<s>
HTTP	B-Protocol
pipelining	I-Protocol
is	O
a	O
feature	O
of	O
HTTP/1.1	B-Protocol
which	O
allows	O
multiple	O
HTTP	B-Protocol
requests	I-Protocol
to	O
be	O
sent	O
over	O
a	O
single	O
TCP	B-Protocol
connection	O
without	O
waiting	O
for	O
the	O
corresponding	O
responses	O
.	O
</s>
<s>
HTTP/1.1	B-Protocol
requires	O
servers	O
to	O
respond	O
to	O
pipelined	O
requests	O
correctly	O
,	O
with	O
non-pipelined	O
but	O
valid	O
responses	O
even	O
if	O
server	O
does	O
not	O
support	O
HTTP	B-Protocol
pipelining	I-Protocol
.	O
</s>
<s>
Despite	O
this	O
requirement	O
,	O
many	O
legacy	O
HTTP/1.1	B-Protocol
servers	O
do	O
not	O
support	O
pipelining	O
correctly	O
,	O
forcing	O
most	O
HTTP	B-Application
clients	I-Application
to	O
not	O
use	O
HTTP	B-Protocol
pipelining	I-Protocol
.	O
</s>
<s>
The	O
technique	O
was	O
superseded	O
by	O
multiplexing	O
via	O
HTTP/2	B-Protocol
,	O
which	O
is	O
supported	O
by	O
most	O
modern	O
browsers	B-Application
.	O
</s>
<s>
In	O
HTTP/3	B-Protocol
,	O
multiplexing	O
is	O
accomplished	O
via	O
QUIC	B-Protocol
which	O
replaces	O
TCP	B-Protocol
.	O
</s>
<s>
This	O
further	O
reduces	O
loading	O
time	O
,	O
as	O
there	O
is	O
no	O
head-of-line	B-Protocol
blocking	I-Protocol
even	O
if	O
some	O
packets	O
are	O
lost	O
.	O
</s>
<s>
The	O
speedup	O
is	O
less	O
apparent	O
on	O
broadband	O
connections	O
,	O
as	O
the	O
limitation	O
of	O
HTTP	B-Protocol
1.1	I-Protocol
still	O
applies	O
:	O
the	O
server	O
must	O
send	O
its	O
responses	O
in	O
the	O
same	O
order	O
that	O
the	O
requests	O
were	O
received	O
—	O
so	O
the	O
entire	O
connection	O
remains	O
first-in-first-out	O
and	O
HOL	B-Protocol
blocking	I-Protocol
can	O
occur	O
.	O
</s>
<s>
The	O
asynchronous	O
operation	O
of	O
HTTP/2	B-Protocol
and	O
SPDY	B-Protocol
are	O
solutions	O
for	O
this	O
.	O
</s>
<s>
By	O
2017	O
most	O
browsers	B-Application
supported	O
HTTP/2	B-Protocol
by	O
default	O
which	O
uses	O
multiplexing	O
instead	O
.	O
</s>
<s>
HTTP	B-Protocol
pipelining	I-Protocol
requires	O
both	O
the	O
client	O
and	O
the	O
server	O
to	O
support	O
it	O
.	O
</s>
<s>
HTTP/1.1	B-Protocol
conforming	O
servers	O
are	O
required	O
to	O
produce	O
valid	O
responses	O
to	O
pipelined	O
requests	O
,	O
but	O
may	O
not	O
actually	O
process	O
requests	O
concurrently	O
.	O
</s>
<s>
Most	O
pipelining	O
problems	O
happen	O
in	O
HTTP	B-Protocol
intermediate	O
nodes	O
(	O
hop-by-hop	O
)	O
,	O
i.e.	O
</s>
<s>
in	O
proxy	B-Application
servers	I-Application
,	O
especially	O
in	O
transparent	O
proxy	B-Application
servers	I-Application
(	O
if	O
one	O
of	O
them	O
along	O
the	O
HTTP	B-Protocol
chain	O
does	O
not	O
handle	O
pipelined	O
requests	O
properly	O
then	O
nothing	O
works	O
as	O
it	O
should	O
)	O
.	O
</s>
<s>
Using	O
pipelining	O
