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Network Working Group                                      Y. Rekhter
Request for Comments: 2008                                      T. Li
BCP: 7                                                  Cisco Systems
Category: Best Current Practice                          October 1996

              Implications of Various Address Allocation
                     Policies for Internet Routing

Status of this Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

IESG Note:

   The addressing constraints described in this document are largely the
   result of the interaction of existing router technology, address
   assignment, and architectural history.  After extensive review and
   discussion, the authors of this document, the IETF working group that
   reviewed it, and the IESG have concluded that there are no other
   currently deployable technologies available to overcome these
   limitations.  In the event that routing or router technology develops
   to the point that adequate routing aggregation can be achieved by
   other means or that routers can deal with larger routing and more
   dynamic tables, it may be appropriate to review these constraints.

1 Abstract

   IP unicast address allocation and management are essential
   operational functions for the Public Internet. The exact policies for
   IP unicast address allocation and management continue to be the
   subject of many discussions. Such discussions cannot be pursued in a
   vacuum - the participants must understand the technical issues and
   implications associated with various address allocation and
   management policies.

   The purpose of this document is to articulate certain relevant
   fundamental technical issues that must be considered in formulating
   unicast address allocation and management policies for the Public
   Internet, and to provide recommendations with respect to these

   The major focus of this document is on two possible policies,
   "address ownership" and "address lending," and the technical
   implications of these policies for the Public Internet.  For the
   organizations that could provide reachability to a sufficiently large

   fraction of the total destinations in the Internet, and could express
   such reachability through a single IP address prefix the document
   suggests to use the "address ownership" policy. However, applying the
   "address ownership" policy to every individual site or organization
   that connects to the Internet results in a non-scalable routing.

   Consequently, this document also recomments that the "address
   lending" policy should be formally added to the set of address
   allocation policies in the Public Internet. The document also
   recommends that organizations that do not provide a sufficient degree
   of routing information aggregation, but wish to obtain access to the
   Internet routing services should be strongly encouraged to use this
   policy to gain access to the services.

2 On the intrinsic value of IP addresses

   Syntactically, the set of IPv4 unicast addresses is the (finite) set
   of integers in the range 0x00000000 - 0xDFFFFFFF. IP addresses are
   used for Network Layer (IP) routing. An IP address is the sole piece
   of information about the node injected into the routing system.

   The notable semantics of an IP unicast address is its ability to
   interact with the Public Internet routing service and thereby
   exchange data with the remainder of the Internet. In other words, for
   the Public Internet, it is the reachability of an IP address that
   gives it an intrinsic value. Observe, however, that IP addresses are
   used outside of the Public Internet. This document does not cover the
   value of addresses in other than the Public Internet context.

   The above implies that in the Public Internet it is the service
   environment (the Internet) and its continued operation, including its
   routing system, which gives an IP address its intrinsic value, rather
   than the inverse. Consequently, if the Public Internet routing system
   ceases to be operational, the service disappears, and the addresses
   cease to have any functional value in the Internet. At this point,
   for the Public Internet, all address allocation and management
   policies, including existing policies, are rendered meaningless.

3 Hierarchical routing and its implication on address allocation

   Hierarchical routing [Kleinrock 77] is a mechanism that improves the
   scaling properties of a routing system. It is the only proven
   mechanism for scaling routing to the current size of the Internet.

   Hierarchical routing requires that addresses be assigned to reflect
   the actual network topology. Hierarchical routing works by taking the
   set of addresses covered by a portion of the topology, and generating
   a single routing advertisement (route) for the entire set. Further,

   hierarchical routing allows this to be done recursively: multiple
   advertisements (routes) can be combined into a single advertisement
   (route). By exercising this recursion, the amount of information
   necessary to provide routing can be decreased substantially.

   A common example of hierarchical routing is the phone network, where
   country codes, area codes, exchanges, and finally subscriber lines
   are different levels in the hierarchy. In the phone network, a switch
   need not keep detailed routing information about every possible
   subscriber in a distant area code. Instead, the switch usually knows
   one routing entry for the entire area code.

