Network Working Group                                        Y. Rekhter
Request for Comments: 1887                                cisco Systems
Category: Informational                                           T. Li
                                                          cisco Systems
                                                                Editors
                                                          December 1995

An Architecture for IPv6 Unicast Address Allocation

Status of this Memo

   This document provides information for the Internet community.  This
   memo does not specify an Internet standard of any kind.  Distribution
   of this memo is unlimited.

Abstract

   This document provides an architecture for allocating IPv6 [1]
   unicast addresses in the Internet. The overall IPv6 addressing
   architecture is defined in [2].  This document does not go into the
   details of an addressing plan.

1. Scope

   The global internet can be modeled as a collection of hosts
   interconnected via transmission and switching facilities.  Control
   over the collection of hosts and the transmission and switching
   facilities that compose the networking resources of the global
   internet is not homogeneous, but is distributed among multiple
   administrative authorities. Resources under control of a single
   administration within a contiguous segment of network topology form a
   domain.  For the rest of this paper, `domain' and `routing domain'
   will be used interchangeably.

   Domains that share their resources with other domains are called
   network service providers (or just providers). Domains that utilize
   other domain's resources are called network service subscribers (or
   just subscribers).  A given domain may act as a provider and a
   subscriber simultaneously.

   There are two aspects of interest when discussing IPv6 unicast
   address allocation within the Internet. The first is the set of
   administrative requirements for obtaining and allocating IPv6
   addresses; the second is the technical aspect of such assignments,
   having largely to do with routing, both within a routing domain
   (intra-domain routing) and between routing domains (inter-domain
   routing). This paper focuses on the technical issues.

   In the current Internet many routing domains (such as corporate and
   campus networks) attach to transit networks (such as regionals) in
   only one or a small number of carefully controlled access points.
   The former act as subscribers, while the latter act as providers.

   Addressing solutions which require substantial changes or constraints
   on the current topology are not considered.

   The architecture and recommendations in this paper are oriented
   primarily toward the large-scale division of IPv6 address allocation
   in the Internet.  Topics covered include:

      - Benefits of encoding some topological information in IPv6
        addresses to significantly reduce routing protocol overhead;

      - The anticipated need for additional levels of hierarchy in
        Internet addressing to support network growth;

      - The recommended mapping between Internet topological entities
        (i.e., service providers, and service subscribers) and IPv6
        addressing and routing components;

      - The recommended division of IPv6 address assignment among
        service providers (e.g., backbones, regionals), and service
        subscribers (e.g., sites);

      - Allocation of the IPv6 addresses by the Internet Registry;

      - Choice of the high-order portion of the IPv6 addresses in leaf
        routing domains that are connected to more than one service
        provider (e.g., backbone or a regional network).

   It is noted that there are other aspects of IPv6 address allocation,
   both technical and administrative, that are not covered in this
   paper.  Topics not covered or mentioned only superficially include:

      - A specific plan for address assignment;

      - Embedding address spaces from other network layer protocols
        (including IPv4) in the IPv6 address space and the addressing

        architecture for such embedded addresses;

      - Multicast addressing;

      - Address allocation for mobile hosts;

      - Identification of specific administrative domains in the
        Internet;

      - Policy or mechanisms for making registered information known to
        third parties (such as the entity to which a specific IPv6
        address or a potion of the IPv6 address space has been
        allocated);

      - How a routing domain (especially a site) should organize its
        internal topology or allocate portions of its IPv6 address
        space; the relationship between topology and addresses is
        discussed, but the method of deciding on a particular topology
        or internal addressing plan is not; and,

      - Procedures for assigning host IPv6 addresses.

2. Background

   Some background information is provided in this section that is
   helpful in understanding the issues involved in IPv6 address
   allocation. A brief discussion of IPv6 routing is provided.

   IPv6 partitions the routing problem into three parts:

      - Routing exchanges between end systems and routers,

      - Routing exchanges between routers in the same routing domain,
        and,

      - Routing among routing domains.

3. IPv6 Addresses and Routing

   For the purposes of this paper, an IPv6 address prefix is defined as
   an IPv6 address and some indication of the leftmost contiguous
   significant bits within this address portion.  Throughout this paper
   IPv6 address prefixes will be represented as X/Y, where X is a prefix
   of an IPv6 address in length greater than or equal to that specified

   by Y and Y is the (decimal) number of the leftmost contiguous
   significant bits within this address.  In the notation, X, the prefix
   of an IPv6 address [2] will have trailing insignificant digits
   removed.  Thus, an IPv6 prefix might appear to be 43DC:0A21:76/40.

   When determining an administrative policy for IPv6 address
   assignment, it is important to understand the technical consequences.
   The objective behind the use of hierarchical routing is to achieve
   some level of routing data abstraction, or summarization, to reduce
   the cpu, memory, and transmission bandwidth consumed in support of
   routing.

   While the notion of routing data abstraction may be applied to
   various types of routing information, this paper focuses on one
   particular type, namely reachability information. Reachability
   information describes the set of reachable destinations.  Abstraction
   of reachability information dictates that IPv6 addresses be assigned
   according to topological routing structures. However in practice
   administrative assignment falls along organizational or political
   boundaries. These may not be congruent to topological boundaries and
   therefore the requirements of the two may collide. It is necessary to
   find a balance between these two needs.

   Reachability information abstraction occurs at the boundary between
   hierarchically arranged topological routing structures. An element
   lower in the hierarchy reports summary reachability information to
   its parent(s).

   At routing domain boundaries, IPv6 address information is exchanged
   (statically or dynamically) with other routing domains. If IPv6
   addresses within a routing domain are all drawn from non-contiguous
   IPv6 address spaces (allowing no abstraction), then the address
   information exchanged at the boundary consists of an enumerated list
   of all the IPv6 addresses.

