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Network Working Group                                         T. BradleyRequest for Comments: 2390                           Avici Systems, Inc.Obsoletes:1293                                                 C. BrownCategory: Standards Track                                     Consultant                                                                A. Malis                                             Ascend Communications, Inc.                                                          September 1998Inverse Address Resolution ProtocolStatus 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 replacesRFC 1293.  The changes fromRFC 1293 are minor   changes to formalize the language, the additions of a packet diagram   and an example insection 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 19984.  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 19986.  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      ar$tpa    mbytes         target protocol address   Possible values for hardware and protocol types are the same as those   for ARP and may be found in the current Assigned Numbers RFC [2].   Length of the hardware and protocol address are dependent on the   environment in which InARP is running.  For example, if IP is running   over Frame Relay, the hardware address length is either 2, 3, or 4,   and the protocol address length is 4.   The operation code indicates the type of message, request or   response.      InARP request  = 8      InARP response = 9   These values were chosen so as not to conflict with other ARP   extensions.7.  Protocol Operation   Basic InARP operates essentially the same as ARP with the exception   that InARP does not broadcast requests.  This is because the hardware   address of the destination station is already known.   When an interface supporting InARP becomes active, it should initiate   the InARP protocol and format InARP requests for each active PVC for   which InARP is active.  To do this, a requesting station simply   formats a request by inserting its source hardware, source protocol   addresses and the known target hardware address.  It then zero fills   the target protocol address field.  Finally, it will encapsulate the   packet for the specific network and send it directly to the target   station.Bradley, et. al.            Standards Track                     [Page 3]

RFC 2390          Inverse Address Resolution Protocol     September 1998   Upon receiving an InARP request, a station may put the requester's   protocol address/hardware address mapping into its ARP cache as it   would any ARP request.  Unlike other ARP requests, however, the   receiving station may assume that any InARP request it receives is   destined for it.  For every InARP request, the receiving station   should format a proper response using the source addresses from the   request as the target addresses of the response.  If the station is   unable or unwilling to reply, it ignores the request.   When the requesting station receives the InARP response, it may   complete the ARP table entry and use the provided address   information.  Note: as with ARP, information learned via InARP may be   aged or invalidated under certain circumstances.7.1.  Operation with Multi-Addressed Hosts   In the context of this discussion, a multi-addressed host will refer   to a host that has multiple protocol addresses assigned to a single   interface.  If such a station receives an InARP request, it must   choose one address with which to respond.  To make such a selection,   the receiving station must first look at the protocol address of the   requesting station, and then respond with the protocol address   corresponding to the network of the requester.  For example, if the   requesting station is probing for an IP address, the responding   multi-addressed station should respond with an IP address which   corresponds to the same subnet as the requesting station.  If the   station does not have an address that is appropriate for the request   it should not respond.  In the IP example, if the receiving station   does not have an IP address assigned to the interface that is a part   of the requested subnet, the receiving station would not respond.   A multi-addressed host should send an InARP request for each of the   addresses defined for the given interface.  It should be noted,   however, that the receiving side may answer some or none of the   requests depending on its configuration.7.2.  Protocol Operation Within Frame Relay   One case where Inverse ARP can be used is on a frame relay interface   which supports signaling of DLCIs via a data link management   interface.  An InARP equipped station connected to such an interface   will format an InARP request and address it to the new virtual   circuit.  If the other side supports InARP, it may return a response   indicating the protocol address requested.   In a frame relay environment, InARP packets are encapsulated using   the NLPID/SNAP format defined in [3] which indicates the ARP   protocol.  Specifically, the packet encapsulation will be as follows:Bradley, et. al.            Standards Track                     [Page 4]

