API	B-Application
.	O
</s>
<s>
Development	O
of	O
application	O
programs	O
that	O
utilize	O
this	O
API	B-Application
is	O
called	O
socket	B-Operating_System
programming	I-Operating_System
or	O
network	B-Operating_System
programming	I-Operating_System
.	O
</s>
<s>
Internet	O
socket	O
APIs	B-Application
are	O
usually	O
based	O
on	O
the	O
Berkeley	B-Protocol
sockets	I-Protocol
standard	O
.	O
</s>
<s>
In	O
the	O
Berkeley	B-Protocol
sockets	I-Protocol
standard	O
,	O
sockets	O
are	O
a	O
form	O
of	O
file	B-Application
descriptor	I-Application
,	O
due	O
to	O
the	B-Operating_System
Unix	I-Operating_System
philosophy	I-Operating_System
that	O
"	O
everything	O
is	O
a	O
file	O
"	O
,	O
and	O
the	O
analogies	O
between	O
sockets	O
and	O
files	O
.	O
</s>
<s>
In	O
inter-process	B-Operating_System
communication	I-Operating_System
,	O
each	O
end	O
generally	O
has	O
its	O
own	O
socket	O
.	O
</s>
<s>
In	O
the	O
standard	O
Internet	B-Protocol
protocols	I-Protocol
TCP	B-Protocol
and	O
UDP	O
,	O
a	O
socket	O
address	O
is	O
the	O
combination	O
of	O
an	O
IP	B-Protocol
address	I-Protocol
and	O
a	O
port	B-Protocol
number	I-Protocol
,	O
much	O
like	O
one	O
end	O
of	O
a	O
telephone	O
connection	O
is	O
the	O
combination	O
of	O
a	O
phone	O
number	O
and	O
a	O
particular	O
extension	B-Application
.	O
</s>
<s>
Based	O
on	O
this	O
address	O
,	O
Internet	O
sockets	O
deliver	O
incoming	O
data	B-Protocol
packets	I-Protocol
to	O
the	O
appropriate	O
application	O
process	B-Operating_System
.	O
</s>
<s>
Socket	O
often	O
refers	O
specifically	O
to	O
an	O
internet	O
socket	O
or	O
TCP	B-Protocol
socket	O
.	O
</s>
<s>
protocol	O
:	O
A	O
transport	B-Architecture
protocol	I-Architecture
,	O
e.g.	O
,	O
TCP	B-Protocol
,	O
UDP	O
,	O
raw	O
IP	B-Protocol
.	O
</s>
<s>
This	O
means	O
that	O
(	O
local	O
or	O
remote	O
)	O
endpoints	O
with	O
TCP	B-Protocol
port	I-Protocol
53	O
and	O
UDP	B-Protocol
port	I-Protocol
53	O
are	O
distinct	O
sockets	O
,	O
while	O
IP	B-Protocol
does	O
not	O
have	O
ports	O
.	O
</s>
<s>
A	O
socket	O
that	O
has	O
been	O
connected	O
to	O
another	O
socket	O
,	O
e.g.	O
,	O
during	O
the	O
establishment	O
of	O
a	O
TCP	B-Protocol
connection	O
,	O
also	O
has	O
a	O
remote	O
socket	O
address	O
.	O
</s>
<s>
a	O
"	O
combination	O
of	O
an	O
IP	B-Protocol
address	I-Protocol
and	O
a	O
port	B-Protocol
number	I-Protocol
"	O
.	O
</s>
<s>
Within	O
the	O
operating	B-General_Concept
system	I-General_Concept
and	O
the	O
application	O
that	O
created	O
a	O
socket	O
,	O
a	O
socket	O
is	O
referred	O
to	O
by	O
a	O
unique	O
integer	O
value	O
called	O
a	O
socket	O
descriptor	O
.	O
</s>
<s>
On	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
and	O
Microsoft	B-Application
Windows	I-Application
,	O
the	O
command-line	O
tools	O
netstat	B-Application
or	O
ss	B-Protocol
are	O
used	O
to	O
list	O
established	O
sockets	O
and	O
related	O
information	O
.	O
</s>
<s>
This	O
example	O
,	O
modeled	O
according	O
to	O
the	O
Berkeley	B-Protocol
socket	I-Protocol
interface	I-Protocol
,	O
sends	O
the	O
string	O
"	O
Hello	O
,	O
world	O
!	O
"	O
</s>
<s>
via	O
TCP	B-Protocol
to	O
port	B-Protocol
80	O
of	O
the	O
host	O
with	O
address	O
1.2.3.4	O
.	O
</s>
<s>
Connectionless	B-Protocol
sockets	O
,	O
which	O
use	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
.	O
</s>
<s>
Each	O
packet	B-Protocol
sent	O
or	O
received	O
on	O
a	O
datagram	O
socket	O
is	O
individually	O
addressed	O
and	O
routed	O
.	O
</s>
<s>
