<s>
The	O
Hypertext	B-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
(	O
HTTP	B-Protocol
)	O
is	O
an	O
application	B-Application
layer	I-Application
protocol	O
in	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
model	O
for	O
distributed	O
,	O
collaborative	O
,	O
hypermedia	O
information	O
systems	O
.	O
</s>
<s>
HTTP	B-Protocol
is	O
the	O
foundation	O
of	O
data	O
communication	O
for	O
the	O
World	O
Wide	O
Web	O
,	O
where	O
hypertext	O
documents	O
include	O
hyperlinks	O
to	O
other	O
resources	O
that	O
the	O
user	O
can	O
easily	O
access	O
,	O
for	O
example	O
by	O
a	O
mouse	B-Device
click	O
or	O
by	O
tapping	O
the	O
screen	O
in	O
a	O
web	B-Application
browser	I-Application
.	O
</s>
<s>
Development	O
of	O
HTTP	B-Protocol
was	O
initiated	O
by	O
Tim	O
Berners-Lee	O
at	O
CERN	O
in	O
1989	O
and	O
summarized	O
in	O
a	O
simple	O
document	O
describing	O
the	O
behavior	O
of	O
a	O
client	O
and	O
a	O
server	O
using	O
the	O
first	O
HTTP	B-Protocol
protocol	I-Protocol
version	O
that	O
was	O
named	O
0.9	O
.	O
</s>
<s>
That	O
first	O
version	O
of	O
HTTP	B-Protocol
protocol	I-Protocol
soon	O
evolved	O
into	O
a	O
more	O
elaborated	O
version	O
that	O
was	O
the	O
first	O
draft	O
toward	O
a	O
far	O
future	O
version	O
1.0	O
.	O
</s>
<s>
Development	O
of	O
early	O
HTTP	B-Protocol
Requests	I-Protocol
for	O
Comments	O
(	O
RFCs	O
)	O
started	O
a	O
few	O
years	O
later	O
and	O
it	O
was	O
a	O
coordinated	O
effort	O
by	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
and	O
the	O
World	O
Wide	O
Web	O
Consortium	O
(	O
W3C	O
)	O
,	O
with	O
work	O
later	O
moving	O
to	O
the	O
IETF	O
.	O
</s>
<s>
HTTP/1	O
was	O
finalized	O
and	O
fully	O
documented	O
(	O
as	O
version	O
1.0	O
)	O
in	O
1996	O
.	O
</s>
<s>
Its	O
secure	O
variant	O
named	O
HTTPS	B-Protocol
is	O
used	O
by	O
more	O
than	O
80%	O
of	O
websites	O
.	O
</s>
<s>
HTTP/2	B-Protocol
,	O
published	O
in	O
2015	O
,	O
provides	O
a	O
more	O
efficient	O
expression	O
of	O
HTTP	B-Protocol
's	O
semantics	O
"	O
on	O
the	O
wire	O
"	O
.	O
</s>
<s>
As	O
of	O
April	O
2023	O
,	O
it	O
is	O
used	O
by	O
39%	O
of	O
websites	O
and	O
supported	O
by	O
almost	O
all	O
web	B-Application
browsers	I-Application
(	O
over	O
97%	O
of	O
users	O
)	O
.	O
</s>
<s>
It	O
is	O
also	O
supported	O
by	O
major	O
web	B-Application
servers	I-Application
over	O
Transport	B-Protocol
Layer	I-Protocol
Security	I-Protocol
(	O
TLS	B-Protocol
)	O
using	O
an	O
Application-Layer	B-Protocol
Protocol	I-Protocol
Negotiation	I-Protocol
(	O
ALPN	B-Protocol
)	O
extension	O
where	O
TLS	B-Protocol
1.2	O
or	O
newer	O
is	O
required	O
.	O
</s>
<s>
HTTP/3	B-Protocol
,	O
the	O
successor	O
to	O
HTTP/2	B-Protocol
,	O
was	O
published	O
in	O
2022	O
.	O
</s>
<s>
It	O
is	O
now	O
used	O
by	O
over	O
25%	O
of	O
websites	O
and	O
is	O
supported	O
by	O
many	O
web	B-Application
browsers	I-Application
(	O
over	O
75%	O
of	O
users	O
)	O
.	O
</s>
<s>
HTTP/3	B-Protocol
uses	O
QUIC	B-Protocol
instead	O
of	O
TCP	B-Protocol
for	O
the	O
underlying	O
transport	B-Architecture
protocol	I-Architecture
.	O
</s>
<s>
Like	O
HTTP/2	B-Protocol
,	O
it	O
does	O
not	O
obsolesce	O
previous	O
major	O
versions	O
of	O
the	O
protocol	O
.	O
</s>
<s>
Support	O
for	O
HTTP/3	B-Protocol
was	O
added	O
to	O
Cloudflare	B-Application
and	O
Google	B-Application
Chrome	I-Application
first	O
,	O
and	O
is	O
also	O
enabled	O
in	O
Firefox	B-Application
.	O
</s>
<s>
HTTP/3	B-Protocol
has	O
lower	O
latency	O
for	O
real-world	O
web	O
pages	O
,	O
if	O
enabled	O
on	O
the	O
server	O
,	O
load	O
faster	O
than	O
with	O
HTTP/2	B-Protocol
,	O
and	O
even	O
faster	O
than	O
HTTP/1.1	B-Protocol
,	O
in	O
some	O
cases	O
over	O
3×	O
faster	O
than	O
HTTP/1.1	B-Protocol
(	O
which	O
is	O
still	O
commonly	O
only	O
enabled	O
)	O
.	O
</s>
<s>
HTTP	B-Protocol
functions	O
as	O
a	O
request	B-Protocol
–	I-Protocol
response	I-Protocol
protocol	O
in	O
the	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
model	I-Operating_System
.	O
</s>
<s>
A	O
web	B-Application
browser	I-Application
,	O
for	O
example	O
,	O
may	O
be	O
the	O
client	O
whereas	O
a	O
process	B-Operating_System
,	O
named	O
web	B-Application
server	I-Application
,	O
running	O
on	O
a	O
computer	O
hosting	B-Device
one	O
or	O
more	O
websites	O
may	O
be	O
the	O
server	O
.	O
</s>
<s>
The	O
client	O
submits	O
an	O
HTTP	B-Protocol
request	O
message	O
to	O
the	O
server	O
.	O
</s>
<s>
The	O
server	O
,	O
which	O
provides	O
resources	O
such	O
as	O
HTML	B-Language
files	I-Language
and	O
other	O
content	O
or	O
performs	O
other	O
functions	O
on	O
behalf	O
of	O
the	O
client	O
,	O
returns	O
a	O
response	O
message	O
to	O
the	O
client	O
.	O
</s>
<s>
The	O
response	O
contains	O
completion	O
status	O
information	O
about	O
the	O
request	O
and	O
may	O
also	O
contain	O
requested	O
content	O
in	O
its	O
message	B-Protocol
body	I-Protocol
.	O
</s>
<s>
A	O
web	B-Application
browser	I-Application
is	O
an	O
example	O
of	O
a	O
user	B-Protocol
agent	I-Protocol
(	O
UA	O
)	O
.	O
</s>
<s>
Other	O
types	O
of	O
user	B-Protocol
agent	I-Protocol
include	O
the	O
indexing	O
software	O
used	O
by	O
search	O
providers	O
(	O
web	B-Application
crawlers	I-Application
)	O
,	O
voice	B-Protocol
browsers	I-Protocol
,	O
mobile	B-Application
apps	I-Application
,	O
and	O
other	O
software	O
that	O
accesses	O
,	O
consumes	O
,	O
or	O
displays	O
web	O
content	O
.	O
</s>
<s>
HTTP	B-Protocol
is	O
designed	O
to	O
permit	O
intermediate	O
network	O
elements	O
to	O
improve	O
or	O
enable	O
communications	O
between	O
clients	O
and	O
servers	O
.	O
</s>
<s>
High-traffic	O
websites	O
often	O
benefit	O
from	O
web	B-Protocol
cache	I-Protocol
servers	O
that	O
deliver	O
content	O
on	O
behalf	O
of	O
upstream	O
servers	O
to	O
improve	O
response	O
time	O
.	O
</s>
<s>
Web	B-Application
browsers	I-Application
cache	O
previously	O
accessed	O
web	O
resources	O
and	O
reuse	O
them	O
,	O
whenever	O
possible	O
,	O
to	O
reduce	O
network	O
traffic	O
.	O
</s>
<s>