RFC 2390          Inverse Address Resolution Protocol     September 1998               +----------+----------+               |    Q.922 address    |               +----------+----------+               |ctrl 0x03 | pad 00   |               +----------+----------+               |nlpid 0x80| oui 0x00 |               +----------+          +               | oui (cont) 0x00 00  |               +----------+----------+               | pid 0x08 06         |               +----------+----------+               |          .          |               |          .          |   The format for an InARP request itself is defined by the following:      ar$hrd - 0x000F the value assigned to Frame Relay      ar$pro - protocol type for which you are searching                  (i.e.  IP = 0x0800)      ar$hln - 2,3, or 4 byte addressing length      ar$pln - byte length of protocol address for which you                  are searching (for IP = 4)      ar$op  - 8; InARP request      ar$sha - Q.922 [6] address of requesting station      ar$spa - protocol address of requesting station      ar$tha - Q.922 address of newly announced virtual circuit      ar$tpa - 0; This is what is being requested   The InARP response will be completed similarly.      ar$hrd - 0x000F the value assigned to Frame Relay      ar$pro - protocol type for which you are searching                 (i.e.  IP = 0x0800)      ar$hln - 2,3, or 4 byte addressing length      ar$pln - byte length of protocol address for which you                 are searching (for IP = 4)      ar$op  - 9; InARP response      ar$sha - Q.922 address of responding station      ar$spa - protocol address requested      ar$tha - Q.922 address of requesting station      ar$tpa - protocol address of requesting station   Note that the Q.922 addresses specified have the C/R, FECN, BECN, and   DE bits set to zero.Bradley, et. al.            Standards Track                     [Page 5]

RFC 2390          Inverse Address Resolution Protocol     September 1998   Procedures for using InARP over a Frame Relay network are as follows:   Because DLCIs within most Frame Relay networks have only local   significance, an end station will not have a specific DLCI assigned   to itself.  Therefore, such a station does not have an address to put   into the InARP request or response.  Fortunately, the Frame Relay   network does provide a method for obtaining the correct DLCIs. The   solution proposed for the locally addressed Frame Relay network below   will work equally well for a network where DLCIs have global   significance.   The DLCI carried within the Frame Relay header is modified as it   traverses the network.  When the packet arrives at its destination,   the DLCI has been set to the value that, from the standpoint of the   receiving station, corresponds to the sending station.  For example,   in figure 1 below, if station A were to send a message to station B,   it would place DLCI 50 in the Frame Relay header.  When station B   received this message, however, the DLCI would have been modified by   the network and would appear to B as DLCI 70.                           ~~~~~~~~~~~~~~~                          (                )        +-----+          (                  )             +-----+        |     |-50------(--------------------)---------70-|     |        |  A  |        (                      )           |  B  |        |     |-60-----(---------+            )           |     |        +-----+         (        |           )            +-----+                         (       |          )                          (      |         )  <---Frame Relay                           ~~~~~~~~~~~~~~~~         network                                 80                                 |                              +-----+                              |     |                              |  C  |                              |     |                              +-----+                              Figure 1      Lines between stations represent data link connections (DLCs).      The numbers indicate the local DLCI associated with each      connection.Bradley, et. al.            Standards Track                     [Page 6]

RFC 2390          Inverse Address Resolution Protocol     September 1998              DLCI to Q.922 Address Table for Figure 1              DLCI (decimal)  Q.922 address (hex)                   50              0x0C21                   60              0x0CC1                   70              0x1061                   80              0x1401      For authoritative description of the correlation between DLCI and      Q.922 [6] addresses, the reader should consult that specification.      A summary of the correlation is included here for convenience. The      translation between DLCI and Q.922 address is based on a two byte      address length using the Q.922 encoding format.  The format is:                8   7   6   5   4   3    2   1              +------------------------+---+--+              |  DLCI (high order)     |C/R|EA|              +--------------+----+----+---+--+              | DLCI (lower) |FECN|BECN|DE |EA|              +--------------+----+----+---+--+      For InARP, the FECN, BECN, C/R and DE bits are assumed to be 0.   When an InARP message reaches a destination, all hardware addresses   will be invalid.  The address found in the frame header will,   however, be correct. Though it does violate the purity of layering,   Frame Relay may use the address in the header as the sender hardware   address.  It should also be noted that the target hardware address,   in both the InARP request and response, will also be invalid.  This   should not cause problems since InARP does not rely on these fields   and in fact, an implementation may zero fill or ignore the target   hardware address field entirely.   Using figure 1 as an example, station A may use Inverse ARP to   discover the protocol address of the station associated with its DLCI   50.  The Inverse ARP request would be as follows:              InARP Request from A (DLCI 50)              ar$op   8       (InARP request)              ar$sha  unknown              ar$spa  pA              ar$tha  0x0C21  (DLCI 50)              ar$tpa  unknown   When Station B receives this packet, it will modify the source   hardware address with the Q.922 address from the Frame Relay header.   This way, the InARP request from A will become:Bradley, et. al.            Standards Track                     [Page 7]