   Notice that the effect on scaling is dramatic. If we look at the
   space complexity of the different schemes, the switch that knows
   about every subscriber in the world needs O(n) space for n worldwide
   subscribers.  Now consider the case of hierarchical routing. We can
   break n down into the number of subscribers in the local area (l),
   the other exchanges in the area code (e), the other area codes in the
   local country code (a) and other country codes (c). Using this
   notation, hierarchical routing has space complexity O(l + e + a + c).
   Notice that each of these factors is much, much less than n, and
   grows very slowly, if at all. This implies that a phone switch can be
   built today that has some hope of not running out of space when it is

   The fundamental property of hierarchical routing that makes this
   scalability possible is the ability to form abstractions: here, the
   ability to group subscribers into exchanges, area codes and country
   codes. Further, such abstractions must provide useful information for
   the ability to do routing. Some abstractions, such as the group of
   users with green phones, are not useful when it comes time to route a

   Since the information that the routing system really needs is the
   location of the address within the topology, for hierarchical
   routing, the useful abstraction must capture the topological location
   of an address within the network. In principle this could be
   accomplished in one of two ways.  Either (a) constrain the topology
   (and allowed topology changes) to match address assignment. Or, (b)
   avoid constraints on the topology (and topology changes), but require
   that as the topology changes, an entity's address change as well. The
   process of changing an entity's address is known as "renumbering."

4 Scaling the Internet routing system

   The enormous growth of the Public Internet places a heavy load on the
   Internet routing system. Before the introduction of CIDR the growth
   rate had doubled the size of the routing table roughly every nine
   months. Capacity of computer technology doubles roughly every 24
   months. Even if we could double the capacities of the routers in the
   Internet every 24 months, inevitably the size of the routing tables
   is going to exceed the limit of the routers. Therefore, to preserve
   uninterrupted continuous growth of the Public Internet, deploying
   mechanisms that contain the growth rate of the routing information is

   Lacking mechanisms to contain the growth rate of the routing
   information, the growth of the Internet would have to be either
   limited or frozen, or the Internet routing system would become
   overloaded. The result of overloading routing is that the routing
   subsystem will fail: either equipment (routers) could not maintain
   enough routes to insure global connectivity, or providers will simply
   exclude certain routes to insure that other routes provide
   connectivity to particular sites. This document assumes that neither
   of the outcomes mentioned in this paragraph is acceptable.

   Classless Inter-Domain Routing (CIDR) [RFC1518, RFC1519] has been
   deployed since late 1992 in the Public Internet as the primary
   mechanism to contain the growth rate of the routing information -
   without CIDR the Internet routing system would have already
   collapsed. For example, in October 1995, within AlterNet (one of the
   major Internet Service Providers) there were 3194 routes. Thanks to
   aggregation, AlterNet advertised only 799 routes to the rest of the
   Internet - a saving of 2395 routes (75%) [Partan 95]. In October 1995
   the Internet Routing Registry (IRR) contained 61,430 unique prefixes
   listed, not counting prefixes marked as withdrawn (or 65,191 prefixes
   with prefixes marked as withdrawn). That is roughly a lower bound
   since many prefixes are not registered in the IRR. CIDR aggregation
   resulted in less than 30,000 routes in the default-free part of the
   Internet routing system [Villamizar 95].

   CIDR is an example of the application of hierarchical routing in the
   Public Internet, where subnets, subscribers, and finally providers
   are some possible levels in the hierarchy. For example, a router
   within a site need not keep detailed routing information about every
   possible host in that site. Instead, the router maintains routing
   information on a per subnet basis. Likewise, a router within a
   provider need not keep detailed routing information about individual
   subnets within its subscribers. Instead, the router could maintain
   routing information on a per subscriber basis. Moreover, a router
   within a provider need not keep detailed routing information about

   stub (single home) subscribers of other providers by maintaining
   routing information on a per provider basis.

   Because of pre-CIDR address allocation, many routes in the Internet
   are not suitable for hierarchical aggregation. Moreover, unconnected
   sites with pre-CIDR address allocations exist. If these sites connect
   to the Internet at some point in the future, the routes to these
   sites are unlikely to be suitable for hierarchical aggregation. Also,
   when a site uses addresses obtain from its provider, but then later
   switches to a different provider (while continuing to use the same
   addresses), the route to the site may no longer be suitable for
   hierarchical aggregation.

