IAB/IESG Recommendations on IPv6 Address Delegation

[Fred Baker]

Folks:

The RIR community asked the IETF community for advice regarding the
assignment of IPv6 prefixes to service providers and edge networks, both
with a view to topological address assignment and to multihomed networks.
The IPv6 Directorate prepared a statement, which the IESG and IAB have
reviewed and approved. This is attached.

I trust that this answers the questions you asked, and serves as a basis for
IPv6 deployment in the near term. If you have questions or issues concerning
it, I would suggest that you address them to the IPv6 Directorate copying
the IESG and IAB.

We intend to publish an Informational RFC in the near future documenting the
contents of this note.

Fred Baker

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	IAB/IESG Recommendations on IPv6 Address Allocations
			   September 1, 2000

Introduction

During a discussion between IETF and RIR experts at the Adelaide IETF,
a suggestion was made that it might be appropriate to allocate /56
prefixes instead of /48 for homes and small businesses.  However,
subsequent analysis has revealed significant advantages in using /48
uniformly.  This note is an update following further discussions at
the Pittsburgh IETF.

This document was developed by the IPv6 Directorate, IAB and IESG, and
is a recommendation from the IAB and IESG to the RIRs.

Background

The technical principles that apply to address allocation seek to
balance healthy conservation practices and wisdom with a certain ease
of access.  On the one hand, when managing the use of a potentially
limited resource, one must conserve wisely to prevent exhaustion
within an expected lifetime.  On the other hand, the IPv6 address
space is in no sense as precious a resource as the IPv4 address space,
and unwarranted conservatism acts as a disincentive in a marketplace
already dampened by other factors.  So from a market development
perspective, we would like to see it be very easy for a user or an ISP
to obtain as many IPv6 addresses as they really need without a
prospect of immediate renumbering or of scaling inefficiencies.

The IETF makes no comment on business issues or relationships.
However, in general, we observe that technical delegation policy can
have strong business impacts.  A strong requirement of the address
delegation plan is that it not be predicated on or unduly bias
business relationships or models.

The IPv6 address, as currently defined, consists of 64 bits of
"network number" and 64 bits of "host number".  The technical reasons
for this are several.  The requirements for IPv6 agreed to in 1993
included a plan to be able to address approximately 2^40 networks and
2^50 hosts; the 64/64 split effectively accomplishes this.  Procedures
used in host address assignment, such as the router advertisement of a
network's prefix to hosts [RFC 2462], which in turn place a locally
unique number in the host portion, depend on this split.  Examples of
obvious choices of host number (IEEE Mac Address, E.164 number, E.214
IMSI, etc) suggest that no assumption should be made that bits may be
stolen from that range for subnet numbering; current generation MAC
layers and E.164 numbers specify up to 64 bit objects.  Therefore,
subnet numbers must be assumed to come from the network part.  This is
not to preclude routing protocols such as IS-IS level 1 (intra-area)
routing, which routes individual host addresses, but says that it may
not be depended upon in the world outside that zone.

The IETF has also gone to a great deal of effort to minimize the
impacts of network renumbering.  None-the-less, renumbering of IPv6
networks is neither invisible nor completely painless.  Therefore,
renumbering should be considered an acceptable event, but to be
avoided if reasonably avoidable.

The IETF's IPNG working group has recommended that the address block
given to a single edge network which may be recursively subnetted be a
48 bit prefix.  This gives each such network 2^16 subnet numbers to
use in routing, and a very large number of unique host numbers within
each network.  This is deemed to be large enough for most enterprises,
and to leave plenty of room for delegation of address blocks to
aggregating entities.

It is not obvious, however, that all edge networks are likely to be
recursively subnetted; an individual PC in a home, or a single cell in
a mobile telephone network, for example, may or may not be further
subnetted (depending whether they are acting as, e.g., gateways to
personal, home, or vehicular networks).  When a network number is
delegated to a place that will not require subnetting, therefore, it
might be acceptable for an ISP to give a single 64 bit prefix -
perhaps shared among the dial-in connections to the same ISP router.
However this decision may be taken in the knowledge that there is
objectively no shortage of /48s, and the expectation that personal,
home and vehicle networks will become the norm.  Indeed, it is widely
expected that all IPv6 subscribers, whether domestic (homes), mobile
(vehicles or individuals), or enterprises of any size, will eventually
possess multiple always-on hosts, at least one subnet with the
potential for additional subnetting, and therefore some internal
routing capability.  Note that in the mobile environment, the device
connecting a mobile site to the network may in fact be a third
generation cellular telephone.  In other words the subscriber
allocation unit is not always a host; it is always potentially a site.

