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rfc:rfc1338

Network Working Group V. Fuller Request for Comments: 1338 BARRNet

                                                                T. Li
                                                                cisco
                                                                J. Yu
                                                                MERIT
                                                          K. Varadhan
                                                               OARnet
                                                            June 1992
    Supernetting: an Address Assignment and Aggregation Strategy

Status of this Memo

 This memo provides information for the Internet community. It does
 not specify an Internet standard.  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
 run by transit routing domain providers.

Table of Contents

 Acknowledgements .................................................  2
 1.  Problem, goal, and motivation ................................  2
 2.  Scheme plan ..................................................  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 .................... 11
 4.1.  General 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
 5.  Example of new allocation and routing ........................ 15
 5.1.  Address allocation ......................................... 15
 5.2.  Routing advertisements ..................................... 17
 6.  Transitioning to a long term solution ........................ 18

Fuller, Li, Yu, & Varadhan [Page 1] RFC 1338 Supernetting June 1992

 7.  Conclusions .................................................. 18
 8.  Recommendations .............................................. 18
 9.  Bibliography ................................................. 19
 10. Security Considerations ...................................... 19
 11. Authors' Addresses ........................................... 19

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 painfully 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 to large to be widely allocated.
      2.   Growth of routing tables in Internet routers beyond the
           ability of current software (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.
 The proposed solution is to hierarchically allocate future IP address
 assignment, by delegating control of segments of the IP address space
 to the various network service providers.
 It is proposed that this scheme of allocating IP addresses be
 undertaken as soon as possible.  It is also believed that the scheme
 will suffice as a short term strategy, to fill the gap between now

Fuller, Li, Yu, & Varadhan [Page 2] RFC 1338 Supernetting June 1992

 and the time when a viable long term plan can be put into place and
 deployed effectively.  It is believed that this scheme would be
 viable for at least three (3) years, in which time frame, a suitable
 long term solution would be expected to be deployed.
 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.
 This scheme 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. Scheme Plan

 There are two basic components of this addressing and routing scheme:
 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
 information along topological lines. For simple, single-homed
 clients, the allocation of their address space out of a service
 provider's space will accomplish this automatically - rather than
 advertise a separate route for each such client, the service provider
 may advertise a single, aggregate, route which describes all of the
 destinations contained within it. Unfortunately, not all sites are
 singly-connected to the network, so some loss of ability to aggregate
 is realized for the non simple 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 service provider'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

Fuller, Li, Yu, & Varadhan [Page 3] RFC 1338 Supernetting June 1992

        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 non-primary basis).  For this reason, the
        routing cost for these organizations will typically be about
        the same as it is today.
   o    Organizations which move from one service provider to another.
        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
   block of network numbers to the client (as opposed to multiple,
   independently represented network numbers) the client's routing
   information may be aggregated into a single (net, mask) pair. Also,
   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 the supernet-capable Inter-
   Domain protocols without deployment of the new address plan will
   not allow useful aggregation to occur (in other words, the

Fuller, Li, Yu, & Varadhan [Page 4] RFC 1338 Supernetting June 1992

   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+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.
   Note that in contrast to a previously described scheme of
   subnetting a class-A network number, this plan should not require
   difficult host changes to work around domain system limitations -
   since each sub-allocated piece of the address space looks like a
   class-C network number, delegation of authority for the IN-
   ADDR.ARPA domain works much the same as it does today - there will
   just be a lot of class-C network numbers whose IN-ADDR.ARPA
   delegations all point to the same servers (the same will be true of
   the root delegating a large block of class-Cs to the network
   provider, unless the delegation just happens to fall on a byte
   boundary). It is also the case that this method of aggregating
   class-C's is somewhat easier to deploy, since it does not require
   the ability to split a class-A across a routing domain boundary
   (i.e., non-contiguous subnets).
   It is also worthy to mention that once Inter-Domain protocols which
   support classless network destinations are widely deployed, the
   rules described by the "supernetting" 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 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
   (there may, however, be further implementation considerations
   before doing this).

Fuller, Li, Yu, & Varadhan [Page 5] RFC 1338 Supernetting June 1992

   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.

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+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 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 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

Fuller, Li, Yu, & Varadhan [Page 6] RFC 1338 Supernetting June 1992

 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 happy side effect
 of distributing the address allocation procedure, greatly reducing
 the load on the central authority.
 3.1.  Present Allocation Figures
    A back-of-the-envelope 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 16256 class-B numbers have been allocated, leaving 10789.
    Assuming that recent trends continue, the number of allocated
    class-B's will continue to double approximately once a year. At
    this rate of grown, all class-B's will be exhausted within about
    15 months.