Order	O
and	O
reliability	O
are	O
not	O
guaranteed	O
with	O
datagram	O
sockets	O
,	O
so	O
multiple	O
packets	B-Protocol
sent	O
from	O
one	O
machine	O
or	O
process	B-Operating_System
to	O
another	O
may	O
arrive	O
in	O
any	O
order	O
or	O
might	O
not	O
arrive	O
at	O
all	O
.	O
</s>
<s>
Special	O
configuration	O
may	O
be	O
required	O
to	O
send	O
broadcasts	B-Operating_System
on	O
a	O
datagram	O
socket	O
.	O
</s>
<s>
In	O
order	O
to	O
receive	O
broadcast	B-Operating_System
packets	I-Operating_System
,	O
a	O
datagram	O
socket	O
should	O
not	O
be	O
bound	O
to	O
a	O
specific	O
address	O
,	O
though	O
in	O
some	O
implementations	O
,	O
broadcast	B-Operating_System
packets	I-Operating_System
may	O
also	O
be	O
received	O
when	O
a	O
datagram	O
socket	O
is	O
bound	O
to	O
a	O
specific	O
address	O
.	O
</s>
<s>
Connection-oriented	B-Protocol
sockets	O
,	O
which	O
use	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	B-Protocol
)	O
,	O
Stream	B-Protocol
Control	I-Protocol
Transmission	I-Protocol
Protocol	I-Protocol
(	O
SCTP	B-Protocol
)	O
or	O
Datagram	B-Protocol
Congestion	I-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
DCCP	B-Protocol
)	O
.	O
</s>
<s>
A	O
stream	O
socket	O
transmits	O
data	O
reliably	B-Protocol
,	O
in	O
order	O
,	O
and	O
with	O
out-of-band	B-Protocol
capabilities	O
.	O
</s>
<s>
On	O
the	O
Internet	O
,	O
stream	O
sockets	O
are	O
typically	O
implemented	O
using	O
TCP	B-Protocol
so	O
that	O
applications	O
can	O
run	O
across	O
any	O
networks	O
using	O
TCP/IP	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
Allow	O
direct	O
sending	O
and	O
receiving	O
of	O
IP	B-Protocol
packets	B-Protocol
without	O
any	O
protocol-specific	O
transport	B-Architecture
layer	I-Architecture
formatting	O
.	O
</s>
<s>
With	O
other	O
types	O
of	O
sockets	O
,	O
the	O
payload	O
is	O
automatically	O
encapsulated	B-Protocol
according	O
to	O
the	O
chosen	O
transport	B-Architecture
layer	I-Architecture
protocol	O
(	O
e.g.	O
</s>
<s>
TCP	B-Protocol
,	O
UDP	O
)	O
,	O
and	O
the	O
socket	O
user	O
is	O
unaware	O
of	O
the	O
existence	O
of	O
protocol	O
headers	B-General_Concept
that	O
are	O
broadcast	B-Operating_System
with	O
the	O
payload	O
.	O
</s>
<s>
When	O
reading	O
from	O
a	O
raw	B-Protocol
socket	I-Protocol
,	O
the	O
headers	B-General_Concept
are	O
usually	O
included	O
.	O
</s>
<s>
When	O
transmitting	O
packets	B-Protocol
from	O
a	O
raw	B-Protocol
socket	I-Protocol
,	O
the	O
automatic	O
addition	O
of	O
a	O
header	B-General_Concept
is	O
optional	O
.	O
</s>
<s>
Most	O
socket	O
application	B-Application
programming	I-Application
interfaces	I-Application
(	O
APIs	B-Application
)	O
,	O
for	O
example	O
,	O
those	O
based	O
on	O
Berkeley	B-Protocol
sockets	I-Protocol
,	O
support	O
raw	B-Protocol
sockets	I-Protocol
.	O
</s>
<s>
Windows	B-Application
XP	I-Application
was	O
released	O
in	O
2001	O
with	O
raw	B-Protocol
socket	I-Protocol
support	O
implemented	O
in	O
the	O
Winsock	B-Application
interface	O
,	O
but	O
three	O
years	O
later	O
,	O
Microsoft	O
limited	O
Winsock	B-Application
's	O
raw	B-Protocol
socket	I-Protocol
support	O
because	O
of	O
security	O
concerns.Ian	O
Griffiths	O
for	O
IanG	O
on	O
Tap	O
.	O
</s>
<s>
Raw	B-Protocol
sockets	I-Protocol
are	O
used	O
in	O
security-related	O
applications	O
like	O
Nmap	B-Language
.	O
</s>
<s>
One	O
use	O
case	O
for	O
raw	B-Protocol
sockets	I-Protocol
is	O
the	O
implementation	O
of	O
new	O
transport-layer	B-Architecture