HTTP	B-Protocol
proxy	B-Application
servers	I-Application
at	O
private	B-Protocol
network	I-Protocol
boundaries	O
can	O
facilitate	O
communication	O
for	O
clients	O
without	O
a	O
globally	O
routable	O
address	O
,	O
by	O
relaying	O
messages	O
with	O
external	O
servers	O
.	O
</s>
<s>
To	O
allow	O
intermediate	O
HTTP	B-Protocol
nodes	O
(	O
proxy	B-Application
servers	I-Application
,	O
web	B-Protocol
caches	I-Protocol
,	O
etc	O
.	O
)	O
</s>
<s>
to	O
accomplish	O
their	O
functions	O
,	O
some	O
of	O
the	O
HTTP	B-Protocol
headers	I-Protocol
(	O
found	O
in	O
HTTP	O
requests/responses	O
)	O
are	O
managed	O
hop-by-hop	B-Protocol
whereas	O
other	O
HTTP	B-Protocol
headers	I-Protocol
are	O
managed	O
end-to-end	B-Application
(	O
managed	O
only	O
by	O
the	O
source	O
client	O
and	O
by	O
the	O
target	O
web	B-Application
server	I-Application
)	O
.	O
</s>
<s>
HTTP	B-Protocol
is	O
an	O
application	B-Application
layer	I-Application
protocol	O
designed	O
within	O
the	O
framework	O
of	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
.	O
</s>
<s>
Its	O
definition	O
presumes	O
an	O
underlying	O
and	O
reliable	O
transport	B-Architecture
layer	I-Architecture
protocol	O
,	O
thus	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	B-Protocol
)	O
is	O
commonly	O
used	O
.	O
</s>
<s>
However	O
,	O
HTTP	B-Protocol
can	O
be	O
adapted	O
to	O
use	O
unreliable	O
protocols	O
such	O
as	O
the	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
,	O
for	O
example	O
in	O
HTTPU	O
and	O
Simple	B-Protocol
Service	I-Protocol
Discovery	I-Protocol
Protocol	I-Protocol
(	O
SSDP	O
)	O
.	O
</s>
<s>
HTTP	B-Protocol
resources	O
are	O
identified	O
and	O
located	O
on	O
the	O
network	O
by	O
Uniform	O
Resource	O
Locators	O
(	O
URLs	O
)	O
,	O
using	O
the	O
Uniform	B-Protocol
Resource	I-Protocol
Identifiers	I-Protocol
(	O
URI	B-Protocol
's	O
)	O
schemes	O
http	B-Protocol
and	O
https	B-Protocol
.	O
</s>
<s>
As	O
defined	O
in	O
,	O
URIs	B-Protocol
are	O
encoded	O
as	O
hyperlinks	O
in	O
HTML	B-Language
documents	O
,	O
so	O
as	O
to	O
form	O
interlinked	O
hypertext	O
documents	O
.	O
</s>
<s>
In	O
HTTP/1.0	B-Protocol
a	O
separate	O
connection	B-Protocol
to	O
the	O
same	O
server	O
is	O
made	O
for	O
every	O
resource	O
request	O
.	O
</s>
<s>
In	O
HTTP/1.1	B-Protocol
instead	O
a	O
TCP	B-Protocol
connection	B-Protocol
can	O
be	O
reused	O
to	O
make	O
multiple	O
resource	O
requests	O
(	O
i.e.	O
</s>
<s>
of	O
HTML	B-Language
pages	O
,	O
frames	O
,	O
images	O
,	O
scripts	O
,	O
stylesheets	B-Language
,	O
etc	O
.	O
</s>
<s>
HTTP/1.1	B-Protocol
communications	O
therefore	O
experience	O
less	O
latency	O
as	O
the	O
establishment	O
of	O
TCP	B-Protocol
connections	O
presents	O
considerable	O
overhead	O
,	O
specially	O
under	O
high	O
traffic	O
conditions	O
.	O
</s>
<s>
HTTP/2	B-Protocol
is	O
a	O
revision	O
of	O
previous	O
HTTP/1.1	B-Protocol
in	O
order	O
to	O
maintain	O
the	O
same	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
model	I-Operating_System
and	O
the	O
same	O
protocol	O
methods	O
but	O
with	O
these	O
differences	O
in	O
order	O
:	O
</s>
<s>
to	O
use	O
a	O
compressed	B-Protocol
binary	O
representation	O
of	O
metadata	O
(	O
HTTP	B-Protocol
headers	I-Protocol
)	O
instead	O
of	O
a	O
textual	O
one	O
,	O
so	O
that	O
headers	O
require	O
much	O
less	O
space	O
;	O
</s>
<s>
to	O
use	O
a	O
single	O
TCP/IP	B-Protocol
(	O
usually	O
encrypted	O
)	O
connection	B-Protocol
per	O
accessed	O
server	O
domain	O
instead	O
of	O
2	O
to	O
8	O
TCP/IP	B-Protocol
connections	O
;	O
</s>
<s>
to	O
use	O
one	O
or	O
more	O
bidirectional	O
streams	O
per	O
TCP/IP	B-Protocol
connection	B-Protocol
in	O
which	O
HTTP	B-Protocol
requests	I-Protocol
and	O
responses	O
are	O
broken	O
down	O
and	O
transmitted	O
in	O
small	O
packets	O
to	O
almost	O
solve	O
the	O
problem	O
of	O
the	O
HOLB	O
(	O
head	B-Protocol
of	I-Protocol
line	I-Protocol
blocking	I-Protocol
)	O
.	O
</s>
<s>
to	O
add	O
a	O
push	O
capability	O
to	O
allow	O
server	O
application	O
to	O
send	O
data	O
to	O
clients	O
whenever	O
new	O
data	O
is	O
available	O
(	O
without	O
forcing	O
clients	O
to	O
request	O
periodically	O
new	O
data	O
to	O
server	O
by	O
using	O
polling	B-General_Concept
methods	O
)	O
.	O
</s>
<s>
HTTP/2	B-Protocol
communications	O
therefore	O
experience	O
much	O
less	O
latency	O
and	O
,	O
in	O
most	O
cases	O
,	O
even	O
more	O
speed	O
than	O
HTTP/1.1	B-Protocol
communications	O
.	O
</s>
<s>
HTTP/3	B-Protocol
is	O
a	O
revision	O
of	O
previous	O
HTTP/2	B-Protocol
in	O
order	O
to	O
use	O
QUIC	B-Protocol
+	O
UDP	O
transport	B-Architecture
protocols	I-Architecture
instead	O
of	O
TCP/IP	B-Protocol
connections	O
also	O
to	O
slightly	O
improve	O
the	O
average	O
speed	O
of	O
communications	O
and	O
to	O
avoid	O
the	O
occasional	O
(	O
very	O
rare	O
)	O
problem	O
of	O
TCP/IP	B-Protocol
connection	B-Protocol
congestion	O
that	O
can	O
temporarily	O
block	O
or	O
slow	O
down	O
the	O
data	O
flow	O
of	O
all	O
its	O
streams	O
(	O
another	O
form	O
of	O
"	O
head	B-Protocol
of	I-Protocol
line	I-Protocol
blocking	I-Protocol
"	O
)	O
.	O
</s>
<s>
The	O
term	O
hypertext	O
was	O
coined	O
by	O
Ted	O
Nelson	O
in	O
1965	O
in	O
the	O
Xanadu	B-Protocol
Project	I-Protocol
,	O
which	O
was	O
in	O
turn	O
inspired	O
by	O
Vannevar	O
Bush	O
's	O
1930s	O
vision	O
of	O
the	O
microfilm-based	O
information	O
retrieval	O
and	O
management	O
"	O
memex	B-General_Concept
"	O
system	O
described	O
in	O
his	O
1945	O
essay	O
"	O
As	O
We	O
May	O
Think	O
"	O
.	O
</s>
<s>
Tim	O
Berners-Lee	O
and	O
his	O
team	O
at	O
CERN	O
are	O
credited	O
with	O
inventing	O
the	O
original	O
HTTP	B-Protocol
,	O
along	O
with	O
HTML	B-Language
and	O
the	O
associated	O
technology	O
for	O
a	O
web	B-Application
server	I-Application
and	O
a	O
client	O
user	B-Application
interface	I-Application
called	O
web	B-Application
browser	I-Application
.	O
</s>
<s>
Berners-Lee	O
designed	O
HTTP	B-Protocol
in	O
order	O
to	O
help	O
with	O
the	O
adoption	O