   Hierarchical routing requires that aggregation boundaries for the
   addressing information be formed along some hierarchy. As a result,
   many exceptions will be injected into the routing system in the
   future, besides those exceptions that currently exist. Each exception
   added to the routing system deters the scalability of the routing
   system. The exact number of exceptions that can be tolerated is
   dependent on the technology used to support routing. Unbridled growth
   in the number of such exceptions will cause the routing system to

5 Address allocation and management policies

   IP address allocation and management policy is a complex,
   multifaceted issue. It covers a broad range of issues, such as who
   formulates the policies, who executes the policies, what is the role
   of various registries, what is the role of various organizations
   (e.g., ISOC, IAB, IESG, IETF, IEPG, various government bodies, etc.),
   the participation of end users in requesting addresses, and so on.
   Address allocation and management and the scalability of the routing
   system are interrelated - only certain address allocation and
   management policies yield scalable routing. The Internet routing
   system is subject to both technological and fundamental constraints.
   These constraints restrict the choices of address allocation policies
   that are practical.

5.1 The "address ownership" allocation policy and its implications on
   the Public Internet

   "Address ownership" is one possible address allocation and management
   policy. The "address ownership" policy means that part of the address
   space, once allocated to an organization, remains allocated to the
   organization as long as that organization wants it. Further, that
   portion of the address space would not be allocated to any other
   organization.  Often, such addresses are called "portable." It was
   assumed that if an organization acquires its addresses via the

   "address ownership" policy, the organization would be able to use
   these addresses to gain access to the Internet routing services,
   regardless of where the organization connects to the Internet.

   While it has never been explicitly stated that various Internet
   Registries use the "address ownership" allocation policy, it has
   always been assumed (and practiced).

   To understand the implications of the "address ownership" policy
   ("portable" addresses) on the scalability of the Internet routing
   system, one must observe that:

     (a) By definition, address ownership assumes that addresses, once
     assigned, fall under the control of the assignee. It is the
     assignee that decides when to relinquish the ownership (although
     the decision could be influenced by various factors).
     Specifically, the assignee is not required (but may be influenced)
     to relinquish the ownership as the connectivity of the assignee to
     the Internet changes.

     (b) By definition, hierarchical routing assumes that addresses
     reflect the network topology as much as possible.

   Therefore, the only presently known practical way to satisfy both
   scalable hierarchical routing and address ownership for everyone is
   to assume that the topology (or at least certain pieces of it) will
   be permanently fixed. Given the distributed, decentralized, largely
   unregulated, and global (international) nature of the Internet,
   constraining the Internet topology (or even certain parts of it) may
   have broad technical, social, economical, and political implications.
   To date, little is known of what these implications are; even less is
   known whether these implications would be acceptable (feasible) in
   practice. Therefore, at least for now, we have to support an Internet
   with an unconstrained topology (and unconstrained topological

   Since the Internet does not constrain its topology (or allowed
   topology changes), we can either have address ownership for everyone
   or a routable Internet, but not both, or we need to develop and
   deploy new mechanisms (e.g., by decoupling the address owned by the
   end users from those used by the Internet routing, and provide
   mechanisms to translate between the two). In the absence of new
   mechanisms, if we have address ownership ("portable" addresses) for
   everyone, then the routing overhead will lead to a breakdown of the
   routing system resulting in a fragmented (partitioned) Internet.
   Alternately, we can have a routable Internet, but without address
    ownership ("portable" addresses) for everyone.

5.2 The "address lending" allocation policy and its implications for the
   Public Internet

   Recently, especially since the arrival of CIDR, some subscribers and
   providers have followed a model in which address space is not owned
   (not portable), but is bound to the topology. This model suggests an
   address allocation and management policy that differs from the
   "address ownership" policy. The following describes a policy, called
   "address lending," that provides a better match (as compared to the
   "address ownership" policy) to the model.

   An "address lending" policy means that an organization gets its
   addresses on a "loan" basis. For the length of the loan, the lender
   cannot lend the addresses to any other borrower. Assignments and
   allocations based on the "address lending" policy should explicitly
   include the conditions of the loan. Such conditions must specify that
   allocations are returned if the borrower is no longer contractually
   bound to the lender, and the lender can no longer provide aggregation
   for the allocation. If a loan ends, the organization can no longer
   use the borrowed addresses, and therefore must get new addresses and
   renumber to use them. The "address lending" policy does not constrain
   how the new addresses could be acquired.

