<s>
Happy	B-Protocol
Eyeballs	I-Protocol
(	O
also	O
called	O
Fast	B-Protocol
Fallback	I-Protocol
)	O
is	O
an	O
algorithm	O
published	O
by	O
the	O
IETF	O
that	O
makes	O
dual-stack	O
applications	O
(	O
those	O
that	O
understand	O
both	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
)	O
more	O
responsive	O
to	O
users	O
by	O
attempting	O
to	O
connect	O
using	O
both	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
at	O
the	O
same	O
time	O
(	O
preferring	O
IPv6	B-Protocol
)	O
,	O
thus	O
minimizing	O
common	B-Protocol
problems	I-Protocol
experienced	O
by	O
users	O
with	O
imperfect	O
IPv6	B-Protocol
connections	O
or	O
setups	O
.	O
</s>
<s>
The	O
name	O
"	O
happy	B-Protocol
eyeballs	I-Protocol
"	O
derives	O
from	O
the	O
term	O
"	O
eyeball	O
"	O
to	O
describe	O
endpoints	O
which	O
represent	O
human	O
Internet	O
end-users	O
,	O
as	O
opposed	O
to	O
servers	O
.	O
</s>
<s>
Happy	B-Protocol
Eyeballs	I-Protocol
is	O
designed	O
to	O
address	O
the	O
problem	O
that	O
many	O
IPv6	B-Protocol
networks	O
are	O
unreachable	O
from	O
parts	O
of	O
the	O
Internet	O
,	O
and	O
applications	O
trying	O
to	O
reach	O
those	O
networks	O
will	O
appear	O
unresponsive	O
,	O
thus	O
frustrating	O
users	O
.	O
</s>
<s>
Happy	B-Protocol
Eyeballs	I-Protocol
solves	O
this	O
problem	O
by	O
determining	O
which	O
transport	O
would	O
be	O
better	O
used	O
for	O
a	O
particular	O
connection	O
by	O
trying	O
them	O
both	O
in	O
parallel	O
.	O
</s>
<s>
An	O
application	O
that	O
uses	O
a	O
Happy	B-Protocol
Eyeballs	I-Protocol
algorithm	O
checks	O
both	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
connectivity	O
(	O
with	O
a	O
preference	O
for	O
IPv6	B-Protocol
)	O
and	O
uses	O
the	O
first	O
connection	O
that	O
is	O
returned	O
.	O
</s>
<s>
The	O
addresses	O
are	O
often	O
chosen	O
from	O
the	O
DNS	O
with	O
a	O
round-robin	B-Protocol
algorithm	I-Protocol
.	O
</s>
<s>
Implementations	O
of	O
Happy	B-Protocol
Eyeballs	I-Protocol
stacks	O
exist	O
in	O
Google	O
's	O
Chrome	B-Application
web	I-Application
browser	I-Application
,	O
Opera	B-Application
12.10	O
,	O
Firefox	B-Application
version	O
13	O
,	O
OS	B-Operating_System
X	I-Operating_System
,	O
cURL	B-Language
and	O
OpenBSD	B-Operating_System
.	O
</s>
<s>
Happy	O
Eyeball	O
testing	O
was	O
part	O
of	O
World	O
IPv6	B-Protocol
Day	O
in	O
2011	O
.	O
</s>
<s>
The	O
Happy	B-Protocol
Eyeballs	I-Protocol
algorithm	O
may	O
be	O
extended	O
for	O
choosing	O
between	O
types	O
of	O
transport	O
protocols	O
as	O
well	O
,	O
such	O
as	O
TCP	B-Protocol
and	O
SCTP	B-Protocol
,	O
but	O
development	O
is	O
still	O
in	O
an	O
experimental	O
phase	O
.	O
</s>
