GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc1519

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

Fuller, Li, Yu & Varadhan [Page 1] 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.

Fuller, Li, Yu & Varadhan [Page 2] 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

Fuller, Li, Yu & Varadhan [Page 3] RFC 1519 CIDR Address Strategy September 1993

 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,

Fuller, Li, Yu & Varadhan [Page 4] 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

Fuller, Li, Yu & Varadhan [Page 5] 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

Fuller, Li, Yu & Varadhan [Page 6] RFC 1519 CIDR Address Strategy September 1993

 of this expected drop and the permanent reduction in rate of growth
 is given in the next section.
 In should also be noted that the present method of flat address
 allocations imposes a large bureaucratic cost on the central address
 allocation authority. For scaling reasons unrelated to address space
 exhaustion or routing table overflow, this should be changed. Using
 the mechanism proposed in this paper will have the fortunate side
 effect of distributing the address allocation procedure, greatly
 reducing the load on the central authority.
 3.1  Present Allocation Figures
 An informal analysis of "network-contacts.txt" (available from the
 DDN NIC) indicates that as of 2/25/92, 46 of 126 class A network
 numbers have been allocated (leaving 81) and 5467 of 16382 class B
 numbers have been allocated, leaving 10915. Assuming that recent
 trends continue, the number of allocated class B's will continue to
 double approximately once a year.  At this rate of growth, all class
 B's will be exhausted within about 15 months.  As of 1/13/93, 52
 class A network numbers have been allocated and 7133 class B's have
 been allocated.  We suggest that the change in the class B allocation
 rate is due to the initial deployment of this address allocation
 plan.

Fuller, Li, Yu & Varadhan [Page 7] RFC 1519 CIDR Address Strategy September 1993

 3.2  Historic growth rates
    MM/YY     ROUTES                        MM/YY     ROUTES
              ADVERTISED                              ADVERTISED
    ------------------------                -----------------------
    Dec-92    8561                          Sep-90    1988
    Nov-92    7854                          Aug-90    1894
    Oct-92    7354                          Jul-90    1727
    Sep-92    6640                          Jun-90    1639
    Aug-92    6385                          May-90    1580
    Jul-92    6031                          Apr-90    1525
    Jun-92    5739                          Mar-90    1038
    May-92    5515                          Feb-90    997
    Apr-92    5291                          Jan-90    927
    Mar-92    4976                          Dec-89    897
    Feb-92    4740                          Nov-89    837
    Jan-92    4526                          Oct-89    809
    Dec-91    4305                          Sep-89    745
    Nov-91    3751                          Aug-89    650
    Oct-91    3556                          Jul-89    603
    Sep-91    3389                          Jun-89    564
    Aug-91    3258                          May-89    516
    Jul-91    3086                          Apr-89    467
    Jun-91    2982                          Mar-89    410
    May-91    2763                          Feb-89    384
    Apr-91    2622                          Jan-89    346
    Mar-91    2501                          Dec-88    334
    Feb-91    2417                          Nov-88    313
    Jan-91    2338                          Oct-88    291
    Dec-90    2190                          Sep-88    244
    Nov-90    2125                          Aug-88    217
    Oct-90    2063                          Jul-88    173
          Table I : Growth in routing table size, total numbers
                    Source for the routing table size data is MERIT
 3.3   Detailed Analysis
 There is a small technical cost and minimal administrative cost
 associated with deployment of the new address assignment plan. The
 administrative cost is basically that of convincing the NIC, the
 IANA, and the network service providers to agree to this plan, which
 is not expected to be too difficult.  In addition, administrative
 cost for the central numbering authorities (the NIC and the IANA)
 will be greatly decreased by the deployment of this plan.  To take
 advantage of aggregation of routing information, however, it is
 necessary that the capability to represent routes as arbitrary
 network and mask fields (as opposed to the current class A/B/C