   Alternatively, should the routing domain draw IPv6 addresses for all
   the hosts within the domain from a single IPv6 address prefix,
   boundary routing information can be summarized into the single IPv6
   address prefix.  This permits substantial data reduction and allows
   better scaling (as compared to the uncoordinated addressing discussed
   in the previous paragraph).

   If routing domains are interconnected in a more-or-less random (i.e.,
   non-hierarchical) scheme, it is quite likely that no further
   abstraction of routing data can occur. Since routing domains would
   have no defined hierarchical relationship, administrators would not
   be able to assign IPv6 addresses within the domains out of some
   common prefix for the purpose of data abstraction. The result would

   be flat inter-domain routing; all routing domains would need explicit
   knowledge of all other routing domains that they route to.  This can
   work well in small and medium sized internets.  However, this does
   not scale to very large internets.  For example, we expect IPv6 to
   grow to hundreds of thousands of routing domains in North America
   alone.  This requires a greater degree of the reachability
   information abstraction beyond that which can be achieved at the
   `routing domain' level.

   In the Internet, it should be possible to significantly constrain the
   volume and the complexity of routing information by taking advantage
   of the existing hierarchical interconnectivity. This is discussed
   further in Section 5. Thus, there is the opportunity for a group of
   routing domains each to be assigned an address prefix from a shorter
   prefix assigned to another routing domain whose function is to
   interconnect the group of routing domains. Each member of the group
   of routing domains now has its (somewhat longer) prefix, from which
   it assigns its addresses.

   The most straightforward case of this occurs when there is a set of
   routing domains which are all attached to a single service provider
   domain (e.g., regional network), and which use that provider for all
   external (inter-domain) traffic.  A short prefix may be given to the
   provider, which then gives slightly longer prefixes (based on the
   provider's prefix) to each of the routing domains that it
   interconnects. This allows the provider, when informing other routing
   domains of the addresses that it can reach, to abstract the
   reachability information for a large number of routing domains into a
   single prefix. This approach therefore can allow a great deal of
   reduction of routing information, and thereby can greatly improve the
   scalability of inter-domain routing.

   Clearly, this approach is recursive and can be carried through
   several iterations. Routing domains at any `level' in the hierarchy
   may use their prefix as the basis for subsequent suballocations,
   assuming that the IPv6 addresses remain within the overall length and
   structure constraints.

   At this point, we observe that the number of nodes at each lower
   level of a hierarchy tends to grow exponentially. Thus the greatest
   gains in the reachability information abstraction (for the benefit of
   all higher levels of the hierarchy) occur when the reachability
   information aggregation occurs near the leaves of the hierarchy; the
   gains drop significantly at each higher level. Therefore, the law of
   diminishing returns suggests that at some point data abstraction
   ceases to produce significant benefits.  Determination of the point
   at which data abstraction ceases to be of benefit requires a careful
   consideration of the number of routing domains that are expected to

   occur at each level of the hierarchy (over a given period of time),
   compared to the number of routing domains and address prefixes that
   can conveniently and efficiently be handled via dynamic inter-domain
   routing protocols.

3.1 Efficiency versus Decentralized Control.

   If the Internet plans to support a decentralized address
   administration, then there is a balance that must be sought between
   the requirements on IPv6 addresses for efficient routing and the need
   for decentralized address administration.  A coherent addressing plan
   at any level within the Internet must take the alternatives into
   careful consideration.

   As an example of administrative decentralization, suppose the IPv6
   address prefix 43/8 identifies part of the IPv6 address space
   allocated for North America. All addresses within this prefix may be
   allocated along topological boundaries in support of increased data
   abstraction.  Within this prefix, addresses may be allocated on a
   per-provider bases, based on geography or some other topologically
   significant criteria.  For the purposes of this example, suppose that
   this prefix is allocated on a per-provider basis.  Subscribers within
   North America use parts of the IPv6 address space that is underneath
   the IPv6 address space of their service providers.  Within a routing
   domain addresses for subnetworks and hosts are allocated from the
   unique IPv6 prefix assigned to the domain according to the addressing
   plan for that domain.

4. IPv6 Address Administration and Routing in the Internet

   Internet routing components -- service providers (e.g., backbones,
   regional networks), and service subscribers (e.g., sites or campuses)
   -- are arranged hierarchically for the most part. A natural mapping
   from these components to IPv6 routing components is for providers and
   subscribers to act as routing domains.

   Alternatively, a subscriber (e.g., a site) may choose to operate as a
   part of a domain formed by a service provider. We assume that some,
   if not most, sites will prefer to operate as part of their provider's
   routing domain, exchanging routing information directly with the
   provider.  The site is still allocated a prefix from the provider's
   address space, and the provider will advertise its own prefix into
   inter-domain routing.

   Given such a mapping, where should address administration and
   allocation be performed to satisfy both administrative
   decentralization and data abstraction? The following possibilities
   are considered:

     1) At some part within a routing domain,

     2) At the leaf routing domain,

     3) At the transit routing domain (TRD), and

     4) At some other, more general boundaries, such as at the
        continental boundary.

   A part within a routing domain corresponds to any arbitrary connected
   set of subnetworks. If a domain is composed of multiple subnetworks,
   they are interconnected via routers.  Leaf routing domains correspond
   to sites, where the primary purpose is to provide intra-domain
   routing services.  Transit routing domains are deployed to carry
   transit (i.e., inter-domain) traffic; backbones and providers are
   TRDs.  More general boundaries can be seen as topologically
   significant collections of TRDs.

   The greatest burden in transmitting and operating on reachability
   information is at the top of the routing hierarchy, where
   reachability information tends to accumulate. In the Internet, for
   example, providers must manage reachability information for all
   subscribers directly connected to the provider. Traffic destined for
   other providers is generally routed to the backbones (which act as
   providers as well).  The backbones, however, must be cognizant of the
   reachability information for all attached providers and their
   associated subscribers.

   In general, the advantage of abstracting routing information at a
   given level of the routing hierarchy is greater at the higher levels
   of the hierarchy. There is relatively little direct benefit to the
   administration that performs the abstraction, since it must maintain
   routing information individually on each attached topological routing
   structure.

