<s>
Round-robin	B-Protocol
DNS	I-Protocol
is	O
a	O
technique	O
of	O
load	B-Application
distribution	I-Application
,	O
load	B-Application
balancing	I-Application
,	O
or	O
fault-tolerance	B-General_Concept
provisioning	O
multiple	O
,	O
redundant	O
Internet	B-Protocol
Protocol	I-Protocol
service	O
hosts	O
,	O
e.g.	O
,	O
Web	B-Application
server	I-Application
,	O
FTP	B-Protocol
servers	I-Protocol
,	O
by	O
managing	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
's	O
(	O
DNS	B-Protocol
)	O
responses	O
to	O
address	O
requests	O
from	O
client	O
computers	O
according	O
to	O
an	O
appropriate	O
statistical	O
model	O
.	O
</s>
<s>
In	O
its	O
simplest	O
implementation	O
,	O
round-robin	B-Protocol
DNS	I-Protocol
works	O
by	O
responding	O
to	O
DNS	B-Protocol
requests	O
not	O
only	O
with	O
a	O
single	O
potential	O
IP	B-Protocol
address	I-Protocol
,	O
but	O
with	O
a	O
list	O
of	O
potential	O
IP	B-Protocol
addresses	I-Protocol
corresponding	O
to	O
several	O
servers	O
that	O
host	O
identical	O
services	O
.	O
</s>
<s>
The	O
order	O
in	O
which	O
IP	B-Protocol
addresses	I-Protocol
from	O
the	O
list	O
are	O
returned	O
is	O
the	O
basis	O
for	O
the	O
term	O
round	O
robin	O
.	O
</s>
<s>
With	O
each	O
DNS	B-Protocol
response	O
,	O
the	O
IP	B-Protocol
address	I-Protocol
sequence	O
in	O
the	O
list	O
is	O
permuted	B-Algorithm
.	O
</s>
<s>
Traditionally	O
,	O
IP	O
clients	O
initially	O
attempt	O
connections	O
with	O
the	O
first	O
address	O
returned	O
from	O
a	O
DNS	B-Protocol
query	I-Protocol
,	O
so	O
that	O
on	O
different	O
connection	O
attempts	O
,	O
clients	O
would	O
receive	O
service	O
from	O
different	O
providers	O
,	O
thus	O
distributing	O
the	O
overall	O
load	O
among	O
servers	O
.	O
</s>
<s>
Round-robin	B-Protocol
DNS	I-Protocol
is	O
often	O
used	O
to	O
load	O
balance	O
requests	O
among	O
a	O
number	O
of	O
Web	B-Application
servers	I-Application
.	O
</s>
<s>
For	O
example	O
,	O
a	O
company	O
has	O
one	O
domain	O
name	O
and	O
three	O
identical	O
copies	O
of	O
the	O
same	O
web	O
site	O
residing	O
on	O
three	O
servers	O
with	O
three	O
IP	B-Protocol
addresses	I-Protocol
.	O
</s>
<s>
The	O
DNS	B-Protocol
server	O
will	O
be	O
set	O
up	O
so	O
that	O
domain	O
name	O
has	O
multiple	O
A	O
records	O
,	O
one	O
for	O
each	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
When	O
one	O
user	O
accesses	O
the	O
home	O
page	O
it	O
will	O
be	O
sent	O
to	O
the	O
first	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
The	O
second	O
user	O
who	O
accesses	O
the	O
home	O
page	O
will	O
be	O
sent	O
to	O
the	O
next	O
IP	B-Protocol
address	I-Protocol
,	O
and	O
the	O
third	O
user	O
will	O
be	O
sent	O
to	O
the	O
third	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
In	O
each	O
case	O
,	O
once	O
the	O
IP	B-Protocol
address	I-Protocol
is	O
given	O
out	O
,	O
it	O
goes	O
to	O
the	O
end	O
of	O
the	O
list	O
.	O
</s>
<s>
The	O
fourth	O
user	O
,	O
therefore	O
,	O
will	O
be	O
sent	O
to	O
the	O
first	O
IP	B-Protocol
address	I-Protocol
,	O
and	O
so	O
forth	O
.	O
</s>
<s>
A	O
round-robin	B-Protocol
DNS	I-Protocol
name	O
is	O
,	O
on	O
rare	O
occasions	O
,	O
referred	O
to	O
as	O
a	O
"	O
rotor	O
"	O
due	O
to	O
the	O
rotation	O
among	O
alternative	O
A	O
records	O
.	O
</s>
<s>
Although	O
easy	O
to	O
implement	O
,	O
round-robin	B-Protocol
DNS	I-Protocol
has	O
a	O
number	O
of	O
drawbacks	O
,	O
such	O
as	O
those	O
arising	O
from	O
record	O
caching	O
in	O
the	O
DNS	B-Protocol
hierarchy	O
itself	O
,	O
as	O
well	O
as	O
client-side	O
address	O
caching	O
and	O
reuse	O
,	O
the	O
combination	O
of	O
which	O
can	O
be	O
difficult	O
to	O
manage	O
.	O
</s>
<s>
Round-robin	B-Protocol
DNS	I-Protocol
should	O
not	O
solely	O
be	O
relied	O
upon	O
for	O
service	O
availability	O
.	O
</s>
<s>
If	O
a	O
service	O
at	O
one	O
of	O
the	O
addresses	O
in	O
the	O
list	O
fails	O
,	O
the	O
DNS	B-Protocol
will	O
continue	O
to	O
hand	O
out	O
that	O
address	O
and	O
clients	O
will	O
still	O
attempt	O
to	O
reach	O
the	O
inoperable	O
service	O
.	O
</s>
<s>
Round-robin	B-Protocol
DNS	I-Protocol
may	O
not	O
be	O
the	O
best	O
choice	O
for	O
load	B-Application
balancing	I-Application
on	O
its	O
own	O
,	O
since	O
it	O
merely	O
alternates	O
the	O
order	O
of	O
the	O
address	O
records	O
each	O
time	O
a	O
name	O
server	O
is	O
queried	O
.	O
</s>
<s>
Otherwise	O
,	O
it	O
just	O
does	O
load	B-Application
distribution	I-Application
.	O
</s>
<s>
For	O
example	O
,	O
modified	O
DNS	B-Protocol
servers	O
(	O
such	O
as	O
lbnamed	O
)	O
can	O
routinely	O
poll	O
mirrored	O
servers	O
for	O
availability	O
and	O
load	O
factor	O
.	O
</s>
<s>
If	O
a	O
server	O
does	O
not	O
reply	O
as	O
required	O
,	O
the	O
server	O
can	O
be	O
temporarily	O
removed	O
from	O
the	O
DNS	B-Protocol
pool	O
,	O
until	O
it	O
reports	O
that	O
it	O
is	O
once	O
again	O
operating	O
within	O
specs	O
.	O
</s>