of	O
his	O
other	O
idea	O
:	O
the	O
"	O
WorldWideWeb	O
"	O
project	O
,	O
which	O
was	O
first	O
proposed	O
in	O
1989	O
,	O
now	O
known	O
as	O
the	O
World	O
Wide	O
Web	O
.	O
</s>
<s>
The	B-Application
first	I-Application
web	I-Application
server	I-Application
went	O
live	O
in	O
1990	O
.	O
</s>
<s>
The	O
response	O
from	O
the	O
server	O
was	O
always	O
an	O
HTML	B-Language
page	O
.	O
</s>
<s>
In	O
1991	O
,	O
the	O
first	O
documented	O
official	O
version	O
of	O
HTTP	B-Protocol
was	O
written	O
as	O
a	O
plain	O
document	O
,	O
less	O
than	O
700	O
words	O
long	O
,	O
and	O
this	O
version	O
was	O
named	O
HTTP/0.9	B-Protocol
.	O
</s>
<s>
HTTP/0.9	B-Protocol
supported	O
only	O
GET	O
method	O
,	O
allowing	O
clients	O
to	O
only	O
retrieve	O
HTML	B-Language
documents	O
from	O
the	O
server	O
,	O
but	O
not	O
supporting	O
any	O
other	O
file	B-Operating_System
formats	O
or	O
information	O
upload	O
.	O
</s>
<s>
It	O
supported	O
both	O
the	O
simple	O
request	O
method	O
of	O
the	O
0.9	O
version	O
and	O
the	O
full	O
GET	O
request	O
that	O
included	O
the	O
client	O
HTTP	B-Protocol
version	O
.	O
</s>
<s>
This	O
was	O
the	O
first	O
of	O
the	O
many	O
unofficial	O
HTTP/1.0	B-Protocol
drafts	O
that	O
preceded	O
the	O
final	O
work	O
on	O
HTTP/1.0	B-Protocol
.	O
</s>
<s>
After	O
having	O
decided	O
that	O
new	O
features	O
of	O
HTTP	B-Protocol
protocol	I-Protocol
were	O
required	O
and	O
that	O
they	O
had	O
to	O
be	O
fully	O
documented	O
as	O
official	O
RFCs	O
,	O
in	O
early	O
1995	O
the	O
HTTP	B-Protocol
Working	O
Group	O
(	O
HTTP	B-Protocol
WG	O
,	O
led	O
by	O
Dave	O
Raggett	O
)	O
was	O
constituted	O
with	O
the	O
aim	O
to	O
standardize	O
and	O
expand	O
the	O
protocol	O
with	O
extended	O
operations	O
,	O
extended	O
negotiation	O
,	O
richer	O
meta-information	O
,	O
tied	O
with	O
a	O
security	O
protocol	O
which	O
became	O
more	O
efficient	O
by	O
adding	O
additional	O
methods	O
and	O
header	B-Protocol
fields	I-Protocol
.	O
</s>
<s>
The	O
HTTP	B-Protocol
WG	O
planned	O
to	O
revise	O
and	O
publish	O
new	O
versions	O
of	O
the	O
protocol	O
as	O
HTTP/1.0	B-Protocol
and	O
HTTP/1.1	B-Protocol
within	O
1995	O
,	O
but	O
,	O
because	O
of	O
the	O
many	O
revisions	O
,	O
that	O
timeline	O
lasted	O
much	O
more	O
than	O
one	O
year	O
.	O
</s>
<s>
The	O
HTTP	B-Protocol
WG	O
planned	O
also	O
to	O
specify	O
a	O
far	O
future	O
version	O
of	O
HTTP	B-Protocol
called	O
HTTP-NG	O
(	O
HTTP	B-Protocol
Next	O
Generation	O
)	O
that	O
would	O
have	O
solved	O
all	O
remaining	O
problems	O
,	O
of	O
previous	O
versions	O
,	O
related	O
to	O
performances	O
,	O
low	O
latency	O
responses	O
,	O
etc	O
.	O
</s>
<s>
In	O
May	O
1996	O
,	O
was	O
published	O
as	O
a	O
final	O
HTTP/1.0	B-Protocol
revision	O
of	O
what	O
had	O
been	O
used	O
in	O
previous	O
4	O
years	O
as	O
a	O
pre-standard	O
HTTP/1.0	B-Protocol
-draft	O
which	O
was	O
already	O
used	O
by	O
many	O
web	B-Application
browsers	I-Application
and	O
web	B-Application
servers	I-Application
.	O
</s>
<s>
In	O
early	O
1996	O
developers	O
started	O
to	O
even	O
include	O
unofficial	O
extensions	O
of	O
the	O
HTTP/1.0	B-Protocol
protocol	O
(	O
i.e.	O
</s>
<s>
into	O
their	O
products	O
by	O
using	O
drafts	O
of	O
the	O
upcoming	O
HTTP/1.1	B-Protocol
specifications	O
.	O
</s>
<s>
Since	O
early	O
1996	O
,	O
major	O
web	B-Application
browsers	I-Application
and	O
web	B-Application
server	I-Application
developers	O
also	O
started	O
to	O
implement	O
new	O
features	O
specified	O
by	O
pre-standard	O
HTTP/1.1	B-Protocol
drafts	O
specifications	O
.	O
</s>
<s>
End-user	O
adoption	O
of	O
the	O
new	O
versions	O
of	O
browsers	B-Application
and	O
servers	O
was	O
rapid	O
.	O
</s>
<s>
In	O
March	O
1996	O
,	O
one	O
web	O
hosting	B-Device
company	O
reported	O
that	O
over	O
40%	O
of	O
browsers	B-Application
in	O
use	O
on	O
the	O
Internet	O
used	O
the	O
new	O
HTTP/1.1	B-Protocol
header	O
"	O
Host	O
"	O
to	O
enable	O
virtual	O
hosting	B-Device
.	O
</s>
<s>
That	O
same	O
web	O
hosting	B-Device
company	O
reported	O
that	O
by	O
June	O
1996	O
,	O
65%	O
of	O
all	O
browsers	B-Application
accessing	O
their	O
servers	O
were	O
pre-standard	O
HTTP/1.1	B-Protocol
compliant	O
.	O
</s>
<s>
In	O
January	O
1997	O
,	O
was	O
officially	O
released	O
as	O
HTTP/1.1	B-Protocol
specifications	O
.	O
</s>
<s>
In	O
June	O
1999	O
,	O
was	O
released	O
to	O
include	O
all	O
improvements	O
and	O
updates	O
based	O
on	O
previous	O
(	O
obsolete	O
)	O
HTTP/1.1	B-Protocol
specifications	O
.	O
</s>
<s>
Resuming	O
the	O
old	O
1995	O
plan	O
of	O
previous	O
HTTP	B-Protocol
Working	O
Group	O
,	O
in	O
1997	O
an	O
HTTP-NG	O
Working	O
Group	O
was	O
formed	O
to	O
develop	O
a	O
new	O
HTTP	B-Protocol
protocol	I-Protocol
named	O
HTTP-NG	O
(	O
HTTP	B-Protocol
New	O
Generation	O
)	O
.	O
</s>
<s>
A	O
few	O
proposals	O
/	O
drafts	O
were	O
produced	O
for	O
the	O
new	O
protocol	O
to	O
use	O
multiplexing	B-Architecture
of	O
HTTP	B-Protocol
transactions	O
inside	O
a	O
single	O
TCP/IP	B-Protocol
connection	B-Protocol
,	O
but	O
in	O
1999	O
,	O
the	O
group	O
stopped	O
its	O
activity	O
passing	O
the	O
technical	O
problems	O
to	O
IETF	O
.	O
</s>
<s>
In	O
2007	O
,	O
the	O
IETF	O
(	O
HTTP	B-Protocol
WG	O
bis	O
or	O
HTTPbis	B-Protocol
)	O
was	O
restarted	O
firstly	O
to	O
revise	O
and	O
clarify	O
previous	O
HTTP/1.1	B-Protocol
specifications	O
and	O
secondly	O
to	O
write	O
and	O
refine	O
future	O
HTTP/2	B-Protocol
specifications	O
(	O
named	O
httpbis	B-Protocol
)	O
.	O
</s>
<s>
In	O
2009	O
,	O
Google	B-Application
,	O
a	O
private	O
company	O
,	O
announced	O
that	O
it	O
had	O
developed	O
and	O
tested	O
a	O
new	O
HTTP	B-Protocol
binary	O
protocol	O
named	O
SPDY	B-Protocol
.	O
</s>
<s>
The	O
implicit	O
aim	O
was	O
to	O
greatly	O
speed	O
up	O
web	O
traffic	O
(	O
specially	O
between	O
future	O
web	B-Application
browsers	I-Application
and	O
its	O
servers	O
)	O
.	O
</s>
<s>
SPDY	B-Protocol
was	O
indeed	O
much	O
faster	O
than	O
HTTP/1.1	B-Protocol
in	O
many	O