   This document expects that the "address lending" policy would be used
   primarily by Internet Registries associated with providers; however,
   this document does not preclude the use of the "address lending"
   policy by an Internet Registry that is not associated with a

   This document expects that when the "address lending" policy is used
   by an Internet Registry associated with a provider, the provider is
   responsible for arranging aggregation of these addresses to a degree
   that is sufficient to achieve Internet-wide IP connectivity.

   This document expects that when the "address lending" policy is used
   by an Internet Registry associated with a provider, the terms and
   conditions of the loan would be coupled to the service agreement
   between the provider and the subscribers. That is, if the subscriber
   moves to another provider, the loan would be canceled.

   To reduce disruptions when a subscriber changes its providers, this
   document strongly recommends that the terms and conditions of the
   loan should include provision for a grace period. This provision
   would allow a subscriber that disconnects from its provider a certain
   grace period after the disconnection. During this grace period, the
   borrower (the subscriber) may continue to use the addresses obtained
   under the loan. This document recommends a grace period of at least
   30 days. Further, to contain the routing information overhead, this
   document suggests that a grace period be no longer than six months.

   To understand the scalability implications of the "address lending"
   policy, observe that if a subscriber borrows its addresses from its
   provider's block, then the provider can advertise a single address
   prefix. This reduces the routing information that needs to be carried
   by the Internet routing system (for more information, see Section
   5.3.1 of RFC1518). As the subscriber changes its provider, the loan
   from the old provider would be returned, and the loan from the new
   provider would be established. As a result, the subscriber would
   renumber to the new addresses. Once the subscriber renumbers into the
   new provider's existing blocks, no new routes need to be introduced
   into the routing system.

   Therefore, the "address lending" policy, if applied appropriately, is
   consistent with the constraints on address allocation policies
   imposed by hierarchical routing, and thus promotes a scalable routing
   system.  Thus, the "address lending" policy, if applied
   appropriately, could play an important role in enabling the
   continuous uninterrupted growth of the Internet.

   To be able to scale routing in other parts of the hierarchy, the
   "lending" policy may also be applied hierarchically, so that
   addresses may in turn be lent to other organizations. The implication
   here is that the end of a single loan may have effects on
   organizations that have recursively borrowed parts of the address
   space from the main allocation. In this case, the exact effects are
   difficult to determine a priori.

5.3 In the absence of an explicit "address lending" policy

   Organizations connecting to the Internet should be aware that even if
   their current provider, and the provider they switch to in the future
   do not require renumbering, renumbering may still be needed to
   achieve Internet-wide IP connectivity. For example, an organization
   may now receive Internet service from some provider and allocate its
   addresses out of the CIDR block associated with the provider. Later
   the organization may switch to another provider. The previous
   provider may still be willing to allow the organization to retain
   part of the provider's CIDR block, and accept a more specific prefix

   for that organization from the new provider. Likewise, the new
   provider may be willing to accept that organization without
   renumbering and advertise the more specific prefix (that covers
   destinations within the organization) to the rest of the Internet.
   However, if one or more other providers exist, that are unwilling or
   unable to accept the longer prefix advertised by the new provider,
   then the organization would not have IP connectivity to part of the
   Internet. Among the possible solutions open to the organization may
   be either to renumber, or for others to acquire connectivity to
   providers that are willing and able to accept the prefix.

   The above shows that the absence of an explicit "address lending"
   policy from a current provider in no way ensures that renumbering
   will not be required in the future when changing providers.
   Organizations should be aware of this fact should they encounter a
   provider making claims to the contrary.

6 Recommendations

   Observe that the goal of hierarchical routing in the Internet is not
   to reduce the total amount of routing information in the Internet to
   the theoretically possible minimum, but just to contain the volume of
   routing information within the limits of technology,
   price/performance, and human factors.  Therefore, organizations that
   could provide reachability to a sufficiently large fraction of the
   total destinations in the Internet and could express such
   reachability through a single IP address prefix could expect that a
   route with this prefix will be maintained throughout the default-free
   part of the Internet routing system, regardless of where they connect
   to the Internet.  Therefore, using the "address ownership" policy
   when allocating addresses to such organizations is a reasonable
   choice.  Within such organizations this document suggests the use of
   the "address lending" policy.

