A Reverse Address Resolution Protocol
RFC 903

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Network Working Group                    Finlayson, Mann, Mogul, Theimer
Request for Comments: 903                            Stanford University
                                                               June 1984

                 A Reverse Address Resolution Protocol

      Ross Finlayson, Timothy Mann, Jeffrey Mogul, Marvin Theimer
                      Computer Science Department
                          Stanford University
                               June 1984

Status of this Memo

   This RFC suggests a method for workstations to dynamically find their
   protocol address (e.g., their Internet Address), when they know only
   their hardware address (e.g., their attached physical network
   address).

   This RFC specifies a proposed protocol for the ARPA Internet
   community, and requests discussion and suggestions for improvements.

I. Introduction

   Network hosts such as diskless workstations frequently do not know
   their protocol addresses when booted; they often know only their
   hardware interface addresses.  To communicate using higher-level
   protocols like IP, they must discover their protocol address from
   some external source.  Our problem is that there is no standard
   mechanism for doing so.

   Plummer's "Address Resolution Protocol" (ARP) [1] is designed to
   solve a complementary problem, resolving a host's hardware address
   given its protocol address.  This RFC proposes a "Reverse Address
   Resolution Protocol" (RARP).  As with ARP, we assume a broadcast
   medium, such as Ethernet.

II. Design Considerations

   The following considerations guided our design of the RARP protocol.

   A. ARP and RARP are different operations.  ARP assumes that every
   host knows the mapping between its own hardware address and protocol
   address(es).  Information gathered about other hosts is accumulated
   in a small cache.  All hosts are equal in status; there is no
   distinction between clients and servers.

   On the other hand, RARP requires one or more server hosts to maintain
   a database of mappings from hardware address to protocol address and
   respond to requests from client hosts.

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RFC 903                                                        June 1984

   B. As mentioned, RARP requires that server hosts maintain large
   databases. It is undesirable and in some cases impossible to maintain
   such a database in the kernel of a host's operating system.  Thus,
   most implementations will require some form of interaction with a
   program outside the kernel.

   C. Ease of implementation and minimal impact on existing host
   software are important.  It would be a mistake to design a protocol
   that required modifications to every host's software, whether or not
   it intended to participate.

   D. It is desirable to allow for the possibility of sharing code with
   existing software, to minimize overhead and development costs.

III.  The Proposed Protocol

   We propose that RARP be specified as a separate protocol at the
   data-link level.  For example, if the medium used is Ethernet, then
   RARP packets will have an Ethertype (still to be assigned) different
   from that of ARP.  This recognizes that ARP and RARP are two
   fundamentally different operations, not supported equally by all
   hosts.  The impact on existing systems is minimized; existing ARP
   servers will not be confused by RARP packets. It makes RARP a general
   facility that can be used for mapping hardware addresses to any
   higher level protocol address.

   This approach provides the simplest implementation for RARP client
   hosts, but also provides the most difficulties for RARP server hosts.
   However, these difficulties should not be insurmountable, as is shown
   in Appendix A, where we sketch two possible implementations for
   4.2BSD Unix.

   RARP uses the same packet format that is used by ARP, namely:

      ar$hrd (hardware address space) -  16 bits
      ar$pro (protocol address space) -  16 bits
      ar$hln (hardware address length) - 8 bits
      ar$pln (protocol address length) - 8 bits
      ar$op  (opcode) - 16 bits
      ar$sha (source hardware address) - n bytes,
                                       where n is from the ar$hln field.
      ar$spa (source protocol address) - m bytes,
                                       where m is from the ar$pln field.
      ar$tha (target hardware address) - n bytes
      ar$tpa (target protocol address) - m bytes

   ar$hrd, ar$pro, ar$hln and ar$pln are the same as in regular ARP
   (see [1]).

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RFC 903                                                        June 1984

   Suppose, for example, that 'hardware' addresses are 48-bit Ethernet
   addresses, and 'protocol' addresses are 32-bit Internet Addresses.
   That is, we wish to determine Internet Addresses corresponding to