<s>
WebSocket	B-Protocol
is	O
a	O
computer	O
communications	O
protocol	O
,	O
providing	O
full-duplex	O
communication	O
channels	O
over	O
a	O
single	O
TCP	B-Protocol
connection	O
.	O
</s>
<s>
The	O
WebSocket	B-Protocol
protocol	O
was	O
standardized	O
by	O
the	O
IETF	O
as	O
in	O
2011	O
.	O
</s>
<s>
The	O
current	O
API	O
specification	O
allowing	O
web	O
applications	O
to	O
use	O
this	O
protocol	O
is	O
known	O
as	O
WebSockets	B-Protocol
.	O
</s>
<s>
It	O
is	O
a	O
living	O
standard	O
maintained	O
by	O
the	O
WHATWG	B-Language
and	O
a	O
successor	O
to	O
The	O
WebSocket	B-Protocol
API	I-Protocol
from	O
the	O
W3C	O
.	O
</s>
<s>
WebSocket	B-Protocol
is	O
distinct	O
from	O
HTTP	B-Protocol
.	I-Protocol
</s>
<s>
Both	O
protocols	O
are	O
located	O
at	O
layer	O
7	O
in	O
the	O
OSI	O
model	O
and	O
depend	O
on	O
TCP	B-Protocol
at	O
layer4	O
.	O
</s>
<s>
Although	O
they	O
are	O
different	O
,	O
states	O
that	O
WebSocket	B-Protocol
"	O
is	O
designed	O
to	O
work	O
over	O
HTTP	B-Protocol
ports	O
443	O
and	O
80	O
as	O
well	O
as	O
to	O
support	O
HTTP	B-Protocol
proxies	O
and	O
intermediaries	O
"	O
,	O
thus	O
making	O
it	O
compatible	O
with	O
HTTP	B-Protocol
.	I-Protocol
</s>
<s>
To	O
achieve	O
compatibility	O
,	O
the	O
WebSocket	B-Protocol
handshake	B-Protocol
uses	O
the	O
HTTP	B-Protocol
Upgrade	I-Protocol
header	I-Protocol
to	O
change	O
from	O
the	O
HTTP	B-Protocol
protocol	I-Protocol
to	O
the	O
WebSocket	B-Protocol
protocol	O
.	O
</s>
<s>
The	O
WebSocket	B-Protocol
protocol	O
enables	O
interaction	O
between	O
a	O
web	B-Application
browser	I-Application
(	O
or	O
other	O
client	B-Protocol
application	I-Protocol
)	O
and	O
a	O
web	B-Application
server	I-Application
with	O
lower	O
overhead	O
than	O
half-duplex	O
alternatives	O
such	O
as	O
HTTP	B-Protocol
polling	B-General_Concept
,	O
facilitating	O
real-time	O
data	O
transfer	O
from	O
and	O
to	O
the	O
server	O
.	O
</s>
<s>
This	O
is	O
made	O
possible	O
by	O
providing	O
a	O
standardized	O
way	O
for	O
the	O
server	O
to	O
send	O
content	O
to	O
the	O
client	B-Protocol
without	O
being	O
first	O
requested	O
by	O
the	O
client	B-Protocol
,	O
and	O
allowing	O
messages	O
to	O
be	O
passed	O
back	O
and	O
forth	O
while	O
keeping	O
the	O
connection	O
open	O
.	O
</s>
<s>
In	O
this	O
way	O
,	O
a	O
two-way	O
ongoing	O
conversation	O
can	O
take	O
place	O
between	O
the	O
client	B-Protocol
and	O
the	O
server	O
.	O
</s>
<s>
The	O
communications	O
are	O
usually	O
done	O
over	O
TCP	B-Protocol
port	I-Protocol
number	O
443	O
(	O
or	O
80	O
in	O
the	O
case	O
of	O
unsecured	O
connections	O
)	O
,	O
which	O
is	O
beneficial	O
for	O
environments	O
that	O
block	O
non-web	O
Internet	O
connections	O
using	O
a	O
firewall	B-Application
.	O
</s>
<s>
Similar	O
two-way	O
browser	B-Application
–	O
server	O
communications	O
have	O
been	O
achieved	O
in	O
non-standardized	O
ways	O
using	O
stopgap	O