tests	O
and	O
so	O
it	O
was	O
quickly	O
adopted	O
by	O
Chromium	B-Language
and	O
then	O
by	O
other	O
major	O
web	B-Application
browsers	I-Application
.	O
</s>
<s>
Some	O
of	O
the	O
ideas	O
about	O
multiplexing	B-Architecture
HTTP	B-Protocol
streams	O
over	O
a	O
single	O
TCP/IP	B-Protocol
connection	B-Protocol
were	O
taken	O
from	O
various	O
sources	O
,	O
including	O
the	O
work	O
of	O
W3C	O
HTTP-NG	O
Working	O
Group	O
.	O
</s>
<s>
In	O
January	O
–	O
March	O
2012	O
,	O
HTTP	B-Protocol
Working	O
Group	O
(	O
HTTPbis	B-Protocol
)	O
announced	O
the	O
need	O
to	O
start	O
to	O
focus	O
on	O
a	O
new	O
HTTP/2	B-Protocol
protocol	O
(	O
while	O
finishing	O
the	O
revision	O
of	O
HTTP/1.1	B-Protocol
specifications	O
)	O
,	O
maybe	O
taking	O
in	O
consideration	O
ideas	O
and	O
work	O
done	O
for	O
SPDY	B-Protocol
.	O
</s>
<s>
After	O
a	O
few	O
months	O
about	O
what	O
to	O
do	O
to	O
develop	O
a	O
new	O
version	O
of	O
HTTP	B-Protocol
,	O
it	O
was	O
decided	O
to	O
derive	O
it	O
from	O
SPDY	B-Protocol
.	O
</s>
<s>
In	O
May	O
2015	O
,	O
HTTP/2	B-Protocol
was	O
published	O
as	O
and	O
quickly	O
adopted	O
by	O
all	O
web	B-Application
browsers	I-Application
already	O
supporting	O
SPDY	B-Protocol
and	O
more	O
slowly	O
by	O
web	B-Application
servers	I-Application
.	O
</s>
<s>
In	O
June	O
2014	O
,	O
the	O
HTTP	B-Protocol
Working	O
Group	O
released	O
an	O
updated	O
six-part	O
HTTP/1.1	B-Protocol
specification	O
obsoleting	O
:	O
</s>
<s>
In	O
Appendix-A	O
,	O
HTTP/0.9	B-Protocol
was	O
deprecated	O
for	O
servers	O
supporting	O
HTTP/1.1	B-Protocol
version	O
(	O
and	O
higher	O
)	O
:	O
</s>
<s>
Since	O
2016	O
many	O
product	O
managers	O
and	O
developers	O
of	O
user	B-Protocol
agents	I-Protocol
(	O
browsers	B-Application
,	O
etc	O
.	O
)	O
</s>
<s>
and	O
web	B-Application
servers	I-Application
have	O
begun	O
planning	O
to	O
gradually	O
deprecate	O
and	O
dismiss	O
support	O
for	O
HTTP/0.9	B-Protocol
protocol	O
,	O
mainly	O
for	O
the	O
following	O
reasons	O
:	O
</s>
<s>
it	O
has	O
no	O
HTTP	B-Protocol
headers	I-Protocol
and	O
lacks	O
many	O
other	O
features	O
that	O
nowadays	O
are	O
required	O
for	O
minimal	O
security	O
reasons	O
;	O
</s>
<s>
it	O
has	O
not	O
been	O
widespread	O
since	O
1999	O
..	O
2000	O
(	O
because	O
of	O
HTTP/1.0	B-Protocol
and	O
HTTP/1.1	B-Protocol
)	O
and	O
is	O
commonly	O
used	O
only	O
by	O
some	O
very	O
old	O
network	O
hardware	O
,	O
i.e.	O
</s>
<s>
routers	B-Protocol
,	O
etc	O
.	O
</s>
<s>
In	O
2020	O
,	O
the	O
first	O
drafts	O
HTTP/3	B-Protocol
were	O
published	O
and	O
major	O
web	B-Application
browsers	I-Application
and	O
web	B-Application
servers	I-Application
started	O
to	O
adopt	O
it	O
.	O
</s>
<s>
On	O
6	O
June	O
2022	O
,	O
IETF	O
standardized	O
HTTP/3	B-Protocol
as	O
.	O
</s>
<s>
In	O
June	O
2022	O
,	O
a	O
batch	O
of	O
RFCs	O
was	O
published	O
,	O
deprecating	O
many	O
of	O
the	O
previous	O
documents	O
and	O
introducing	O
a	O
few	O
minor	O
changes	O
and	O
a	O
refactoring	O
of	O
HTTP	B-Protocol
semantics	O
description	O
into	O
a	O
separate	O
document	O
.	O
</s>
<s>
HTTP	B-Protocol
is	O
a	O
stateless	B-Protocol
application-level	O
protocol	O
and	O
it	O
requires	O
a	O
reliable	O
network	O
transport	O
connection	B-Protocol
to	O
exchange	O
data	O
between	O
client	O
and	O
server	O
.	O
</s>
<s>
In	O
HTTP	B-Protocol
implementations	O
,	O
TCP/IP	B-Protocol
connections	O
are	O
used	O
using	O
well	O
known	O
ports	O
(	O
typically	O
port	B-Protocol
80	I-Protocol
if	O
the	O
connection	B-Protocol
is	O
unencrypted	O
or	O
port	B-Protocol
443	O
if	O
the	O
connection	B-Protocol
is	O
encrypted	O
,	O
see	O
also	O
List	B-Protocol
of	I-Protocol
TCP	I-Protocol
and	I-Protocol
UDP	I-Protocol
port	I-Protocol
numbers	I-Protocol
)	O
.	O
</s>
<s>
In	O
HTTP/2	B-Protocol
,	O
a	O
TCP/IP	B-Protocol
connection	B-Protocol
plus	O
multiple	O
protocol	O
channels	O
are	O
used	O
.	O
</s>
<s>
In	O
HTTP/3	B-Protocol
,	O
the	O
application	O
transport	B-Architecture
protocol	I-Architecture
QUIC	B-Protocol
over	O
UDP	O
is	O
used	O
.	O
</s>
<s>
Data	O
is	O
exchanged	O
through	O
a	O
sequence	O
of	O
request	B-Protocol
–	I-Protocol
response	I-Protocol
messages	I-Protocol
which	O
are	O
exchanged	O
by	O
a	O
session	B-Application
layer	I-Application
transport	O
connection	B-Protocol
.	O
</s>
<s>
An	O
HTTP	B-Application
client	I-Application
initially	O
tries	O
to	O
connect	O
to	O
a	O
server	O
establishing	O
a	O
connection	B-Protocol
(	O
real	O
or	O
virtual	O
)	O
.	O
</s>
<s>
An	O
HTTP(S )	O
server	O
listening	O
on	O
that	O
port	B-Protocol
accepts	O
the	O
connection	B-Protocol
and	O
then	O
waits	O
for	O
a	O
client	O
's	O
request	O
message	O
.	O
</s>
<s>
Upon	O
receiving	O
the	O
request	O
,	O
the	O
server	O
sends	O
back	O
an	O
HTTP	B-Protocol
response	O
message	O
(	O
header	O
plus	O
a	O
body	O
if	O
it	O
is	O
required	O
)	O
.	O
</s>
<s>
At	O
any	O
time	O
(	O
for	O
many	O
reasons	O
)	O
client	O
or	O
server	O
can	O
close	O
the	O
connection	B-Protocol
.	O
</s>
<s>
Closing	O
a	O
connection	B-Protocol
is	O
usually	O
advertised	O
in	O
advance	O
by	O
using	O
one	O
or	O
more	O
HTTP	B-Protocol
headers	I-Protocol
in	O
the	O
last	O
request/response	B-Protocol
message	O
sent	O
to	O
server	O
or	O
client	O
.	O
</s>
<s>
In	O
HTTP/0.9	B-Protocol
,	O
the	O
TCP/IP	B-Protocol
connection	B-Protocol
is	O
always	O
closed	O
after	O
server	O
response	O
has	O
been	O
sent	O
,	O
so	O
it	O
is	O
never	O
persistent	O
.	O
</s>
<s>
In	O
HTTP/1.0	B-Protocol
,	O
as	O
stated	O
in	O
RFC	O
1945	O
,	O
the	O
TCP/IP	B-Protocol
connection	B-Protocol
should	O
always	O
be	O
closed	O
by	O
server	O
after	O
a	O
response	O
has	O
been	O
sent	O
.	O
</s>
<s>
In	O
HTTP/1.1	B-Protocol
a	O
keep-alive-mechanism	O
was	O
officially	O
introduced	O
so	O
that	O
a	O
connection	B-Protocol
could	O
be	O
reused	O
for	O
more	O
than	O
one	O
request/response	B-Protocol
.	O
</s>
<s>
Such	O
persistent	O
connections	O
reduce	O
request	O
latency	O
perceptibly	O