Fuller, Li, Yu & Varadhan [Page 8] RFC 1519 CIDR Address Strategy September 1993

 distinction) be added to the common Internet inter-domain routing
 protocol(s).  Thus, the technical cost is in the implementation of
 classless interdomain routing protocols.
 3.3.1 Benefits of the new addressing plan
 There are two benefits to be had by deploying this plan:
    o    The current problem with depletion of the available class B
         address space can be ameliorated by assigning more-
         appropriately sized blocks of class C's to mid-sized
         organizations (in the 200-4000 host range).
    o    When the improved inter-domain routing protocol is deployed,
         an immediate decrease in the number routing table entries
         should occur, followed by a significant reduction in the rate
         growth of routing table size (for default-free routers).
 3.3.2 Growth rate projections
 As of Jan '92, a default-free routing table (for example, the routing
 tables maintained by the routers in the NSFNET backbone) contained
 approximately 4700 entries. This number reflects the current size of
 the NSFNET routing database. Historic data shows that this number, on
 average, has doubled every 10 months between 1988 and 1991. Assuming
 that this growth rate is going to persist in the foreseeable future
 (and there is no reason to assume otherwise), we expect the number of
 entries in a default-free routing table to grow to approximately
 30000 in two years time.  In the following analysis, we assume that
 the growth of the Internet has been, and will continue to be,
 exponential.
 It should be stressed that these projections do not consider that the
 current shortage of class B network numbers may increase the number
 of instances where many class C's are used rather than a class B.
 Using an assumption that new organizations which formerly obtained
 class B's will now obtain somewhere between 4 and 16 class C's, the
 rate of routing table growth can conservatively be expected to at
 least double and probably quadruple. This means the number of entries
 in a default-free routing table may well exceed 10,000 entries within
 six months and 20,000 entries in less than a year.
 As of Dec '92, the routing table contains 8500 routes.  The original
 growth curves would predict over 9400 routes.  At this time, it is
 not clear if this would indicate a significant change in the rate of
 growth.
 Under the proposed plan, growth of the routing table in a default-

Fuller, Li, Yu & Varadhan [Page 9] RFC 1519 CIDR Address Strategy September 1993

 free router is greatly reduced since most new address assignment will
 come from one of the large blocks allocated to the service providers.
 For the sake of this analysis, we assume prompt implementation of
 this proposal and deployment of the revised routing protocols. We
 make the initial assumption that any initial block given to a
 provider is sufficient to satisfy its needs for two years.
 Since under this plan, multi-homed networks must continue to be
 explicitly advertised throughout the system (according to Rule #1
 described in section 4.2), the number multi-homed routes is expected
 to be the dominant factor in future growth of routing table size,
 once the supernetting plan is applied.
 Presently, it is estimated that there are fewer than 100 multi-homed
 organizations connected to the Internet. Each such organization's
 network is comprised of one or more network numbers.  In many cases
 (and in all future cases under this plan), the network numbers used
 by an organization are consecutive, meaning that aggregation of those
 networks during route advertisement may be possible. This means that
 the number of routes advertised within the Internet for multi-homed
 networks may be approximated as the total number of multi-homed
 organizations.  Assuming that the number of multi-homed organization
 will double every year (which may be a over-estimation, given that
 every connection costs money), the number of routes for multi-homed
 networks would be expected to grow to approximately 800 in three
 years.
 If we further assume that there are approximately 100 service
 providers, then each service provider will also need to advertise its
 block of addresses.  However, due to aggregation, these
 advertisements will be reduced to only 100 additional routes.  We
 assume that after the initial two years, new service providers
 combined with additional requests from existing providers will
 require an additional 50 routes per year.  Thus, the total is 4700 +
 800 + 150 = 5650.  This represents an annual growth rate of
 approximately 6%.  This is in clear contrast to the current annual
 growth of 130%.  This analysis also assumes an immediate deployment
 of this plan with full compliance. Note that this analysis assumes
 only a single level of route aggregation in the current Internet -
 intelligent address allocation should significantly improve this.
 Clearly, this is not a very conservative assumption in the Internet
 environment nor can 100% adoption of this proposal be expected.
 Still, with only a 90% participation in this proposal by service
 providers, at the end of the target three years, global routing table
 size will be "only" 4700 + 800 + 145 + 7500 = 13145 routes -- without
 any action, the routing table will grow to approximately 75000 routes
 during that time period.

Fuller, Li, Yu & Varadhan [Page 10] RFC 1519 CIDR Address Strategy September 1993

