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rfc:rfc1887

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.

Rekhter & Li Informational [Page 1] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

 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:
  1. Benefits of encoding some topological information in IPv6

addresses to significantly reduce routing protocol overhead;

  1. The anticipated need for additional levels of hierarchy in

Internet addressing to support network growth;

  1. The recommended mapping between Internet topological entities

(i.e., service providers, and service subscribers) and IPv6

      addressing and routing components;
  1. The recommended division of IPv6 address assignment among

service providers (e.g., backbones, regionals), and service

      subscribers (e.g., sites);
  1. Allocation of the IPv6 addresses by the Internet Registry;
  1. 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:
  1. A specific plan for address assignment;
  1. Embedding address spaces from other network layer protocols

(including IPv4) in the IPv6 address space and the addressing

Rekhter & Li Informational [Page 2] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

      architecture for such embedded addresses;
  1. Multicast addressing;
  1. Address allocation for mobile hosts;
  1. Identification of specific administrative domains in the

Internet;

  1. 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);
  1. 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,
  1. 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:
  1. Routing exchanges between end systems and routers,
  1. Routing exchanges between routers in the same routing domain,

and,

  1. 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

Rekhter & Li Informational [Page 3] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

 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

Rekhter & Li Informational [Page 4] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

 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

Rekhter & Li Informational [Page 5] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

 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.

Rekhter & Li Informational [Page 6] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

 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

Rekhter & Li Informational [Page 7] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

 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.

Rekhter & Li Informational [Page 8] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

 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

Rekhter & Li Informational [Page 9] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

 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.

Rekhter & Li Informational [Page 10] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

 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

Rekhter & Li Informational [Page 11] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

 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

Rekhter & Li Informational [Page 12] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

 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

Rekhter & Li Informational [Page 13] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

 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

Rekhter & Li Informational [Page 14] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

 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.

Rekhter & Li Informational [Page 15] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

 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.

Rekhter & Li Informational [Page 16] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

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

Rekhter & Li Informational [Page 17] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

 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.

Rekhter & Li Informational [Page 18] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

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

Rekhter & Li Informational [Page 19] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

 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.

Rekhter & Li Informational [Page 20] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

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

Rekhter & Li Informational [Page 21] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

 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:
  1. Route optimality
  1. Routing algorithm efficiency
  1. Ease and administrative efficiency of address registration
  1. 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

Rekhter & Li Informational [Page 22] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

 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):
  1. Backbone networks;
  1. A number of regional or national networks; and,
  1. 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

Rekhter & Li Informational [Page 23] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

 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

Rekhter & Li Informational [Page 24] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

 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:
  1. Resources used within routers within the TRDs;
  1. Administrative cost on TRD personnel; and,
  1. 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.

Rekhter & Li Informational [Page 25] RFC 1887 IPv6 Unicast Address Allocation Architecture December 1995

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

Rekhter & Li Informational [Page 26]

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