IPv6ユニキャストアドレス割り当て体系（IPv6アドレスの管理とインターネットにおけるルーティング）

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

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

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

     1) At some part within a routing domain,

     2) At the leaf routing domain,

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

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

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

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

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

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

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

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

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

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