4. Changes to inter-domain routing protocols and practices

 In order to support supernetting efficiently, it is clear that some
 changes will need to be made to both routing protocols themselves and
 to the way in which routing information is interpreted. In the case
 of "new" inter-domain protocols, the actual protocol syntax changes
 should be relatively minor. This mechanism will not work with older
 inter-domain protocols such as EGP2; the only ways to interoperate
 with old systems using such protocols are either to use existing
 mechanisms for providing "default" routes or b) require that new
 routers talking to old routers "explode" supernet information into
 individual network numbers.  Since the first of these is trivial
 while the latter is cumbersome (at best -- consider the memory
 requirements it imposes on the receiver of the exploded information),
 it is recommended that the first approach be used -- that older
 systems to continue to the mechanisms they currently employ for
 default handling.
 Note that a basic assumption of this plan is that those organizations
 which need to import "supernet" information into their routing
 systems must run IGPs (such as OSPF [1]) which support classless
 routes. Systems running older IGPs may still advertise and receive
 "supernet" information, but they will not be able to propagate such
 information through their routing domains.
 4.1  Protocol-independent semantic changes
 There are two fundamental changes which must be applied to Inter-
 Domain routing protocols in order for this plan to work. First, the
 concept of network "class" needs to be deprecated - this plan assumes
 that routing destinations are represented by network and mask pairs
 and that routing is done on a longest-match basis (i.e., for a given
 destination which matches multiple network+mask pairs, the match with
 the longest mask is used).  Second, current inter-domain protocols
 generally do not support the concept of route aggregation, so the new
 semantics need to be implemented in a new set of inter-domain
 protocols. In particular, when doing aggregation, dealing with
 multi-homed sites or destinations which change service providers is
 difficult. Fortunately, it is possible to define several fairly
 simple rules for dealing with such cases.
 4.2.  Rules for route advertisement
    1.   Routing to all destinations must be done on a longest-match
         basis only.  This implies that destinations which are multi-
         homed relative to a routing domain must always be explicitly
         announced into that routing domain - they cannot be summarized
         (this makes intuitive sense - if a network is multi-homed, all

Fuller, Li, Yu & Varadhan [Page 11] RFC 1519 CIDR Address Strategy September 1993

         of its paths into a routing domain which is "higher" in the
         hierarchy of networks must be known to the "higher" network).
    2.   A routing domain which performs summarization of multiple
         routes must discard packets which match the summarization but
         do not match any of the explicit routes which makes up the
         summarization. This is necessary to prevent routing loops in
         the presence of less-specific information (such as a default
         route).  Implementation note - one simple way to implement
         this rule would be for the border router to maintain a "sink"
         route for each of its aggregations. By the rule of longest
         match, this would cause all traffic destined to components of
         the aggregation which are not explicitly known to be
         discarded.
 Note that during failures, partial routing of traffic to a site which
 takes its address space from one service provider but which is
 actually reachable only through another (i.e., the case of a site
 which has change service providers) may occur because such traffic
 will be routed along the path advertised by the aggregated route.
 Rule #2 will prevent any real problem from occurring by forcing such
 traffic to be discarded by the advertiser of the aggregated route,
 but the output of "traceroute" and other similar tools will suggest
 that a problem exists within the service provider advertising the
 aggregate, which may be confusing to network operators (see the
 example in section 5.2 for details). Solutions to this problem appear
 to be challenging and not likely to be implementable by current
 Inter-Domain protocols within the time-frame suggested by this
 document. This decision may need to be revisited as Inter-Domain
 protocols evolve.
 An implementation following these rules should also be generalized,
 so that an arbitrary network number and mask are accepted for all
 routing destinations.  The only outstanding constraint is that the
 mask must be left contiguous.  Note that the degenerate route 0.0.0.0
 mask 0.0.0.0 is used as a default route and MUST be accepted by all
 implementations.  Further, to protect against accidental
 advertisements of this route via the inter-domain protocol, this
 route should never be advertised unless there is specific
 configuration information indicating to do so.
 Systems which process route announcements must also be able to verify
 that information which they receive is correct. Thus, implementations
 of this plan which filter route advertisements must also allow masks
 in the filter elements.  To simplify administration, it would be
 useful if filter elements automatically allowed more specific network
 numbers and masks to pass in filter elements given for a more general
 mask.  Thus, filter elements which looked like:

Fuller, Li, Yu & Varadhan [Page 12] RFC 1519 CIDR Address Strategy September 1993