because	O
the	O
client	O
does	O
not	O
need	O
to	O
re-negotiate	O
the	O
TCP	B-Protocol
3-Way-Handshake	O
connection	B-Protocol
after	O
the	O
first	O
request	O
has	O
been	O
sent	O
.	O
</s>
<s>
Another	O
positive	O
side	O
effect	O
is	O
that	O
,	O
in	O
general	O
,	O
the	O
connection	B-Protocol
becomes	O
faster	O
with	O
time	O
due	O
to	O
TCP	B-Protocol
's	O
slow-start-mechanism	O
.	O
</s>
<s>
HTTP/1.1	B-Protocol
added	O
also	O
HTTP	B-Protocol
pipelining	I-Protocol
in	O
order	O
to	O
further	O
reduce	O
lag	O
time	O
when	O
using	O
persistent	O
connections	O
by	O
allowing	O
clients	O
to	O
send	O
multiple	O
requests	O
before	O
waiting	O
for	O
each	O
response	O
.	O
</s>
<s>
This	O
optimization	O
was	O
never	O
considered	O
really	O
safe	O
because	O
a	O
few	O
web	B-Application
servers	I-Application
and	O
many	O
proxy	B-Application
servers	I-Application
,	O
specially	O
transparent	O
proxy	B-Application
servers	I-Application
placed	O
in	O
Internet	O
/	O
Intranets	B-General_Concept
between	O
clients	O
and	O
servers	O
,	O
did	O
not	O
handle	O
pipelined	O
requests	O
properly	O
(	O
they	O
served	O
only	O
the	O
first	O
request	O
discarding	O
the	O
others	O
,	O
they	O
closed	O
the	O
connection	B-Protocol
because	O
they	O
saw	O
more	O
data	O
after	O
the	O
first	O
request	O
or	O
some	O
proxies	O
even	O
returned	O
responses	O
out	O
of	O
order	O
etc	O
.	O
)	O
.	O
</s>
<s>
limited	O
to	O
real	O
file	B-Operating_System
requests	O
and	O
so	O
with	O
URLs	O
without	O
query	O
string	O
used	O
as	O
a	O
command	O
,	O
etc	O
.	O
)	O
</s>
<s>
After	O
many	O
years	O
of	O
struggling	O
with	O
the	O
problems	O
introduced	O
by	O
enabling	O
pipelining	O
,	O
this	O
feature	O
was	O
first	O
disabled	O
and	O
then	O
removed	O
from	O
most	O
browsers	B-Application
also	O
because	O
of	O
the	O
announced	O
adoption	O
of	O
HTTP/2	B-Protocol
.	O
</s>
<s>
HTTP/2	B-Protocol
extended	O
the	O
usage	O
of	O
persistent	O
connections	O
by	O
multiplexing	B-Architecture
many	O
concurrent	O
requests/responses	O
through	O
a	O
single	O
TCP/IP	B-Protocol
connection	B-Protocol
.	O
</s>
<s>
HTTP/3	B-Protocol
does	O
not	O
use	O
TCP/IP	B-Protocol
connections	O
but	O
QUIC	B-Protocol
+	O
UDP	O
(	O
see	O
also	O
:	O
technical	O
overview	O
)	O
.	O
</s>
<s>
HTTP/1.0	B-Protocol
added	O
headers	O
to	O
manage	O
resources	O
cached	O
by	O
client	O
in	O
order	O
to	O
allow	O
conditional	O
GET	O
requests	O
;	O
in	O
practice	O
a	O
server	O
has	O
to	O
return	O
the	O
entire	O
content	O
of	O
the	O
requested	O
resource	O
only	O
if	O
its	O
last	O
modified	O
time	O
is	O
not	O
known	O
by	O
client	O
or	O
if	O
it	O
changed	O
since	O
last	O
full	O
response	O
to	O
GET	O
request	O
.	O
</s>
<s>
One	O
of	O
these	O
headers	O
,	O
"	O
Content-Encoding	O
"	O
,	O
was	O
added	O
to	O
specify	O
whether	O
the	O
returned	O
content	O
of	O
a	O
resource	O
was	O
or	O
was	O
not	O
compressed	B-Protocol
.	O
</s>
<s>
then	O
the	O
header	O
"	O
Content-Length	O
:	O
number	O
"	O
was	O
not	O
present	O
in	O
HTTP	B-Protocol
headers	I-Protocol
and	O
the	O
client	O
assumed	O
that	O
when	O
server	O
closed	O
the	O
connection	B-Protocol
,	O
the	O
content	O
had	O
been	O
entirely	O
sent	O
.	O
</s>
<s>
HTTP/1.1	B-Protocol
introduced	O
:	O
</s>
<s>
chunked	B-Protocol
transfer	I-Protocol
encoding	I-Protocol
to	O
allow	O
content	O
to	O
be	O
streamed	O
in	O
chunks	O
in	O
order	O
to	O
reliably	O
send	O
it	O
even	O
when	O
the	O
server	O
does	O
not	O
know	O
in	O
advance	O
its	O
length	O
(	O
i.e.	O
</s>
<s>
byte	B-Protocol
range	I-Protocol
serving	I-Protocol
,	O
where	O
a	O
client	O
can	O
request	O
only	O
one	O
or	O
more	O
portions	O
(	O
ranges	O
of	O
bytes	O
)	O
of	O
a	O
resource	O
(	O
i.e.	O
</s>
<s>
This	O
is	O
useful	O
to	O
resume	O
an	O
interrupted	O
download	O
(	O
when	O
a	O
file	B-Operating_System
is	O
really	O
big	O
)	O
,	O
when	O
only	O
a	O
part	O
of	O
a	O
content	O
has	O
to	O
be	O
shown	O
or	O
dynamically	O
added	O
to	O
the	O
already	O
visible	O
part	O
by	O
a	O
browser	B-Application
(	O
i.e.	O
</s>
<s>
Both	O
HTTP/2	B-Protocol
and	O
HTTP/3	B-Protocol
have	O
kept	O
the	O
above	O
mentioned	O
features	O
of	O
HTTP/1.1	B-Protocol
.	O
</s>
<s>
HTTP	B-Protocol
provides	O
multiple	O
authentication	O
schemes	O
such	O
as	O
basic	B-Protocol
access	I-Protocol
authentication	I-Protocol
and	O
digest	B-Protocol
access	I-Protocol
authentication	I-Protocol
which	O
operate	O
via	O
a	O
challenge	O
–	O
response	O
mechanism	O
whereby	O
the	O
server	O
identifies	O
and	O
issues	O
a	O
challenge	O
before	O
serving	O
the	O
requested	O
content	O
.	O
</s>
<s>
HTTP	B-Protocol
provides	O
a	O
general	O
framework	O
for	O
access	O
control	O
and	O
authentication	O
,	O
via	O
an	O
extensible	O
set	O
of	O
challenge	O
–	O
response	O
authentication	O
schemes	O
,	O
which	O
can	O
be	O
used	O
by	O
a	O
server	O
to	O
challenge	O
a	O
client	O
request	O
and	O
by	O
a	O
client	O
to	O
provide	O
authentication	O
information	O
.	O
</s>
<s>
The	O
authentication	O
mechanisms	O
described	O
above	O
belong	O
to	O
the	O
HTTP	B-Protocol
protocol	I-Protocol
and	O
are	O
managed	O
by	O
client	O
and	O
server	O
HTTP	B-Protocol
software	O
(	O
if	O
configured	O
to	O
require	O
authentication	O
before	O
allowing	O
client	O
access	O
to	O
one	O
or	O
more	O
web	O
resources	O
)	O
,	O
and	O
not	O
by	O
the	O
web	B-Application
applications	I-Application
using	O
a	O
web	B-Application
application	I-Application
session	O
.	O
</s>
<s>
The	O
HTTP	B-Protocol
Authentication	O
specification	O
also	O
provides	O
an	O
arbitrary	O
,	O
implementation-specific	O
construct	O
for	O
further	O
dividing	O
resources	O
common	O
to	O
a	O
given	O
root	O
URI	B-Protocol
.	O
</s>
<s>
The	O
realm	O
value	O
string	O
,	O
if	O
present	O
,	O
is	O
combined	O
with	O
the	O
canonical	O
root	O
URI	B-Protocol
to	O
form	O
the	O
protection	O
space	O
component	O
of	O
the	O
challenge	O
.	O
</s>
<s>
This	O
in	O
effect	O
allows	O
the	O
server	O
to	O
define	O
separate	O
authentication	O
