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Network Working Group                                          J. Postel
Request for Comments: 925                                            ISI
                                                            October 1984

                      Multi-LAN Address Resolution


STATUS OF THIS MEMO

   This memo is prompted by RFC-917 by Jeffery Mogul on "Internet
   Subnets".   In that memo, Mogul makes a case for the use of "explicit
   subnets" in a multi-LAN environment.  In this memo, I attempt to make
   a case for "transparent subnets".  This RFC suggests a proposed
   protocol for the ARPA-Internet community, and requests discussion and
   suggestions for improvements.  Distribution of this memo is
   unlimited.

INTRODUCTION

   The problem of treating a set of local area networks (LANs) as one
   Internet network has generated some interest and concern.  It is
   inappropriate to give each LAN within an site a distinct Internet
   network number.  It is desirable to hide the details of the
   interconnections between the LANs within an site from people,
   gateways, and hosts outside the site.  The question arises on how to
   best do this, and even how to do it at all.  One proposal is to use
   "explicit subnets" [1].  The explicit subnet scheme is a call to
   recursively apply the mechanisms the Internet uses to manage networks
   to the problem of managing LANs within one network.  In this note I
   urge another approach: the use of "transparent subnets" supported by
   a multi-LAN extension of the Address Resolution Protocol [2].

OVERVIEW

   To quickly review the Address Resolution Protocol (ARP).  Each host
   on a broadcast LAN knows both its own physical hardware address (HA)
   on the LAN and its own Internet Address (IA).  When Host-A is given
   the IA of Host-B and told to send a datagram to it, Host-A must find
   the HA that corresponds to Host-B's IA.  To do this Host-A forms an
   ARP packet that contains its own HA and IA and the IA of the
   destination host (Host-B).  Host-A broadcasts this ARP packet.  The
   hosts that receive this ARP packet check to see if they are
   destination sought.  If so, they (it should be only Host-B) send a
   reply specifically addressed to the originator of the query (Host-A)
   and supplying the HA that was needed.  The Host-A now has both the HA
   and the IA of the destination (Host-B).  The Host-A adds this
   information to a local cache for future use.

      Note:  The ARP is actually more general purpose than this brief
      sketch indicates.



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   The idea in this memo is to extend the ARP to work in an environment
   of multiple interconnected LANs.

   To see how this could work let us imagine a "magic box" (BOX) that is
   connected as if it were an ordinary host to two (or more) LANs.

   Hosts continue to behave exactly as they do with the basic ARP.

   When an ARP query is broadcast by any host the BOX reads it (as do
   all the hosts on that LAN).  In addition to checking whether it is
   the host sought (and replying if it is), the BOX checks its cache of
   IA:HA address mappings in the cache that it keeps for each LAN it is
   attached to.

      Case 1: If the mapping for the host is found in the cache for the
      LAN that the query came from, the BOX does not respond (letting
      the sought host respond for itself).

      Case 2: If the mapping for the host is found in the cache for a
      different LAN than the query came from, the BOX sends a reply
      giving its own HA on the LAN the query came from.  The BOX acts as
      an agent for the destination host.

      Case 3: If the mapping is not found in any of the caches then, the
      BOX must try to find out the the address, and then respond as in
      case 1 or 2.

      In case 3, the BOX has to do some magic.

         The BOX keeps a search list of sought hosts.  Each entry
         includes the IA of the host sought, the interface the ARP was
         received on, and the source addresses of the original request.
         When case 3 occurs, the search list is checked.  If the sought
         host is already listed the search is terminated, if not the
         search is propagated.

         To propagate the search, an entry is first made on the search
         list, then the BOX composes and sends an ARP packet on each of
         its interfaces except the interface the instigating ARP packet
         was received on.  If a reply is received, the information is
         entered into the appropriate cache, the entry is deleted from
         the search list and a response to the search instigating ARP is
         made as in case 1 or 2.  If no reply is received, give up and
         do nothing -- no response is sent to the instigating host (the
         entry stays on the search list).




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         To terminate the search, give up and do nothing -- no response
         is sent to the instigating host (the entry stays on the search
         list).

   The entries in the caches and the search list must time out.

   For every ARP request that is received, the BOX must also put the
   sending host's IA:HA address mapping into the cache for the LAN it
   was received on.

