PI vs PA Address Space
Daniel Karrenberg
____________________________________________________
Provider Independent
vs
Provider Aggregatable
Address Space
Daniel Karrenberg
RIPE NCC
Document: ripe-127
Updates: ripe-126
Scope
This memo provides guidance to European Internet
registries concerning the assignment of Internet
address space allocated by the RIPE NCC.
Introduction
One can currently distinguish two kinds of globally
unique, unicast IPv4 addresses: provider independent
(PI) and provider aggregatable (PA) addresses. These
addresses are assigned to end-users by Internet Reg-
istries (IRs).
There are also non globally unique e.g. private uni-
cast addresses as described in RFC1597 which are
suitable for many applications.
Provider Aggregatable Address Space
With the introduction of classless interdomain rout-
ing (CIDR) [RFC1519] in the Internet, address space
is typically assigned by an Internet service
provider (ISP) to a customer. The service provider
assigns this address space in such a way that rout-
ing information for many customers can be aggregated
once it leaves the provider's routing domain. This
keeps the number of routes and state changes in the
interdomain routing system at an acceptable level.
The cost of propagating a relatively small number of
aggregated routes is much lower than that of propa-
gating each end-site's individual routes throughout
the whole interdomain routing system.
____________________________________________________
ripe-127.txt Page 1
PI vs PA Address Space
Daniel Karrenberg
____________________________________________________
After a customer leaves the service provider who
assigned the address space, it can be assigned to
another customer. As a consequence the customer
will have to reconfigure all their hosts and routers
if they continue to require globally unique address
space. This requires a clear, preferably contrac-
tual, understanding between the assigning service
provider and the customer, that the assignment of
the address space ends when the provider no longer
provides Internet connectivity to the customer or
soon thereafter.
The goal of this arrangement is to minimise the load
on the interdomain routing system. If the customer
used the address space assigned by and aggregatable
by their previous service provider when connecting
to another service provider, their routing informa-
tion could not be aggregated and would have to be
propagated separately throughout the whole interdo-
main routing system.
Provider Independent Address Space
Contrary to PA address space, PI address space
remains assigned to its user as long as the criteria
for the original assignment are met independently of
the use of a particular provider's services. Fre-
quently PI addresses are not even assigned by
providers but by other Internet registries. The
apparent advantage of PI address space is that the
user does not have to reconfigure their hosts and
routers if they decide to leave a particular service
provider. However, PI addresses are expensive to
route because no use can be made of aggregation. All
early Internet address space assignments were
provider independent. Many assignments made by ISPs
are also formally provider independent because they
lack the clear prior understanding between ISP and
customer that the assignment will end with the ter-
mination of the service.
Current Issues
At the time of this writing there is growing concern
among the operators of major transit routing domains
in the Internet that the number of individual routes
and their associated information is growing faster
than the deployed routing technology will be able to
handle. Parts of the interdomain routing system are
already operating at capacity.
It has been argued that PI addresses will quickly
____________________________________________________
ripe-127.txt Page 2
PI vs PA Address Space
Daniel Karrenberg
____________________________________________________
become be totally useless since the Internet routing
system will not be able to support them any longer.
Consequently it has been suggested that the regional
IRs should immediately stop allocating and assigning
PI space and only allocate PA space to service
providers.
The RIPE NCC cannot do this because it would have to
determine who is a service provider and who is not
as well as refusing address space allocations to
anyone not meeting minimum size criteria. This
would amount to nothing less than the RIPE NCC regu-
lating Internet service provision in Europe. So far
no practical policies for these determinations have
been suggested let alone met with community consen-
sus. If possible at all, the process to define such
policies and to establish Internet community consen-
sus is expected to take considerable time.
On the other hand the RIPE NCC receives an increas-
ing amount of requests for guidance on these issues
from European Internet registries. This memo is
intended to provide that guidance by spelling out a
number of policies. It does not represent a major
change in policy, but refinement and clarification.
Assignment Policies
European Internet Registries continue to assign both
PA and PI address space to users.
IRs will clearly warn users about the issues w.r.t.
their choice of a particular type of address space.
IRs will promote the use of PA address space as much
as possible.
Assignment criteria for both kinds of address space
will be exactly identical w.r.t. the amount of
address space assigned, the registration require-
ments etc.. This also implies that assigning PI
space prefixes longer than 24 bits is perfectly
acceptable if the request does not merit 8 bits of
address space to be assigned.
IRs will mark all PA space assignments in the RIPE
database. See below for details.
It is then up to the service providers to decide
whether they will route particular prefixes or not.
