rfc2008.txt

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   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
   collapse.

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



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   "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
   changes).

   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.




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RFC 2008                                                    October 1996


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
   provider.

   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.












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   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



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   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.