technologies	O
such	O
as	O
Comet	B-General_Concept
or	O
Adobe	B-Operating_System
Flash	I-Operating_System
Player	I-Operating_System
.	O
</s>
<s>
Most	O
browsers	B-Application
support	O
the	O
protocol	O
,	O
including	O
Google	B-Application
Chrome	I-Application
,	O
Firefox	B-Application
,	O
Microsoft	B-Application
Edge	I-Application
,	O
Internet	B-Application
Explorer	I-Application
,	O
Safari	B-Application
and	O
Opera	B-Application
.	O
</s>
<s>
Unlike	O
HTTP	B-Protocol
,	O
WebSocket	B-Protocol
provides	O
full-duplex	O
communication	O
.	O
</s>
<s>
Additionally	O
,	O
WebSocket	B-Protocol
enables	O
streams	O
of	O
messages	O
on	O
top	O
of	O
TCP	B-Protocol
.	O
</s>
<s>
TCP	B-Protocol
alone	O
deals	O
with	O
streams	O
of	O
bytes	O
with	O
no	O
inherent	O
concept	O
of	O
a	O
message	O
.	O
</s>
<s>
Before	O
WebSocket	B-Protocol
,	O
port	B-Protocol
80	O
full-duplex	O
communication	O
was	O
attainable	O
using	O
Comet	B-General_Concept
channels	O
;	O
however	O
,	O
Comet	B-General_Concept
implementation	O
is	O
nontrivial	O
,	O
and	O
due	O
to	O
the	O
TCP	B-Protocol
handshake	B-Protocol
and	O
HTTP	B-Protocol
header	O
overhead	O
,	O
it	O
is	O
inefficient	O
for	O
small	O
messages	O
.	O
</s>
<s>
The	O
WebSocket	B-Protocol
protocol	O
aims	O
to	O
solve	O
these	O
problems	O
without	O
compromising	O
the	O
security	O
assumptions	O
of	O
the	O
web	O
.	O
</s>
<s>
The	O
WebSocket	B-Protocol
protocol	O
specification	O
defines	O
ws	O
(	O
WebSocket	B-Protocol
)	O
and	O
wss	O
(	O
WebSocket	B-Protocol
Secure	I-Protocol
)	O
as	O
two	O
new	O
uniform	B-Protocol
resource	I-Protocol
identifier	I-Protocol
(	O
URI	B-Protocol
)	O
schemes	O
that	O
are	O
used	O
for	O
unencrypted	O
and	O
encrypted	O
connections	O
respectively	O
.	O
</s>
<s>
#	O
is	O
not	O
supported	O
)	O
,	O
the	O
rest	O
of	O
the	O
URI	B-Protocol
components	O
are	O
defined	O
to	O
use	O
URI	B-Protocol
generic	O
syntax	O
.	O
</s>
<s>
Using	O
browser	B-Application
developer	O
tools	O
,	O
developers	O
can	O
inspect	O
the	O
WebSocket	B-Protocol
handshake	B-Protocol
as	O
well	O
as	O
the	O
WebSocket	B-Protocol
frames	O
.	O
</s>
<s>
WebSocket	B-Protocol
was	O
first	O
referenced	O
as	O
TCPConnection	O
in	O
the	O
HTML5	B-Application
specification	O
,	O
as	O
a	O
placeholder	O
for	O
a	O
TCP-based	O
socket	B-Protocol
API	I-Protocol
.	O
</s>
<s>
In	O
June	O
2008	O
,	O
a	O
series	O
of	O
discussions	O
were	O
led	O
by	O
Michael	B-Application
Carter	I-Application
that	O
resulted	O
in	O
the	O
first	O
version	O
of	O
the	O
protocol	O
known	O
as	O
WebSocket	B-Protocol
.	O
</s>
<s>
The	O
name	O
"	O
WebSocket	B-Protocol
"	O
was	O
coined	O
by	O
Ian	O
Hickson	O
and	O
Michael	B-Application
Carter	I-Application
shortly	O
thereafter	O
through	O
collaboration	O
on	O
the	O
#whatwg	O
IRC	O
chat	O
room	O
,	O
and	O
subsequently	O
authored	O
for	O
inclusion	O
in	O
the	O
HTML5	B-Application