protocols	I-Architecture
in	O
user	B-Operating_System
space	I-Operating_System
.	O
</s>
<s>
Raw	B-Protocol
sockets	I-Protocol
are	O
typically	O
available	O
in	O
network	O
equipment	O
,	O
and	O
used	O
for	O
routing	B-Protocol
protocols	I-Protocol
such	O
as	O
the	O
Internet	B-Protocol
Group	I-Protocol
Management	I-Protocol
Protocol	I-Protocol
(	O
IGMP	B-Protocol
)	O
and	O
Open	B-Protocol
Shortest	I-Protocol
Path	I-Protocol
First	I-Protocol
(	O
OSPF	B-Protocol
)	O
,	O
and	O
in	O
the	O
Internet	B-Protocol
Control	I-Protocol
Message	I-Protocol
Protocol	I-Protocol
(	O
ICMP	O
)	O
used	O
,	O
among	O
other	O
things	O
,	O
by	O
the	O
ping	B-Protocol
utility	I-Protocol
.	O
</s>
<s>
Other	O
socket	O
types	O
are	O
implemented	O
over	O
other	O
transport	B-Architecture
protocols	I-Architecture
,	O
such	O
as	O
Systems	B-Architecture
Network	I-Architecture
Architecture	I-Architecture
and	O
Unix	B-Protocol
domain	I-Protocol
sockets	I-Protocol
for	O
internal	O
inter-process	B-Operating_System
communication	I-Operating_System
.	O
</s>
<s>
Computer	O
processes	O
that	O
provide	O
application	O
services	O
are	O
referred	O
to	O
as	O
servers	B-Application
,	O
and	O
create	O
sockets	O
on	O
startup	O
that	O
are	O
in	O
the	O
listening	O
state	O
.	O
</s>
<s>
These	O
sockets	O
are	O
waiting	O
for	O
initiatives	O
from	O
client	B-Protocol
programs	I-Protocol
.	O
</s>
<s>
A	O
TCP	B-Protocol
server	O
may	O
serve	O
several	O
clients	O
concurrently	O
by	O
creating	O
a	O
unique	O
dedicated	O
socket	O
for	O
each	O
client	B-Protocol
connection	O
in	O
a	O
new	O
child	O
process	B-Operating_System
or	O
processing	O
thread	O
for	O
each	O
client	B-Protocol
.	O
</s>
<s>
These	O
are	O
in	O
the	O
established	O
state	O
when	O
a	O
socket-to-socket	O
virtual	B-Protocol
connection	I-Protocol
or	O
virtual	B-Protocol
circuit	I-Protocol
(	O
VC	O
)	O
,	O
also	O
known	O
as	O
a	O
TCP	B-Protocol
session	B-Protocol
,	O
is	O
established	O
with	O
the	O
remote	O
socket	O
,	O
providing	O
a	O
duplex	O
byte	O
stream	O
.	O
</s>
<s>
A	O
server	O
may	O
create	O
several	O
concurrently	O
established	O
TCP	B-Protocol
sockets	O
with	O
the	O
same	O
local	O
port	B-Protocol
number	I-Protocol
and	O
local	O
IP	B-Protocol
address	I-Protocol
,	O
each	O
mapped	O
to	O
its	O
own	O
server-child	O
process	B-Operating_System
,	O
serving	O
its	O
own	O
client	B-Protocol
process	B-Operating_System
.	O
</s>
<s>
They	O
are	O
treated	O
as	O
different	O
sockets	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
since	O
the	O
remote	O
socket	O
address	O
(	O
the	O
client	B-Protocol
IP	B-Protocol
address	I-Protocol
or	O
port	B-Protocol
number	I-Protocol
)	O
is	O
different	O
;	O
i.e.	O
</s>
<s>
UDP	O
sockets	O
do	O
not	O
have	O
an	O
established	O
state	O
,	O
because	O
the	O
protocol	O
is	O
connectionless	B-Protocol
.	O
</s>
<s>
A	O
UDP	O
server	O
process	B-Operating_System
handles	O
incoming	O
datagrams	O
from	O
all	O
remote	O
clients	O
sequentially	O
through	O
the	O
same	O
socket	O
.	O
</s>
<s>
UDP	O
sockets	O
are	O
not	O
identified	O
by	O
the	O
remote	O
address	O
,	O
but	O
only	O
by	O
the	O
local	O
address	O
,	O
although	O
each	O
message	O
has	O
an	O
associated	O
remote	O
address	O
that	O
can	O
be	O
retrieved	O
from	O
each	O
datagram	O
with	O
the	O
networking	O
application	B-Application
programming	I-Application
interface	I-Application
(	O
API	B-Application
)	O
.	O
</s>
<s>
Each	O
socket	O
pair	O
is	O
described	O
by	O
