RFC 925 (rfc925) - Page 2 of 15

Multi-LAN address resolution

RFC 925                                                     October 1984
Multi-LAN Address Resolution


   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).




Postel