<s>
Reliable	B-Protocol
Server	I-Protocol
Pooling	I-Protocol
(	O
RSerPool	B-Protocol
)	O
is	O
a	O
computer	O
protocol	O
framework	B-Architecture
for	O
management	O
of	O
and	O
access	O
to	O
multiple	O
,	O
coordinated	O
(	O
pooled	O
)	O
servers	B-Application
.	O
</s>
<s>
RSerPool	B-Protocol
is	O
an	O
IETF	O
standard	O
,	O
which	O
has	O
been	O
developed	O
by	O
the	O
IETF	O
RSerPool	B-Protocol
Working	O
Group	O
and	O
documented	O
in	O
RFC	O
5351	O
,	O
RFC	O
5352	O
,	O
RFC	O
5353	O
,	O
RFC	O
5354	O
,	O
RFC	O
5355	O
and	O
RFC	O
5356	O
.	O
</s>
<s>
In	O
the	O
terminology	O
of	O
RSerPool	B-Protocol
a	O
server	O
is	O
denoted	O
as	O
a	O
Pool	B-Protocol
Element	I-Protocol
(	O
PE	O
)	O
.	O
</s>
<s>
In	O
its	O
Pool	O
,	O
it	O
is	O
identified	O
by	O
its	O
Pool	B-Protocol
Element	I-Protocol
Identifier	O
(	O
PE	O
ID	O
)	O
,	O
a	O
32-bit	O
number	O
.	O
</s>
<s>
This	O
denomination	O
has	O
been	O
dropped	O
in	O
order	O
to	O
avoid	O
confusion	O
with	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
.	O
</s>
<s>
It	O
is	O
explicitly	O
not	O
a	O
goal	O
of	O
RSerPool	B-Protocol
to	O
manage	O
the	O
global	O
Internet	O
's	O
pools	O
within	O
a	O
single	O
handlespace	O
.	O
</s>
<s>
That	O
is	O
,	O
PHs	O
do	O
not	O
have	O
any	O
hierarchy	O
in	O
contrast	O
to	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
with	O
its	O
top-level	O
and	O
sub-domains	O
.	O
</s>
<s>
Within	O
an	O
operation	O
scope	O
,	O
the	O
handlespace	O
is	O
managed	O
by	O
redundant	O
Pool	B-Protocol
Registrars	I-Protocol
(	O
PR	O
)	O
,	O
also	O
denoted	O
as	O
ENRP	B-Protocol
servers	B-Application
or	O
Name	O
Servers	B-Application
(	O
NS	O
)	O
.	O
</s>
<s>
PRs	O
have	O
to	O
be	O
redundant	O
in	O
order	O
to	O
avoid	O
a	O
PR	O
to	O
become	O
a	O
Single	B-Architecture
Point	I-Architecture
of	I-Architecture
Failure	I-Architecture
(	O
SPoF	B-Architecture
)	O
.	O
</s>
<s>
PRs	O
of	O
an	O
operation	O
scope	O
synchronize	O
their	O
view	O
of	O
the	O
handlespace	O
using	O
the	O
Endpoint	B-Protocol
Handlespace	I-Protocol
Redundancy	I-Protocol
Protocol	I-Protocol
(	O
ENRP	B-Protocol
)	O
.	O
</s>
<s>
Older	O
versions	O
of	O
this	O
protocol	O
use	O
the	O
term	O
Endpoint	O
Namespace	O
Redundancy	O
Protocol	O
;	O
this	O
naming	O
has	O
been	O
replaced	O
to	O
avoid	O
confusion	O
with	O
DNS	B-Protocol
,	O
but	O
the	O
abbreviation	O
has	O
been	O
kept	O
.	O
</s>
<s>
Due	O
to	O
handlespace	O
synchronization	O
by	O
ENRP	B-Protocol
,	O
PRs	O
of	O
an	O
operation	O
scope	O
are	O
functionally	O
equal	O
.	O
</s>
<s>
Using	O
the	O
Aggregate	B-Protocol
Server	I-Protocol
Access	I-Protocol
Protocol	I-Protocol
(	O
ASAP	O
)	O
,	O
a	O
PE	O
can	O
add	O
itself	O
to	O
a	O
pool	O
or	O
remove	O
it	O
from	O
by	O
requesting	O
a	O
registration	O
or	O
deregistration	O
from	O
an	O
arbitrary	O
PR	O
of	O
the	O
operation	O
scope	O
.	O
</s>
<s>
To	O
use	O
the	O
service	O
of	O
a	O
pool	O
,	O
a	O
client	O
—	O
called	O
Pool	B-Protocol
User	I-Protocol
(	O
PU	O
)	O
in	O
RSerPool	B-Protocol
terminology	O
—	O
first	O
has	O
to	O
request	O
the	O
resolution	O
of	O
the	O
pool	O
's	O
PH	O
to	O
a	O
list	O
of	O
PE	O
identities	O
at	O
an	O
arbitrary	O
PR	O
of	O
the	O
operation	O
scope	O
.	O
</s>
<s>
HTTP	B-Protocol
over	O
SCTP	B-Protocol
or	O
TCP	B-Protocol
in	O
case	O
of	O
a	O
web	O
server	O
.	O
</s>
<s>
For	O
an	O
FTP	B-Protocol
download	O
for	O
example	O
,	O
the	O
failover	O
procedure	O
could	O
mean	O
to	O
tell	O
the	O
new	O
FTP	B-Protocol
server	O
the	O
file	O
name	O
and	O
the	O
last	O
received	O
data	O
position	O
.	O
</s>
<s>
By	O
that	O
,	O
the	O
FTP	B-Protocol
server	O
will	O
be	O
able	O
to	O
resume	O
the	O
download	O
session	O
.	O
</s>
<s>
Since	O
the	O
failover	O
procedure	O
is	O
highly	O
application-dependent	O
,	O
it	O
is	O
not	O
part	O
of	O
RSerPool	B-Protocol
itself	O
,	O
though	O
RSerPool	B-Protocol
provides	O
far	O
reaching	O
support	O
for	O
the	O
implementation	O
of	O
arbitrary	O
failover	O
schemes	O
by	O
its	O
Session	O
Layer	O
mechanisms	O
.	O
</s>
<s>
To	O
make	O
it	O
possible	O
for	O
RSerPool	B-Protocol
components	O
to	O
configure	O
automatically	O
,	O
PRs	O
can	O
announce	O
themselves	O
via	O
UDP	B-Protocol
over	O
IP	B-Protocol
multicast	O
.	O
</s>
<s>
The	O
advantage	O
of	O
using	O
IP	B-Protocol
multicast	O
instead	O
of	O
broadcast	O
is	O
that	O
this	O
mechanism	O
will	O
also	O
work	O
over	O
routers	O
(	O
e.g.	O
</s>
<s>
LANs	B-General_Concept
inter-connected	O
via	O
a	O
VPN	B-Application
)	O
and	O
the	O
announces	O
will	O
—	O
for	O
the	O
case	O
of	O
e.g.	O
</s>
<s>
For	O
the	O
case	O
that	O
IP	B-Protocol
multicast	O
is	O
not	O
available	O
,	O
it	O
is	O
of	O
course	O
possible	O
to	O
statically	O
configure	O
PR	O
addresses	O
.	O
</s>
