Movatterモバイル変換

Network Working Group                                        Y. RekhterRequest for Comments: 1887                                cisco SystemsCategory: Informational                                           T. Li                                                          cisco Systems                                                                Editors                                                          December 1995An Architecture for IPv6 Unicast Address AllocationStatus 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:      - 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 addressingRekhter & Li                 Informational                      [Page 2]

RFC 1887      IPv6 Unicast Address Allocation Architecture December 1995        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 specifiedRekhter & 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 wouldRekhter & 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 inSection 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 toRekhter & 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 informationRekhter & 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 discussedRekhter & 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 someRekhter & 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 inSection 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. ThereRekhter & 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 aRekhter & 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 stayRekhter & 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 19954.4.5 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 canRekhter & 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 fromSection 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 19954.6   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 forRekhter & 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 19954.8   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 thatRekhter & 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:     - 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 notRekhter & 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):     - 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, aRekhter & 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 strictRekhter & 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 inSection 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 onRFC 1518.  The section on Private   Addresses borrowed heavily fromRFC 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 1995REFERENCES   [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.comRekhter & Li                 Informational                     [Page 26]