specification	O
by	O
Ian	O
Hickson	O
.	O
</s>
<s>
In	O
December	O
2009	O
,	O
Google	B-Application
Chrome	I-Application
4	I-Application
was	O
the	O
first	O
browser	B-Application
to	O
ship	O
full	O
support	O
for	O
the	O
standard	O
,	O
with	O
WebSocket	B-Protocol
enabled	O
by	O
default	O
.	O
</s>
<s>
Development	O
of	O
the	O
WebSocket	B-Protocol
protocol	O
was	O
subsequently	O
moved	O
from	O
the	O
W3C	O
and	O
WHATWG	B-Language
group	O
to	O
the	O
IETF	O
in	O
February	O
2010	O
,	O
and	O
authored	O
for	O
two	O
revisions	O
under	O
Ian	O
Hickson	O
.	O
</s>
<s>
After	O
the	O
protocol	O
was	O
shipped	O
and	O
enabled	O
by	O
default	O
in	O
multiple	O
browsers	B-Application
,	O
the	O
was	O
finalized	O
under	O
Ian	O
Fette	O
in	O
December	O
2011	O
.	O
</s>
<s>
introduced	O
compression	O
extension	O
to	O
WebSocket	B-Protocol
using	O
the	O
DEFLATE	B-Algorithm
algorithm	O
on	O
a	O
per-message	O
basis	O
.	O
</s>
<s>
A	O
secure	O
version	O
of	O
the	O
WebSocket	B-Protocol
protocol	O
is	O
implemented	O
in	O
Firefox	B-Application
6	O
,	O
Safari	B-Application
6	O
,	O
Google	B-Application
Chrome	I-Application
14	I-Application
,	O
Opera	B-Application
12.10	O
and	O
Internet	B-Application
Explorer	I-Application
10	O
.	O
</s>
<s>
A	O
detailed	O
protocol	O
test	O
suite	O
report	O
lists	O
the	O
conformance	O
of	O
those	O
browsers	B-Application
to	O
specific	O
protocol	O
aspects	O
.	O
</s>
<s>
An	O
older	O
,	O
less	O
secure	O
version	O
of	O
the	O
protocol	O
was	O
implemented	O
in	O
Opera	B-Application
11	O
and	O
Safari	B-Application
5	O
,	O
as	O
well	O
as	O
the	O
mobile	O
version	O
of	O
Safari	B-Application
in	O
iOS	O
4.2	O
.	O
</s>
<s>
The	O
BlackBerry	O
Browser	B-Application
in	O
OS7	O
implements	O
WebSockets	B-Protocol
.	O
</s>
<s>
Because	O
of	O
vulnerabilities	O
,	O
it	O
was	O
disabled	O
in	O
Firefox	B-Application
4	O
and	O
5	O
,	O
and	O
Opera	B-Application
11	O
.	O
</s>
<s>
Nginx	B-Operating_System
has	O
supported	O
WebSockets	B-Protocol
since	O
2013	O
,	O
implemented	O
in	O
version	O
1.3.13	O
including	O
acting	O
as	O
a	O
reverse	B-General_Concept
proxy	I-General_Concept
and	O
load	B-Application
balancer	I-Application
of	O
WebSocket	B-Protocol
applications	O
.	O
</s>
<s>
Internet	B-Application
Information	I-Application
Services	I-Application
added	O
support	O
for	O
WebSockets	B-Protocol
in	O
version	O
8	O
which	O
was	O
released	O
with	O
Windows	B-Device
Server	I-Device
2012	I-Device
.	O
</s>
<s>
lighttpd	B-Protocol
has	O
supported	O
WebSockets	B-Protocol
since	O
2017	O
,	O
implemented	O
in	O
version	O
1.4.46	O
.	O
lighttpd	B-Protocol
mod_proxy	O
can	O
act	O
as	O
a	O
reverse	B-General_Concept
proxy	I-General_Concept
and	O
load	B-Application
balancer	I-Application
of	O
WebSocket	B-Protocol
applications	O
.	O
</s>
<s>
lighttpd	B-Protocol
mod_wstunnel	O
can	O
construct	O
WebSocket	B-Protocol
tunnels	O