   For all other organizations that expect Internet-wide IP
   connectivity, the reachability information they inject into the
   Internet routing system should be subject to hierarchical
   aggregation. For such organizations, allocating addresses based on
   the "address ownership" policy makes hierarchical aggregation
   difficult, if not impossible. This, in turn, has a very detrimental
   effect on the Internet routing system. To prevent the collapse of the
   Internet routing system, for such organizations, this document
   recommends using the "address lending" policy. Consequently, when
   such an organization first connects to the Public Internet or changes
   its topological attachment to the Public Internet, the organization
   eventually needs to renumber. Renumbering allows the organization to
   withdraw any exceptional prefixes that the organization would
   otherwise inject into the Internet routing system. This applies to

   the case where the organization takes its addresses out of its direct
   provider's block and the organization changes its direct provider.
   This may also apply to the case where the organization takes its
   addresses out of its indirect provider's block, and the organization
   changes its indirect provider, or the organization's direct provider
   changes its provider.

   Carrying routing information has a cost associated with it. This
   cost, at some point, may be passed back in full to the organizations
   that inject the routing information. Aggregation of addressing
   information (via CIDR) could reduce the cost, as it allows an
   increase in the number of destinations covered by a single route.
   Organizations whose addresses are allocated based on the "address
   ownership" policy (and thus may not be suitable for aggregation)
   should be prepared to absorb the cost completely on their own.

   Observe that neither the "address ownership," nor the "address
   lending" policy, by itself, is sufficient to guarantee Internet-wide
   IP connectivity. Therefore, we recommend that sites with addresses
   allocated based on either policy should consult their providers about
   the reachability scope that could be achieved with these addresses,
   and associated costs that result from using these addresses.

   If an organization doesn't require Internet-wide IP connectivity,
   then address allocation for the organization could be done based on
   the "address ownership" policy. Here, the organization may still
   maintain limited IP connectivity (e.g., with all the subscribers of
   its direct provider) by limiting the distribution scope of its
   routing information to its direct provider. Connectivity to the rest
   of the Internet can be handled by mediating gateways (e.g.,
   application layer gateways, Network Address Translators (NATs)). Note
   that use of mediating gateways eliminates the need for renumbering,
   and avoids burdening the Internet routing system with non-
   aggregatable addressing information; however they have other
   drawbacks which may prove awkward in certain situations.

   Both renumbering (due to the "address lending" policy), and non-
   aggregated routing information (due to the "address ownership"
   policy), and the use of mediating gateways result in some costs.
   Therefore, an organization needs to analyze its own connectivity
   requirements carefully and compare the tradeoffs associated with
   addresses acquired via either policy vs. having connectivity via
   mediating gateways (possibly augmented by limited IP connectivity)
   using addresses acquired via "address ownership." To reduce the cost
   of renumbering, organizations should be strongly encouraged to deploy
   tools that simplify renumbering (e.g., Dynamic Host Configuration
   Protocol [RFC 1541]). Use of the DNS should be strongly encouraged.

7 Summary

   Any address allocation and management policy for IP addresses used
   for Internet connectivity must take into account its impact on the
   scalability of the Public Internet routing system. Among all of the
   possible address allocation and management policies only the ones
   that yield a scalable routing system are feasible. All other policies
   are self-destructive in nature, as they lead to a collapse of the
   Internet routing system, and therefore to the fragmentation
   (partitioning) of the Public Internet.

   Within the context of the current Public Internet, address allocation
   and management policies that assume unrestricted address ownership
   have an extremely negative impact on the scalability of the Internet
   routing system. Such policies are almost certain to exhaust the
   scalability of the Internet routing system well before we approach
   the exhaustion of the IPv4 address space and before we can make
   effective use of the IPv6 address space. Given the Internet's growth
   rate and current technology, the notion that everyone can own address
   space and receive Internet-wide routing services, despite where they
   connect to the Internet, is currently technically infeasible.
   Therefore, this document makes two recommendations. First, the
   "address lending" policy should be formally added to the set of
   address allocation policies in the Public Internet. Second,
   organizations that do not provide a sufficient degree of routing
   information aggregation to obtain access to the Internet routing
   services should be strongly encouraged to use this policy to gain
   access to the services.

