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Network Working Group G. Malkin Request For Comments: 1868 Xylogics, Inc. Category: Experimental November 1995

                       ARP Extension - UNARP

Status of this Memo

 This memo defines an Experimental Protocol for the Internet
 community.  This memo does not specify an Internet standard of any
 kind.  Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Abstract

 The Address Resolution Protocol allows an IP node to determine the
 hardware (datalink) address of a neighboring node on a broadcast
 network.  The protocol depends on timers to age away old ARP entries.
 This document specifies a trivial modification to the ARP mechanism,
 not the packet format, which allows a node to announce that it is
 leaving the network and that all other nodes should modify their ARP
 tables accordingly.

Acknowledgements

 Thanks to James Carlson/Xylogics for reviewing this document and
 proposing the backwards compatibility mechanism.

1. Introduction

 The primary purpose of the Address Resolution Protocol, as defined in
 [1], is to determine a node's hardware address based on its network
 address (protocol address in ARPspeak).  The ARP protocol
 specifically states that nodes should not periodically advertise
 their existence for two reasons: first, this would generate a lot of
 network traffic and table maintenance overhead; second, it is highly
 unlikely that all nodes will need to communicate to all other nodes.
 Since a node does not advertise its existence, neither does it
 advertise its imminent departure.  This is not a serious problem
 since most ARP implementations maintain timers to age away old
 entries, and departing nodes seldom depart gracefully in any case.

 Over time, an additional use has been found for ARP: Proxy ARP.
 While there are those who believe Proxy ARP is an evil thing, it does
 serve a purpose; that is, it allows for communication in ways never
 considered in the original IP architecture.  For example, allows
 dial-in hosts to connect to a network without consuming a large

Malkin Experimental [Page 1]  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.

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3. The Solution

 When a server, as described above, disconnects from a remote host for
 which it has responded to a Proxy ARP, it broadcasts an UNARP.  An
 UNARP is an unsolicited ARP Reply with the following field values:

    Hardware Address Space       as appropriate
    Protocol Address Space       0x800 (IP)
    Hardware Address Length      0 (see Backwards Compatibility)
    Protocol Address Length      4 (length of an IP address)
    Opcode                       2 (Reply)
    Source Hardware Address      Not Included
    Source Protocol Address      IP address of detaching host
    Target Hardware Address      Not Included
    Target Protocol Address      255.255.255.255 (IP broadcast)

    NOTE: this is a 16-byte packet (not including MAC header)

 On receiving an UNARP, a node deletes the ARP cache entry associated
 with the IP address.

 It is not strictly necessary that a server keep state information
 about whether or not it has actually sent a Proxy ARP Reply; it would
 be sufficient if a server always sends an UNARP when a remote host
 disconnects.

 Of course, there is no reason why a host which gracefully detaches
 from a LAN cannot also send an UNARP for itself.  This would be
 especially useful if, upon re-attaching, it might have a different
 hardware address.

4. Backwards Compatibility

 The modifications to support UNARP are trivial, so there is every
 expectation that it will be widely supported.  Of course, there will
 be a period of time during which nodes which support UNARP will
 coexist with nodes which do not support UNARP.  To prevent
 unenlightened nodes from adding spurious ARP cache entries with
 hardware addresses of zero, UNARP packets specify a hardware address
 length of zero.  This should be rejected by nodes which do not
 support UNARP.  As a consequence of this, the source and target
 hardware address fields do not exist in UNARP packets (as previously
 described).

 It is recommended that implementors include a configuration switch to
 disable UNARP in the event that some vendor's ARP implementation
 might take offense at the abbreviated UNARP packet format.

Malkin Experimental [Page 3]  RFC 1868 UNARP November 1995

5. Security Considerations

 Security issues are not discussed in this memo.

References

 [1] Plummer, D., "An Ethernet Address Resolution Protocol", STD 37,
     RFC 826, MIT, November 1982.

Author's Address

 Gary Scott Malkin
 Xylogics, Inc.
 53 Third Avenue
 Burlington, MA  01803

 Phone:  (617) 272-8140
 EMail:  gmalkin@xylogics.com

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