THE MULTI-LAN ADDRESS RESOLUTION PROTOCOL

   The plan is to use ARP just as it is.  The new element is the "magic
   box" ("ARP-based bridge") that relays the ARP request into
   neighboring LANs and acts as an agent for relaying datagrams to hosts
   on other LANs.

   The Details

      Hosts continue to behave exactly as they do with the basic ARP.

      The LANs are connected together by BOXes (computers that are
      attached to two or more LANs exactly as hosts are attached to
      LANs).  The BOXes implement the following procedure.

      Each BOX keeps a table for each LAN it is connected to (or for
      each LAN interface).  Entries in these tables time out, so these
      tables are caches of recent information.  The entries in these
      caches are the IA:HA address pairs for that LAN.

      When an ARP query is broadcast by any host the BOX reads it (as do
      all the hosts on that LAN).  In addition to checking to see if it
      is the host sought (and replying if it is), the BOX checks its
      cache of IA:HA address mappings in the table it keeps for each LAN
      it is attached to.

         Case 1: If the mapping for the host is found in the cache for
         the LAN that the query came from, the BOX does not respond
         (letting the sought host respond for itself).  The time out on
         this entry is not reinitialized.

         Case 2: If the mapping for the host is found in the cache for a
         different LAN than the query came from, the BOX sends a reply
         giving its own HA on the LAN the query came from.  The time out
         on this entry is not reinitialized.

            In this case the BOX is indicating that it will act as an


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            agent for the destination host.  When an IP datagram arrives
            at the BOX, the BOX must attempt to forward it using the
            information in its address mapping caches.

         Case 3: If the mapping is not found in any of the caches, then
         the BOX must try to find out the the address, and then respond
         as in case 1 or 2.  In this case, the BOX has to do some magic.

            The BOX keeps a search list of sought (but not yet found)
            hosts.  Each entry includes the IA of the host sought, the
            interface the ARP was received on, and the source addresses
            of the original request.

            When case 3 occurs, the search list is checked.  If the
            sought host is already listed the search is terminated, if
            not the search is propagated.

            To propagate the search, an entry is first made on the
            search list, then the BOX composes and sends an ARP packet
            on each of its interfaces.  These ARP requests contain the
            IA and HA of the BOX and the IA of the sought host, and
            request the HA of the sought host.  If a reply is received
            to the ARP request, the information is entered into the
            appropriate cache, the entry is deleted from the search list
            and a response to the search instigating ARP requests is
            made as in case 1 or 2 above.  If no reply is received, give
            up and do nothing -- no response is sent to the instigating
            host (the entry stays on the search list).

               Note that the BOX must make a reasonable effort with its
               ARP requests,  if it is normal for ordinary hosts to
               retry ARP requests five times, then a BOX must also retry
               it's ARP requests five times.

            To terminate the search, give up and do nothing -- no
            response is sent to the instigating host (the entry stays on
            the search list).

            There is no negative feedback from an ARP request, so there
            is no way to decide that a search was unsuccessful except by
            means of a time out.

      For every ARP request that is received, the BOX must also put the
      sending hosts IA:HA address mapping into the cache for the LAN it
      was received on.

      The entries in the caches and the search list must time out.


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      The search list must be kept and the termination rule followed to
      avoid an infinite relaying of an ARP request for a host that does
      not respond.  Once a host is listed in the search list, ARP
      requests will not be relayed.  If a host that is down (or
      otherwise not responding to ARP requests), comes up (or otherwise
      begins responding to ARP requests) it will still not become
      available to hosts in other LANs until the search list entry times
      out.

         There are two approaches to this problem: first, to have a
         relatively short time out on the search list entries; or
         second, to have the BOX periodically send ARPs for each entry
         on the search list.

      There are several time outs involved in this scheme.

         First, the hosts try to get the address resolved using ARP.
         They may actually make several attempts before giving up if a
         host is not responding.  One must have an good estimate of the
         length of time that a host may keep trying.  Call this time T1.

         Second, there is the time that an entry stays on the search
         list, or the time between BOX generated ARPs to resolve these
         addresses.  Call this time T2.

            Note that this time (T2) must be greater than the sum of the
            T1s for the longest loop of LANs.

         Third, there is the time that entries stay in the cache for
         each LAN.  Call this time T3.

         The relationship must be  T1 < T2 < T3.

            One suggestion is that T1 be less than one minute, T2 be ten
            minutes, and T3 be one hour.

