rfc1519.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                          V. Fuller
Request for Comments: 1519                                       BARRNet
Obsoletes: 1338                                                    T. Li
Category: Standards Track                                          cisco
                                                                   J. Yu
                                                                   MERIT
                                                             K. Varadhan
                                                                  OARnet
                                                          September 1993


                 Classless Inter-Domain Routing (CIDR):
             an Address Assignment and Aggregation Strategy

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.

Abstract

   This memo discusses strategies for address assignment of the existing
   IP address space with a view to conserve the address space and stem
   the explosive growth of routing tables in default-route-free routers.

Table of Contents

   Acknowledgements .................................................  2
   1.  Problem, Goal, and Motivation ................................  2
   2.  CIDR address allocation ......................................  3
   2.1  Aggregation and its limitations .............................  3
   2.2  Distributed network number allocation .......................  5
   3.  Cost-benefit analysis ........................................  6
   3.1  Present allocation figures ..................................  7
   3.2  Historic growth rates .......................................  8
   3.3  Detailed analysis ...........................................  8
   3.3.1  Benefits of new addressing plan ...........................  9
   3.3.2  Growth rate projections ...................................  9
   4.  Changes to inter-domain routing protocols and practices ...... 11
   4.1  Protocol-independent semantic changes ....................... 11
   4.2  Rules for route advertisement ............................... 11
   4.3  How the rules work .......................................... 13
   4.4  Responsibility for and configuration of aggregation ......... 14
   4.5  Intra-domain protocol considerations ........................ 15
   5.  Example of new allocation and routing ........................ 15



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RFC 1519                 CIDR Address Strategy            September 1993


   5.1  Address allocation .......................................... 15
   5.2  Routing advertisements ...................................... 17
   6.  Extending CIDR to class A addresses .......................... 18
   7.  Domain Naming Service considerations ......................... 20
   7.1 Procedural changes for class-C "supernets" ................... 20
   7.2 Procedural changes for class-A subnetting .................... 21
   8.  Transitioning to a long term solution ........................ 22
   9.  Conclusions .................................................. 22
   10.  Recommendations ............................................. 22
   11.  References .................................................. 23
   12.  Security Considerations ..................................... 23
   13.  Authors' Addresses .......................................... 24

Acknowledgements

   The authors wish to express their appreciation to the members of the
   ROAD group with whom many of the ideas contained in this document
   were inspired and developed.

1.  Problem, Goal, and Motivation

   As the Internet has evolved and grown over in recent years, it has
   become evident that it is soon to face several serious scaling
   problems. These include:

      1.   Exhaustion of the class B network address space. One
           fundamental cause of this problem is the lack of a network
           class of a size which is appropriate for mid-sized
           organization; class C, with a maximum of 254 host
           addresses, is too small, while class B, which allows up to
           65534 addresses, is too large for most organizations.

      2.   Growth of routing tables in Internet routers beyond the
           ability of current software, hardware, and people to
           effectively manage.

      3.   Eventual exhaustion of the 32-bit IP address space.

   It has become clear that the first two of these problems are likely
   to become critical within the next one to three years.  This memo
   attempts to deal with these problems by proposing a mechanism to slow
   the growth of the routing table and the need for allocating new IP
   network numbers. It does not attempt to solve the third problem,
   which is of a more long-term nature, but instead endeavors to ease
   enough of the short to mid-term difficulties to allow the Internet to
   continue to function efficiently while progress is made on a longer-
   term solution.




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RFC 1519                 CIDR Address Strategy            September 1993


   The proposed solution is to topologically allocate future IP address
   assignment, by allocating segments of the IP address space to the
   transit routing domains.

   This plan for allocating IP addresses should be undertaken as soon as
   possible.  We believe that this will suffice as a short term
   strategy, to fill the gap between now and the time when a viable long
   term plan can be put into place and deployed effectively.  This plan
   should be viable for at least three (3) years, after which time,
   deployment of a suitable long term solution is expected to occur.

   This plan is primarily directed at the first two problems listed
   above.  We believe that the judicious use of variable-length
   subnetting techniques should help defer the onset of the last problem
   problem, the exhaustion of the 32-bit address space. Note also that
   improved tools for performing address allocation in a "supernetted"
   and variably-subnetted world would greatly help the user community in
   accepting these sometimes confusing techniques. Efforts to create
   some simple tools for this purpose should be encouraged by the
   Internet community.

   Note that this plan neither requires nor assumes that already
   assigned addresses will be reassigned, though if doing so were
   possible, it would further reduce routing table sizes. It is assumed
   that routing technology will be capable of dealing with the current
   routing table size and with some reasonably small rate of growth.
   The emphasis of this plan is on significantly slowing the rate of
   this growth.

   Note that this plan does not require domains to renumber if they
   change their attached transit routing domain.  Domains are encouraged
   to renumber so that their individual address allocations do not need
   to be advertised.

   This plan will not affect the deployment of any specific long term
   plan, and therefore, this document will not discuss any long term
   plans for routing and address architectures.

2.  CIDR address allocation

   There are two basic components of this addressing and routing plan:
   one, to distribute the allocation of Internet address space and two,
   to provide a mechanism for the aggregation of routing information.

   2.1  Aggregation and its limitations

   One major goal of this addressing plan is to allocate Internet
   address space in such a manner as to allow aggregation of routing



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   information along topological lines.  For simple, single-homed
   clients, the allocation of their address space out of a transit
   routing domain's space will accomplish this automatically - rather
   than advertise a separate route for each such client, the transit
   domain may advertise a single aggregate route which describes all of
   the destinations connected to it. Unfortunately, not all sites are
   singly-connected to the network, so some loss of ability to aggregate
   is realized for the non-trivial cases.

