rfc925.txt

RFC 相关的技术文档

RFC 925                                                     October 1984
Multi-LAN Address Resolution


            some other LAN.  If on this LAN then use some procedure to
            find the HA.  If on some other LAN then use some procedure
            to find the HA of a BOX.

         Extended ARP Scheme

            In every case the host uses ARP to get a IA:HA mapping.

      What do the BOXes do?

         Explicit Subnet Scheme

            The BOX must be able to decide which LAN within the site the
            destination host is on.  The BOXes must have some routing
            table that tells for each LAN in the site which interface to
            send datagrams on.  This routing table must be kept up to
            date, probably by a BOX-to-BOX protocol much like the
            Internet Gateway-to-Gateway protocol.

         Extended ARP Scheme

            The BOX must keep caches for each LAN it is attached to of
            IA:HA mappings, and it must keep a search list.  It does not
            run any BOX-to-BOX protocol, It does not even know if any
            other BOXes exist.

   Topology and Implementation Complexity

      Trees

         If the organization of the LANs and the BOXes is tree
         structured, the BOXes may be very simple, they don't have to
         keep the search lists at all, since there won't be any loops
         for the ARP-request to traverse.

      Loops

         If the organization has loops then the search lists are
         essential.  If the topology is kept balanced so that there are
         no long loops (all loops are about the same size), and the LANs
         are reasonably compatible in delay characteristics, then the
         procedure described here will work well.

      Complex

         If the organization is very complex, topologically unbalanced,



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RFC 925                                                     October 1984
Multi-LAN Address Resolution


         and/or composed of mix of different types of LANS with vastly
         different delay characteristics, then it may be better to use a
         BOX-to-BOX routing protocol.

SUMMARY

   It would be useful if the Internet community could come to some
   agreement on a solution to the multi-LAN network problem and could
   with a unified voice urge work station manufacturers to provide that
   solution built in.

   I urge consideration of the extended ARP scheme expounded on here.

   I think that most work stations will be connected to LANs that have a
   broadcast capability.  I think that most work stations will be used
   in situations that do not require explicit subnets, and most will be
   used in situations where a class C Internet addresses would be
   appropriate (and explicit subnets impossible).  Thus, i think it
   would be best to ask manufacturers to include support for ARP in work
   stations off the shelf.  I also think we ought to get busy and
   create, develop, test, and produce the magic boxes I suggest so that
   they too are available off the shelf.

   Please note that neither this note nor [1] proposes a specific
   routing procedure or BOX-to-BOX protocol.  This is because such a
   routing procedure is a very hard problem.  The plan proposed here
   will let us get started on using multi-LAN environments in a
   reasonable way.  If we later decide on a routing procedure to be used
   between the BOXes we can redo the BOXes without having to redo the
   hosts.



















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RFC 925                                                     October 1984
Multi-LAN Address Resolution


GLOSSARY

   ARP

      Address Resolution Protocol (see [2]).

   BOX

      Magic Box.  A box (computer) connected to two or more LANs of the
      same Network.  Also called an "ARP-based bridge".

   Bridge

      A node (computer) connected to two or more administratively
      indistinguishable but physically distinct subnets, that
      automatically forwards datagrams when necessary, but whose
      existence is not know to other hosts.  Also called a "software
      repeater".

   Datagram

      The unit of communication at the IP level.

   Explicit Subnet

      A Subnet explicitly identified in the the Internet Address by a
      subnet address field, and so visible to others both in side and
      out side the Network.

   Gateway

      A node (computer) connected to two or more administratively
      distinct networks and/or subnets, to which hosts send datagrams to
      be forwarded.

   HA

      Hardware Address, the address used in a packet on a LAN.

   Host Number

      The address of a host within an Network, the low-order part of an
      IA.

   IA

      Internet Address, as defined in IP.


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RFC 925                                                     October 1984
Multi-LAN Address Resolution


   Internet

      The collection of connected Internet Networks (also known as the
      Catenet).  A set of interconnected networks using IP.

   IP

      Internet Protocol (see [3]).

   LAN

      Local Area Network.

   Multi-LAN Network

      A set of LANs treated as one Network, i.e., using one Network
      Number in common.  The individual LANs may be either Explicit
      Subnets or Transparent Subnets.

   Network

      A single Internet Network (possibly divided into subnets or
      composed of multiple LANs), identified by an individual Network
      Number.

   Network Number

      An IP Network Number, the high-order part of an IA.

   Packet

      The unit of communication at the LAN hardware level.

   Subnet

      A subnet of Network. A portion of a Network (either logical or
      physical).

   Transparent Subnet

      A Subnet not identified in the Internet Address, and so invisible
      to others, (see Multi-LAN Network).

   TTL

      The IP Time-To-Live field.



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RFC 925                                                     October 1984
Multi-LAN Address Resolution


REFERENCES

   [1]  J. Mogul, "Internet Subnets",  RFC-917, Stanford University,
        October 1984.

   [2]  D. Plummer, "An Ethernet Address Resolution Protocol or
        Converting Network Protocol Addresses to 48-bit Ethernet
        Addresses for Transmission on Ethernet Hardware",  RFC-826,
        Symbolics, November 1982.

   [3]  J. Postel, "Internet Protocol",  RFC-791, USC-ISI,
        September 1981.





































Postel                                                         [Page 15]