with	O
HTTP	B-Protocol
proxy	B-Application
servers	I-Application
is	O
usually	O
not	O
recommended	O
also	O
because	O
the	O
HOL	B-Protocol
blocking	I-Protocol
problem	O
may	O
really	O
slow	O
down	O
proxy	B-Application
server	I-Application
responses	O
(	O
as	O
the	O
server	O
responses	O
must	O
be	O
in	O
the	O
same	O
order	O
of	O
the	O
received	O
requests	O
)	O
.	O
</s>
<s>
If	O
instead	O
a	O
client	O
opens	O
4	O
connections	O
to	O
a	O
proxy	O
and	O
sends	O
1	O
GET	O
request	O
per	O
connection	O
(	O
without	O
using	O
pipelining	O
)	O
the	O
proxy	O
can	O
send	O
the	O
three	O
cached	O
responses	O
to	O
client	O
in	O
parallel	O
before	O
the	O
response	O
from	O
server	O
is	O
received	O
,	O
decreasing	O
the	O
overall	O
completion	O
time	O
(	O
because	O
requests	O
are	O
served	O
in	O
parallel	O
with	O
no	O
head-of-line	B-Protocol
blocking	I-Protocol
problem	O
)	O
.	O
</s>
<s>
The	O
same	O
advantage	O
exists	O
in	O
HTTP/2	B-Protocol
multiplexed	O
streams	O
.	O
</s>
<s>
Pipelining	O
was	O
introduced	O
in	O
HTTP/1.1	B-Protocol
and	O
was	O
not	O
present	O
in	O
HTTP/1.0	B-Protocol
.	O
</s>
<s>
There	O
have	O
always	O
been	O
complaints	O
about	O
browsers	B-Application
,	O
proxy	B-Application
servers	I-Application
,	O
etc	O
.	O
</s>
<s>
Of	O
all	O
the	O
major	O
browsers	B-Application
,	O
only	O
Opera	B-Application
had	O
a	O
fully	O
working	O
implementation	O
that	O
was	O
enabled	O
by	O
default	O
.	O
</s>
<s>
In	O
other	O
browsers	B-Application
HTTP	B-Protocol
pipelining	I-Protocol
was	O
disabled	O
or	O
not	O
implemented	O
.	O
</s>
<s>
Internet	B-Application
Explorer	I-Application
8	I-Application
does	O
not	O
support	O
pipeline	O
requests	O
,	O
due	O
to	O
concerns	O
regarding	O
buggy	O
proxies	O
and	O
head-of-line	B-Protocol
blocking	I-Protocol
.	O
</s>
<s>
Internet	B-Application
Explorer	I-Application
11	I-Application
does	O
not	O
support	O
pipelining	O
.	O
</s>
<s>
Mozilla	B-Operating_System
browsers	B-Application
(	O
such	O
as	O
Mozilla	B-Application
Firefox	I-Application
,	O
SeaMonkey	B-Language
and	O
Camino	B-Language
)	O
support	O
pipelining	O
;	O
however	O
,	O
it	O
is	O
disabled	O
by	O
default	O
.	O
</s>
<s>
When	O
pipelining	O
is	O
enabled	O
,	O
Mozilla	B-Operating_System
browsers	B-Application
use	O
some	O
heuristics	O
,	O
especially	O
to	O
turn	O
pipelining	O
off	O
for	O
older	O
IIS	B-Application
servers	I-Application
.	O
</s>
<s>
Support	O
for	O
H1	O
Pipeline	O
was	O
removed	O
from	O
Mozilla	B-Application
Firefox	I-Application
in	O
Version	O
54	O
.	O
</s>
<s>
Konqueror	B-Application
2.0	O
supports	O
pipelining	O
,	O
but	O
it	O
is	O
disabled	O
by	O
default	O
.	O
</s>
<s>
Google	B-Application
Chrome	I-Application
previously	O
supported	O
pipelining	O
,	O
but	O