   For example, suppose that a given site is trying to decide whether to
   obtain an IPv6 address prefix directly from the IPv6 address space
   allocated for North America, or from the IPv6 address space allocated
   to its service provider. If considering only their own self-interest,
   the site itself and the attached provider have little reason to
   choose one approach or the other. The site must use one prefix or
   another; the source of the prefix has little effect on routing
   efficiency within the site. The provider must maintain information

   about each attached site in order to route, regardless of any
   commonality in the prefixes of the sites.

   However, there is a difference when the provider distributes routing
   information to other providers (e.g., backbones or TRDs).  In the
   first case, the provider cannot aggregate the site's address into its
   own prefix; the address must be explicitly listed in routing
   exchanges, resulting in an additional burden to other providers which
   must exchange and maintain this information.

   In the second case, each other provider (e.g., backbone or TRD) sees
   a single address prefix for the provider, which encompasses the new
   site. This avoids the exchange of additional routing information to
   identify the new site's address prefix. Thus, the advantages
   primarily accrue to other providers which maintain routing
   information about this site and provider.

   One might apply a supplier/consumer model to this problem: the higher
   level (e.g., a backbone) is a supplier of routing services, while the
   lower level (e.g., a TRD) is the consumer of these services. The
   price charged for services is based upon the cost of providing them.
   The overhead of managing a large table of addresses for routing to an
   attached topological entity contributes to this cost.

   At present the Internet, however, is not a market economy.  Rather,
   efficient operation is based on cooperation.  The recommendations
   discussed below describe simple and tractable ways of managing the
   IPv6 address space that benefit the entire community.

4.1 Administration of IPv6 addresses within a domain.

   If individual hosts take their IPv6 addresses from a myriad of
   unrelated IPv6 address spaces, there will be effectively no data
   abstraction beyond what is built into existing intra-domain routing
   protocols.  For example, assume that within a routing domain uses
   three independent prefixes assigned from three different IPv6 address
   spaces associated with three different attached providers.

   This has a negative effect on inter-domain routing, particularly on
   those other domains which need to maintain routes to this domain.
   There is no common prefix that can be used to represent these IPv6
   addresses and therefore no summarization can take place at the
   routing domain boundary. When addresses are advertised by this
   routing domain to other routing domains, an enumerated list of the
   three individual prefixes must be used.

   The number of IPv6 prefixes that leaf routing domains would advertise
   is on the order of the number of prefixes assigned to the domain; the
   number of prefixes a provider's routing domain would advertise is
   approximately the number of prefixes attached to the client leaf
   routing domains; and for a backbone this would be summed across all
   attached providers.  This situation is just barely acceptable in the
   current Internet, and is intractable for the IPv6 Internet.  A
   greater degree of hierarchical information reduction is necessary to
   allow continued growth in the Internet.

4.2 Administration at the Leaf Routing Domain

   As mentioned previously, the greatest degree of data abstraction
   comes at the lowest levels of the hierarchy. Providing each leaf
   routing domain (that is, site) with a contiguous block of addresses
   from its provider's address block results in the biggest single
   increase in abstraction. From outside the leaf routing domain, the
   set of all addresses reachable in the domain can then be represented
   by a single prefix.  Further, all destinations reachable within the
   provider's prefix can be represented by a single prefix.

   For example, consider a single campus which is a leaf routing domain
   which would currently require 4 different IPv6 prefixes.  Instead,
   they may be given a single prefix which provides the same number of
   destination addresses.  Further, since the prefix is a subset of the
   provider's prefix, they impose no additional burden on the higher
   levels of the routing hierarchy.

   There is a close relationship between hosts and routing domains.  The
   routing domain represents the only path between a host and the rest
   of the internetwork. It is reasonable that this relationship also
   extend to include a common IPv6 addressing space. Thus, the hosts
   within the leaf routing domain should take their IPv6 addresses from
   the prefix assigned to the leaf routing domain.

4.3 Administration at the Transit Routing Domain

   Two kinds of transit routing domains are considered, direct providers
   and indirect providers. Most of the subscribers of a direct provider
   are domains that act solely as service subscribers (they carry no
   transit traffic). Most of the subscribers of an indirect provider are
   domains that, themselves, act as service providers. In present
   terminology a backbone is an indirect provider, while an NSFnet
   regional is an example of a direct provider. Each case is discussed

separately below.

4.3.1 Direct Service Providers

   In a provider-based addressing plan, direct service providers should
   use their IPv6 address space for assigning IPv6 addresses from a
   unique prefix to the leaf routing domains that they serve. The
   benefits derived from data abstraction are greater than in the case
   of leaf routing domains, and the additional degree of data
   abstraction provided by this may be necessary in the short term.

   As an illustration consider an example of a direct provider that
   serves 100 clients. If each client takes its addresses from 4
   independent address spaces then the total number of entries that are
   needed to handle routing to these clients is 400 (100 clients times 4
   providers).  If each client takes its addresses from a single address
   space then the total number of entries would be only 100. Finally, if
   all the clients take their addresses from the same address space then
   the total number of entries would be only 1.

   We expect that in the near term the number of routing domains in the
   Internet will grow to the point that it will be infeasible to route
   on the basis of a flat field of routing domains. It will therefore be
   essential to provide a greater degree of information abstraction with
   IPv6.

   Direct providers may give part of their address space (prefixes) to
   leaf domains, based on an address prefix given to the provider.  This
   results in direct providers advertising to other providers a small
   fraction of the number of address prefixes that would be necessary if
   they enumerated the individual prefixes of the leaf routing domains.
   This represents a significant savings given the expected scale of
   global internetworking.

   The efficiencies gained in inter-domain routing clearly warrant the
   adoption of IPv6 address prefixes derived from the IPv6 address space
   of the providers.

