IAB/IESG Recommendations on IPv6 Address Allocations

Fred Baker, IETF Chair

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:

1. ISPs no longer need to ask for details of their customers' network architecture and growth plans
2. ISPs and registries no longer have to judge rates of address consumption by customer type
3. registry operations will be made more efficient by reducing the need for evaluations and judgements
4. 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.

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