The way this determination is made is beyond the
____________________________________________________
ripe-127.txt Page 3
PI vs PA Address Space
Daniel Karrenberg
____________________________________________________
scope of this document, but we expect that accepting
and charging routing announcements based on whether
or not they can be aggregated is a distinct possi-
bility.
We therefore urgently recommend that service
providers shall inform their current and prospective
customers as clearly as possible about the issues
involved in using PI vs PA space with their service
offerings. It is recommended that ISPs clearly
specify present and future differences in their ser-
vice offerings w.r.t. usage of PI vs PA addresses.
Detailed Recommendations
The remainder of this document spells out some of
the details concerning the policy.
IRs will give those customers requesting PA space
this or a similar warning:
Assignment of this address space is valid
as long as the criteria for the original
assignment are still met and only for the
duration of the service agreement between
yourself and ISP XXXX who will have the
right to re-assign the address space to
another user upon termination of the
agreement or an agreed period thereafter.
This means that you will have to re-
configure the addresses of all equipment
using this address space if you continue
to require global uniqueness of those
addresses. Note that some Internet ser-
vices do not require globally unique
addresses if accessed through a NAT or
application layer gateway/firewall.
____________________________________________________
ripe-127.txt Page 4
PI vs PA Address Space
Daniel Karrenberg
____________________________________________________
IRs will give those end-users requesting PI space
this or a similar warning:
Assignment of this address space is valid
as long as the criteria for the original
assignment are still met. However,
assignment of address space does NOT imply
that this address space will be ROUTABLE
ON ANY PART OF THE INTERNET. It is
expected that users will have to pay a
premium for actual routing of PI addresses
as opposed to PA addresses. It may even-
tually become impossible to get relatively
small amounts of PI space routed on most
of the Internet. We strongly suggest you
contact any prospective service provider
for information about the possibility and
pricing of service when using PI
addresses.
IRs will recommend that end-users use PA space as
much as possible.
____________________________________________________
ripe-127.txt Page 5
PI vs PA Address Space
Daniel Karrenberg
____________________________________________________
IRs will register the type of any assigned address
space using the "status:" attribute of the "inetnum"
object in the RIPE database. The possible values of
this attribute are
ASSIGNED PA
This address space has been assigned to an end-
user and is Provider Aggregateable.
ASSIGNED PI
This address space has been assigned to an end-
user and is Provider Independent.
ALLOCATED PA
This address space has been allocated to an IR
and all assignments made from it are Provider
Aggregateable.
ALLOCATED PI
This address space has been allocated to a
local IR and all assignments made from it are
Provider Independent.
ALLOCATED UNSPECIFIED
This address space has been allocated to an IR
and assignments made from it are either
Provider Aggregateable or Provider Independent.
This is intended to document past allocations
from with assignments of both types or unknown
type. It should be avoided for new alloca-
tions.
Address space without an explicit type in the status
attribute is assumed to be PI space.
IRs will clearly mark all new assignments of address
space in the RIPE database with either PA or PI as
appropriate.
Wherever possible IRs will work to mark all past
assignments in the assignment database(s) with
either PA or PI as appropriate. Priority should be
given to mark all PA space as such.
____________________________________________________
ripe-127.txt Page 6
PI vs PA Address Space
Daniel Karrenberg
____________________________________________________
IRs will inform end-users about private address
space as defined in RFC1597 where appropriate. They
will advise them to carefully consider the potential
advantages and possible problems as discussed in
RFCs 1597 and 1627.
Local IRs may decide which kind they of address
space they will assign: PA, PI or both.
If an end-user requests address space of a type
which an IR does not assign, the IR will refer the
end-user to an appropriate registry. In particular
ISP IRs not offering PI space shall support the IR
that does assign this space to their customers con-
cerning assignments to those customers. This sup-
port includes supporting the customer in preparing a
properly documented request and furnishing back-
ground information to the IR.
Local IRs which do not normally assign large amounts
of a particular type of address space need not hold
an allocation of that type of address space. They
can get it as needed from the RIPE NCC. These
assignments will typically be not aggregateable.
Local IRs will make it clear to the user which
type of address space is assigned. Clear contractual
arrangements are recommended in general and manda-
tory for PA space.
IRs have assigned address space in the past which is
de-facto aggregated but not formally PA type because
there are no clear contractual arrangements about
termination of the assignment. IRs will ask leaving
customers to voluntarily release this address space
upon termination of service. Where possible IRs
will work to make contractual arrangements to con-
vert PI addresses into PA addresses.
____________________________________________________
ripe-127.txt Page 7