   The mechanics of this scenario are straightforward. Each direct
   provider is given a unique small set of IPv6 address prefixes, from
   which its attached leaf routing domains can allocate slightly longer
   IPv6 address prefixes.  For example assume that NIST is a leaf
   routing domain whose inter-domain link is via SURANet. If SURANet is
   assigned an unique IPv6 address prefix 43DC:0A21/32, NIST could use a
   unique IPv6 prefix of 43DC:0A21:7652:34/56.

   If a direct service provider is connected to another provider(s)
   (either direct or indirect) via multiple attachment points, then in
   certain cases it may be advantageous to the direct provider to exert
   a certain degree of control over the coupling between the attachment
   points and flow of the traffic destined to a particular subscriber.
   Such control can be facilitated by first partitioning all the
   subscribers into groups, such that traffic destined to all the
   subscribers within a group should flow through a particular
   attachment point. Once the partitioning is done, the address space of
   the provider is subdivided along the group boundaries. A leaf routing
   domain that is willing to accept prefixes derived from its direct
   provider gets a prefix from the provider's address space subdivision
   associated with the group the domain belongs to.

   At the attachment point (between the direct and indirect providers)
   the direct provider advertises both an address prefix that
   corresponds to the address space of the provider, and one or more
   address prefixes that correspond to the address space associated with
   each subdivision.  The latter prefixes match the former prefix, but
   are longer than the former prefix. Use of the "longest match"
   forwarding algorithm by the recipients of these prefixes (e.g., a
   router within the indirect provider) results in forcing the flow of
   the traffic to destinations depicted by the longer address prefixes
   through the attachment point where these prefixes are advertised to
   the indirect provider.

   For example, assume that SURANet is connected to another regional
   provider, NEARNet, at two attachment points, A1 and A2. SURANet is
   assigned a unique IPv6 address prefix 43DC:0A21/32. To exert control
   over the traffic flow destined to a particular subscriber within
   SURANet, SURANet may subdivide the address space assigned to it into
   two groups, 43DC:0A21:8/34 and 43DC:0A21:C/34. The former group may
   be used for sites attached to SURANet that are closer (as determined
   by the topology within SURANet) to A1, while the latter group may be
   used for sites that are closer to A2.  The SURANet router at A1
   advertises both 43DC:0A21/32 and 43DC:0A21:8/34 address prefixes to
   the router in NEARNet. Likewise, the SURANet router at A2 advertises
   both 43DC:0A21/32 and 43DC:0A21:C/34 address prefixes to the router
   in NEARNet. Traffic that flows through NEARNet to destinations that
   match 43DC:0A21:8/34 address prefix would enter SURANet at A1, while
   traffic to destinations that match 43DC:0A21:C/34 address prefix
   would enter SURANet at A2.

   Note that the advertisement by the direct provider of the routing
   information associated with each subdivision must be done with care
   to ensure that such an advertisement would not result in a global
   distribution of separate reachability information associated with
   each subdivision, unless such distribution is warranted for some

other purposes (e.g., supporting certain aspects of policy-based
routing).

4.3.2 Indirect Providers (Backbones)

   There does not at present appear to be a strong case for direct
   providers to take their address spaces from the the IPv6 space of an
   indirect provider (e.g., backbone). The benefit in routing data
   abstraction is relatively small. The number of direct providers today
   is in the tens and an order of magnitude increase would not cause an
   undue burden on the backbones.  Also, it may be expected that as time
   goes by there will be increased direct interconnection of the direct
   providers, leaf routing domains directly attached to the backbones,
   and international links directly attached to the providers. Under
   these circumstances, the distinction between direct and indirect
   providers may become blurred.

   An additional factor that discourages allocation of IPv6 addresses
   from a backbone prefix is that the backbones and their attached
   providers are perceived as being independent. Providers may take
   their long-haul service from one or more backbones, or may switch
   backbones should a more cost-effective service be provided elsewhere.
   Having IPv6 addresses derived from a backbone is inconsistent with
   the nature of the relationship.

4.4 Multi-homed Routing Domains

   The discussions in Section 4.3 suggest methods for allocating IPv6
   addresses based on direct or indirect provider connectivity. This
   allows a great deal of information reduction to be achieved for those
   routing domains which are attached to a single TRD. In particular,
   such routing domains may select their IPv6 addresses from a space
   delegated to them by the direct provider. This allows the provider,
   when announcing the addresses that it can reach to other providers,
   to use a single address prefix to describe a large number of IPv6
   addresses corresponding to multiple routing domains.

   However, there are additional considerations for routing domains
   which are attached to multiple providers. Such `multi-homed' routing
   domains may, for example, consist of single-site campuses and
   companies which are attached to multiple backbones, large
   organizations which are attached to different providers at different
   locations in the same country, or multi-national organizations which
   are attached to backbones in a variety of countries worldwide. There

are a number of possible ways to deal with these multi-homed routing
domains.

4.4.1 Solution 1

   One possible solution is for each multi-homed organization to obtain
   its IPv6 address space independently of the providers to which it is
   attached.  This allows each multi-homed organization to base its IPv6
   assignments on a single prefix, and to thereby summarize the set of
   all IPv6 addresses reachable within that organization via a single
   prefix.  The disadvantage of this approach is that since the IPv6
   address for that organization has no relationship to the addresses of
   any particular TRD, the TRDs to which this organization is attached
   will need to advertise the prefix for this organization to other
   providers.  Other providers (potentially worldwide) will need to
   maintain an explicit entry for that organization in their routing
   tables.

   For example, suppose that a very large North American company `Mega
   Big International Incorporated' (MBII) has a fully interconnected
   internal network and is assigned a single prefix as part of the North
   American prefix.  It is likely that outside of North America, a
   single entry may be maintained in routing tables for all North
   American Destinations.  However, within North America, every provider
   will need to maintain a separate address entry for MBII. If MBII is
   in fact an international corporation, then it may be necessary for
   every provider worldwide to maintain a separate entry for MBII
   (including backbones to which MBII is not attached). Clearly this may
   be acceptable if there are a small number of such multi-homed routing
   domains, but would place an unacceptable load on routers within
   backbones if all organizations were to choose such address
   assignments.  This solution may not scale to internets where there
   are many hundreds of thousands of multi-homed organizations.