   Since the current IPv6 address allocation architecture is based on
   CIDR, recommendations presented in this document apply to IPv6
   address allocation and management policies as well.

8 Security Considerations

   Renumbering a site has several possible implications on the security
   policies of both the site itself and sites that regularly communicate
   with the renumbering sites.

   Many sites currently use "firewall" systems to provide coarse-grained
   access control from external networks, such as The Internet, to their
   internal systems.  Such firewalls might include access control
   decisions based on the claimed source address of packets arriving at
   such firewall systems.  When the firewall policy relates to packets
   arriving on the firewall from inside the site, then that firewall
   will need to be reconfigured at the same time that the site itself
   renumbers.  When the firewall policy relates to packets arriving at
   the firewall from outside the site, then such firewalls will need to

   be reconfigured whenever an outside site that is granted any access
   inside the site through the firewall is renumbered.

   It is highly inadvisable to rely upon unauthenticated source or
   destination IP addresses for security policy decisions. [Bellovin89]
   IP address spoofing is not difficult with widely available systems,
   such as personal computers.  A better approach would probably involve
   the use of IP Security techniques, such as the IP Authentication
   Header [RFC-1826] or IP Encapsulating Security Payload [RFC-1827], at
   the firewall so that the firewall can rely on cryptographic
   techniques for identification when making its security policy

   It is strongly desirable that authentication be present in any
   mechanism used to renumber IP nodes.  A renumbering mechanism that
   lacks authentication could be used by an adversary to renumber
   systems that should not have been renumbered, for example.

   There may be other security considerations that are not covered in
   this document.

9 Acknowledgments

   This document borrows heavily from various postings on various
   mailing lists. Special thanks to Noel Chiappa, Dennis Ferguson, Eric
   Fleischman, Geoff Huston, and Jon Postel whose postings were used in
   this document.

   Most of the Section 5.3 was contributed by Curtis Villamizar.  The
   Security section was contributed by Ran Atkinson.

   Many thanks to Scott Bradner, Randy Bush, Brian Carpenter, Noel
   Chiappa, David Conrad, John Curran, Sean Doran, Dorian Kim, Thomas
   Narten, Andrew Partan, Dave Piscitello, Simon Poole, Curtis
   Villamizar, and Nicolas Williams for their review, comments, and
   contributions to this document.

   Finally, we like to thank all the members of the CIDR Working Group
   for their review and comments.

9 References

   [Bellovin89] Bellovin, S., "Security Problems in the TCP/IP Protocol
   Suite", ACM Computer Communications Review, Vol. 19, No. 2, March

   [Kleinrock 77] Kleinrock, L., and K. Farouk, K., "Hierarchical
   Routing for Large Networks," Computer Networks 1 (1977), North-
   Holland Publishing Company.

   [Partan 95] Partan, A., private communications, October 1995.

   [RFC 1541] Droms, R., "Dynamic Host Configuration Protocol", October

   [RFC 1519] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless
   Inter-Domain Routing (CIDR): an Address Assignment and Aggregation
   Strategy", September 1993.

   [RFC 1518] Rekhter, Y., and T. Li, "An Architecture for IP Address
   Allocation with CIDR", September 1993.

   [RFC 1825] Atkinson, R., "IP Security Architecture", RFC 1825, August

   [RFC 1826] Atkinson, R., "IP Authentication Header (AH), RFC 1826,
   August 1995.

   [RFC 1827] Atkinson, R., "IP Encapsulating Security Payload (ESP)",
   RFC 1827, August 1995.

   [Villamizar 95] Villamizar, C., private communications, October 1995.

10 Authors' Addresses

      Yakov Rekhter
      cisco Systems, Inc.
      170 Tasman Dr.
      San Jose, CA 95134
      Phone: (914) 528-0090
      EMail: yakov@cisco.com

      Tony Li
      cisco Systems, Inc.
      170 Tasman Dr.
      San Jose, CA 95134
      Phone: (408) 526-8186
      EMail: tli@cisco.com


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