<s>
Application-Layer	B-Protocol
Protocol	I-Protocol
Negotiation	I-Protocol
(	O
ALPN	B-Protocol
)	O
is	O
a	O
Transport	B-Protocol
Layer	I-Protocol
Security	I-Protocol
(	O
TLS	O
)	O
extension	O
that	O
allows	O
the	O
application	O
layer	O
to	O
negotiate	O
which	O
protocol	O
should	O
be	O
performed	O
over	O
a	O
secure	O
connection	O
in	O
a	O
manner	O
that	O
avoids	O
additional	O
round	O
trips	O
and	O
which	O
is	O
independent	O
of	O
the	O
application-layer	O
protocols	O
.	O
</s>
<s>
It	O
is	O
used	O
to	O
establish	O
HTTP/2	B-Protocol
connections	O
without	O
additional	O
round	O
trips	O
(	O
client	O
and	O
server	O
can	O
communicate	O
over	O
to	O
ports	O
previously	O
assigned	O
to	O
HTTPS	B-Protocol
with	O
HTTP/1.1	B-Protocol
and	O
upgrade	O
to	O
use	O
HTTP/2	B-Protocol
or	O
continue	O
with	O
HTTP/1.1	B-Protocol
without	O
closing	O
the	O
initial	O
connection	O
)	O
.	O
</s>
<s>
ALPN	B-Protocol
is	O
supported	O
by	O
these	O
libraries	O
:	O
</s>
<s>
s2n	B-Protocol
since	O
its	O
original	O
public	O
release	O
in	O
June	O
2015	O
.	O
</s>
<s>
This	O
extension	O
was	O
used	O
to	O
negotiate	O
experimental	O
SPDY	O
connections	O
between	O
Google	B-Application
Chrome	I-Application
and	O
some	O
of	O
Google	O
's	O
servers	O
.	O
</s>
<s>
As	O
SPDY	O
evolved	O
,	O
NPN	O
was	O
replaced	O
with	O
ALPN	B-Protocol
.	O
</s>
<s>
On	O
July	O
11	O
,	O
2014	O
,	O
ALPN	B-Protocol
was	O
published	O
as	O
.	O
</s>
<s>
ALPN	B-Protocol
replaces	O
Next	O
Protocol	O
Negotiation	O
(	O
NPN	O
)	O
extension	O
.	O
</s>
<s>
TLS	O
False	O
Start	O
was	O
disabled	O
in	O
Google	B-Application
Chrome	I-Application
from	O
version	O
20	O
(	O
2012	O
)	O
onward	O
except	O
for	O
websites	O
with	O
the	O
earlier	O
NPN	O
extension	O
.	O
</s>
<s>
ALPN	B-Protocol
is	O
a	O
TLS	O
extension	O
which	O
is	O
sent	O
on	O
the	O
initial	O
TLS	O
handshake	O
'	O
Client	O
Hello	O
 '	O
,	O
and	O
it	O
lists	O
the	O
protocols	O
that	O
the	O
client	O
(	O
for	O
example	O
the	O
web	O
browser	O
)	O
supports	O
:	O
</s>
<s>
The	O
resulting	O
'	O
Server	O
Hello	O
 '	O
from	O
the	O
web	O
server	O
will	O
also	O
contain	O
the	O
ALPN	B-Protocol
extension	O
,	O
and	O
it	O
confirms	O
which	O
protocol	O
will	O
be	O
used	O
for	O
the	O
HTTP	B-Protocol
request	O
:	O
</s>