         If the environment is very stable, making T3 longer will result
         in fewer searches (less overhead in ARP traffic).  If the
         environment is very dynamic making T3 shorter will result in
         more rapid adaptation to the changes.

         Another possibility is to restart the timer on the cache
         entries each time they are referenced, and have a small value
         for T3.  This would result in entries that are frequently used
         staying in the cache, but infrequently used information being
         discarded quickly.  Unfortunately there is no necessary
         relationship between frequency of use and correctness.  This


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         method could result in an out-of-date entry persisting in a
         cache for a very long time if ARP requests for that address
         mapping were received at just less than the time out period.

      When handling regular datagrams, the BOXes must decrement the IP
      datagram Time-To-Live field (TTL) and update the IP header check
      sum.  If the TTL becomes zero the datagram is discarded (not
      forwarded).

      ARP, as currently defined, will take the most recent information
      as the best and most up-to-date.  In a complicated multi-LAN
      environment where there are loops in the connectivity it is likely
      that one will get two (or more) responses to an ARP request for a
      host on some other LAN.  It is probable that the first response
      will be from the BOX that is the most efficient path.

      The one change to the host implementation of ARP that is suggested
      here is to prevent later responses from replacing the mapping
      recorded from the first response.

   Potential Problems

      Bad Cache Entries

         If some wrong information get into a cache entry, it will stay
         there for time T3.  The persistence of old information could
         prevent communication (for a time) if a host changed its IA:HA
         mapping.

         One way to replace bad or out-of-date entries in a cache would
         be to have the BOXes explicitly interpret a broadcast ARP reply
         to require an entry with either this IA or HA to be replaced
         with this new IA:HA mapping.  One could have important servers
         send a broadcast ARP reply when they come up.

      Non-ARP Hosts

         It seems unrealistic to expect to use both ARP hosts and
         non-ARP hosts on the same LAN and expect them to communicate.
         If all the non-ARP hosts are on the same LAN the situation is
         considered with under the next heading (Non-Broadcast LANs).

         Hosts that do not implement ARP must use some other means of
         address mapping.  Either they hold a complete table of all
         hosts, or they access some such table in a server via some
         protocol; or they expect to make all routing decisions based on
         analysis of address fields.


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      Non-Broadcast LANs

         BOXes that are connected to LANs that do not have broadcast
         capability and/or LANs where the hosts do not respond to ARP
         may have a static or dynamic table of the IA:HA mappings for
         that LAN (or the addresses may be computed from one another).
         All the hosts on that LAN must be in the table.

         When a BOX must find the address mapping and would otherwise
         send an ARP request into a non-broadcast LAN (this can only
         happen when the sought host is not the non-broadcast LAN since
         all the hosts are in the table), it must instead send an ARP
         type request specifically to each of the other BOXes on that
         LAN.

      Size of Tables

         The worst case of the size of the tables in the BOXes is the
         number of hosts in the set of LANs for each table.  That is,
         the table kept for each LAN interface may (in the worst case)
         grow to have an entry for each host in the entire set of LANs.
         However, these tables are really caches of the entries needed
         for current communication activity and the typical case will be
         far from the worst case.  Most hosts will communicate mostly
         with other hosts on their own LAN and with a few hosts on other
         LANs.  Most communication on LANs is between work station hosts
         and server hosts.  It can be expected that there will be
         frequent communication involving the main server hosts and that
         these server hosts will be entered in the tables of most of the
         BOXes most of the time.

      Infinite Transmission Loops

         The possibility of infinite transmission loops through an
         interconnected set of LANs is prevented by keeping search lists
         in the BOXes and terminating the search when a request is
         received for an address already on the list.

         Transmission loops of regular datagrams can not persist because
         them the BOXes must decrement the TTL, and discard the datagram
         if the TTL is reduced to zero.  For debugging purposes it would
         be useful for a BOX to report to the implementer any datagrams
         discarded for this reason.






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      Broadcast

         Note that broadcast does not really have anything to do with
         either transparent subnets or explicit subnets.  Since it was
         discussed in [1], it will be discussed here, too.  Two of the
         three broadcast functions suggested in [1] work just the same
         and have the same effects, the third can be supported, too.

         It is also argued that the support for a broadcast
         interpretation of IAs is a bigger issue that the question of
         explicit subnets versus transparent subnets and it should be
         decided separately.

         It is also suggested that broadcast is not really what is
         desired, but rather multicast is the better function.  It may
         make sense to understand how to do an Internet multicast before
         adopting a broadcast scheme.