      accept 128.32.0.0
      accept 128.120.0.0
      accept 134.139.0.0
      deny 36.2.0.0
      accept 36.0.0.0
 would look something like:
      accept 128.32.0.0 255.255.0.0
      accept 128.120.0.0 255.255.0.0
      accept 134.139.0.0 255.255.0.0
      deny 36.2.0.0 255.255.0.0
      accept 36.0.0.0 255.0.0.0
 This is merely making explicit the network mask which was implied by
 the class A/B/C classification of network numbers.
 4.3.  How the rules work
 Rule #1 guarantees that the routing algorithm used is consistent
 across implementations and consistent with other routing protocols,
 such as OSPF. Multi-homed networks are always explicitly advertised
 by every service provider through which they are routed even if they
 are a specific subset of one service provider's aggregate (if they
 are not, they clearly must be explicitly advertised). It may seem as
 if the "primary" service provider could advertise the multi-homed
 site implicitly as part of its aggregate, but the assumption that
 longest-match routing is always done causes this not to work.
 Rule #2 guarantees that no routing loops form due to aggregation.
 Consider a mid-level network which has been allocated the 2048 class
 C networks starting with 192.24.0.0 (see the example in section 5 for
 more on this).  The mid-level advertises to a "backbone"
 192.24.0.0/255.248.0.0. Assume that the "backbone", in turn, has been
 allocated the block of networks 192.0.0.0/255.0.0.0. The backbone
 will then advertise this aggregate route to the mid-level. Now, if
 the mid-level loses internal connectivity to the network
 192.24.1.0/255.255.255.0 (which is part of its aggregate), traffic
 from the "backbone" to the mid-level to destination 192.24.1.1 will
 follow the mid-level's advertised route. When that traffic gets to
 the mid-level, however, the mid-level *must not* follow the route
 192.0.0.0/255.0.0.0 it learned from the backbone, since that would
 result in a routing loop. Rule #2 says that the mid-level may not
 follow a less-specific route for a destination which matches one of
 its own aggregated routes. Note that handling of the "default" route
 (0.0.0.0/0.0.0.0) is a special case of this rule - a network must not
 follow the default to destinations which are part of one of it's
 aggregated advertisements.

Fuller, Li, Yu & Varadhan [Page 13] RFC 1519 CIDR Address Strategy September 1993

 4.4.  Responsibility for and configuration of aggregation
 The domain which has been allocated a range of addresses has the sole
 authority for aggregation of its address space.  In the usual case,
 the AS will install manual configuration commands in its border
 routers to aggregate some portion of its address space.  An domain
 can also delegate aggregation authority to another domain.  In this
 case, aggregation is done in the other domain by one of its border
 routers.
 When an inter-domain border router performs route aggregation, it
 needs to know the range of the block of IP addresses to be
 aggregated.  The basic principle is that it should aggregate as much
 as possible but not to aggregate those routes which cannot be treated
 as part of a single unit due to multi-homing, policy, or other
 constraints.
 One mechanism is to do aggregation solely based on dynamically
 learned routing information. This has the danger of not specifying a
 precise enough range since when a route is not present, it is not
 always possible to distinguish whether it is temporarily unreachable
 or that it does not belong in the aggregate. Purely dynamic routing
 also does not allow the flexibility of defining what to aggregate
 within a range. The other mechanism is to do all aggregation based on
 ranges of blocks of IP addresses preconfigured in the router.  It is
 recommended that preconfiguration be used, since it more flexible and
 allows precise specification of the range of destinations to
 aggregate.
 Preconfiguration does require some manually-maintained configuration
 information, but not excessively more so than what router
 administrators already maintain today. As an addition to the amount
 of information that must be typed in and maintained by a human,
 preconfiguration is just a line or two defining the range of the
 block of IP addresses to aggregate. In terms of gathering the
 information, if the advertising router is doing the aggregation, its
 administrator knows the information because the aggregation ranges
 are assigned to its domain.  If the receiving domain has been granted
 the authority to and task of performing aggregation, the information
 would be known as part of the agreement to delegate aggregation.
 Given that it is common practice that a network administrator learns
 from its neighbor which routes it should be willing to accept,
 preconfiguration of aggregation information does not introduce
 additional administrative overhead.
 Implementation note: aggregates which encompass the class D address
 space (multicast addresses) are currently not well understood.  At
 present, it appears that the optimal strategy is to consider

Fuller, Li, Yu & Varadhan [Page 14] RFC 1519 CIDR Address Strategy September 1993

 aggregates to never encompass class D space, even if they do so
 numerically.
 4.5  Intra-domain protocol considerations
 While no changes need be made to internal routing protocols to
 support the advertisement of aggregated routing information between
 autonomous systems, it is often the case that external routing
 information is propagated within interior protocols for policy
 reasons or to aid in the propagation of information through a transit
 network. At the point when aggregated routing information starts to
 appear in the new exterior protocols, this practice of importing
 external information will have to be modified.  A transit network
 which imports external information will have to do one of:
    a) use an interior protocol which supports aggregated routing
    b) find some other method of propagating external information
       which does not involve flooding it through the interior
       protocol (i.e., by the use of internal BGP, for example).
    c) stop the importation of external information and flood a
       "default" route through the internal protocol for discovery
       of paths to external destinations.
 For case (a), the modifications necessary to a routing protocol to
 allow it to support aggregated information may not be simple. For
 protocols such as OSPF and IS-IS, which represent routing information
 as either a destination+mask (OSPF) or as a prefix+prefix-length
 (IS-IS) changes to support aggregated information are conceptually
 fairly simple; for protocols which are dependent on the class-A/B/C
 nature of networks or which support only fixed-sized subnets, the
 changes are of a more fundamental nature. Even in the "conceptually
 simple" cases of OSPF and IS-IS, an implementation may need to be
 modified to support supernets in the database or in the forwarding
 table.

