RFC1518

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.

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.

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.

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:

  - 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.

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).

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.

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

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 may use their prefix as the basis for subsequent suballocations, assuming that the IP addresses remain within the overall length and structure constraints.

At this point, we observe that the number of nodes at each lower level of a hierarchy tends to grow exponentially. Thus the greatest gains in the reachability information abstraction (for the benefit of all higher levels of the hierarchy) occur when the reachability information aggregation occurs near the leaves of the hierarchy; the gains drop significantly at each higher level. Therefore, the law of diminishing returns suggests that at some point data abstraction ceases to produce significant benefits. Determination of the point at which data abstraction ceases to be of benefit requires a careful consideration of the number of routing domains that are expected to 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.

Efficiency versus Decentralized Control

If the Internet plans to support a decentralized address administration [4], then there is a balance that must be sought between the requirements on IP addresses for efficient routing and the need for decentralized address administration. A proposal described in [3] offers an example of how these two needs might be met.

The IP address prefix <198.0.0.0 254.0.0.0> provides for administrative decentralization. This prefix identifies part of the IP address space allocated for North America. The lower order part of that prefix allows allocation of IP addresses along topological boundaries in support of increased data abstraction. Clients within North America use parts of the IP address space that is underneath the IP address space of their service providers. Within a routing domain addresses for subnetworks and hosts are allocated from the unique IP prefix assigned to the domain.

IP Address Administration and Routing in the Internet

The basic Internet routing components are service providers (e.g., backbones, regional networks), and service subscribers (e.g., sites or campuses). These components are arranged hierarchically for the most part. A natural mapping from these components to IP routing components is that providers and subscribers 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. Such sites can exchange routing information with their provider via interior routing protocol route leaking or via an exterior routing protocol. For the purposes of this discussion, the choice is not significant. 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:

  - at some part within a routing domain,

  - at the leaf routing domain,

  - at the transit routing domain (TRD), and

  - at the continental boundaries.

  A point within a routing domain corresponds to a subnetwork. 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.

  The greatest burden in transmitting and operating on routing
  information is at the top of the routing hierarchy, where routing
  information tends to accumulate. In the Internet, for example,
  providers must manage the set of network numbers for all networks
  reachable through 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 network numbers for all attached providers and their
  associated networks.

  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 IP address prefix directly from the IP address space
  allocated for North America, or from the IP 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.

  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 IP
  address space that benefit the entire community.

Administration of IP addresses within a domain

  If individual subnetworks take their IP addresses from a myriad of
  unrelated IP 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 IP 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 IP 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.

  This situation is roughly analogous to the present dissemination
  of routing information in the Internet, where each domain may have
  non-contiguous network numbers assigned to it.  The result of
  allowing subnetworks within a routing domain to take their IP
  addresses from unrelated IP address spaces is flat routing at the
  A/B/C class network level.  The number of IP prefixes that leaf
  routing domains would advertise is on the order of the number of
  attached network numbers; the number of prefixes a provider's
  routing domain would advertise is approximately the number of
  network numbers 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 as the Internet grows this will quickly become intractable. A
  greater degree of hierarchical information reduction is necessary
  to allow continued growth in the Internet.

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 prefix from its provider's
  prefix 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 IP networks.
  Under the new allocation scheme, they might instead 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 subnetworks and routing
  domains implicit in the fact that they operate a common routing
  protocol and are under the control of a single administration. The
  routing domain administration subdivides the domain into
  subnetworks.  The routing domain represents the only path between
  a subnetwork and the rest of the internetwork. It is reasonable
  that this relationship also extend to include a common IP
  addressing space. Thus, the subnetworks within the leaf routing
  domain should take their IP addresses from the prefix assigned to
  the leaf routing domain.

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 a TRD is a direct provider. Each case is discussed
  separately below.

Direct Service Providers

  It is interesting to consider whether direct service providers'
  routing domains should use their IP address space for assigning IP
  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.

  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 backbones 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.

  Are leaf routing domains willing to accept prefixes derived from
  the direct providers? In the supplier/consumer model, the direct
  provider is offering connectivity as the service, priced according
  to its costs of operation. This includes the "price" of obtaining
  service from one or more indirect providers (e.g., backbones). In
  general, indirect providers will want to handle as few address
  prefixes as possible to keep costs low. In the Internet
  environment, which does not operate as a typical marketplace, leaf
  routing domains must be sensitive to the resource constraints of
  the providers (both direct and indirect). The efficiencies gained
  in inter-domain routing clearly warrant the adoption of IP address
  prefixes derived from the IP address space of the providers.