scopes	O
under	O
one	O
root	O
URI	B-Protocol
.	O
</s>
<s>
HTTP	B-Protocol
is	O
a	O
stateless	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
A	O
stateless	B-Protocol
protocol	I-Protocol
does	O
not	O
require	O
the	O
web	B-Application
server	I-Application
to	O
retain	O
information	O
or	O
status	O
about	O
each	O
user	O
for	O
the	O
duration	O
of	O
multiple	O
requests	O
.	O
</s>
<s>
Some	O
web	B-Application
applications	I-Application
need	O
to	O
manage	O
user	O
sessions	O
,	O
so	O
they	O
implement	O
states	O
,	O
or	O
server	B-Protocol
side	I-Protocol
sessions	I-Protocol
,	O
using	O
for	O
instance	O
HTTP	B-Application
cookies	I-Application
or	O
hidden	O
variables	O
within	O
web	B-General_Concept
forms	I-General_Concept
.	O
</s>
<s>
To	O
start	O
an	O
application	O
user	O
session	O
,	O
an	O
interactive	O
authentication	O
via	O
web	B-Application
application	I-Application
login	B-General_Concept
must	O
be	O
performed	O
.	O
</s>
<s>
To	O
stop	O
a	O
user	O
session	O
a	O
logout	B-General_Concept
operation	O
must	O
be	O
requested	O
by	O
user	O
.	O
</s>
<s>
These	O
kind	O
of	O
operations	O
do	O
not	O
use	O
HTTP	B-Protocol
authentication	O
but	O
a	O
custom	O
managed	O
web	B-Application
application	I-Application
authentication	O
.	O
</s>
<s>
zero	O
or	O
more	O
request	O
header	B-Protocol
fields	I-Protocol
(	O
at	O
least	O
1	O
or	O
more	O
headers	O
in	O
case	O
of	O
HTTP/1.1	B-Protocol
)	O
,	O
each	O
consisting	O
of	O
the	O
case-insensitive	O
field	O
name	O
,	O
a	O
colon	O
,	O
optional	O
leading	O
whitespace	O
,	O
the	O
field	O
value	O
,	O
an	O
optional	O
trailing	O
whitespace	O
and	O
ending	O
with	O
a	O
carriage	O
return	O
and	O
a	O
line	O
feed	O
,	O
e.g.	O
</s>
<s>
an	O
optional	O
message	B-Protocol
body	I-Protocol
.	O
</s>
<s>
In	O
the	O
HTTP/1.1	B-Protocol
protocol	O
,	O
all	O
header	B-Protocol
fields	I-Protocol
except	O
Host	O
:	O
hostname	O
are	O
optional	O
.	O
</s>
<s>
A	O
request	O
line	O
containing	O
only	O
the	O
path	O
name	O
is	O
accepted	O
by	O
servers	O
to	O
maintain	O
compatibility	O
with	O
HTTP	B-Application
clients	I-Application
before	O
the	O
HTTP/1.0	B-Protocol
specification	O
in	O
.	O
</s>
<s>
HTTP	B-Protocol
defines	O
methods	O
(	O
sometimes	O
referred	O
to	O
as	O
verbs	O
,	O
but	O
nowhere	O
in	O
the	O
specification	O
does	O
it	O
mention	O
verb	O
)	O
to	O
indicate	O
the	O
desired	O
action	O
to	O
be	O
performed	O
on	O
the	O
identified	O
resource	O
.	O
</s>
<s>
Often	O
,	O
the	O
resource	O
corresponds	O
to	O
a	O
file	B-Operating_System
or	O
the	O
output	O
of	O
an	O
executable	O
residing	O
on	O
the	O
server	O
.	O
</s>
<s>
The	O
HTTP/1.0	B-Protocol
specification	O
defined	O
the	O
GET	O
,	O
HEAD	O
,	O
and	O
POST	O
methods	O
,	O
and	O
the	O
HTTP/1.1	B-Protocol
specification	O
added	O
five	O
new	O
methods	O
:	O
PUT	O
,	O
DELETE	O
,	O
CONNECT	O
,	O
OPTIONS	O
,	O
and	O
TRACE	O
.	O
</s>
<s>
For	O
example	O
,	O
WebDAV	B-Protocol
defined	O
seven	O
new	O
methods	O
and	O
specified	O
the	O
PATCH	B-Protocol
method	I-Protocol
.	O
</s>
<s>
This	O
is	O
in	O
contrast	O
to	O
HTTP	B-Protocol
header	I-Protocol
field	O
names	O
which	O
are	O
case-insensitive	O
.	O
</s>
<s>
GET	O
requests	O
should	O
only	O
retrieve	B-Application
data	I-Application
and	O
should	O
have	O
no	O
other	O
effect	O
.	O
</s>
<s>
(	O
This	O
is	O
also	O
true	O
of	O
some	O
other	O
HTTP	B-Protocol
methods	O
.	O
)	O
</s>
<s>
This	O
enables	O
bookmarking	O
and	O
sharing	O
and	O
makes	O
GET	O
responses	O
eligible	O
for	O
caching	B-Protocol
,	O
which	O
can	O
save	O
bandwidth	O
.	O
</s>
<s>
The	O
W3C	O
has	O
published	O
guidance	O
principles	O
on	O
this	O
distinction	O
,	O
saying	O
,	O
"	O
Web	B-Application
application	I-Application
design	O
should	O
be	O
informed	O
by	O
the	O
above	O
principles	O
,	O
but	O
also	O
by	O
the	O
relevant	O
limitations.	O
"	O
</s>
<s>
Uses	O
include	O
checking	O
whether	O
a	O
page	O
is	O
available	O
through	O
the	O
status	B-Protocol
code	I-Protocol
and	O
quickly	O
finding	O
the	O
size	O
of	O
a	O
file	B-Operating_System
(	O
Content-Length	O
)	O
.	O
</s>
<s>
POST	O
The	O
POST	B-Protocol
method	I-Protocol
requests	O
that	O
the	O
target	O
resource	O
process	B-Operating_System
the	O
representation	O
enclosed	O
in	O
the	O
request	O
according	O
to	O
the	O
semantics	O
of	O
the	O
target	O
resource	O
.	O
</s>
<s>
For	O
example	O
,	O
it	O
is	O
used	O
for	O
posting	O
a	O
message	O
to	O
an	O
Internet	B-Application
forum	I-Application
,	O
subscribing	O
to	O
a	O
mailing	O
list	O
,	O
or	O
completing	O
an	O
online	O
shopping	O
transaction	O
.	O
</s>
<s>
CONNECT	O
The	O
CONNECT	O
method	O
requests	O
that	O
the	O
intermediary	O
establish	O
a	O
TCP/IP	B-Protocol
tunnel	I-Protocol
to	O
the	O
origin	O
server	O
identified	O
by	O
the	O
request	O
target	O
.	O
</s>
<s>
It	O
is	O
often	O
used	O
to	O
secure	O
connections	O
through	O
one	O
or	O
more	O
HTTP	B-Protocol
proxies	O
with	O
TLS	B-Protocol
.	O
</s>
<s>
See	O
HTTP	B-Protocol
CONNECT	O
method	O
.	O
</s>
<s>
OPTIONS	O
The	O
OPTIONS	O
method	O
requests	O
that	O
the	O
target	O
resource	O
transfer	O
the	O
HTTP	B-Protocol
methods	O
that	O
it	O
supports	O
.	O
</s>
<s>
This	O
can	O
be	O
used	O
to	O
check	O
the	O
functionality	O
of	O
a	O
web	B-Application
server	I-Application
by	O
requesting	O
'	O
*	O
 '	O
instead	O
of	O
a	O
specific	O
resource	O
.	O
</s>
<s>
PATCH	B-Protocol
The	O
PATCH	B-Protocol
method	I-Protocol
requests	O
that	O
the	O
target	O
resource	O
modify	O
its	O
state	O
according	O
to	O
the	O
partial	O
update	O
defined	O
in	O
the	O
representation	O
enclosed	O
in	O
the	O
request	O
.	O
</s>
<s>
This	O
can	O
save	O
bandwidth	O
by	O
updating	O
a	O
part	O
of	O
a	O
file	B-Operating_System
or	O
document	O
without	O
having	O
to	O
transfer	O
it	O
entirely	O
.	O
</s>
<s>
All	O
general-purpose	O
web	B-Application
servers	I-Application
are	O
required	O
to	O
implement	O
at	O
least	O
the	O
GET	O
and	O
HEAD	O
methods	O
,	O
and	O
all	O
