rfc1887.txt

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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.




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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 addressing



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        architecture for such embedded addresses;

      - Multicast addressing;

      - Address allocation for mobile hosts;

      - Identification of specific administrative domains in the
        Internet;

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

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

      - Procedures for assigning host IPv6 addresses.


2.   Background


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

   IPv6 partitions the routing problem into three parts:

      - Routing exchanges between end systems and routers,

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

      - Routing among routing domains.


3.   IPv6 Addresses and Routing


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



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   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



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   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