  The mechanics of this scenario are straightforward. Each direct
  provider is given a unique small set of IP address prefixes, from
  which its attached leaf routing domains can allocates slightly
  longer IP 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 IP address prefix <198.1.0.0
  255.255.0.0>, NIST could use a unique IP prefix of <198.1.0.0
  255.255.240.0>.

  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. Note
  that the advertisement by the direct provider of the routing
  information associated with each subdivision must be done with
  care to ensure that such an advertisement would not result in a
  global distribution of separate reachability information
  associated with each subdivision, unless such distribution is
  warranted for some other purposes (e.g., supporting certain
  aspects of policy-based routing).

Indirect Providers (Backbones)

  There does not appear to be a strong case for direct providers to
  take their address spaces from the the IP 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 IP 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 IP addresses derived from a backbone is
  inconsistent with the nature of the relationship.

Multi-homed Routing Domains

  The discussions in Section 5.3 suggest methods for allocating IP
  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 IP 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 IP addresses corresponding to multiple routing

  domains.

  However, there are additional considerations for routing domains
  which are attached to multiple providers. Such "multi-homed"
  routing domains may, for example, consist of single-site campuses
  and companies which are attached to multiple backbones, large
  organizations which are attached to different providers at
  different locations in the same country, or multi-national
  organizations which are attached to backbones in a variety of
  countries worldwide. There are a number of possible ways to deal
  with these multi-homed routing domains.

  One possible solution is for each multi-homed organization to
  obtain its IP address space independently from the providers to
  which it is attached.  This allows each multi-homed organization
  to base its IP assignments on a single prefix, and to thereby
  summarize the set of all IP addresses reachable within that
  organization via a single prefix.  The disadvantage of this
  approach is that since the IP 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.

  A second possible approach would be for multi-homed organizations
  to be assigned a separate IP address space for each connection to
  a TRD, and to assign a single prefix to some subset of its
  domain(s) based on the closest interconnection point. For example,
  if MBII had connections to two providers in the U.S. (one east
  coast, and one west coast), as well as three connections to

  national backbones in Europe, and one in the far east, then MBII
  may make use of six different address prefixes.  Each part of MBII
  would be assigned a single address prefix based on the nearest
  connection.

  For purposes of external routing of traffic from outside MBII to a
  destination inside of MBII, this approach works similarly to
  treating MBII as six separate organizations. For purposes of
  internal routing, or for routing traffic from inside of MBII to a
  destination outside of MBII, this approach works the same as the
  first solution.

  If we assume that incoming traffic (coming from outside of MBII,
  with a destination within MBII) is always to enter via the nearest
  point to the destination, then each TRD which has a connection to
  MBII needs to announce to other TRDs the ability to reach only
  those parts of MBII whose address is taken from its own address
  space. This implies that no additional routing information needs
  to be exchanged between TRDs, resulting in a smaller load on the
  inter-domain routing tables maintained by TRDs when compared to
  the first solution. This solution therefore scales better to
  extremely large internets containing very large numbers of multi-
  homed organizations.

  One problem with the second solution is that backup routes to
  multi-homed organizations are not automatically maintained. With
  the first solution, each TRD, in announcing the ability to reach
  MBII, specifies that it is able to reach all of the hosts within
  MBII. With the second solution, each TRD announces that it can
  reach all of the hosts based on its own address prefix, which only
  includes some of the hosts within MBII. If the connection between
  MBII and one particular TRD were severed, then the hosts within
  MBII with addresses based on that TRD would become unreachable via
  inter-domain routing. The impact of this problem can be reduced
  somewhat by maintenance of additional information within routing
  tables, but this reduces the scaling advantage of the second
  approach.

  The second solution also requires that when external connectivity
  changes, internal addresses also change.

  Also note that this and the previous approach will tend to cause
  packets to take different routes. With the first approach, packets
  from outside of MBII destined for within MBII will tend to enter
  via the point which is closest to the source (which will therefore
  tend to maximize the load on the networks internal to MBII). With
  the second solution, packets from outside destined for within MBII
  will tend to enter via the point which is closest to the

  destination (which will tend to minimize the load on the networks
  within MBII, and maximize the load on the TRDs).