Address Delegation Recommendations

The RIR communities, with the IAB, have determined that reasonable
address prefixes delegated to service providers for initial
allocations should be on the order of 29 to 35 bits in length, giving
individual delegations support for 2^13 (8K) to 2^19 (512K) subscriber
networks.  Allocations are to be given in a manner such that an
initial prefix may be subsumed by subsequent larger allocations
without forcing existing subscriber networks to renumber.  We concur
that this meets the technical requirement for manageable and scalable
backbone routing while simultaneously allowing for managed growth of
individual delegations.

The same type of rule could be used in the allocation of addresses in
edge networks; if there is doubt whether an edge network will in turn
be subnetted, the edge network might be encouraged to allocate the
first 64 bit prefix out of a block of 8..256, preserving room for
growth of that allocation without renumbering up to a point.  However,
for the reasons described below, we recommend use of a fixed boundary
at /48 for all subscribers except the very largest (who could receive
multiple /48's), and those clearly transient or otherwise have no
interest in subnetting (who could receive a /64).  Note that there
seems to be little benefit in not giving a /48 if future growth is
anticipated.  In the following, we give the arguments for a uniform
use of /48 and then demonstrate that it is entirely compatible with
responsible stewardship of the total IPv6 address space.

The arguments for the fixed boundary are:

  - only by having an ISP-independent boundary can we guarantee that a
    change of ISP will not require a costly internal restructuring or
    consolidation of subnets.

  - to enable straightforward site renumbering, i.e., when a site
    renumbers from one prefix to another, the whole process, including
    parallel running of the two prefixes, would be greatly complicated
    if the prefixes had different lengths (depending of course on the
    size and complexity of the site).

  - there are various possible approaches to multihoming for IPv6
    sites, including the techniques already used for IPv4 multihoming.
    The main open issue is finding solutions that scale massively
    without unduly damaging route aggregation and/or optimal route
    selection.  Much more work remains to be done in this area, but it
    seems likely that several approaches will be deployed in practice,
    each with their own advantages and disadvantages.  Some (but not
    all) will work better with a fixed prefix boundary.  (Multihoming
    is discussed in more detail below.)

  - to allow easy growth of the subscribers' networks -- no need to
    keep going back to ISPs for more space (except for that relatively
    small number of subscribers for which a /48 is not enough).

  - remove the burden from the ISPs and registries of judging sites'
    needs for address space, unless the site requests more space than a
    /48, with several advantages:

    - ISPs no longer need to ask for details of their customers'
      network architecture and growth plans
    - ISPs and registries no longer have to judge rates of address
      consumption by customer type
    - registry operations will be made more efficient by reducing the
      need for evaluations and judgements
    - address space will no longer be a precious resource for
      customers, removing the major incentive for subscribers to
      install v6/v6 NATs, which would defeat the ability of IPv6 to
      restore address transparency.

  - to allow the site to maintain a single reverse-DNS zone covering
    all prefixes.

    - If and only if a site can use the same subnetting structure under
      each of its prefixes, then it can use the same zone file for the
      address-to-name mapping of all of them.  And, using the
      conventions of RFC 2874, it can roll the reverse mapping data
      into the "forward" (name-keyed) zone.

Specific advantages of the fixed boundary being at /48 include

  - to leave open the technical option of retro-fitting the GSE
    (Global, Site and End-System Designator, a.k.a "8+8") proposal for
    separating locators and identifiers, which assumes a fixed boundary
    between global and site addressing at /48.  Although the GSE
    technique was deferred a couple of years ago, it still has strong
    proponents.  Also, the IRTF Namespace Research Group is actively
    looking into topics closely related to GSE.  It is still possible
    that GSE or a derivative of GSE will be used with IPv6 in the
    future.

  - since the site local prefix is fec0::/48, global site prefixes of
    /48 will allow sites to easily maintain a simple 1 to 1 mapping
    between the global topology and the site local topology in the SLA
    field.

  - similarly, if the 6to4 proposal is standardized, migration from a
    6to4 prefix, which is /48 by construction, to a native IPv6 prefix
    will be simplified if the native prefix is /48.

Note that none of these reasons imply an expectation that homes,
vehicles, etc. will intrinsically require 16 bits of subnet space.

Conservation of Address Space

The question naturally arises whether giving a /48 to every subscriber
represents a profligate waste of address space.  Objective analysis
shows that this is not the case.  A /48 prefix under the Aggregatable
Global Unicast Address (TLA) format prefix actually contains 45
variable bits, i.e., the number of available prefixes is 2**45 or about
35 trillion (35,184,372,088,832).  If we take the limiting case of
assigning one prefix per human, then the utilization of the TLA space
appears to be limited to approximately 0.03% on reasonable
assumptions.