4.4.2 Solution 2

   A second possible approach would be for multi-homed organizations to
   be assigned a separate IPv6 address space for each connection to a
   TRD, and to assign a single prefix to some subset of its domain(s)
   based on the closest interconnection point. For example, if MBII had
   connections to two providers in the U.S. (one east coast, and one
   west coast), as well as three connections to national backbones in
   Europe, and one in the far east, then MBII may make use of six
   different address prefixes.  Each part of MBII would be assigned a

   single address prefix based on the nearest connection.

   For purposes of external routing of traffic from outside MBII to a
   destination inside of MBII, this approach works similarly to treating
   MBII as six separate organizations. For purposes of internal routing,
   or for routing traffic from inside of MBII to a destination outside
   of MBII, this approach works the same as the first solution.

   If we assume that incoming traffic (coming from outside of MBII, with
   a destination within MBII) is always to enter via the nearest point
   to the destination, then each TRD which has a connection to MBII
   needs to announce to other TRDs the ability to reach only those parts
   of MBII whose address is taken from its own address space. This
   implies that no additional routing information needs to be exchanged
   between TRDs, resulting in a smaller load on the inter-domain routing
   tables maintained by TRDs when compared to the first solution. This
   solution therefore scales better to extremely large internets
   containing very large numbers of multi-homed organizations.

   One problem with the second solution is that backup routes to multi-
   homed organizations are not automatically maintained. With the first
   solution, each TRD, in announcing the ability to reach MBII,
   specifies that it is able to reach all of the hosts within MBII. With
   the second solution, each TRD announces that it can reach all of the
   hosts based on its own address prefix, which only includes some of
   the hosts within MBII. If the connection between MBII and one
   particular TRD were severed, then the hosts within MBII with
   addresses based on that TRD would become unreachable via inter-domain
   routing. The impact of this problem can be reduced somewhat by
   maintenance of additional information within routing tables, but this
   reduces the scaling advantage of the second approach.

   The second solution also requires that when external connectivity
   changes, internal addresses also change.

   Also note that this and the previous approach will tend to cause
   packets to take different routes. With the first approach, packets
   from outside of MBII destined for within MBII will tend to enter via
   the point which is closest to the source (which will therefore tend
   to maximize the load on the networks internal to MBII). With the
   second solution, packets from outside destined for within MBII will
   tend to enter via the point which is closest to the destination
   (which will tend to minimize the load on the networks within MBII,
   and maximize the load on the TRDs).

   These solutions also have different effects on policies. For example,
   suppose that country `X' has a law that traffic from a source within
   country X to a destination within country X must at all times stay

   entirely within the country. With the first solution, it is not
   possible to determine from the destination address whether or not the
   destination is within the country. With the second solution, a
   separate address may be assigned to those hosts which are within
   country X, thereby allowing routing policies to be followed.
   Similarly, suppose that `Little Small Company' (LSC) has a policy
   that its packets may never be sent to a destination that is within
   MBII. With either solution, the routers within LSC may be configured
   to discard any traffic that has a destination within MBII's address
   space. However, with the first solution this requires one entry; with
   the second it requires many entries and may be impossible as a
   practical matter.

4.4.3 Solution 3

   There are other possible solutions as well. A third approach is to
   assign each multi-homed organization a single address prefix, based
   on one of its connections to a TRD. Other TRDs to which the multi-
   homed organization are attached maintain a routing table entry for
   the organization, but are extremely selective in terms of which other
   TRDs are told of this route. This approach will produce a single
   `default' routing entry which all TRDs will know how to reach (since
   presumably all TRDs will maintain routes to each other), while
   providing more direct routing in some cases.

   There is at least one situation in which this third approach is
   particularly appropriate. Suppose that a special interest group of
   organizations have deployed their own provider. For example, lets
   suppose that the U.S. National Widget Manufacturers and Researchers
   have set up a U.S.-wide provider, which is used by corporations who
   manufacture widgets, and certain universities which are known for
   their widget research efforts. We can expect that the various
   organizations which are in the widget group will run their internal
   networks as separate routing domains, and most of them will also be
   attached to other TRDs (since most of the organizations involved in
   widget manufacture and research will also be involved in other
   activities). We can therefore expect that many or most of the
   organizations in the widget group are dual-homed, with one attachment
   for widget-associated communications and the other attachment for
   other types of communications. Let's also assume that the total
   number of organizations involved in the widget group is small enough
   that it is reasonable to maintain a routing table containing one
   entry per organization, but that they are distributed throughout a
   larger internet with many millions of (mostly not widget-associated)
   routing domains.

   With the third approach, each multi-homed organization in the widget
   group would make use of an address assignment based on its other
   attachment(s) to TRDs (the attachments not associated with the widget
   group). The widget provider would need to maintain routes to the
   routing domains associated with the various member organizations.
   Similarly, all members of the widget group would need to maintain a
   table of routes to the other members via the widget provider.
   However, since the widget provider does not inform other general
   worldwide TRDs of what addresses it can reach (since the provider is
   not intended for use by other outside organizations), the relatively
   large set of routing prefixes needs to be maintained only in a
   limited number of places. The addresses assigned to the various
   organizations which are members of the widget group would provide a
   `default route' via each members other attachments to TRDs, while
   allowing communications within the widget group to use the preferred
   path.