other	O
methods	O
are	O
considered	O
optional	O
by	O
the	O
specification	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
safe	O
methods	O
are	O
intended	O
to	O
be	O
read-only	B-Application
.	O
</s>
<s>
They	O
do	O
not	O
exclude	O
side	O
effects	O
though	O
,	O
such	O
as	O
appending	O
request	O
information	O
to	O
a	O
log	B-Application
file	I-Application
or	O
charging	O
an	O
advertising	O
account	O
,	O
since	O
they	O
are	O
not	O
requested	O
by	O
the	O
client	O
,	O
by	O
definition	O
.	O
</s>
<s>
In	O
contrast	O
,	O
the	O
methods	O
POST	O
,	O
PUT	O
,	O
DELETE	O
,	O
CONNECT	O
,	O
and	O
PATCH	B-Protocol
are	O
not	O
safe	O
.	O
</s>
<s>
Such	O
methods	O
are	O
therefore	O
not	O
usually	O
used	O
by	O
conforming	O
web	B-Protocol
robots	I-Protocol
or	O
web	B-Application
crawlers	I-Application
;	O
some	O
that	O
do	O
not	O
conform	O
tend	O
to	O
make	O
requests	O
without	O
regard	O
to	O
context	O
or	O
consequences	O
.	O
</s>
<s>
This	O
is	O
discouraged	O
because	O
of	O
the	O
problems	O
which	O
can	O
occur	O
when	O
web	B-Protocol
caching	I-Protocol
,	O
search	B-Application
engines	I-Application
,	O
and	O
other	O
automated	B-Application
agents	I-Application
make	O
unintended	O
changes	O
on	O
the	O
server	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
website	O
might	O
allow	O
deletion	O
of	O
a	O
resource	O
through	O
a	O
URL	O
such	O
as	O
https://example.com/article/1234/delete	O
,	O
which	O
,	O
if	O
arbitrarily	O
fetched	O
,	O
even	O
using	O
GET	O
,	O
would	O
simply	O
delete	O
the	O
article	O
.	O
</s>
<s>
A	O
properly	O
coded	O
website	O
would	O
require	O
a	O
DELETE	O
or	O
POST	B-Protocol
method	I-Protocol
for	O
this	O
action	O
,	O
which	O
non-malicious	O
bots	B-Protocol
would	O
not	O
make	O
.	O
</s>
<s>
One	O
example	O
of	O
this	O
occurring	O
in	O
practice	O
was	O
during	O
the	O
short-lived	O
Google	B-Application
Web	I-Application
Accelerator	I-Application
beta	O
,	O
which	O
prefetched	O
arbitrary	O
URLs	O
on	O
the	O
page	O
a	O
user	O
was	O
viewing	O
,	O
causing	O
records	O
to	O
be	O
automatically	O
altered	O
or	O
deleted	O
en	O
masse	O
.	O
</s>
<s>
In	O
contrast	O
,	O
the	O
methods	O
POST	O
,	O
CONNECT	O
,	O
and	O
PATCH	B-Protocol
are	O
not	O
necessarily	O
idempotent	O
,	O
and	O
therefore	O
sending	O
an	O
identical	O
POST	B-Protocol
request	I-Protocol
multiple	O
times	O
may	O
further	O
modify	O
the	O
state	O
of	O
the	O
server	O
or	O
have	O
further	O
effects	O
,	O
such	O
as	O
sending	O
multiple	O
emails	O
.	O
</s>
<s>
A	O
user	O
might	O
,	O
for	O
example	O
,	O
inadvertently	O
send	O
multiple	O
POST	B-Protocol
requests	I-Protocol
by	O
clicking	O
a	O
button	O
again	O
if	O
they	O
were	O
not	O
given	O
clear	O
feedback	O
that	O
the	O
first	O
click	O
was	O
being	O
processed	O
.	O
</s>
<s>
While	O
web	B-Application
browsers	I-Application
may	O
show	O
alert	O
dialog	O
boxes	O
to	O
warn	O
users	O
in	O
some	O
cases	O
where	O
reloading	O
a	O
page	O
may	O
re-submit	O
a	O
POST	B-Protocol
request	I-Protocol
,	O
it	O
is	O
generally	O
up	O
to	O
the	O
web	B-Application
application	I-Application
to	O
handle	O
cases	O
where	O
a	O
POST	B-Protocol
request	I-Protocol
should	O
not	O
be	O
submitted	O
more	O
than	O
once	O
.	O
</s>
<s>
Note	O
that	O
whether	O
or	O
not	O
a	O
method	O
is	O
idempotent	O
is	O
not	O
enforced	O
by	O
the	O
protocol	O
or	O
web	B-Application
server	I-Application
.	O
</s>
<s>
It	O
is	O
perfectly	O
possible	O
to	O
write	O
a	O
web	B-Application
application	I-Application
in	O
which	O
(	O
for	O
example	O
)	O
a	O
database	O
insert	O
or	O
other	O
non-idempotent	O
action	O
is	O
triggered	O
by	O
a	O
GET	O
or	O
other	O
request	O
.	O
</s>
<s>
To	O
do	O
so	O
against	O
recommendations	O
,	O
however	O
,	O
may	O
result	O
in	O
undesirable	O
consequences	O
,	O
if	O
a	O
user	B-Protocol
agent	I-Protocol
assumes	O
that	O
repeating	O
the	O
same	O
request	O
is	O
safe	O
when	O
it	O
is	O
not	O
.	O
</s>
<s>
In	O
contrast	O
,	O
the	O
methods	O
PUT	O
,	O
DELETE	O
,	O
CONNECT	O
,	O
OPTIONS	O
,	O
TRACE	O
,	O
and	O
PATCH	B-Protocol
are	O
not	O
cacheable	O
.	O
</s>
<s>
Request	O
header	B-Protocol
fields	I-Protocol
allow	O
the	O
client	O
to	O
pass	O
additional	O
information	O
beyond	O
the	O
request	O
line	O
,	O
acting	O
as	O
request	O
modifiers	O
(	O
similarly	O
to	O
the	O
parameters	O
of	O
a	O
procedure	O
)	O
.	O
</s>
<s>
a	O
status	O
line	O
,	O
consisting	O
of	O
the	O
protocol	O
version	O
,	O
a	O
space	O
,	O
the	O
response	B-Protocol
status	I-Protocol
code	I-Protocol
,	O
another	O
space	O
,	O
a	O
possibly	O
empty	O
reason	O
phrase	O
,	O
a	O
carriage	O
return	O
and	O
a	O
line	O
feed	O
,	O
e.g.	O
</s>
<s>
zero	O
or	O
more	O
response	O
header	B-Protocol
fields	I-Protocol
,	O
each	O
consisting	O
of	O
the	O
case-insensitive	O
field	O
name	O
,	O
a	O
colon	O
,	O
optional	O
leading	O
whitespace	O
,	O
the	O
field	O
value	O
,	O
an	O
optional	O
trailing	O
whitespace	O
and	O
ending	O
with	O
a	O
carriage	O
return	O
and	O
a	O
line	O
feed	O
,	O
e.g.	O
</s>
<s>
an	O
optional	O
message	B-Protocol
body	I-Protocol
.	O
</s>
<s>
In	O
HTTP/1.0	B-Protocol
and	O
since	O
,	O
the	O
first	O
line	O
of	O
the	O
HTTP	B-Protocol
response	O
is	O
called	O
the	O
status	O
line	O
and	O
includes	O
a	O
numeric	O
status	B-Protocol
code	I-Protocol
(	O
such	O
as	O
"	O
404	B-Protocol
"	O
)	O
and	O
a	O
textual	O
reason	O
phrase	O
(	O
such	O
as	O
"	O
Not	B-Protocol
Found	I-Protocol
"	O
)	O
.	O
</s>
<s>
The	O
response	B-Protocol
status	I-Protocol
code	I-Protocol
is	O
a	O
three-digit	O
integer	O
code	O
representing	O
the	O
result	O
of	O
the	O
server	O
's	O
attempt	O
to	O
understand	O
and	O
satisfy	O
the	O
client	O
's	O
corresponding	O
request	O
.	O
</s>
<s>
The	O
way	O
the	O
client	O
handles	O
the	O
response	O
depends	O
primarily	O
on	O
the	O
status	B-Protocol
code	I-Protocol
,	O
and	O
secondarily	O
on	O
the	O
other	O
response	O
header	B-Protocol
fields	I-Protocol
.	O
</s>
<s>
Clients	O
may	O
not	O
understand	O
all	O
