rfc1518.txt

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Network Working Group                                         Y. Rekhter
Request for Comments: 1518        T.J. Watson Research Center, IBM Corp.
Category: Standards Track                                          T. Li
                                                           cisco Systems
                                                                 Editors
                                                          September 1993


          An Architecture for IP Address Allocation with CIDR

Status of this Memo

   This RFC specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" for the standardization state and status
   of this protocol.  Distribution of this memo is unlimited.

1.  Introduction

   This paper provides an architecture and a plan for allocating IP
   addresses in the Internet. This architecture and the plan are
   intended to play an important role in steering the Internet towards
   the Address Assignment and Aggregating Strategy outlined in [1].

   The IP address space is a scarce shared resource that must be managed
   for the good of the community. The managers of this resource are
   acting as its custodians. They have a responsibility to the community
   to manage it for the common good.

2.  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 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                                                    [Page 1]

RFC 1518          CIDR Address Allocation Architecture    September 1993


   There are two aspects of interest when discussing IP address
   allocation within the Internet. The first is the set of
   administrative requirements for obtaining and allocating IP
   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.

   The architecture and recommendations provided in this paper are
   intended for immediate deployment. This paper specifically does not
   address long-term research issues, such as complex policy-based
   routing requirements.

   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 IP address allocation in
   the Internet. Topics covered include:

      - Benefits of encoding some topological information in IP
        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 IP
        addressing and routing components;

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

      - Allocation of the IP addresses by the Internet Registry;

      - Choice of the high-order portion of the IP 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 IP address allocation,
   both technical and administrative, that are not covered in this
   paper.  Topics not covered or mentioned only superficially include:



Rekhter & Li                                                    [Page 2]

RFC 1518          CIDR Address Allocation Architecture    September 1993


      - 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 IP address
        or a portion of the IP address space has been allocated);

      - How a routing domain (especially a site) should organize its
        internal topology or allocate portions of its IP 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 IP addresses.

3.  Background

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

   IP partitions the routing problem into three parts:

      - routing exchanges between end systems and routers (ARP),

      - routing exchanges between routers in the same routing domain
        (interior routing), and,

      - routing among routing domains (exterior routing).

4. IP Addresses and Routing

   For the purposes of this paper, an IP prefix is an IP address and
   some indication of the leftmost contiguous significant bits within
   this address. Throughout this paper IP address prefixes will be
   expressed as <IP-address IP-mask> tuples, such that a bitwise logical
   AND operation on the IP-address and IP-mask components of a tuple
   yields the sequence of leftmost contiguous significant bits that form
   the IP address prefix. For example a tuple with the value <193.1.0.0
   255.255.0.0> denotes an IP address prefix with 16 leftmost contiguous
   significant bits.

   When determining an administrative policy for IP 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.



Rekhter & Li                                                    [Page 3]

RFC 1518          CIDR Address Allocation Architecture    September 1993


   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 IP addresses be assigned
   according to topological routing structures. However, 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.

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

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

   Alternatively, should the routing domain draw IP addresses for all
   the hosts within the domain from a single IP address prefix, boundary
   routing information can be summarized into the single IP 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 IP addresses within the domains out of some common
   prefix for the purpose of data abstraction. The result would be flat
   inter-domain routing; all routing domains would need explicit
   knowledge of all other routing domains that they route to.  This can
   work well in small and medium sized internets.  However, this does
   not scale to very large internets.  For example, we expect growth in
   the future to an Internet which has tens or 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, however, it should be possible to significantly
   constrain the volume and the complexity of routing information by
   taking advantage of the existing hierarchical interconnectivity, as
   discussed in Section 5. Thus, there is the opportunity for a group of



Rekhter & Li                                                    [Page 4]

RFC 1518          CIDR Address Allocation Architecture    September 1993


   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 small 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 abbreviate the
   reachability information for a large number of routing domains as a
   single prefix. This approach therefore can allow a great deal of
   hierarchical abbreviation 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