to	O
transmit	O
arbitrary	O
data	O
,	O
including	O
in	O
JSON	B-General_Concept
format	O
,	O
to	O
a	O
backend	O
application	O
.	O
</s>
<s>
Tempesta	O
FW	O
supports	O
WebSockets	B-Protocol
for	O
HTTP/1.1	B-Protocol
and	O
HTTPS	B-Protocol
connections	O
since	O
2022	O
.	O
</s>
<s>
WebSockets	B-Protocol
over	O
HTTP/2	B-Protocol
by	O
were	O
considered	O
by	O
the	O
developers	O
as	O
not	O
widely	O
enough	O
deployed	O
and	O
were	O
not	O
implemented	O
.	O
</s>
<s>
To	O
establish	O
a	O
WebSocket	B-Protocol
connection	O
,	O
the	O
client	B-Protocol
sends	O
a	O
WebSocket	B-Protocol
handshake	B-Protocol
request	O
,	O
for	O
which	O
the	O
server	O
returns	O
a	O
WebSocket	B-Protocol
handshake	B-Protocol
response	O
,	O
as	O
shown	O
in	O
the	O
example	O
below	O
.	O
</s>
<s>
Client	B-Protocol
request	O
(	O
just	O
like	O
in	O
HTTP	B-Protocol
,	O
each	O
line	O
ends	O
with	O
\r\n	O
and	O
there	O
must	O
be	O
an	O
extra	O
blank	O
line	O
at	O
the	O
end	O
)	O
:	O
</s>
<s>
The	O
handshake	B-Protocol
starts	O
with	O
an	O
HTTP	B-Protocol
request/response	O
,	O
allowing	O
servers	O
to	O
handle	O
HTTP	B-Protocol
connections	O
as	O
well	O
as	O
WebSocket	B-Protocol
connections	O
on	O
the	O
same	O
port	B-Protocol
.	O
</s>
<s>
Once	O
the	O
connection	O
is	O
established	O
,	O
communication	O
switches	O
to	O
a	O
bidirectional	O
binary	O
protocol	O
which	O
does	O
not	O
conform	O
to	O
the	O
HTTP	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
In	O
addition	O
to	O
Upgrade	B-Protocol
headers	I-Protocol
,	O
the	O
client	B-Protocol
sends	O
a	O
Sec-WebSocket-Key	O
header	O
containing	O
base64-encoded	O
random	O
bytes	O
,	O
and	O
the	O
server	O
replies	O
with	O
a	O
hash	B-Error_Name
of	O
the	O
key	O
in	O
the	O
Sec-WebSocket-Accept	O
header	O
.	O
</s>
<s>
This	O
is	O
intended	O
to	O
prevent	O
a	O
caching	B-General_Concept
proxy	O
from	O
re-sending	O
a	O
previous	O
WebSocket	B-Protocol
conversation	O
,	O
and	O
does	O
not	O
provide	O
any	O
authentication	O
,	O
privacy	O
,	O
or	O
integrity	O
.	O
</s>
<s>
The	O
hashing	B-Error_Name
function	I-Error_Name
appends	O
the	O
fixed	O
string	O
258EAFA5-E914-47DA-95CA-C5AB0DC85B11	O
(	O
a	O
UUID	O
)	O
to	O
the	O
value	O
from	O
Sec-WebSocket-Key	O
header	O
(	O
which	O
is	O
not	O
decoded	O
from	O
base64	B-Protocol
)	O
,	O
applies	O
the	O
SHA-1	B-Algorithm
hashing	B-Error_Name
function	I-Error_Name
,	O
and	O
encodes	O
the	O
result	O
using	O
base64	B-Protocol
.	O
</s>
<s>
The	O
RFC6455	B-Protocol
requires	O
the	O
key	O
MUST	O
be	O
a	O
nonce	O
consisting	O
of	O
a	O
randomly	O
selected	O
16-byte	O
value	O
that	O
has	O
been	O
base64-encoded	O
,	O
that	O
is	O
24	O
bytes	O
in	O
base64	B-Protocol
(	O
with	O
last	O
two	O
bytes	O
to	O
be	O
==	O
)	O
.	O
</s>
<s>
Though	O
some	O
relaxed	O
HTTP	B-Application
servers	I-Application
do	O
allow	O
shorter	O
keys	O
to	O
present	O
,	O
many	O
modern	O