registered	O
status	O
codes	O
but	O
they	O
must	O
understand	O
their	O
class	O
(	O
given	O
by	O
the	O
first	O
digit	O
of	O
the	O
status	B-Protocol
code	I-Protocol
)	O
and	O
treat	O
an	O
unrecognized	O
status	B-Protocol
code	I-Protocol
as	O
being	O
equivalent	O
to	O
the	O
x00	O
status	B-Protocol
code	I-Protocol
of	O
that	O
class	O
.	O
</s>
<s>
The	O
standard	O
reason	O
phrases	O
are	O
only	O
recommendations	O
,	O
and	O
can	O
be	O
replaced	O
with	O
"	O
local	O
equivalents	O
"	O
at	O
the	O
web	B-Operating_System
developer	I-Operating_System
's	O
discretion	O
.	O
</s>
<s>
If	O
the	O
status	B-Protocol
code	I-Protocol
indicated	O
a	O
problem	O
,	O
the	O
user	B-Protocol
agent	I-Protocol
might	O
display	O
the	O
reason	O
phrase	O
to	O
the	O
user	O
to	O
provide	O
further	O
information	O
about	O
the	O
nature	O
of	O
the	O
problem	O
.	O
</s>
<s>
The	O
standard	O
also	O
allows	O
the	O
user	B-Protocol
agent	I-Protocol
to	O
attempt	O
to	O
interpret	O
the	O
reason	O
phrase	O
,	O
though	O
this	O
might	O
be	O
unwise	O
since	O
the	O
standard	O
explicitly	O
specifies	O
that	O
status	O
codes	O
are	O
machine-readable	O
and	O
reason	O
phrases	O
are	O
human-readable	O
.	O
</s>
<s>
The	O
first	O
digit	O
of	O
the	O
status	B-Protocol
code	I-Protocol
defines	O
its	O
class	O
:	O
</s>
<s>
1XX	O
(	O
informational	O
)	O
The	O
request	O
was	O
received	O
,	O
continuing	O
process	B-Operating_System
.	O
</s>
<s>
The	O
response	O
header	B-Protocol
fields	I-Protocol
allow	O
the	O
server	O
to	O
pass	O
additional	O
information	O
beyond	O
the	O
status	O
line	O
,	O
acting	O
as	O
response	O
modifiers	O
.	O
</s>
<s>
Each	O
response	O
header	O
field	O
has	O
a	O
defined	O
meaning	O
which	O
can	O
be	O
further	O
refined	O
by	O
the	O
semantics	O
of	O
the	O
request	O
method	O
or	O
response	B-Protocol
status	I-Protocol
code	I-Protocol
.	O
</s>
<s>
Below	O
is	O
a	O
sample	O
HTTP	B-Protocol
transaction	O
between	O
an	O
HTTP/1.1	B-Protocol
client	O
and	O
an	O
HTTP/1.1	B-Protocol
server	O
running	O
on	O
www.example.com	O
,	O
port	B-Protocol
80	I-Protocol
.	O
</s>
<s>
The	O
"	O
Host	O
:	O
hostname	O
"	O
header	O
value	O
distinguishes	O
between	O
various	O
DNS	B-Protocol
names	I-Protocol
sharing	O
a	O
single	O
IP	B-Protocol
address	I-Protocol
,	O
allowing	O
name-based	O
virtual	O
hosting	B-Device
.	O
</s>
<s>
While	O
optional	O
in	O
HTTP/1.0	B-Protocol
,	O
it	O
is	O
mandatory	O
in	O
HTTP/1.1	B-Protocol
.	O
</s>
<s>
The	O
ETag	B-Protocol
(	O
entity	B-Protocol
tag	I-Protocol
)	O
header	O
field	O
is	O
used	O
to	O
determine	O
if	O
a	O
cached	O
version	O
of	O
the	O
requested	O
resource	O
is	O
identical	O
to	O
the	O
current	O
version	O
of	O
the	O
resource	O
on	O
the	O
server	O
.	O
</s>
<s>
"	O
Content-Type	B-Application
"	O
specifies	O
the	O
Internet	B-Application
media	I-Application
type	I-Application
of	O
the	O
data	O
conveyed	O
by	O
the	O
HTTP	B-Protocol
message	O
,	O
while	O
"	O
Content-Length	O
"	O
indicates	O
its	O
length	O
in	O
bytes	O
.	O
</s>
<s>
The	O
HTTP/1.1	B-Protocol
webserver	B-Application
publishes	O
its	O
ability	O
to	O
respond	O
to	O
requests	O
for	O
certain	O
byte	O
ranges	O
of	O
the	O
document	O
by	O
setting	O
the	O
field	O
"	O
Accept-Ranges	O
:	O
bytes	O
"	O
.	O
</s>
<s>
This	O
is	O
useful	O
,	O
if	O
the	O
client	O
needs	O
to	O
have	O
only	O
certain	O
portions	O
of	O
a	O
resource	O
sent	O
by	O
the	O
server	O
,	O
which	O
is	O
called	O
byte	B-Protocol
serving	I-Protocol
.	O
</s>
<s>
When	O
"	O
Connection	B-Protocol
:	O
close	O
"	O
is	O
sent	O
,	O
it	O
means	O
that	O
the	O
web	B-Application
server	I-Application
will	O
close	O
the	O
TCP	B-Protocol
connection	B-Protocol
immediately	O
after	O
the	O
end	O
of	O
the	O
transfer	O
of	O
this	O
response	O
.	O
</s>
<s>
When	O
header	O
"	O
Content-Length	O
:	O
number	O
"	O
is	O
missing	O
in	O
a	O
response	O
with	O
an	O
entity	O
body	O
then	O
this	O
should	O
be	O
considered	O
an	O
error	O
in	O
HTTP/1.0	B-Protocol
but	O
it	O
may	O
not	O
be	O
an	O
error	O
in	O
HTTP/1.1	B-Protocol
if	O
header	O
"	O
Transfer-Encoding	O
:	O
chunked	O
"	O
is	O
present	O
.	O
</s>
<s>
Chunked	B-Protocol
transfer	I-Protocol
encoding	I-Protocol
uses	O
a	O
chunk	O
size	O
of	O
0	O
to	O
mark	O
the	O
end	O
of	O
the	O
content	O
.	O
</s>
<s>
Some	O
old	O
implementations	O
of	O
HTTP/1.0	B-Protocol
omitted	O
the	O
header	O
"	O
Content-Length	O
"	O
when	O
the	O
length	O
of	O
the	O
body	O
entity	O
was	O
not	O
known	O
at	O
the	O
beginning	O
of	O
the	O
response	O
and	O
so	O
the	O
transfer	O
of	O
data	O
to	O
client	O
continued	O
until	O
server	O
closed	O
the	O
socket	O
.	O
</s>
<s>
A	O
"	O
Content-Encoding	O
:	O
gzip	B-Application
"	O
can	O
be	O
used	O
to	O
inform	O
the	O
client	O
that	O
the	O
body	O
entity	O
part	O
of	O
the	O
transmitted	O
data	O
is	O
compressed	B-Protocol
by	O
gzip	B-Application
algorithm	O
.	O
</s>
<s>
The	O
most	O
popular	O
way	O
of	O
establishing	O
an	O
encrypted	O
HTTP	B-Protocol
connection	B-Protocol
is	O
HTTPS	B-Protocol
.	O
</s>
<s>
Two	O
other	O
methods	O
for	O
establishing	O
an	O
encrypted	O
HTTP	B-Protocol
connection	B-Protocol
also	O
exist	O
:	O
Secure	B-Protocol
Hypertext	I-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
,	O
and	O
using	O
the	O
HTTP/1.1	B-Protocol
Upgrade	I-Protocol
header	I-Protocol
to	O
specify	O
an	O
upgrade	O
to	O
TLS	B-Protocol
.	O
</s>
<s>
Browser	B-Application
support	O
for	O
these	O
two	O
is	O
,	O
however	O
,	O
nearly	O
non-existent	O
.	O
</s>
<s>
The	O
Gopher	B-Protocol
protocol	I-Protocol
is	O
a	O
content	O
delivery	O
protocol	O
that	O
was	O
displaced	O
by	O
HTTP	B-Protocol
in	O
the	O
early	O
1990s	O
.	O
</s>
<s>
The	O
SPDY	B-Protocol
protocol	O
is	O
an	O
alternative	O
to	O
HTTP	B-Protocol
developed	O
at	O
Google	B-Application
,	O
superseded	O
by	O
HTTP/2	B-Protocol
.	O
</s>
<s>
The	O
Gemini	B-Protocol
protocol	I-Protocol
is	O
a	O
Gopher-inspired	O
protocol	O
which	O
mandates	O
privacy-related	O
features	O
.	O
</s>
