rfc1335.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                          Z. Wang
Request for Comments: 1335                                J. Crowcroft
                                             University College London
                                                              May 1992


             A Two-Tier Address Structure for the Internet:
         A Solution to the Problem of Address Space Exhaustion

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 RFC presents a solution to problem of address space exhaustion
   in the Internet.  It proposes a two-tier address structure for the
   Internet.  This is an "idea" paper and discussion is strongly
   encouraged.

Introduction

   Address space exhaustion is one of the most serious and immediate
   problems that the Internet faces today [1,2].  The current Internet
   address space is 32-bit.  Each Internet address is divided into two
   parts: a network portion and a host portion.  This division
   corresponds the three primary Internet address classes: Class A,
   Class B and Class C.  Table 1 lists the network number statistics as
   of April 1992.

                      Total       Allocated     Allocated (%)
   Class A              126            48            54%
   Class B            16383          7006            43%
   Class C          2097151         40724             2%

          Table 1: Network Number Statistics (April 1992)

   If recent trends of exponential growth continue, the network numbers
   in Class B will soon run out [1,2].  There are over 2 million Class C
   network numbers and only 2% have been allocated.  However, a Class C
   network number can only accommodate 254 host numbers which is too
   small for most networks.  With the rapid expansion of the Internet
   and drastic increase in personal computers, the time when the 32-bit
   address space is exhausted altogether is also not too distant [1-3].

   Recently several proposals have been put forward to deal with the



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RFC 1335      Two-Tier Address Structure for the Internet       May 1992


   immediate problem [1-4].  The Supernetting and C-sharp schemes
   attempt to make the Class C numbers more usable by re-defining the
   way in which Class C network numbers are classified and assigned
   [3,4].  Both schemes require modifications to the exterior routing
   algorithms and global coordination across the Internet may be
   required for the deployment.  The two schemes do not expand the total
   number of addresses available to the Internet and therefore can only
   be used as a short-term fix for next two or three years.  Schemes
   have also been put forwarded in which the 32-bit address field is
   replaced with a field of the same size but with different meaning and
   the gateways on the boundary re-write the address when the packet
   crossed the boundary [1,2,5].  Such schemes, however, requires
   substantial changes to the gateways and the exterior routing
   algorithm.

   In this paper, we present an alternative solution to the problem of
   address space exhaustion.  The "Dual Network Addressing (DNA)" scheme
   proposed here is based on a two-tier address structure and sharing of
   addresses.  It requires no modifications to the exterior routing
   algorithms and any networks can adopt the scheme individually at any
   time without affecting other networks.

The Scheme

   The DNA scheme attempts to reduce the waste in using the Internet
   addresses.  A useful analogy to our scheme is the extension system
   used in the telephone system.  Many large organizations usually have
   extensive private telephone networks for internal use and at the mean
   time hire a limited number of external lines for communications with
   the outside world.  In such a telephone system, important offices may
   have direct external lines and telephones in the public areas may be
   restricted to internal calls only.  The majority of the telephones
   can usually make both internal calls and external calls.  But they
   must share a limited number of external lines.  When an external call
   is being made, a pre-defined digit has to be pressed so that an
   external line can be allocated from the poll of external lines.

   In the DNA scheme, there are two types of Internet addresses:
   Internal addresses and External addresses.  An internal address is an
   Internet address only used within one network and is unique only
   within that network.  An interface with an internal address can only
   communicate with another interface with an internal address in the
   same network.  An external address is unique in the entire Internet
   and an interface with an external address can communicate directly to
   another interface with an external address over the Internet.  All
   current Internet addresses are external addresses.

   In effect, the external addresses form one global Internet and the



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   internal addresses form many private Internets.  Within one network,
   the external addresses are only used for inter-network communications
   and internal addresses for intra-network communications.  An External
   Address Sharing Service (EASS) is needed to manage the sharing of
   external addresses.  An EASS server reserves a number of external
   addresses.  When a machine that only has an internal address wants to
   communicate a machine with an external address in other networks, it
   can send a request to an EASS server to obtain a temporary external
   address.  After the use, the machine can return the external address
   to the EASS server.

   We believe that, with the DNA scheme, a network can operate with a
   limited number of external addresses.  The reasons are as follows:

   *  In most networks, the majority of the traffic is confined to
      its local area networks.  This is due the nature of
      networking applications and the bandwidth constraints on
      inter-network links.

   *  The number of machines which act as Internet servers, i.e.,
      running programs waiting to be called by machines in other
      networks, is often limited and certainly much smaller than
      the total number of machines.  These machines include mail
      servers, domain name servers, ftp archive servers, directory
      servers, etc.

   *  There are an increasingly large number of personal machines
      entering the Internet.  The use of these machines is
      primarily limited to their local environment.  They may also
      be used as "clients" such as ftp and telnet to access other
      machines.

   *  For security reasons, many large organizations, such as banks,
      government departments, military institution and some
      companies, may only allow a very limited number of their
      machines to have access to the global Internet.  The majority
      of their machines are purely for internal use.

   In the DNA scheme, all machines in a network are assigned a permanent
   internal address and can communicate with any machines within the
   same network.  The allocation of external addresses depends on the
   functions of the machines and as a result it creates three-level
   privileges:

   *  machines which act as servers or used as central computing
      infrastructure are likely to have frequent communications
      with other networks therefore they may require external
      addresses all the time.  These machines are allocated



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      permanent external addresses.

   *  machines which are not allowed to communicate with other
      networks have no external addresses and can only communicate
      with machines within their own network.

   *  the rest of the machines share a number of external
      addresses. The external addresses are allocated by
      the EASS server on request.  These machines can only
      used as clients to call machines in other networks,
      i.e., they can not be called by machines in other networks.

   A network can choose any network number other than its external
   network number as its internal network number.  Different networks
   can use the same network number as their internal number.  We propose
   to reserve one Class A network number as the well-known network
   number for internal use.

The Advantages

   The DNA scheme attempts to tackle the problem from the bottom of the
   Internet, i.e., each individual network, while other schemes
   described in the first section deal with the problem from the top of
   the Internet, i.e., gateways and exterior routing algorithms.  These