4.4.4 Solution 4

   A fourth solution involves assignment of a particular address prefix
   for routing domains which are attached to precisely two (or more)
   specific routing domains. For example, suppose that there are two
   providers `SouthNorthNet' and `NorthSouthNet' which have a very large
   number of customers in common (i.e., there are a large number of
   routing domains which are attached to both). Rather than getting two
   address prefixes these organizations could obtain three prefixes.
   Those routing domains which are attached to NorthSouthNet but not
   attached to SouthNorthNet obtain an address assignment based on one
   of the prefixes. Those routing domains which are attached to
   SouthNorthNet but not to NorthSouthNet would obtain an address based
   on the second prefix. Finally, those routing domains which are
   multi-homed to both of these networks would obtain an address based
   on the third prefix.  Each of these two TRDs would then advertise two
   prefixes to other TRDs, one prefix for leaf routing domains attached
   to it only, and one prefix for leaf routing domains attached to both.

   This fourth solution is likely to be important when use of public
   data networks becomes more common. In particular, it is likely that
   at some point in the future a substantial percentage of all routing
   domains will be attached to public data networks. In this case,
   nearly all government-sponsored networks (such as some current
   regionals) may have a set of customers which overlaps substantially
   with the public networks.

4.4.5 Summary

   There are therefore a number of possible solutions to the problem of
   assigning IPv6 addresses to multi-homed routing domains. Each of
   these solutions has very different advantages and disadvantages.
   Each solution places a different real (i.e., financial) cost on the
   multi-homed organizations, and on the TRDs (including those to which
   the multi-homed organizations are not attached).

   In addition, most of the solutions described also highlight the need
   for each TRD to develop a policy on whether and under what conditions
   to accept addresses that are not based on its own address prefix, and
   how such non-local addresses will be treated. For example, a somewhat
   conservative policy might be that non-local IPv6 address prefixes
   will be accepted from any attached leaf routing domain, but not
   advertised to other TRDs.  In a less conservative policy, a TRD might
   accept such non-local prefixes and agree to exchange them with a
   defined set of other TRDs (this set could be an a priori group of
   TRDs that have something in common such as geographical location, or
   the result of an agreement specific to the requesting leaf routing
   domain). Various policies involve real costs to TRDs, which may be
   reflected in those policies.

4.5 Private Links

   The discussion up to this point concentrates on the relationship
   between IPv6 addresses and routing between various routing domains
   over transit routing domains, where each transit routing domain
   interconnects a large number of routing domains and offers a more-
   or-less public service.

   However, there may also exist a number of links which interconnect
   two routing domains in such a way, that usage of these links may be
   limited to carrying traffic only between the two routing domains.
   We'll refer to such links as "private".

   For example, let's suppose that the XYZ corporation does a lot of
   business with MBII. In this case, XYZ and MBII may contract with a
   carrier to provide a private link between the two corporations, where
   this link may only be used for packets whose source is within one of
   the two corporations, and whose destination is within the other of
   the two corporations. Finally, suppose that the point-to-point link
   is connected between a single router (router X) within XYZ
   corporation and a single router (router M) within MBII. It is
   therefore necessary to configure router X to know which addresses can

   be reached over this link (specifically, all addresses reachable in
   MBII). Similarly, it is necessary to configure router M to know which
   addresses can be reached over this link (specifically, all addresses
   reachable in XYZ Corporation).

   The important observation to be made here is that the additional
   connectivity due to such private links may be ignored for the purpose
   of IPv6 address allocation, and do not pose a problem for routing on
   a larger scale. This is because the routing information associated
   with such connectivity is not propagated throughout the internet, and
   therefore does not need to be collapsed into a TRD's prefix.

   In our example, let's suppose that the XYZ corporation has a single
   connection to a regional, and has therefore uses the IPv6 address
   space from the space given to that regional.  Similarly, let's
   suppose that MBII, as an international corporation with connections
   to six different providers, has chosen the second solution from
   Section 4.4, and therefore has obtained six different address
   allocations. In this case, all addresses reachable in the XYZ
   Corporation can be described by a single address prefix (implying
   that router M only needs to be configured with a single address
   prefix to represent the addresses reachable over this link). All
   addresses reachable in MBII can be described by six address prefixes
   (implying that router X needs to be configured with six address
   prefixes to represent the addresses reachable over the link).

   In some cases, such private links may be permitted to forward traffic
   for a small number of other routing domains, such as closely
   affiliated organizations. This will increase the configuration
   requirements slightly. However, provided that the number of
   organizations using the link is relatively small, then this still
   does not represent a significant problem.

   Note that the relationship between routing and IPv6 addressing
   described in other sections of this paper is concerned with problems
   in scaling caused by large, essentially public transit routing
   domains which interconnect a large number of routing domains.
   However, for the purpose of IPv6 address allocation, private links
   which interconnect only a small number of private routing domains do
   not pose a problem, and may be ignored. For example, this implies
   that a single leaf routing domain which has a single connection to a
   `public' provider (e.g., the Alternet), plus a number of private
   links to other leaf routing domains, can be treated as if it were
   single-homed to the provider for the purpose of IPv6 address
   allocation.  We expect that this is also another way of dealing with
   multi-homed domains.

4.6 Zero-Homed Routing Domains

   Currently, a very large number of organizations have internal
   communications networks which are not connected to any service
   providers.  Such organizations may, however, have a number of private
   links that they use for communications with other organizations. Such
   organizations do not participate in global routing, but are satisfied
   with reachability to those organizations with which they have
   established private links. These are referred to as zero-homed
   routing domains.

   Zero-homed routing domains can be considered as the degenerate case
   of routing domains with private links, as discussed in the previous
   section, and do not pose a problem for inter-domain routing. As
   above, the routing information exchanged across the private links
   sees very limited distribution, usually only to the routing domain at
   the other end of the link. Thus, there are no address abstraction
   requirements beyond those inherent in the address prefixes exchanged
   across the private link.

   However, it is important that zero-homed routing domains use valid
   globally unique IPv6 addresses. Suppose that the zero-homed routing
   domain is connected through a private link to a routing domain.
   Further, this routing domain participates in an internet that
   subscribes to the global IPv6 addressing plan. This domain must be
   able to distinguish between the zero-homed routing domain's IPv6
   addresses and any other IPv6 addresses that it may need to route to.
   The only way this can be guaranteed is if the zero-homed routing
   domain uses globally unique IPv6 addresses.

