Inverse Address Resolution Protocol
RFC 2390

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Network Working Group                                         T. Bradley
Request for Comments: 2390                           Avici Systems, Inc.
Obsoletes: 1293                                                 C. Brown
Category: Standards Track                                     Consultant
                                                                A. Malis
                                             Ascend Communications, Inc.
                                                          September 1998

                  Inverse Address Resolution Protocol

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

2.  Abstract

   This memo describes additions to ARP that will allow a station to
   request a protocol address corresponding to a given hardware address.
   Specifically, this applies to Frame Relay stations that may have a
   Data Link Connection Identifier (DLCI), the Frame Relay equivalent of
   a hardware address, associated with an established Permanent Virtual
   Circuit (PVC), but do not know the protocol address of the station on
   the other side of this connection.  It will also apply to other
   networks with similar circumstances.

   This memo replaces RFC 1293.  The changes from RFC 1293 are minor
   changes to formalize the language, the additions of a packet diagram
   and an example in section 7.2, and a new security section.

3.  Conventions

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [5].

Bradley, et. al.            Standards Track                     [Page 1]
RFC 2390          Inverse Address Resolution Protocol     September 1998

4.  Introduction

   This document will rely heavily on Frame Relay as an example of how
   the Inverse Address Resolution Protocol (InARP) can be useful. It is
   not, however, intended that InARP be used exclusively with Frame
   Relay.  InARP may be used in any network that provides destination
   hardware addresses without indicating corresponding protocol
   addresses.

5.  Motivation

   The motivation for the development of Inverse ARP is a result of the
   desire to make dynamic address resolution within Frame Relay both
   possible and efficient.  Permanent virtual circuits (PVCs) and
   eventually switched virtual circuits (SVCs) are identified by a Data
   Link Connection Identifier (DLCI).  These DLCIs define a single
   virtual connection through the wide area network (WAN) and may be
   thought of as the Frame Relay equivalent to a hardware address.
   Periodically, through the exchange of signaling messages, a network
   may announce a new virtual circuit with its corresponding DLCI.
   Unfortunately, protocol addressing is not included in the
   announcement.  The station receiving such an indication will learn of
   the new connection, but will not be able to address the other side.
   Without a new configuration or a mechanism for discovering the
   protocol address of the other side, this new virtual circuit is
   unusable.

   Other resolution methods were considered to solve the problems, but
   were rejected.  Reverse ARP [4], for example, seemed like a good
   candidate, but the response to a request is the protocol address of
   the requesting station, not the station receiving the request.  IP
   specific mechanisms were limiting since they would not allow
   resolution of other protocols other than IP. For this reason, the ARP
   protocol was expanded.

   Inverse Address Resolution Protocol (InARP) will allow a Frame Relay
   station to discover the protocol address of a station associated with
   the virtual circuit.  It is more efficient than sending ARP messages
   on every VC for every address the system wants to resolve and it is
   more flexible than relying on static configuration.

Bradley, et. al.            Standards Track                     [Page 2]
RFC 2390          Inverse Address Resolution Protocol     September 1998

6.  Packet Format

   Inverse ARP is an extension of the existing ARP.  Therefore, it has
   the same format as standard ARP.

      ar$hrd   16 bits         Hardware type
      ar$pro   16 bits         Protocol type
      ar$hln    8 bits         Byte length of each hardware address (n)
      ar$pln    8 bits         Byte length of each protocol address (m)
      ar$op    16 bits         Operation code
      ar$sha    nbytes         source hardware address
      ar$spa    mbytes         source protocol address
      ar$tha    nbytes         target hardware address