5. Example of new allocation and routing

 5.1  Address allocation
 Consider the block of 2048 class C network numbers beginning with
 192.24.0.0 (0xC0180000 and ending with 192.31.255.0 (0xC01FFF00)
 allocated to a single network provider, "RA". A "supernetted" route
 to this block of network numbers would be described as 192.24.0.0
 with mask of 255.248.0.0 (0xFFF80000).

Fuller, Li, Yu & Varadhan [Page 15] RFC 1519 CIDR Address Strategy September 1993

 Assume this service provider connects six clients in the following
 order (significant because it demonstrates how temporary "holes" may
 form in the service provider's address space):
     "C1" requiring fewer than 2048 addresses (8 class C networks)
     "C2" requiring fewer than 4096 addresses (16 class C networks)
     "C3" requiring fewer than 1024 addresses (4 class C networks)
     "C4" requiring fewer than 1024 addresses (4 class C networks)
     "C5" requiring fewer than 512 addresses (2 class C networks)
     "C6" requiring fewer than 512 addresses (2 class C networks)
 In all cases, the number of IP addresses "required" by each client is
 assumed to allow for significant growth. The service provider
 allocates its address space as follows:
     C1: allocate 192.24.0 through 192.24.7. This block of networks is
         described by the "supernet" route 192.24.0.0 and mask
         255.255.248.0
     C2: allocate 192.24.16 through 192.24.31. This block is described
         by the route 192.24.16.0, mask 255.255.240.0
     C3: allocate 192.24.8 through 192.24.11. This block is described
         by the route 192.24.8.0, mask 255.255.252.0
     C4: allocate 192.24.12 through 192.24.15. This block is described
         by the route 192.24.12.0, mask 255.255.252.0
     C5: allocate 192.24.32 and 192.24.33. This block is described by
         the route 192.24.32.0, mask 255.255.254.0
     C6: allocate 192.24.34 and 192.24.35. This block is described by
         the route 192.24.34.0, mask 255.255.254.0
 Note that if the network provider uses an IGP which can support
 classless networks, he can (but doesn't have to) perform
 "supernetting" at the point where he connects to his clients and
 therefore only maintain six distinct routes for the 36 class C
 network numbers. If not, explicit routes to all 36 class C networks
 will have to be carried by the IGP.
 To make this example more realistic, assume that C4 and C5 are
 multi-homed through some other service provider, "RB". Further assume

Fuller, Li, Yu & Varadhan [Page 16] RFC 1519 CIDR Address Strategy September 1993

 the existence of a client "C7" which was originally connected to "RB"
 but has moved to "RA". For this reason, it has a block of network
 numbers which are allocated out "RB"'s block of (the next) 2048 class
 C network numbers:
     C7: allocate 192.32.0 through 192.32.15. This block is described
         by the route 192.32.0, mask 255.255.240.0
 For the multi-homed clients, we will assume that C4 is advertised as
 primary via "RA" and secondary via "RB"; C5 is primary via "RB" and
 secondary via "RA". To connect this mess together, we will assume
 that "RA" and "RB" are connected via some common "backbone" provider
 "BB".
 Graphically, this simple topology looks something like this:
                     C1

192.24.0.0 – 192.24.7.0 \ _ 192.32.0.0 - 192.32.15.0 192.24.0.0/255.255.248.0 \ / 192.32.0.0/255.255.240.0

                         \     /             C7
                     C2  +----+                                 +----+

192.24.16.0 - 192.24.31.0 \| | | | 192.24.16.0/255.255.240.0 | | _ 192.24.12.0 - 192.24.15.0 _ | |