  These solutions also have different effects on policies. For
  example, suppose that country "X" has a law that traffic from a
  source within country X to a destination within country X must at
  all times stay 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.

  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 backbone. For example, lets
  suppose that the U.S. National Widget Manufacturers and
  Researchers have set up a U.S.-wide backbone, 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.

  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 backbone 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 backbone.  However, since the widget backbone does not
  inform other general worldwide TRDs of what addresses it can reach
  (since the backbone 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.

  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.

  There are therefore a number of possible solutions to the problem
  of assigning IP 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 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 IP 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.

Private Links

  The discussion up to this point concentrates on the relationship
  between IP addresses and routing between various routing domains
  over transit routing domains, where each transit routing domain
  interconnects a large number of routing domains and offers a
  more-or-less public service.

  However, there may also exist a number of links which interconnect
  two routing domains in such a way, that usage of these links may
  be limited to carrying traffic only between the two routing
  domains.  We'll refer to such links as "private".

  For example, let's suppose that the XYZ corporation does a lot of
  business with MBII. In this case, XYZ and MBII may contract with a
  carrier to provide a private link between the two corporations,
  where this link may only be used for packets whose source is
  within one of the two corporations, and whose destination is
  within the other of the two corporations. Finally, suppose that
  the point-to-point link is connected between a single router
  (router X) within XYZ corporation and a single router (router M)
  within MBII. It is therefore necessary to configure router X to
  know which addresses can 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 IP address allocation, and do not pose a problem for
  routing. 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 IP
  address space from the space given to that regional.  Similarly,
  let's suppose that MBII, as an international corporation with
  connections to six different providers, has chosen the second
  solution from Section 5.4, 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 IP 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 IP 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" backbone, plus a number of
  private links to other leaf routing domains, can be treated as if
  it were single-homed to the backbone for the purpose of IP address
  allocation.  We expect that this is also another way of dealing
  with multi-homed domains.

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 IP 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 IP addressing plan. This domain must be
  able to distinguish between the zero-homed routing domain's IP
  addresses and any other IP 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 IP addresses.

Continental aggregation

  Another level of hierarchy may also be used in this addressing
  scheme to further reduce the amount of routing information
  necessary for inter-continental 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 subset of
  the address space.  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.

  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
  <194.0.0.0 254.0.0.0>, such that European routing also contains
  the longer prefixes <194.1.0.0 255.255.0.0> and <194.2.0.0
  255.255.0.0>.  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 <194.0.0.0 255.0.0.0> into North
  American routing.  Packets which are destined for 194.1.1.1 would
  traverse North American routing, but would encounter the North
  American router which performed the European aggregation.  If the
  prefix <194.1.0.0 255.255.0.0> is unreachable, the router would
  drop the packet and send an ICMP Unreachable without using the
  trans-Atlantic link.

Transition Issues

  Allocation of IP addresses based on connectivity to TRDs is
  important to allow scaling of inter-domain routing to an internet
  containing millions of routing domains. However, such address
  allocation based on topology implies that in order to maximize the
  efficiency in routing gained by such allocation, certain changes
  in topology may suggest a change of address.

  Note that an address change need not happen immediately.  A domain
  which has changed service providers may still advertise its prefix
  through its new service provider.  Since upper levels in the
  routing hierarchy will perform routing based on the longest
  prefix, reachability is preserved, although the aggregation and
  scalability of the routing information has greatly diminished.
  Thus, a domain which does change its topology should change
  addresses as soon as convenient.  The timing and mechanics of such
  changes must be the result of agreements between the old service
  provider, the new provider, and the domain.

  This need to allow for change in addresses is a natural,
  inevitable consequence of routing data abstraction. The basic
  notion of routing data abstraction is that there is some
  correspondence between the address and where a system (i.e., a
  routing domain, subnetwork, or end system) is located. Thus if the
  system moves, in some cases the address will have to change. If it

  were possible to change the connectivity between routing domains
  without changing the addresses, then it would clearly be necessary
  to keep track of the location of that routing domain on an
  individual basis.