More precisely,

  - RFC 1715 defines an "H ratio" based on experience in address space
    assignment in various networks.  Applied to a 45 bit address space,
    and projecting a world population of 10.7 billion by 2050 (see
    http://www.popin.org/pop1998/), the required assignment efficiency
    is log_10(1.07*10^10) / 45 = 0.22.  This is less than the
    efficiencies of telephone numbers and DECnetIV or IPv4 addresses
    shown in RFC 1715, i.e., gives no grounds for concern.

  - We are highly confident in the validity of this analysis, based on
    experience with IPv4 and several other address spaces, and on
    extremely ambitious scaling goals for the Internet amounting to an
    80 bit address space *per person*.  Even so, being acutely aware of
    the history of under-estimating demand, we have reserved more than
    85% of the address space (i.e., the bulk of the space not under the
    Aggregatable Global Unicast Address (TLA) format prefix).
    Therefore, if the analysis does one day turn out to be wrong, our
    successors will still have the option of imposing much more
    restrictive allocation policies on the remaining 85%.

  - For transient use by non-routing hosts (e.g., for stand-alone
    dial-up users who prefer transient addresses for privacy reasons),
    a prefix of /64 might be OK.  But a subscriber who wants "static"
    IPv6 address space, or who has or plans to have multiple subnets,
    ought to be provided with a /48, for the reasons given above, even
    if it is a transiently provided /48.

To summarize, we argue that although careful stewardship of IPv6
address space is essential, this is completely compatible with the
convenience and simplicity of a uniform prefix size for IPv6 sites of
any size.  The numbers are such that there seems to be no objective
risk of running out of space, giving an unfair amount of space to
early customers, or of getting back into the over-constrained IPv4
situation where address conservation and route aggregation damage each
other.

Multihoming Issues

In the realm of multi-homed networks, the techniques used in IPv4 can
all be applied, but they have known scaling problems.  Specifically,
if the same prefix is advertised by multiple ISPs, the routing tables
will grow as a function of the number of multihomed sites.  To go
beyond this for IPv6, we only have initial proposals on the table at
this time, and active work is under way in the IETF IPNG working
group.  Until existing or new proposals become more fully developed,
existing techniques known to work in IPv4 will continue to be used in
IPv6.

Key characteristics of an ideal multi-homing proposal include (at
minimum) that it provides routing connectivity to any multi-homed
network globally, conserves address space, produces high quality
routes at least as well as the single-homed host case previously
discussed via any of the network's providers, enables a multi-homed
network to connect to multiple ISPs, does not inherently bias routing
to use any proper subset of those networks, does not unduly damage
route aggregation, and scales to very large numbers of multi-homed
networks.

One class of solution being considered amounts to permanent parallel
running of two (or more) prefixes per site.  In the absence of a fixed
prefix boundary, such a site might be required to have multiple
different internal subnet numbering strategies, (one for each prefix
length) or, if it only wanted one, be forced to use the most
restrictive one as defined by the longest prefix it received from any
of its ISPs.  In this approach, a multi-homed network would have an
address block from each of its upstream providers.  Each host would
either have exactly one address picked from the set of upstream
providers, or one address per host from each of the upstream
providers.  The first case is essentially a variant on RFC 2260, with
known scaling limits.

In the second case (multiple addresses per host), if two multi-homed
networks communicate, having respectively m and n upstream providers,
then the one initiating the connection will select one address pair
from the n*m potential address pairs to connect from and to, and in so
doing will select the providers, and therefore the applicable route,
for the life of the connection.  Given that each path will have a
different ambient bit rate, loss rate, and delay, if neither host is
in possession of any routing or metric information, the initiating
host has only a 1/(m*n) probability of selecting the optimal address
pair.  Work on better-than-random address selection is in progress in
the IETF, but is incomplete.

An existence proof exists in the existing IPv4 Internet that a network
whose address is distinct from and globally advertised to all upstream
providers permits the routing network to select a reasonably good path
within the applicable policy.  Present-day routing policies are not
QoS policies but reachability policies, which means that they will not
necessarily select the optimal delay, bit rate, or loss rate, but the
route will be the best within the metrics that are indeed in use.  One
may therefore conclude that this would work correctly for IPv6
networks as well, apart from scaling issues.
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Fred Baker	| 519 Lado Drive
IETF Chair	| Santa Barbara California 93111
www.ietf.org	| Desk:   +1-408-526-4257
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