a	O
unique	O
4-tuple	O
consisting	O
of	O
source	O
and	O
destination	O
IP	B-Protocol
addresses	I-Protocol
and	O
port	B-Protocol
numbers	I-Protocol
,	O
i.e.	O
</s>
<s>
of	O
local	O
and	O
remote	O
socket	O
addresses.books.google.com	O
-	O
UNIX	B-Application
Network	B-Operating_System
Programming	I-Operating_System
:	O
The	O
sockets	O
networking	O
APIbooks.google.com	O
-	O
Designing	O
BSD	B-Operating_System
Rootkits	O
:	O
An	O
Introduction	O
to	O
Kernel	O
Hacking	O
As	O
discussed	O
above	O
,	O
in	O
the	O
TCP	B-Protocol
case	O
,	O
a	O
socket	O
pair	O
is	O
associated	O
on	O
each	O
end	O
of	O
the	O
connection	O
with	O
a	O
unique	O
4-tuple	O
.	O
</s>
<s>
Most	O
modern	O
implementations	O
of	O
sockets	O
are	O
based	O
on	O
Berkeley	B-Protocol
sockets	I-Protocol
(	O
1983	O
)	O
,	O
and	O
other	O
stacks	O
such	O
as	O
Winsock	B-Application
(	O
1991	O
)	O
.	O
</s>
<s>
The	O
Berkeley	B-Protocol
sockets	I-Protocol
API	B-Application
in	O
the	O
Berkeley	B-Operating_System
Software	I-Operating_System
Distribution	I-Operating_System
(	O
BSD	B-Operating_System
)	O
,	O
originated	O
with	O
the	O
4.2BSD	O
Unix	B-Application
operating	I-Application
system	I-Application
as	O
an	O
API	B-Application
.	O
</s>
<s>
Only	O
in	O
1989	O
,	O
however	O
,	O
could	O
UC	O
Berkeley	O
release	O
versions	O
of	O
its	O
operating	B-General_Concept
system	I-General_Concept
and	O
networking	O
library	O
free	O
from	O
the	O
licensing	O
constraints	O
of	O
T	O
's	O
copyright-protected	O
Unix	B-Application
.	O
</s>
<s>
In	O
c	O
.	O
1987	O
,	O
AT&T	O
introduced	O
the	O
STREAMS-based	O
Transport	B-Application
Layer	I-Application
Interface	I-Application
(	O
TLI	O
)	O
in	O
UNIX	B-Operating_System
System	I-Operating_System
V	I-Operating_System
Release	O
3	O
(	O
SVR3	B-Operating_System
)	O
.	O
</s>
<s>
Other	O
early	O
implementations	O
were	O
written	O
for	O
TOPS-20	B-Operating_System
,	O
MVS	B-Application
,	O
VM	B-Application
,	O
IBM-DOS	B-Operating_System
(	O
PCIP	O
)	O
.	O
</s>
<s>
The	O
socket	O
is	O
primarily	O
a	O
concept	O
used	O
in	O
the	O
transport	B-Architecture
layer	I-Architecture
of	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
or	O
session	B-Application
layer	I-Application
of	O
the	O
OSI	O
model	O
.	O
</s>
<s>
Networking	O
equipment	O
such	O
as	O
routers	B-Protocol
,	O
which	O
operate	O
at	O
the	O
internet	B-Protocol
layer	I-Protocol
,	O
and	O
switches	B-Protocol
,	O
which	O
operate	O
at	O
the	O
link	B-Protocol
layer	I-Protocol
,	O
do	O
not	O
require	O
implementations	O
of	O
the	O
transport	B-Architecture
layer	I-Architecture
.	O
</s>
<s>
However	O
,	O
stateful	O
network	B-Application
firewalls	I-Application
,	O
network	B-Protocol
address	I-Protocol
translators	I-Protocol
,	O
and	O
proxy	O
servers	B-Application
keep	O
track	O
of	O
active	O
socket	O
pairs	O
.	O
</s>
<s>
In	O
multilayer	B-Application
switches	I-Application
and	O
quality	O
of	O
service	O
(	O
QoS	O
)	O
support	O
in	O
routers	B-Protocol
,	O
packet	B-Operating_System
flows	I-Operating_System
may	O
be	O
identified	O
by	O
extracting	O
information	O
about	O
the	O
socket	O
pairs	O
.	O
</s>
<s>
Raw	B-Protocol
sockets	I-Protocol
are	O
typically	O
available	O
in	O
network	O
equipment	O
and	O
are	O
used	O
for	O
routing	B-Protocol
protocols	I-Protocol
such	O
as	O
IGRP	B-Protocol
and	O
OSPF	B-Protocol
,	O
and	O
for	O
Internet	B-Protocol
Control	I-Protocol
Message	I-Protocol
Protocol	I-Protocol
(	O
ICMP	O
)	O
.	O
</s>