HTTP	B-Application
servers	I-Application
will	O
reject	O
the	O
request	O
with	O
error	O
"	O
invalid	O
Sec-WebSocket-Key	O
header	O
"	O
.	O
</s>
<s>
Once	O
the	O
connection	O
is	O
established	O
,	O
the	O
client	B-Protocol
and	O
server	O
can	O
send	O
WebSocket	B-Protocol
data	O
or	O
text	O
frames	O
back	O
and	O
forth	O
in	O
full-duplex	O
mode	O
.	O
</s>
<s>
WebSocket	B-Protocol
transmissions	O
are	O
described	O
as	O
"	O
messages	O
"	O
,	O
where	O
a	O
single	O
message	O
can	O
optionally	O
be	O
split	O
across	O
several	O
data	O
frames	O
.	O
</s>
<s>
Operation	B-Language
code	I-Language
.	O
</s>
<s>
All	O
frames	O
sent	O
from	O
the	O
client	B-Protocol
should	O
be	O
masked	O
by	O
this	O
key	O
.	O
</s>
<s>
The	O
payload	O
data	O
sent	O
from	O
the	O
client	B-Protocol
should	O
be	O
masked	O
by	O
the	O
masking	O
key	O
.	O
</s>
<s>
The	O
masking	O
key	O
is	O
a	O
4	O
bytes	O
random	O
value	O
chosen	O
by	O
the	O
client	B-Protocol
and	O
should	O
be	O
unpredictable	O
.	O
</s>
<s>
Unlike	O
regular	O
cross-domain	O
HTTP	B-Protocol
requests	I-Protocol
,	O
WebSocket	B-Protocol
requests	O
are	O
not	O
restricted	O
by	O
the	O
same-origin	B-Protocol
policy	I-Protocol
.	O
</s>
<s>
Therefore	O
,	O
WebSocket	B-Protocol
servers	O
must	O
validate	O
the	O
"	O
Origin	O
"	O
header	O
against	O
the	O
expected	O
origins	O
during	O
connection	O
establishment	O
,	O
to	O
avoid	O
cross-site	O
WebSocket	B-Protocol
hijacking	O
attacks	O
(	O
similar	O
to	O
cross-site	B-Protocol
request	I-Protocol
forgery	I-Protocol
)	O
,	O
which	O
might	O
be	O
possible	O
when	O
the	O
connection	O
is	O
authenticated	O
with	O
cookies	B-Application
or	O
HTTP	B-Protocol
authentication	O
.	O
</s>
<s>
It	O
is	O
better	O
to	O
use	O
tokens	O
or	O
similar	O
protection	O
mechanisms	O
to	O
authenticate	O
the	O
WebSocket	B-Protocol
connection	O
when	O
sensitive	O
(	O
private	O
)	O
data	O
is	O
being	O
transferred	O
over	O
the	O
WebSocket	B-Protocol
.	O
</s>
<s>
A	O
live	O
example	O
of	O
vulnerability	O
was	O
seen	O
in	O
2020	O
in	O
the	O
form	O
of	O
Cable	B-Language
Haunt	I-Language
.	O
</s>
<s>
WebSocket	B-Protocol
protocol	O
client	B-Protocol
implementations	O
try	O
to	O
detect	O
whether	O
the	O
user	B-Protocol
agent	I-Protocol
is	O
configured	O
to	O
use	O
a	O
proxy	O
when	O
connecting	O
to	O
destination	O
host	O
and	O
port	B-Protocol
,	O
and	O
if	O
it	O
is	O
,	O
uses	O
HTTP	B-Protocol
CONNECT	O
method	O
to	O
set	O
up	O
a	O
persistent	O
tunnel	O
.	O
</s>
<s>
While	O
the	O
WebSocket	B-Protocol
protocol	O
itself	O
is	O
unaware	O
of	O
proxy	O
servers	O
and	O
firewalls	B-Application
,	O
it	O
features	O
an	O
HTTP-compatible	O
handshake	B-Protocol
,	O
thus	O
allowing	O
HTTP	B-Application
servers	I-Application
to	O
share	O
their	O
default	O
HTTP	B-Protocol
and	O
HTTPS	B-Protocol
ports	O
(	O
80	O
and	O
443	O
respectively	O
)	O
with	O
a	O
WebSocket	B-Protocol