   Whereas globally unique addresses are necessary to differentiate
   between destinations in the Internet, globally unique addresses may
   not be sufficient to guarantee global connectivity.  If a zero-homed
   routing domain gets connected to the Internet, the block of addresses
   used within the domain may not be related to the block of addresses
   allocated to the domain's direct provider. In order to maintain the
   gains given by hierarchical routing and address assignment the zero-
   homed domain should under such circumstances change addresses
   assigned to the systems within the domain.

4.7 Continental aggregation

Another level of hierarchy may also be used in this addressing scheme
to further reduce the amount of routing information necessary for

   global routing.  Continental aggregation is useful because
   continental boundaries provide natural barriers to topological
   connection and administrative boundaries.  Thus, it presents a
   natural boundary for another level of aggregation of inter-domain
   routing information.  To make use of this, it is necessary that each
   continent be assigned an appropriate contiguous block of addresses.
   Providers (both direct and indirect) within that continent would
   allocate their addresses from this space.  Note that there are
   numerous exceptions to this, in which a service provider (either
   direct or indirect) spans a continental division.  These exceptions
   can be handled similarly to multi-homed routing domains, as discussed
   above.

   The benefit of continental aggregation is that it helps to absorb the
   entropy introduced within continental routing caused by the cases
   where an organization must use an address prefix which must be
   advertised beyond its direct provider.  In such cases, if the address
   is taken from the continental prefix, the additional cost of the
   route is not propagated past the point where continental aggregation
   takes place.

   Note that, in contrast to the case of providers, the aggregation of
   continental routing information may not be done on the continent to
   which the prefix is allocated.  The cost of inter-continental links
   (and especially trans-oceanic links) is very high.  If aggregation is
   performed on the `near' side of the link, then routing information
   about unreachable destinations within that continent can only reside
   on that continent.  Alternatively, if continental aggregation is done
   on the `far' side of an inter-continental link, the `far' end can
   perform the aggregation and inject it into continental routing.  This
   means that destinations which are part of the continental
   aggregation, but for which there is not a corresponding more specific
   prefix can be rejected before leaving the continent on which they
   originated.

   For example, suppose that Europe is assigned a prefix of 46/8, such
   that European routing also contains the longer prefixes 46DC:0A01/32
   and 46DC:0A02/32 .  All of the longer European prefixes may be
   advertised across a trans-Atlantic link to North America.  The router
   in North America would then aggregate these routes, and only
   advertise the prefix 46/8 into North American routing.  Packets which
   are destined for 46DC:0A01:1234:5678:ABCD:8765:4321:AABB would
   traverse North American routing, but would encounter the North
   American router which performed the European aggregation.  If the
   prefix 46DC:0A01/32 is unreachable, the router would drop the packet
   and send an unreachable message without using the trans-Atlantic
   link.

4.8 Private (Local Use) Addresses

   Many domains will have hosts which, for one reason or another, will
   not require globally unique IPv6 addresses.  A domain which decides
   to use IPv6 addresses out of the private address space is able to do
   so without address allocation from any authority.  Conversely, the
   private address prefix need not be advertised throughout the public
   portion of the Internet.

   In order to use private address space, a domain needs to determine
   which hosts do not need to have network layer connectivity outside
   the domain in the foreseeable future.  Such hosts will be called
   private hosts, and may use the private addresses described above if
   it is topologically convenient.  Private hosts can communicate with
   all other hosts inside the domain, both public and private.  However,
   they cannot have IPv6 connectivity to any external host.  While not
   having external network layer connectivity, a private host can still
   have access to external services via application layer relays.
   Public hosts do not have connectivity to private hosts outside of
   their own domain.

   Because private addresses have no global meaning, reachability
   information associated with the private address space shall not be
   propagated on inter-domain links, and packets with private source or
   destination addresses should not be forwarded across such links.
   Routers in domains not using private address space, especially those
   of Internet service providers, are expected to be configured to
   reject (filter out) routing information that carries reachability
   information associated with private addresses.  If such a router
   receives such information the rejection shall not be treated as a
   routing protocol error.

   In addition, indirect references to private addresses should be
   contained within the enterprise that uses these addresses. Prominent
   example of such references are DNS Resource Records.  A possible
   approach to avoid leaking of DNS RRs is to run two nameservers, one
   external server authoritative for all globally unique IP addresses of
   the enterprise and one internal nameserver authoritative for all IP
   addresses of the enterprise, both public and private.  In order to
   ensure consistency both these servers should be configured from the
   same data of which the external nameserver only receives a filtered
   version.  The resolvers on all internal hosts, both public and
   private, query only the internal nameserver.  The external server
   resolves queries from resolvers outside the enterprise and is linked
   into the global DNS.  The internal server forwards all queries for
   information outside the enterprise to the external nameserver, so all
   internal hosts can access the global DNS.  This ensures that

information about private hosts does not reach resolvers and
nameservers outside the enterprise.

4.9 Interaction with Policy Routing

   We assume that any inter-domain routing protocol will have difficulty
   trying to aggregate multiple destinations with dissimilar policies.
   At the same time, the ability to aggregate routing information while
   not violating routing policies is essential. Therefore, we suggest
   that address allocation authorities attempt to allocate addresses so
   that aggregates of destinations with similar policies can be easily
   formed.

5. Recommendations

   We anticipate that the current exponential growth of the Internet
   will continue or accelerate for the foreseeable future. In addition,
   we anticipate a rapid internationalization of the Internet. The
   ability of routing to scale is dependent upon the use of data
   abstraction based on hierarchical IPv6 addresses.  It is therefore
   essential to choose a hierarchical structure for IPv6 addresses with
   great care.

