Internet Engineering Task Force                            Juha Heinanen
INTERNET DRAFT                                             Telia Finland
Expires September 2001                                       March, 2001

            Inverse ARP over Unidirectional Virtual Circuits

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

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   This memo describes operation of an Inverse Address Resolution
   Protocol (InARP) over unidirectional virtual circuits such as MPLS

1. Introduction

   Inverse Address Resolution Protocol (InARP) [1] is commonly used by
   stations (usually routers) connected via Frame Relay or ATM virtual
   circuits to automatically learn the protocol addresses of their
   peers.  InARP is needed when a station only knows that a virtual
   circuit to another station exists, but doesn't have any knowledge of
   protocol layer identity of the other station.  This can happen either
   if the virtual circuit is network provisioned or if some other
   address than the protocol address of the other station is used in the
   virtual circuit setup.

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INTERNET DRAFT                                               March, 2001

   When a Frame Relay or ATM local station has discovered the hardware
   address (Frame Relay DLCI or ATM VPI/VCI) of a remote station, it
   sends an InARP Request to query the protocol address of the remote
   station.  The remote station learns the protocol address of the local
   station from the source protocol address field of the InARP request
   and the corresponding hardware address from the frame header of the
   InARP request.  The remote station then sends an InARP response
   containing its own protocol address to the learned hardware address.

   The above procedure does not work if the stations are connected via
   unidirectional virtual circuits, such as network provisioned MPLS
   LSPs.  In order to be able migrate from network provisioned Frame
   Relay or ATM virtual circuits to network provisioned MPLS LSPs, a new
   version of InARP is needed.  This memo describes the operation of
   InARP in situations where one or more unidirectional virtual circuits
   are used to implement bidirectional connectivity between two

2. Protocol Operation

   Once the local station (A) learns the hardware address (label) of an
   outgoing unidirectional virtual circuit, it constructs an InARP
   request to find out the protocol address of the remote station (B) to
   which this virtual circuit leads to. The InARP request contains the
   protocol address (pA) and hardware address (hA) of the local station
   in the source protocol and hardware address fields, respectively:

        ar$op   8       (InARP request)
        ar$sha  hA
        ar$spa  pA
        ar$tha  unknown
        ar$tpa  unknown

   When the remote station (B) receives the request, it constructs a
   response by including its own protocol address (pB) in the source
   protocol address field and by copying the source protocol and
   hardware addresses from the request to the target protocol and
   hardware address fields, respectively:

        ar$op   9       (InARP response)
        ar$sha  unknown
        ar$spa  pB
        ar$tha  hA
        ar$tpa  pA

   Because of unidirectionality of the virtual circuits, the remote
   station can't use the either the source hardware address in the
   request or the hardware address in the frame header to send the

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INTERNET DRAFT                                               March, 2001

   response back to the local station.  Instead, the remote station
   first checks if it itself has already learned about a virtual
   circuit, which has the same target protocol address as the source
   protocol address in the request.  If so, the remote station sends the
   response to such a virtual circuit.  If not, the remote station sends
   the response to every virtual circuit whose target protocol address
   is still unknown to it.

   When the local station receives an InARP response, it first checks if
   the target address pair of the response matches an existing outgoing
   virtual circuit.  If so, it creates a new protocol address/hardware
   address mapping for the virtual circuit based on the protocol address
   and hardware address fields of the response.  I not, it silently
   discards the response.

   Once the local station unlearns the hardware address (label) of an
   outgoing virtual circuit, it deletes the protocol address/hardware
   address mapping that was associated with it.  Even if the local
   station doesn't unlearn a hardware address, it may be desirable to
   age the address/hardware address mapping after a time period.  The
   implementation of aging (if any) is outside the scope of this memo.

3. Scalability Considerations

   The above operation could potentially result in generation of a large
   number of simultaneous InARP responses.  The worst case occurs when a
   full mesh of virtual circuits connecting N stations is created
   simultaneously and each local station sends simultaneously N-1 InARP
   requests to each of which every remote station (having not yet
   learned any addresses) replies with N-1 InARP responses.

   Although it is not likely in practice that all virtual circuits are
   created simultaneously, InARP implementations can also help to
   alleviate the problem.  The local stations could wait a random time
   interval after virtual circuit discovery before sending out their
   InARP requests.  That would creating an effect similar to as if the
   stations and their virtual circuits had been added one at a time.

4. 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.

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INTERNET DRAFT                                               March, 2001


   I would like to thank Joel Halpern of Longitude Systems for his
   constructive comments on earlier versions of this memo.


   [1] Bradley, T., Brown, C., and Malis, A., Inverse Address Resolution
   Protocol.  RFC 2390, September 1998.

Author's Address

   Juha Heinanen
   Telia Finland, Inc.
   Hallituskatu 16
   33200 Tampere, Finland

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