         This IP Network

            If the IA of this network number and an all ones host number
            (e.g., 36.255.255.255) is used, an IP level broadcast to all
            hosts on this Network (all LANs) is intended.  A BOX must
            forward this datagram.  A BOX must examine the datagram for
            potential significance to the BOX itself.

            To prevent infinite transmission loops each BOX must keep a
            list of recent broadcasts.  The entries in this list contain
            the source IA and the Identification field from the datagram
            header.  If a broadcast is received and matches an entry on
            the list it is discarded and not forwarded.  The entries on
            this list time out in time T2.

         This LAN Only

            If the IA of all ones (i.e., 255.255.255.255) is used an IP
            level broadcast to all hosts on this LAN only is intended.
            A BOX must not forward this datagram.  A BOX must examine
            the datagram for potential significance to the BOX itself.

         Another LAN Only

            Since the LANs are not individually identified in the IA
            this can not be supported in the same way. Some have also
            argued that this is a silly capability to provide.

            One way to provide it is to establish a specific IA for each


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RFC 925                                                     October 1984
Multi-LAN Address Resolution


            LAN that means "broadcast on this LAN".  For example,
            36.255.255.128 means broadcast on LAN A, and 36.255.255.187
            means broadcast on LAN B, etc.  These addresses would be
            specially interpreted by the BOXes attached to the specific
            LAN where they had the special interpretation, other BOXes
            would treat these address as any other IAs.   Where these
            addresses are specially interpreted they are converted to
            the broadcast on this LAN only address.

DISCUSSION

   The claim for the extended ARP scheme is that the average host need
   not even know it is in a multi-LAN environment.

      If a host took the trouble to analyze its local cache of IA:AH
      address mappings it might discover that several of the IAs mapped
      to the same HA.  And if it took timing measurements it might
      discover that some hosts responded with less delay that others.
      And further, it might be able to find a correlation between these
      discoveries.  But few hosts would take the trouble.

   Address Structure

      In the explicit subnet scheme, some IA bits are devoted to
      identifying the subnet (i.e., the LAN).  The address is broken up
      into network, subnet, and host fields.  Generally, when fields are
      use the density of the assigned addresses in the address space
      goes down.  That is, there is a less efficient use of the address
      space.  Significant implementation problems may arise if more
      subnets than planned are installed and it becomes necessary to
      change the size of the subnet field.  It seems totally impractical
      to use the explicit subnet scheme with a class C IA.

      In the extended ARP scheme the address is simply the network, and
      host fields.  The extended ARP scheme may be used with any class
      of IA.

   Relocating Hosts

      In the explicit subnet scheme when a host is unplugged from one
      LAN and plugged into another its IA must change.

      In the extended ARP scheme it may keep the same IA.






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RFC 925                                                     October 1984
Multi-LAN Address Resolution


   One view of the situation suggests that there are really two
   problems:

      1. How does the host discover if the destination is in this LAN or
      some other LAN?

         This question assumes that a host should know the difference
         and should do something different in the two cases, and further
         that once the host knows the answer it also know how to send
         the data (e.g., directly to the host, or to the box).

            The claim here is that the hosts should not know the
            difference and should always do the same thing.

      2. How do the BOXes that connect LANs know which BOXes are the
      routes to which LANs?

         This question assumes that the BOXes need some kind of
         topological knowledge, and exchange BOX-to-BOX protocol
         information about connectivity.

            The claim here is that the BOXes do not need topological
            knowledge and do not need to explicitly know about the
            existence of other BOXes.

   It has been suggested that there are two problems: first, how the
   hosts do routing; and second, how the BOXes do routing.  A claim has
   been made that the competing strategies each have an approach to each
   problems and one could select a solution made up partly from one
   approach and partly from another.

      For example: use ARP within the LAN and have the BOX send ARP
      replies and act as a agent (as in the extended ARP scheme), but
      use a BOX-to-BOX protocol to get the "which hosts are where"
      information into the BOXes (as in the explicit subnet scheme).

   There are two places where code is involved: a large number of hosts,
   and a small number of BOXes.  In considering the trade off between
   explicit subnet scheme and extended ARP scheme, the work done in the
   hosts should weigh a lot more than the work done in the BOXes.

      What do hosts do?

         Explicit Subnet Scheme

            The host must be able to decide if this IA is on this LAN or



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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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         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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   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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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.





































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