   There are two situations that cause a loss of aggregation efficiency.

      o    Organizations which are multi-homed. Because multi-homed
           organizations must be advertised into the system by each of
           their service providers, it is often not feasible to
           aggregate their routing information into the address space
           any one of those providers. Note that they still may receive
           their address allocation out of a transit domain's address
           space (which has other advantages), but their routing
           information must still be explicitly advertised by most of
           their service providers (the exception being that if the
           site's allocation comes out of its least-preferable service
           provider, then that service provider need not advertise the
           explicit route - longest-match will insure that its
           aggregated route is used to get to the site on a backup
           basis).  For this reason, the routing cost for these
           organizations will typically be about the same as it is
           today.

      o    Organizations which change service provider but do not
           renumber. This has the effect of "punching a hole" in the
           aggregation of the original service provider's advertisement.
           This plan will handle the situation by requiring the newer
           service provider to advertise a specific advertisement for
           the new client, which is preferred by virtue of being the
           longest match.  To maintain efficiency of aggregation, it is
           recommended that organizations which do change service
           providers plan to eventually migrate their address
           assignments from the old provider's space to that of the new
           provider. To this end, it is recommended that mechanisms to
           facilitate such migration, including improved protocols and
           procedures for dynamic host address assignment, be developed.

   Note that some aggregation efficiency gain can still be had for
   multi-homed sites (and, in general, for any site composed of
   multiple, logical IP network numbers) - by allocating a contiguous
   power-of-two block of network numbers to the client (as opposed to
   multiple, independent network numbers) the client's routing
   information may be aggregated into a single (net, mask) pair. Also,



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RFC 1519                 CIDR Address Strategy            September 1993


   since the routing cost associated with assigning a multi-homed site
   out of a service provider's address space is no greater than the
   current method of a random allocation by a central authority, it
   makes sense to allocate all address space out of blocks assigned to
   service providers.

   It is also worthwhile to mention that since aggregation may occur at
   multiple levels in the system, it may still be possible to aggregate
   these anomalous routes at higher levels of whatever hierarchy may be
   present. For example, if a site is multi-homed to two NSFNET regional
   networks both of whom obtain their address space from the NSFNET,
   then aggregation by the NSFNET of routes from the regionals will
   include all routes to the multi-homed site.

   Finally, it should also be noted that deployment of the new
   addressing plan described in this document may (and should) begin
   almost immediately but effective use of the plan to aggregate routing
   information will require changes to some Inter-Domain routing
   protocols. Likewise, deploying classless Inter-Domain protocols
   without deployment of the new address plan will not allow useful
   aggregation to occur (in other words, the addressing plan and routing
   protocol changes are both required for supernetting, and its
   resulting reduction in table growth, to be effective.)  Note,
   however, that during the period of time between deployment of the
   addressing plan and deployment of the new protocols, the size of
   routing tables may temporarily grow very rapidly. This must be
   considered when planning the deployment of the two plans.

   Note: in the discussion and examples which follow, the network and
   mask notation is used to represent routing destinations. This is used
   for illustration only and does not require that routing protocols use
   this representation in their updates.

   2.2  Distributed allocation of address space

   The basic idea of the plan is to allocate one or more blocks of Class
   C network numbers to each network service provider. Organizations
   using the network service provider for Internet connectivity are
   allocated bitmask-oriented subsets of the provider's address space as
   required.

   It is also worthwhile to mention that once inter-domain protocols
   which support classless network destinations are widely deployed, the
   rules described by this plan generalize to permit arbitrary
   super/subnetting of the remaining class A and class B address space
   (the assumption being that classless inter-domain protocols will
   either allow for non-contiguous subnets to exist in the system or
   that all components of a sub-allocated class A/B will be contained



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RFC 1519                 CIDR Address Strategy            September 1993


   within a single routing domain). This will allow this plan to
   continue to be used in the event that the class C space is exhausted
   before implementation of a long-term solution is deployed.  This
   alternative is discussed further below in section 6.

   Hierarchical sub-allocation of addresses in this manner implies that
   clients with addresses allocated out of a given service provider are,
   for routing purposes, part of that service provider and will be
   routed via its infrastructure. This implies that routing information
   about multi-homed organizations, i.e., organizations connected to
   more than one network service provider, will still need to be known
   by higher levels in the hierarchy.

   The advantages of hierarchical assignment in this fashion are

      a)  It is expected to be easier for a relatively small number of
          service providers to obtain addresses from the central
          authority, rather than a much larger, and monotonically
          increasing, number of individual clients.  This is not to be
          considered as a loss of part of the service providers' address
          space.

      b)  Given the current growth of the Internet, a scalable and
          delegatable method of future allocation of network numbers has
          to be achieved.

   For these reasons, and in the interest of providing a consistent
   procedure for obtaining Internet addresses, it is recommended that
   most, if not all, network numbers be distributed through service
   providers.  These issues are discussed in much greater length in [2].

3.  Cost-benefit analysis

   This new method of assigning address through service providers can be
   put into effect immediately and will, from the start, have the
   benefit of distributing the currently centralized process of
   assigning new addresses. Unfortunately, before the benefit of
   reducing the size of globally-known routing destinations can be
   achieved, it will be necessary to deploy an Inter-Domain routing
   protocol capable of handling arbitrary network and mask pairs. Only
   then will it be possible to aggregate individual class C networks
   into larger blocks represented by single routing table entries.

   This means that upon introduction, the new addressing allocation plan
   will not in and of itself help solve the routing table size problem.
   Once the new Inter-Domain routing protocol is deployed, however, an
   immediate drop in the number of destinations which clients of the new
   protocol must carry will occur.  A detailed analysis of the magnitude