   Great attention must be paid to the definition of addressing
   structures to balance the conflicting goals of:

     - Route optimality

     - Routing algorithm efficiency

     - Ease and administrative efficiency of address registration

     - Considerations for what addresses are assigned by what addressing
        authority

   It is in the best interests of the internetworking community that the
   cost of operations be kept to a minimum where possible. In the case
   of IPv6 address allocation, this again means that routing data
   abstraction must be encouraged.

   In order for data abstraction to be possible, the assignment of IPv6
   addresses must be accomplished in a manner which is consistent with
   the actual physical topology of the Internet. For example, in those
   cases where organizational and administrative boundaries are not

   related to actual network topology, address assignment based on such
   organization boundaries is not recommended.

   The intra-domain routing protocols allow for information abstraction
   to be maintained within a domain.  For zero-homed and single-homed
   routing domains (which are expected to remain zero-homed or single-
   homed), we recommend that the IPv6 addresses assigned within a single
   routing domain use a single address prefix assigned to that domain.
   Specifically, this allows the set of all IPv6 addresses reachable
   within a single domain to be fully described via a single prefix.

   We anticipate that the total number of routing domains existing on a
   worldwide Internet to be great enough that additional levels of
   hierarchical data abstraction beyond the routing domain level will be
   necessary.

   In most cases, network topology will have a close relationship with
   national boundaries. For example, the degree of network connectivity
   will often be greater within a single country than between countries.
   It is therefore appropriate to make specific recommendations based on
   national boundaries, with the understanding that there may be
   specific situations where these general recommendations need to be
   modified.

   Further, from experience with IPv4, we feel that continental
   aggregation is beneficial and should be supported where possible as a
   means of limiting the unwarranted spread of routing exceptions.

5.1 Recommendations for an address allocation plan

   We anticipate that public interconnectivity between private routing
   domains will be provided by a diverse set of TRDs, including (but not
   necessarily limited to):

     - Backbone networks;

     - A number of regional or national networks; and,

     - A number of commercial Public Data Networks.

   These networks will not be interconnected in a strictly hierarchical
   manner (for example, there is expected to be direct connectivity
   between regionals, and all of these types of networks may have direct
   international connections).  These TRDs will be used to interconnect
   a wide variety of routing domains, each of which may comprise a
   single corporation, part of a corporation, a university campus, a

   government agency, or other organizational unit.

   In addition, some private corporations may be expected to make use of
   dedicated private TRDs for communication within their own
   corporation.

   We anticipate that the great majority of routing domains will be
   attached to only one of the TRDs. This will permit hierarchical
   address aggregation based on TRD. We therefore strongly recommend
   that addresses be assigned hierarchically, based on address prefixes
   assigned to individual TRDs.

   To support continental aggregation of routes, we recommend that all
   addresses for TRDs which are wholly within a continent be taken from
   the continental prefix.

   For the proposed address allocation scheme, this implies that
   portions of IPv6 address space should be assigned to each TRD
   (explicitly including the backbones and regionals). For those leaf
   routing domains which are connected to a single TRD, they should be
   assigned a prefix value from the address space assigned to that TRD.

   For routing domains which are not attached to any publically
   available TRD, there is not the same urgent need for hierarchical
   address aggregation. We do not, therefore, make any additional
   recommendations for such `isolated' routing domains.  Where such
   domains are connected to other domains by private point-to-point
   links, and where such links are used solely for routing between the
   two domains that they interconnect, again no additional technical
   problems relating to address abbreviation is caused by such a link,
   and no specific additional recommendations are necessary.  We do
   recommend that since such domains may wish to use a private address
   space, that the addressing plan specify a specific prefix for private
   addressing.

   Further, in order to allow aggregation of IPv6 addresses at national
   and continental boundaries into as few prefixes as possible, we
   further recommend that IPv6 addresses allocated to routing domains
   should be assigned based on each routing domain's connectivity to
   national and continental Internet backbones.

5.2 Recommendations for Multi-Homed Routing Domains

   Some routing domains will be attached to multiple TRDs within the
   same country, or to TRDs within multiple different countries. We
   refer to these as `multi-homed' routing domains. Clearly the strict

   hierarchical model discussed above does not neatly handle such
   routing domains.

   There are several possible ways that these multi-homed routing
   domains may be handled, as described in Section 4.4.  Each of these
   methods vary with respect to the amount of information that must be
   maintained for inter-domain routing and also with respect to the
   inter-domain routes. In addition, the organization that will bear the
   brunt of this cost varies with the possible solutions. For example,
   the solutions vary with respect to:

     - Resources used within routers within the TRDs;

     - Administrative cost on TRD personnel; and,

     - Difficulty of configuration of policy-based inter-domain routing
        information within leaf routing domains.

   Also, the solution used may affect the actual routes which packets
   follow, and may effect the availability of backup routes when the
   primary route fails.

   For these reasons it is not possible to mandate a single solution for
   all situations. Rather, economic considerations will require a
   variety of solutions for different routing domains, service
   providers, and backbones.

6. Security Considerations

Security issues are not discussed in this document.

7. Acknowledgments

   This document is largely based on RFC 1518.  The section on Private
   Addresses borrowed heavily from RFC 1597.

   We'd like to thank Havard Eidnes, Bill Manning, Roger Fajman for
   their review and constructive comments.

REFERENCES

   [1]  Deering, S., and R. Hinden, Editors, "Internet Protocol, Version
        6 (IPv6) Specification", RFC 1883, Xerox PARC, Ipsilon Networks,
        December 1995.

   [2]  Hinden, R., and S. Deering, Editors, "IP Version 6 Addressing
        Architecture", RFC 1884, Ipsilon Networks, Xerox PARC, December
        1995.

AUTHORS' ADDRESSES

   Yakov Rekhter
   cisco Systems, Inc.
   470 Tasman Dr.
   San Jose, CA 95134

   Phone: (914) 528-0090
   EMail: yakov@cisco.com

   Tony Li
   cisco Systems, Inc.
   470 Tasman Dr.
   San Jose, CA 95134

   Phone: (408) 526-8186
   EMail: tli@cisco.com