                         |    | /  192.24.12.0/255.255.252.0  \ |    |
                     C3 -|    |/              C4               \|    |

192.24.8.0 - 192.24.11.0 | RA | | RB | 192.24.8.0/255.255.252.0 | |_ 192.24.32.0 - 192.24.33.0 _| |

                        /|    |    192.24.32.0/255.255.254.0    |    |
                     C6  |    |               C5                |    |

192.24.34.0 - 192.24.35.0 | | | | 192.24.34.0/255.255.254.0 | | | |

                         +----+                                 +----+
                            \\                                     \\

192.24.12.0/255.255.252.0 (C4) || 192.24.12.0/255.255.252.0 (C4) || 192.32.0.0/255.255.240.0 (C7) || 192.24.32.0/255.255.254.0 (C5) || 192.24.0.0/255.248.0.0 (RA) || 192.32.0.0/255.248.0.0 (RB) ||

                             ||                                     ||
                             VV                                     VV
                   +--------------- BACKBONE PEER  BB ---------------+
 5.2  Routing advertisements
 To follow rule #1, RA will need to advertise the block of addresses
 that it was given and C7.  Since C4 is multi-homed and primary
 through RA, it must also be advertised.  C5 is multi-homed and
 primary through RB.  It need not be advertised since longest match by
 BB will automatically select RB as primary and the advertisement of

Fuller, Li, Yu & Varadhan [Page 17] RFC 1519 CIDR Address Strategy September 1993

 RA's aggregate will be used as a secondary.
 Advertisements from "RA" to "BB" will be:
     192.24.12.0/255.255.252.0 primary    (advertises C4)
     192.32.0.0/255.255.240.0 primary     (advertises C7)
     192.24.0.0/255.248.0.0 primary       (advertises remainder of RA)
 For RB, the advertisements must also include C4 and C5 as well as
 it's block of addresses.  Further, RB may advertise that C7 is
 unreachable.
 Advertisements from "RB" to "BB" will be:
     192.24.12.0/255.255.252.0 secondary  (advertises C4)
     192.24.32.0/255.255.254.0 primary    (advertises C5)
     192.32.0.0/255.248.0.0 primary       (advertises remainder of RB)
 To illustrate the problem alluded to by the "note" in section 4.2,
 consider what happens if RA loses connectivity to C7 (the client
 which is allocated out of RB's space). In a stateful protocol, RA
 will announce to BB that 192.32.0.0/255.255.240.0 has become
 unreachable. Now, when BB flushes this information out of its routing
 table, any future traffic sent through it for this destination will
 be forwarded to RB (where it will be dropped according to Rule #2) by
 virtue of RB's less specific match 192.32.0.0/255.248.0.0.  While
 this does not cause an operational problem (C7 is unreachable in any
 case), it does create some extra traffic across "BB" (and may also
 prove confusing to a network manager debugging the outage with
 "traceroute"). A mechanism to cache such unreachability information
 would help here, but is beyond the scope of this document (such a
 mechanism is also not implementable in the near-term).

6. Extending CIDR to class A addresses

 At some point, it is expected that this plan will eventually consume
 all of the remaining class C address space.  As of this writing, the
 upper half of the class A address space has already been reserved for
 future expansion.  This section describes how the CIDR plan can be
 used to utilize this portion of the class A space efficiently.  It is
 expected that this contingency would only be used if no long term
 solution has become apparent by the time that the class C address
 space is consumed.
 Fundamentally, there are two differences between using a class A
 address and a block of class C's.  First, the configuration of DNS
 becomes somewhat more complicated than it is without the aggregation
 of class A subnets.  The second difference is that the routers within

Fuller, Li, Yu & Varadhan [Page 18] RFC 1519 CIDR Address Strategy September 1993