  In the short term, due to the rapid growth and increased
  commercialization of the Internet, it is possible that the
  topology may be relatively volatile. This implies that planning
  for address transition is very important. Fortunately, there are a
  number of steps which can be taken to help ease the effort
  required for address transition. A complete description of address
  transition issues is outside of the scope of this paper. However,
  a very brief outline of some transition issues is contained in
  this section.

  Also note that the possible requirement to transition addresses
  based on changes in topology imply that it is valuable to
  anticipate the future topology changes before finalizing a plan
  for address allocation. For example, in the case of a routing
  domain which is initially single-homed, but which is expecting to
  become multi-homed in the future, it may be advantageous to assign
  IP addresses based on the anticipated future topology.

  In general, it will not be practical to transition the IP
  addresses assigned to a routing domain in an instantaneous "change
  the address at midnight" manner. Instead, a gradual transition is
  required in which both the old and the new addresses will remain
  valid for a limited period of time. During the transition period,
  both the old and new addresses are accepted by the end systems in
  the routing domain, and both old and new addresses must result in
  correct routing of packets to the destination.

  During the transition period, it is important that packets using
  the old address be forwarded correctly, even when the topology has
  changed.  This is facilitated by the use of "longest match"
  inter-domain routing.

  For example, suppose that the XYZ Corporation was previously
  connected only to the NorthSouthNet regional. The XYZ Corporation
  therefore went off to the NorthSouthNet administration and got an
  IP address prefix assignment based on the IP address prefix value
  assigned to the NorthSouthNet regional. However, for a variety of
  reasons, the XYZ Corporation decided to terminate its association
  with the NorthSouthNet, and instead connect directly to the
  NewCommercialNet public data network. Thus the XYZ Corporation now
  has a new address assignment under the IP address prefix assigned
  to the NewCommercialNet. The old address for the XYZ Corporation
  would seem to imply that traffic for the XYZ Corporation should be

  routed to the NorthSouthNet, which no longer has any direct
  connection with XYZ Corporation.

  If the old TRD (NorthSouthNet) and the new TRD (NewCommercialNet)
  are adjacent and cooperative, then this transition is easy to
  accomplish.  In this case, packets routed to the XYZ Corporation
  using the old address assignment could be routed to the
  NorthSouthNet, which would directly forward them to the
  NewCommercialNet, which would in turn forward them to XYZ
  Corporation. In this case only NorthSouthNet and NewCommercialNet
  need be aware of the fact that the old address refers to a
  destination which is no longer directly attached to NorthSouthNet.

  If the old TRD and the new TRD are not adjacent, then the
  situation is a bit more complex, but there are still several
  possible ways to forward traffic correctly.

  If the old TRD and the new TRD are themselves connected by other
  cooperative transit routing domains, then these intermediate
  domains may agree to forward traffic for XYZ correctly. For
  example, suppose that NorthSouthNet and NewCommercialNet are not
  directly connected, but that they are both directly connected to
  the BBNet backbone.  In this case, all three of NorthSouthNet,
  NewCommercialNet, and the BBNet backbone would need to maintain a
  special entry for XYZ corporation so that traffic to XYZ using the
  old address allocation would be forwarded via NewCommercialNet.
  However, other routing domains would not need to be aware of the
  new location for XYZ Corporation.

  Suppose that the old TRD and the new TRD are separated by a non-
  cooperative routing domain, or by a long path of routing domains.
  In this case, the old TRD could encapsulate traffic to XYZ
  Corporation in order to deliver such packets to the correct
  backbone.

  Also, those locations which do a significant amount of business
  with XYZ Corporation could have a specific entry in their routing
  tables added to ensure optimal routing of packets to XYZ. For
  example, suppose that another commercial backbone
  "OldCommercialNet" has a large number of customers which exchange
  traffic with XYZ Corporation, and that this third TRD is directly
  connected to both NorthSouthNet and NewCommercialNet. In this case
  OldCommercialNet will continue to have a single entry in its
  routing tables for other traffic destined for NorthSouthNet, but
  may choose to add one additional (more specific) entry to ensure
  that packets sent to XYZ Corporation's old address are routed
  correctly.

  Whichever method is used to ease address transition, the goal is
  that knowledge relating XYZ to its old address that is held
  throughout the global internet would eventually be replaced with
  the new information.  It is reasonable to expect this to take
  weeks or months and will be accomplished through the distributed
  directory system.  Discussion of the directory, along with other
  address transition techniques such as automatically informing the
  source of a changed address, are outside the scope of this paper.

