Network Working Group                                         J. Luciani
Request for Comments: 2336                                  Bay Networks
Category: Informational                                        July 1998




           Classical IP and ARP over ATM to NHRP Transition


Status of this Memo

  This memo provides information for the Internet community.  It does
  not specify an Internet standard of any kind.  Distribution of this
  memo is unlimited.

Copyright Notice

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

Abstract

  This document describes methods and procedures for the graceful
  transition from an ATMARP LIS[1] to an NHRP LIS[2] network model over
  ATM.

1. Introduction

  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 [6].

  ATMARP defines an initial application of classical IP and ARP in an
  ATM network environment configured as a LIS[1].  ATMARP only
  considers application of ATM as a direct replacement for the "wires"
  and local LAN segments connecting IP end-stations and routers
  operating in the "classical" LAN-based paradigm.

  The NBMA Next Hop Resolution Protocol (NHRP) allows a source station
  (a host or router), wishing to communicate over a Non-Broadcast,
  Multi-Access (NBMA) subnetwork, to determine the internetworking
  layer addresses and NBMA addresses of suitable "NBMA next hops"
  toward a destination station. If the destination is connected to the
  NBMA subnetwork and direct communication is administratively allowed,
  then the NBMA next hop is the destination station itself.  Otherwise,
  the NBMA next hop is the egress router from the NBMA subnetwork that
  is "nearest" to the destination station.  For the purposes of this
  document, the NBMA network is of type ATM.



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  It is reasonable to expect that ATMARP Clients and NHRP Clients will
  initially coexist within a LIS.  Thus, it is necessary to define a
  graceful transition, including a period of coexistance, from the use
  of ATMARP to the use of NHRP for address resolution in the LIS
  [1][2]. In short, NHSs will be required to respond to ATMARP Client
  queries in a fashion which will permit continued use of the ATMARP
  Client within the LIS during the ATMARP to NHRP transition period.
  Note that this document places no protocol requirements upon
  ATMARP[1] servers.

  For the following, it will be assumed that the reader is familiar
  with the terminology as described in [1][2][3].

2. Service Requirements

  If NHRP is to be used in a LIS then only NHSs will be used in the
  LIS; that is, there will not be a mixture of NHSs and ATMARP servers
  within the same LIS.  Since ATMARP servers will not be able to
  understand NHCs and since, as described below, NHSs will respond to
  ATMARP Clients, this is a reasonable simplifying restriction.

  This document will only address SVC based environments and will not
  address PVC environments.  This document will refer only to ATM AAL5
  as the NBMA and IP as the protocol layer since ATMARP only addresses
  these protocols.

2.1 NHRP Server Requirements

  If NHRP Servers (NHS) are to be deployed in a LIS which contains both
  ATMARP Clients and NHRP Clients then NHSs MUST respond to
  ATMARP_Requests sent by ATMARP Clients in the same fashion that an
  ATMARP Server would respond as described in [1].  To do this, the NHS
  MUST first recognize the LLC/SNAP ATMARP code point with LLC=0xAA-
  AA-03, OUI=0x00-00-00, and ethertype=0x08-06.  Further, the NHS MUST
  recognize the packet formats described in Section 8.7 of [1].
  However, since this document does not extend to PVC environments,

  NHSs MUST only receive/respond to values of ar$op of 1,2,10
  (Decimal).  If an NHS receives an ATMARP message with ar$op values
  other than those previously noted then the NHS MUST discard the
  packet and MUST NOT take any further action.

  When an NHS receives a valid (as defined in the previous paragraph)
  ATMARP_Request packet, the NHS MUST follow the rules described in
  Section 8.4 of [1] with the following additional processing:






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    1) When an ATMARP_Request causes a new table entry in the NHS for
       an ATMARP Client, that table entry MUST be marked as being of
       type "ATMARP" so that it can be differentiated from an NHRP
       sourced entry.

    2) An ATMARP_Request MUST NOT cause an ATMARP_Reply to be sent if
       that ATMARP_Request contains an off-LIS protocol address.  This
       should never happen because the IP stack on the requesting
       machine should automatically send the packet to the default
       router.  If this does occur then the ATMARP_Request MUST cause
       an ATMARP_NAK to be sent to the originator.

  In [1], an ATMARP_Request packet also serves as a
  registraion/registration-update packet which would cause a server to
  add an entry to a server's cache or to update a previously existing
  entry.  When an NHS receives an ATMARP_Request which causes the
  creation of a new cache entry in the NHS or updates an existing entry
  then that cache entry will have a holding time of 20 minutes (this is
  the default value in [1]).

  An NHS receiving an NHRP Resolution Request MUST NOT send a positive
  NHRP Resolution Reply for a station which registered via ATMARP if
  the station sending the NHRP Resolution Request is outside the LIS of
  the station which registered itself via ATMARP.  This is because the
  station which registered via ATMARP is almost certainly not prepared
  to accept a cut-through.   When this occurs, the replying NHS must
  send NHRP Resolution Reply which contains a CIE code of "4 -
  Administratively Prohibited" as described in [2].  This type of reply
  does not preclude the station sending the NHRP Resolution Request
  from sending its data packets along the routed path but it does
  preclude that station from setting up a cut-through VC.

2.2 Multi-server environments

  Since NHRP servers may work in a multi-server environment on a per
  LIS basis during the transition, it is necessary to know how cache
  synchronization occurs. These rules may be found in [5].

3. Security Considerations

  Not all of the security issues relating to IP over ATM are clearly
  understood at this time, due to the fluid state of ATM
  specifications, newness of the technology, and other factors.








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  It is believed that ATM and IP facilities for authenticated call
  management, authenticated end-to-end communications, and data
  encryption will be needed in globally connected ATM networks.  Such
  future security facilities and their use by IP networks are beyond
  the scope of this memo.

  There are known security issues relating to host impersonation via
  the address resolution protocols used in the Internet [4].  No
  special security mechanisms have been added to ATMARP.  While NHRP
  supplies some mechanisms for authentication, ATMARP does not.  Since
  any security mechanism is only as good as its weakest link, it should
  be assumed that when NHRP and ATMARP exist with a given LIS, the
  security of a combination is only as good as that supplied by ATMARP.

References

  [1] Laubach, M. and J. Halpern, "Classical IP and ARP over ATM", RFC
  2225, April 1998.

  [2] Luciani, J., Katz, D., Piscitello, D., Cole, B. and N. Doraswamy,
  "NBMA Next Hop Resolution Protocol (NHRP)", RFC 2332, April 1998.

  [3] Luciani, J., Armitage, G., Halpern, J. and N. Doraswamy, "Server
  Cache Synchronization Protocol (SCSP)", RFC 2334, April 1998.

  [4] Security Problems in the TCP/IP Protocol Suite, Bellovin, ACM
  Computer Communications Review, Vol. 19, Issue 2, pp. 32-48, 1989.

  [5] Luciani, J., "A Distributed NHRP Service Using SCSP", RFC 2335,
  April 1998.

  [6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
  Levels", RFC 2119, March 1997.

Acknowledgments

  Thanks to Andy Malis for his input on this draft.

Author's Addresses

  James V. Luciani
  Bay Networks
  3 Federal Street
  Mail Stop: BL3-03
  Billerica, MA 01821
  Phone:  +1 978 916 4734
  Email:  [email protected]




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