 the class A address would need to support and use a classless IGP.
 Maintenance of DNS with a subnetted class A is somewhat painful.  As
 part of the mechanism for providing reverse address lookups, DNS
 maintains a "IN-ADDR.ARPA" reverse domain.  This is configured by
 reversing the dotted decimal network number, appending "IN-ADDR.ARPA"
 and using this as a type of pseudo-domain.  Individual hosts then end
 up pointing back to a host name.  Thus, for example, 131.108.1.111
 has a DNS record "111.1.108.131.IN-ADDR.ARPA."  Since the pseudo-
 domains can only be delegated on a byte boundary, this becomes
 painful if a stub domain receives a block of address space that does
 not fall on a byte boundary.  The solution in this case is to
 enumerate all of the possible byte combinations involved.  This is
 painful, but workable.  This is discussed further below.
 Routing within a class A used for CIDR is also an interesting
 challenge.  The usual case will be that a domain will be assigned a
 portion of the class A address space.  The domain can either use an
 IGP which allows variable length subnets or it can pick a single
 subnet mask to be used throughout the domain.  In the latter case,
 difficulties arise because other domains have been allocated other
 parts of the class A address space and may be using a different
 subnet mask.  If the domain is itself a transit, it may also need to
 allocate some portion of its space to a client, which might also use
 a different subnet mask.  The client would then need routing
 information about the remainder of the class A.
 If the client's IGP does not support variable length subnet masks,
 this could be done by advertising the remainder of the class A's
 address space in appropriately sized subnets.  However, unless the
 client has a very large portion of the class A space, this is likely
 to result in a large number of subnets (for example, a mask of
 255.255.255.0 would require a total of 65535 subnets, including those
 allocated to the client).  For this reason, it may be preferable to
 simply use an IGP that supports variable length subnet masks within
 the client's domain.
 Similarly, if a transit has been assigned address space from a class
 A network number, it is likely that it was not assigned the entire
 class A, and that other transit domains will get address space from
 this class A.  In this case, the transit would also have to inject
 routing information about the remainder of the class A into it's IGP.
 This is analogous to the situation above, with the same
 complications.  For this reason, we recommend that the use of a class
 A for CIDR only be attempted if IGP's with variable length subnet
 mask support be used throughout the class A.  Note that the IGP's
 need not support supernetting, as discussed above.

Fuller, Li, Yu & Varadhan [Page 19] RFC 1519 CIDR Address Strategy September 1993

 Note that the technique here could also apply to class B addresses.
 However, the limited number of available class B addresses and their
 usage for multihomed networks suggests that this address space should
 only be reserved for those large single organizations that warrant
 this type of address. [2]

7. Domain Service considerations

 One aspect of Internet services which will be notably affected by a
 move to either "supernetted" class-C network numbers or subdivided
 class-A's will be the mechanism used for address-to-name translation:
 the IN-ADDR.ARPA zone of the domain system. Because this zone is
 delegated on octet boundaries only, any address allocation plan which
 uses bitmask-oriented addressing will cause some degree of difficulty
 for those which maintain parts of the IN-ADDR.ARPA zone.
 7.1  Procedural changes for class-C "supernets"
 At the present time, parts of the IN-ADDR.ARPA zone are delegated
 only on network boundaries which happen to fall on octet boundaries.
 To aid in the use of blocks of class-C networks, it is recommended
 that this policy be relaxed and allow the delegation of arbitrary,
 octet-oriented pieces of the IN-ADDR.ARPA zone.
 As an example of this policy change, consider a hypothetical large
 network provider named "BigNet" which has been allocated the 1024
 class-C networks 199.0.0 through 199.3.255. Under current policies,
 the root domain servers would need to have 1024 entries of the form:
         0.0.199.IN-ADDR.ARPA.   IN      NS      NS1.BIG.NET.
         1.0.199.IN-ADDR.ARPA.   IN      NS      NS1.BIG.NET.
                 ....
         255.3.199.IN-ADDR.ARPA. IN      NS      NS1.BIG.NET.
 By revising the policy as described above, this is reduced only four
 delegation records:
         0.199.IN-ADDR.ARPA.     IN      NS      NS1.BIG.NET.
         1.199.IN-ADDR.ARPA.     IN      NS      NS1.BIG.NET.
         2.199.IN-ADDR.ARPA.     IN      NS      NS1.BIG.NET.
         3.199.IN-ADDR.ARPA.     IN      NS      NS1.BIG.NET.

Fuller, Li, Yu & Varadhan [Page 20] RFC 1519 CIDR Address Strategy September 1993