Fuller, Li, Yu, & Varadhan [Page 7] RFC 1338 Supernetting June 1992

 3.2.  Historic growth rates
    MM/YY     ROUTES                        MM/YY     ROUTES
              ADVERTISED                              ADVERTISED
    ------------------------                -----------------------
    Feb-92    4775                          Apr-90    1525
    Jan-92    4526                          Mar-90    1038
    Dec-91    4305                          Feb-90    997
    Nov-91    3751                          Jan-90    927
    Oct-91    3556                          Dec-89    897
    Sep-91    3389                          Nov-89    837
    Aug-91    3258                          Oct-89    809
    Jul-91    3086                          Sep-89    745
    Jun-91    2982                          Aug-89    650
    May-91    2763                          Jul-89    603
    Apr-91    2622                          Jun-89    564
    Mar-91    2501                          May-89    516
    Feb-91    2417                          Apr-89    467
    Jan-91    2338                          Mar-89    410
    Dec-90    2190                          Feb-89    384
    Nov-90    2125                          Jan-89    346
    Oct-90    2063                          Dec-88    334
    Sep-90    1988                          Nov-88    313
    Aug-90    1894                          Oct-88    291
    Jul-90    1727                          Sep-88    244
    Jun-90    1639                          Aug-88    217
    May-90    1580                          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 no 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+mask fields (as opposed to the current
    class-A/B/C distinction) be added to the common Internet inter-
    domain routing protocol(s).

Fuller, Li, Yu, & Varadhan [Page 8] RFC 1338 Supernetting June 1992

 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
         followed by a significant reduction in the rate growth of
         routing table size should occur (for default-free routers).
 3.3.2. Growth rate projections
    Currently, a default-free routing table (for example, the routing
    tables maintained by the routers in the NSFNET backbone) contains
    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(2) 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.
    Under the proposed plan, growth of the routing table in a
    default-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.

Fuller, Li, Yu, & Varadhan [Page 9] RFC 1338 Supernetting June 1992

    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 grown rate of
    approximately 6%.  This is in clear contrast to the current annual
    growth of 150%.  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 1338 Supernetting June 1992

4. Changes to Inter-Domain routing protocols

 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[RFC1267]) 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+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 mechanisms that routers use to
 interpret routing information returned by the 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

Fuller, Li, Yu, & Varadhan [Page 11] RFC 1338 Supernetting June 1992

        (this makes intuitive sense - if a network is multi-homed, all
        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 make the
 implementation be generalized, so that 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.

Fuller, Li, Yu, & Varadhan [Page 12] RFC 1338 Supernetting June 1992

 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:
      accept 128.32.0.0
      accept 128.120.0.0
      accept 134.139.0.0
      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

Fuller, Li, Yu, & Varadhan [Page 13] RFC 1338 Supernetting June 1992

 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.
 4.4.  Responsibility for and configuration of aggregation
 The AS which owns 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.  As AS can also delegate
 aggregation authority to another AS.  In this case, aggregation is
 done in the other AS 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

Fuller, Li, Yu, & Varadhan [Page 14] RFC 1338 Supernetting June 1992

 from its neighbor which routes it should be willing to accept,
 preconfiguration of aggregation information does not introduce
 additional administrative overhead.

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).
 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

Fuller, Li, Yu, & Varadhan [Page 15] RFC 1338 Supernetting June 1992

         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 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:

Fuller, Li, Yu, & Varadhan [Page 16] RFC 1338 Supernetting June 1992

                     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) || 192.32.12.0/255.255.252.0 (C4) || 192.24.32.0/255.255.254.0 (C5) || 192.32.32.0/255.255.192.0 (C5) || 192.32.0.0/255.255.240.0 (C7) || 192.32.0.0/255.248.0.0 (RB) || 192.24.0.0/255.248.0.0 (RA) || ||

                             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 and C5 are multi-homed, they must
 also be advertised.
 Advertisements from "RA" to "BB" will be:
     192.24.12.0/255.255.252.0 primary    (advertises C4)
     192.24.32.0/255.255.254.0 secondary  (advertises C5)
     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)

Fuller, Li, Yu, & Varadhan [Page 17] RFC 1338 Supernetting June 1992

 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. 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.

7. 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.  It is hoped that having the IAB and the Internet society bless
 this plan would go a long way in the wide deployment, and hence
 benefit of this plan.
 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.

8. 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.

Fuller, Li, Yu, & Varadhan [Page 18] RFC 1338 Supernetting June 1992

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

9. Bibliography

 [RFC1247]  Moy, J, "The OSPF Specification  Version 2", January 1991.

10. Security Considerations

 Security issues are not discussed in this memo.

11. 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

Fuller, Li, Yu, & Varadhan [Page 19] RFC 1338 Supernetting June 1992

    Kannan Varadhan
    Internet Engineer, OARnet
    1224, Kinnear Road,
    Columbus, OH 43212
    email: kannan@oar.net

Fuller, Li, Yu, & Varadhan [Page 20]

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