RFC 1868 (rfc1868) - Page 2 of 4


ARP Extension - UNARP



Alternative Format: Original Text Document



RFC 1868                         UNARP                     November 1995


   amount of the IP address space (i.e., all of the hosts contain
   addresses on the same subnet, even though they are not directly
   attached to the physical network associated with that subnet address.
   It is this use of Proxy ARP which produces the problem addressed by
   this document.

2. The Problem

   Consider the following topology:

                    +--------+
                    | Host A |
                    +--------+
                        |
      ======================================== LAN
          |                             |
      +--------+                    +--------+
      |  CS1   |   comm. servers    |  CS2   |
      +--------+                    +--------+
        |    |                        |    |
       +-+  +-+                      +-+  +-+
       | |  | |       modems         | |  | |
       +-+  +-+                      +-+  +-+

   Assume that all of the modems are on the same rotary; that is, when a
   remote host dials in, it may be assigned a modem on either of the
   communication servers.  Further assume that all of the remote hosts'
   IP addresses have the same subnet address as the servers and Host A,
   this in order to conserve address space.

   To begin, a remote host dials into CS1 and attempts to communicate
   with Host A.  Host A will assume, based on the subnet mask, that the
   remote host is actually attached to the LAN and will issue an ARP
   Request to determine its hardware address.  Naturally, the remote
   host will not hear this request.  CS1, knowing this, will respond in
   the remote host's place with its own hardware address.  Host A, on
   receiving the ARP Reply, will then communicate with the remote host,
   transparently through CS1.  So far everything is just fine.

   Now, the remote host disconnects and, before Host A can age its ARP
   cache, reconnects through CS2.  Herein lies the problem.  Whenever
   Host A attempts to send a packet to the remote host, it will send it
   to CS1 because it cannot know that its ARP cache entry is invalid.
   If, when the remote host disconnects, the server to which it was
   attached could inform other nodes on the LAN that the protocol
   address/hardware address mapping was no longer valid, the problem
   would not occur.




Malkin                        Experimental