RFC 2390          Inverse Address Resolution Protocol     September 1998              ar$op   8       (InARP request)              ar$sha  0x1061  (DLCI 70)              ar$spa  pA              ar$tha  0x0C21  (DLCI 50)              ar$tpa  unknown.   Station B will format an Inverse ARP response and send it to station   A:              ar$op   9       (InARP response)              ar$sha  unknown              ar$spa  pB              ar$tha  0x1061  (DLCI 70)              ar$tpa  pA   The source hardware address is unknown and when the response is   received, station A will extract the address from the Frame Relay   header and place it in the source hardware address field.  Therefore,   the response will become:              ar$op   9       (InARP response)              ar$sha  0x0C21  (DLCI 50)              ar$spa  pB              ar$tha  0x1061  (DLCI 70)              ar$tpa  pA   This means that the Frame Relay interface must only intervene in the   processing of incoming packets.   Also, see [3] for a description of similar procedures for using ARP   [1] and RARP [4] with Frame Relay.8.  Security Considerations   This document specifies a functional enhancement to the ARP family of   protocols, and is subject to the same security constraints that   affect ARP and similar address resolution protocols.  Because   authentication is not a part of ARP, there are known security issues   relating to its use (e.g., host impersonation).  No additional   security mechanisms have been added to the ARP family of protocols by   this document.Bradley, et. al.            Standards Track                     [Page 8]

RFC 2390          Inverse Address Resolution Protocol     September 19989.  References   [1] Plummer, D., "An Ethernet Address Resolution Protocol - or -       Converting Network Protocol Addresses to 48.bit Ethernet Address       for Transmission on Ethernet Hardware", STD 37,RFC 826, November       1982.   [2] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,RFC 1700,       October 1994.  See also:http://www.iana.org/numbers.html   [3] Bradley, T., Brown, C., and A. Malis, "Multiprotocol Interconnect       over Frame Relay",RFC 1490, July 1993.   [4] Finlayson, R., Mann, R., Mogul, J., and M. Theimer, "A Reverse       Address Resolution Protocol", STD 38,RFC 903, June 1984.   [5] Bradner, S., "Key words for use in RFCs to Indicate Requirement       Levels",BCP 14,RFC 2119, March 1997.   [6] Information technology - Telecommunications and Information       Exchange between systems - Protocol Identification in the Network       Layer, ISO/IEC TR 9577: 1992.10.  Authors' Addresses   Terry Bradley   Avici Systems, Inc.   12 Elizabeth Drive   Chelmsford, MA  01824   Phone: (978) 250-3344   EMail: tbradley@avici.com   Caralyn Brown   Consultant   EMail:  cbrown@juno.com   Andrew Malis   Ascend Communications, Inc.   1 Robbins Road   Westford, MA  01886   Phone:  (978) 952-7414   EMail:  malis@ascend.comBradley, et. al.            Standards Track                     [Page 9]

RFC 2390          Inverse Address Resolution Protocol     September 199811.  Full Copyright Statement   Copyright (C) The Internet Society (1998).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Bradley, et. al.            Standards Track                    [Page 10]