  Another significant transition difficulty is the establishment of
  appropriate addressing authorities.  In order not to delay the
  deployment of this addressing scheme, if no authority has been
  created at an appropriate level, a higher level authority may
  allocated addresses instead of the lower level authority.  For
  example, suppose that the continental authority has been allocated
  a portion of the address space and that the service providers
  present on that continent are clear, but have not yet established
  their addressing authority.  The continental authority may foresee
  (possibly with information from the provider) that the provider
  will eventually create an authority.  The continental authority
  may then act on behalf of that provider until the provider is
  prepared to assume its addressing authority duties.

  Finally, it is important to emphasize, that a change of addresses
  due to changes in topology is not mandated by this document.  The
  continental level addressing hierarchy, as discussed in Section
  5.7, is intended to handle the aggregation of reachability
  information in the cases where addresses do not directly reflect
  the connectivity between providers and subscribers.

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.

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 IP addresses. As
  CIDR [1] is introduced in the Internet, it is therefore essential

  to choose a hierarchical structure for IP addresses with great
  care.

  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 IP address allocation, this again means that routing data
  abstraction must be encouraged.

  In order for data abstraction to be possible, the assignment of IP
  addresses must be accomplished in a manner which is consistent
  with the actual physical topology of the Internet. For example, in
  those cases where organizational and administrative boundaries are
  not 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 IP addresses
  assigned within a single routing domain use a single address
  prefix assigned to that domain.  Specifically, this allows the set
  of all IP 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.

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 (Alternet, ANSnet, CIX, EBone, PSI,
    SprintLink);

  - 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). However, the total number of such TRDs is expected to remain (for the foreseeable future) small enough to allow addressing of this set of TRDs via a flat address space. These TRDs will be used to interconnect a wide variety of routing domains, each of which may comprise a single corporation, part of a corporation, a university campus, a 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 IP 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 abbreviation. 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.

Further, in order to allow aggregation of IP addresses at national and continental boundaries into as few prefixes as possible, we further recommend that IP addresses allocated to routing domains should be assigned based on each routing domain's connectivity to national and continental Internet backbones.

Recommendations for Multi-Homed Routing Domains

There are several possible ways that these multi-homed routing domains may be handled, as described in Section 5.4. 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.

Recommendations for the Administration of IP addresses

A companion document [3] provides recommendations for the administrations of IP addresses.

Acknowledgments

The authors would like to acknowledge the substantial contributions made by the authors of RFC 1237 [2], Richard Colella, Ella Gardner, and Ross Callon. The significant concepts (and a large portion of the text) in this document are taken directly from their work.

The authors would like to acknowledge the substantial contributions made by the members of the following two groups, the Federal Engineering Planning Group (FEPG) and the International Engineering Planning Group (IEPG). This document also reflects many concepts expressed at the IETF Addressing BOF which took place in Cambridge, MA in July 1992.

We would also like to thank Peter Ford (Los Alamos National Laboratory), Elise Gerich (MERIT), Steve Kent (BBN), Barry Leiner (ADS), Jon Postel (ISI), Bernhard Stockman (NORDUNET/SUNET), Claudio

Topolcic (CNRI), and Kannan Varadhan (OARnet) for their review and constructive comments.

References

[1] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Supernetting: an

   Address Assignment and Aggregation Strategy", RFC 1338, BARRNet,
   cicso, Merit, OARnet, June 1992.

[2] Colella, R., Gardner, E, and R. Callon, "Guidelines for OSI NSAP

   Allocation in the Internet", RFC 1237, JuNIST, Mitre, DEC, July
   1991.

[3] Gerich, E., "Guidelines for Management of IP Address Space", RFC

   1466, Merit, May 1993.

[4] Cerf, V., "IAB Recommended Policy on Distributing Internet

   Identifier Assignment and IAB Recommended Policy Change to
   Internet "Connected" Status", RFC 1174, CNRI, August 1990.

Security Considerations

Security issues are not discussed in this memo.

10. Authors' Addresses

Yakov Rekhter T.J. Watson Research Center, IBM Corporation P.O. Box 218 Yorktown Heights, NY 10598

Phone: (914) 945-3896 EMail: [email protected]

Tony Li cisco Systems, Inc. 1525 O'Brien Drive Menlo Park, CA 94025

EMail: [email protected]