gateway	O
or	O
server	O
.	O
</s>
<s>
The	O
WebSocket	B-Protocol
protocol	O
defines	O
a	O
ws://	O
and	O
wss://	O
prefix	O
to	O
indicate	O
a	O
WebSocket	B-Protocol
and	O
a	O
WebSocket	B-Protocol
Secure	I-Protocol
connection	O
respectively	O
.	O
</s>
<s>
Both	O
schemes	O
use	O
an	O
HTTP	B-Protocol
upgrade	I-Protocol
mechanism	I-Protocol
to	O
upgrade	O
to	O
the	O
WebSocket	B-Protocol
protocol	O
.	O
</s>
<s>
Some	O
proxy	O
servers	O
are	O
transparent	O
and	O
work	O
fine	O
with	O
WebSocket	B-Protocol
;	O
others	O
will	O
prevent	O
WebSocket	B-Protocol
from	O
working	O
correctly	O
,	O
causing	O
the	O
connection	O
to	O
fail	O
.	O
</s>
<s>
In	O
some	O
cases	O
,	O
additional	O
proxy-server	O
configuration	O
may	O
be	O
required	O
,	O
and	O
certain	O
proxy	O
servers	O
may	O
need	O
to	O
be	O
upgraded	O
to	O
support	O
WebSocket	B-Protocol
.	O
</s>
<s>
If	O
unencrypted	O
WebSocket	B-Protocol
traffic	O
flows	O
through	O
an	O
explicit	O
or	O
a	O
transparent	O
proxy	O
server	O
without	O
WebSockets	B-Protocol
support	O
,	O
the	O
connection	O
will	O
likely	O
fail	O
.	O
</s>
<s>
If	O
an	O
encrypted	O
WebSocket	B-Protocol
connection	O
is	O
used	O
,	O
then	O
the	O
use	O
of	O
Transport	B-Protocol
Layer	I-Protocol
Security	I-Protocol
(	O
TLS	O
)	O
in	O
the	O
WebSocket	B-Protocol
Secure	I-Protocol
connection	O
ensures	O
that	O
an	O
HTTP	B-Protocol
CONNECT	O
command	O
is	O
issued	O
when	O
the	O
browser	B-Application
is	O
configured	O
to	O
use	O
an	O
explicit	O
proxy	O
server	O
.	O
</s>
<s>
This	O
sets	O
up	O
a	O
tunnel	O
,	O
which	O
provides	O
low-level	O
end-to-end	O
TCP	B-Protocol
communication	O
through	O
the	O
HTTP	B-Protocol
proxy	O
,	O
between	O
the	O
WebSocket	B-Protocol
Secure	I-Protocol
client	B-Protocol
and	O
the	O
WebSocket	B-Protocol
server	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
transparent	O
proxy	O
servers	O
,	O
the	O
browser	B-Application
is	O
unaware	O
of	O
the	O
proxy	O
server	O
,	O
so	O
no	O
HTTP	B-Protocol
CONNECT	O
is	O
sent	O
.	O
</s>
<s>
However	O
,	O
since	O
the	O
wire	O
traffic	O
is	O
encrypted	O
,	O
intermediate	O
transparent	O
proxy	O
servers	O
may	O
simply	O
allow	O
the	O
encrypted	O
traffic	O
through	O
,	O
so	O
there	O
is	O
a	O
much	O
better	O
chance	O
that	O
the	O
WebSocket	B-Protocol
connection	O
will	O
succeed	O
if	O
WebSocket	B-Protocol
Secure	I-Protocol
is	O
used	O
.	O
</s>
<s>
A	O
mid-2010	O
draft	O
(	O
version	O
hixie-76	O
)	O
broke	O
compatibility	O
with	O
reverse	B-General_Concept
proxies	I-General_Concept
and	O
gateways	O
by	O
including	O
eight	O
bytes	O
of	O
key	O
data	O
after	O
the	O
headers	O
,	O
but	O
not	O
advertising	O
that	O
data	O
in	O
a	O
Content-Length	O
:	O
8	O
header	O
.	O
</s>
<s>
More	O
recent	O
drafts	O
(	O
e.g.	O
,	O
hybi-09	O
)	O
put	O
the	O
key	O
data	O
in	O
a	O
Sec-WebSocket-Key	O
header	O
,	O
solving	O
this	O
problem	O
.	O
</s>