 The provider would then maintain further delegations of naming
 authority for each individual class-C network which it assigns,
 rather than having each registered separately. Note that due to the
 way the DNS is designed, it is still possible for the root
 nameservers to maintain the delegation information for individual
 networks for which the provider is unwilling or unable to do so. This
 should greatly reduce the load on the domain servers for the "top"
 levels of the IN-ADDR.ARPA domain.  The example above illustrates
 only the records for a single nameserver.  In the normal case, there
 are usually several nameservers for each domain, thus the size of the
 examples will double or triple in the common cases.
 7.2  Procedural changes for class-A subnetting
 Should it be the case the class-A network numbers are subdivided into
 blocks allocated to transit network providers, it will be similarly
 necessary to relax the restriction on how IN-ADDR.ARPA naming works
 for them. As an example, take a provider is allocated the 19-bit
 portion of address space which matches 10.8.0.0 with mask
 255.248.0.0. This represents all addresses which begin with the
 prefixes 10.8, 10.9, 10.10, 10.11, 10.12, 10.13, 10.14, an 10.15 and
 requires the following IN-ADDR.ARPA delegations:
         8.10.IN-ADDR.ARPA.      IN      NS      NS1.MOBY.NET.
         9.10.IN-ADDR.ARPA.      IN      NS      NS1.MOBY.NET.
                 ....
         15.10.IN-ADDR.ARPA.     IN      NS      NS1.MOBY.NET.
 To further illustrate how IN-ADDR.ARPA sub-delegation will work,
 consider a company named "FOO" connected to this provider which has
 been allocated the 14-bit piece of address space which matches
 10.10.64.0 with mask 255.255.192.0. This represents all addresses in
 the range 10.10.64.0 through 10.10.127.255 and will require that the
 provider implement the following IN-ADDR.ARPA delegations:
         64.10.10.IN-ADDR.ARPA.  IN      NS      NS1.FOO.COM.
         65.10.10.IN-ADDR.ARPA.  IN      NS      NS1.FOO.COM.
                 ....
         127.10.10.IN-ADDR.ARPA. IN      NS      NS1.FOO.COM.
 with the servers for "FOO.COM" containing the individual PTR records
 for all of the addresses on each of these subnets.

Fuller, Li, Yu & Varadhan [Page 21] RFC 1519 CIDR Address Strategy September 1993

8. Transitioning to a long term solution

 This solution does not change the Internet routing and addressing
 architectures.  Hence, transitioning to a more long term solution is
 not affected by the deployment of this plan.

9. Conclusions

 We are all aware of the growth in routing complexity, and the rapid
 increase in allocation of network numbers.  Given the rate at which
 this growth is being observed, we expect to run out in a few short
 years.
 If the inter-domain routing protocol supports carrying network routes
 with associated masks, all of the major concerns demonstrated in this
 paper would be eliminated.
 One of the influential factors which permits maximal exploitation of
 the advantages of this plan is the number of people who agree to use
 it.
 If service providers start charging networks for advertising network
 numbers, this would be a very great incentive to share the address
 space, and hence the associated costs of advertising routes to
 service providers.

10. Recommendations

 The NIC should begin to hand out large blocks of class C addresses to
 network service providers.  Each block must fall on bit boundaries
 and should be large enough to serve the provider for two years.
 Further, the NIC should distribute very large blocks to continental
 and national network service organizations to allow additional levels
 of aggregation to take place at the major backbone networks.  In
 addition, the NIC should modify its procedures for the IN-ADDR.ARPA
 domain to permit delegation along arbitrary octet boundaries.
 Service providers will further allocate power-of-two blocks of class
 C addresses from their address space to their subscribers.
 All organizations, including those which are multi-homed, should
 obtain address space from their provider (or one of their providers,
 in the case of the multi-homed).  These blocks should also fall on
 bit boundaries to permit easy route aggregation.
 To allow effective use of this new addressing plan to reduce
 propagated routing information, appropriate IETF WGs will specify the
 modifications needed to Inter-Domain routing protocols.

Fuller, Li, Yu & Varadhan [Page 22] RFC 1519 CIDR Address Strategy September 1993

 Implementation and deployment of these modifications should occur as
 quickly as possible.

11 References

 [1] Moy, J, "The OSPF Specification  Version 2", RFC 1247, Proteon,
     Inc., January 1991.
 [2] Rekhter, Y., and T. Li, "An Architecture for IP Address
     Allocation with CIDR", RFC 1518, T.J. Watson Research Center, IBM
     Corp., cisco Systems, September 1993.

12. Security Considerations

 Security issues are not discussed in this memo.

Fuller, Li, Yu & Varadhan [Page 23] RFC 1519 CIDR Address Strategy September 1993

13. Authors' Addresses

 Vince Fuller
 BARRNet
 Pine Hall 115
 Stanford, CA, 94305-4122
 EMail: vaf@Stanford.EDU
 Tony Li
 cisco Systems, Inc.
 1525 O'Brien Drive
 Menlo Park, CA 94025
 EMail: tli@cisco.com
 Jessica (Jie Yun) Yu
 Merit Network, Inc.
 1071 Beal Ave.
 Ann Arbor, MI 48109
 EMail: jyy@merit.edu
 Kannan Varadhan
 Internet Engineer, OARnet
 1224, Kinnear Road,
 Columbus, OH 43212
 EMail: kannan@oar.net

Fuller, Li, Yu & Varadhan [Page 24]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1519.txt · Last modified: 1993/09/23 19:35 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki