<s>
HTTP/2	B-Protocol
Server	I-Protocol
Push	I-Protocol
is	O
an	O
optional	O
feature	O
of	O
the	O
HTTP/2	B-Protocol
and	O
HTTP/3	B-Protocol
network	O
protocols	O
which	O
allows	O
servers	O
to	O
send	O
resources	O
to	O
a	O
client	O
before	O
the	O
client	O
requests	O
them	O
.	O
</s>
<s>
HTTP/2	B-Protocol
Server	I-Protocol
Push	I-Protocol
is	O
not	O
a	O
notification	O
mechanism	O
from	O
server	O
to	O
client	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
the	O
case	O
of	O
a	O
website	O
with	O
three	O
resources	O
:	O
index.html	O
,	O
styles.css	O
and	O
script.js	O
,	O
when	O
a	O
user	O
,	O
through	O
their	O
browser	O
,	O
connects	O
to	O
Wikipedia	O
webserver	O
to	O
get	O
the	O
home	O
page	O
,	O
they	O
automatically	O
retrieve	O
index.html	O
.	O
</s>
<s>
As	O
the	O
browser	O
parses	O
the	O
HTML	B-Language
text	O
in	O
index.html	O
,	O
it	O
finds	O
instructions	O
that	O
will	O
require	O
styles.css	O
and	O
script.js	O
.	O
</s>
<s>
With	O
HTTP/2	B-Protocol
Push	O
,	O
the	O
server	O
can	O
take	O
the	O
initiative	O
by	O
having	O
rules	O
that	O
trigger	O
content	O
to	O
be	O
sent	O
even	O
before	O
it	O
is	O
requested	O
.	O
</s>
<s>
In	O
this	O
example	O
scenario	O
,	O
the	O
server	O
knows	O
that	O
anyone	O
requesting	O
index.html	O
will	O
need	O
styles.css	O
and	O
script.js	O
,	O
so	O
it	O
can	O
push	O
them	O
to	O
the	O
client	O
immediately	O
without	O
waiting	O
for	O
the	O
client	O
to	O
request	O
them	O
.	O
</s>
<s>
If	O
done	O
correctly	O
,	O
by	O
the	O
time	O
the	O
browser	O
finishes	O
parsing	O
index.html	O
,	O
the	O
transfer	O
of	O
styles.css	O
and	O
script.js	O
would	O
have	O
already	O
started	O
,	O
or	O
even	O
completed	O
,	O
removing	O
the	O
latency	O
of	O
having	O
to	O
request	O
these	O
and	O
wait	O
for	O
them	O
to	O
arrive	O
.	O
</s>
<s>
Push	O
works	O
over	O
HTTP/2	B-Protocol
,	O
which	O
at	O
its	O
core	O
is	O
a	O
frames	O
protocol	O
,	O
meaning	O
that	O
information	O
is	O
exchanged	O
in	O
groups	O
of	O
bytes	O
called	O
frames	O
.	O
</s>
<s>
the	O
response	O
to	O
request	O
GET	O
/index.html	O
at	O
stream	O
3	O
must	O
also	O
be	O
at	O
stream	O
3	O
.	O
</s>
<s>
HTTP/2	B-Protocol
has	O
only	O
a	O
few	O
of	O
these	O
types	O
,	O
and	O
we	O
do	O
n't	O
need	O
all	O
of	O
them	O
to	O
explain	O
the	O
basics	O
.	O
</s>
<s>
The	O
request	O
URL	O
,	O
for	O
example	O
,	O
is	O
present	O
in	O
the	O
PUSH_PROMISE	O
frame	O
as	O
the	O
HTTP/2	B-Protocol
-specific	O
:path	O
pseudo-header	O
,	O
as	O
is	O
the	O
:authority	O
pseudo-header	O
to	O
indicate	O
a	O
host	O
.	O
</s>
<s>
If	O
the	O
server	O
is	O
HTTP/2	B-Protocol
PUSH	O
ready	O
,	O
when	O
it	O
receives	O
a	O
request	O
to	O
index.html	O
it	O
can	O
forecast	O
that	O
requests	O
to	O
styles.css	O
and	O
script.js	O
are	O
following	O
close	O
in	O
tail	O
.	O
</s>
<s>
Server	O
receives	O
HEADERS	O
frame	O
asking	O
for	O
index.html	O
in	O
stream	O
3	O
,	O
and	O
it	O
can	O
forecast	O
the	O
need	O
for	O
styles.css	O
and	O
script.js	O
.	O
</s>
<s>
Server	O
sends	O
a	O
HEADERS	O
frame	O
in	O
stream	O
3	O
for	O
responding	O
to	O
the	O
request	O
for	O
index.html	O
.	O
</s>
<s>
Server	O
sends	O
DATA	O
frame(s )	O
with	O
the	O
contents	O
of	O
index.html	O
,	O
still	O
in	O
stream	O
3	O
.	O
</s>
<s>
HTTP/2	B-Protocol
Server	I-Protocol
push	I-Protocol
is	O
being	O
progressively	O
implemented	O
,	O
for	O
example	O
Nginx	B-Operating_System
web	O
server	O
implemented	O
it	O
in	O
version	O
1.13.9	O
in	O
February	O
2018	O
.	O
</s>
<s>
According	O
to	O
the	O
Google	B-Application
Chrome	I-Application
team	O
,	O
Server	O
Push	O
in	O
HTTP/2	B-Protocol
and	O
gQUIC	O
is	O
rarely	O
used	O
,	O
and	O
the	O
pushed	O
resources	O
are	O
more	O
often	O
unused	O
than	O
used	O
.	O
</s>
<s>
They	O
have	O
proposed	O
to	O
remove	O
the	O
feature	O
from	O
Chrome	B-Application
and	O
Chromium	O
.	O
</s>