it	O
has	O
been	O
disabled	O
due	O
to	O
bugs	O
and	O
problems	O
with	O
poorly	O
behaving	O
servers	O
.	O
</s>
<s>
Pale	B-Protocol
Moon	I-Protocol
(	O
web	B-Application
browser	I-Application
)	O
supports	O
pipelining	O
,	O
and	O
is	O
enabled	O
by	O
default	O
.	O
</s>
<s>
Most	O
HTTP	B-Protocol
proxies	O
do	O
not	O
pipeline	O
outgoing	O
requests	O
.	O
</s>
<s>
Some	O
HTTP	B-Protocol
proxies	O
,	O
including	O
transparent	O
HTTP	B-Protocol
proxies	O
,	O
may	O
manage	O
pipelined	O
requests	O
very	O
badly	O
(	O
e.g.	O
</s>
<s>
Some	O
versions	O
of	O
the	O
Squid	B-Protocol
web	O
proxy	O
will	O
pipeline	O
up	O
to	O
two	O
outgoing	O
requests	O
.	O
</s>
<s>
Squid	B-Protocol
supports	O
multiple	O
requests	O
from	O
clients	O
.	O
</s>
<s>
The	O
Polipo	B-Protocol
proxy	O
pipelines	O
outgoing	O
requests	O
.	O
</s>
<s>
Tempesta	O
FW	O
,	O
an	O
open	O
source	O
application	B-Protocol
delivery	I-Protocol
controller	I-Protocol
,	O
also	O
pipelines	O
requests	O
to	O
backend	O
servers	O
.	O
</s>
<s>
The	O
libwww	B-Protocol
library	B-Library
made	O
by	O
the	O
World	O
Wide	O
Web	O
Consortium	O
(	O
W3C	O
)	O
,	O
supports	O
pipelining	O
since	O
version	O
5.1	O
released	O
at	O
18	O
February	O
1997	O
.	O
</s>
<s>
Other	O
application	O
development	O
libraries	O
that	O
support	O
HTTP	B-Protocol
pipelining	I-Protocol
include	O
:	O
</s>
<s>
Perl	O
modules	O
providing	O
client	O
support	O
for	O
HTTP	B-Protocol
pipelining	I-Protocol
are	O
HTTP::Async	O
and	O
the	O
LWPng	O
(	O
libwww-perl	B-Language
New	O
Generation	O
)	O
library	B-Library
.	O
</s>
<s>
The	O
Microsoft	B-Application
.NET	I-Application
Framework	I-Application
3.5	O
supports	O
HTTP	B-Protocol
pipelining	I-Protocol
in	O
the	O
module	O
System.Net.HttpWebRequest	O
.	O
</s>
<s>
Qt	B-Language
class	O
QNetworkRequest	O
,	O
introduced	O
in	O
4.4	O
.	O
</s>
<s>
Advanced	B-Application
Packaging	I-Application
Tool	I-Application
(	O
APT	B-Application
)	O
supports	O
pipelining	O
.	O
</s>
<s>
Subversion	B-Application
(	O
SVN	B-Application
)	O
has	O
optional	O
support	O
for	O
HTTP	B-Protocol
pipelining	I-Protocol
with	O
the	O
serf	O
WebDAV	O
access	O
module	O
(	O
the	O
default	O
module	O
,	O
neon	O
,	O
does	O
not	O
have	O
pipelining	O
support	O
)	O
.	O
</s>
<s>
Microsoft	B-Application
Message	I-Application
Queuing	I-Application
on	O
Windows	B-Application
Server	I-Application
2003	I-Application
utilises	O
pipelining	O
on	O
HTTP	B-Protocol
by	O
default	O
,	O
and	O
can	O
be	O
configured	O
to	O
use	O
it	O
on	O
HTTPS	B-Protocol
.	O
</s>
<s>
IBM	O
CICS	B-Application
3.1	O
supports	O
HTTP	B-Protocol
pipelining	I-Protocol
within	O
its	O
client	O
.	O
</s>
<s>
Testing	O
tools	O
which	O
support	O
HTTP	B-Protocol
pipelining	I-Protocol
include	O
:	O
</s>
