rfc1338.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

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]