rfc2333.txt

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   At the LIS level, the main issue is to maintain and deliver a sizable
   number of NBMA to Network layer address mappings within large LISs.
   To this goal, NHRP implementations can use the services of the Server
   Cache Synchronization Protocol (SCSP) [8] that allows multiple
   synchronized NHSs within an LIS, and hence resolve the associated
   scalability issue.

   At the NHRP Domain level, network layer routing is used in resolving
   the NBMA address of a destination outside the LIS.  As such, the
   scalability of NHRP is closely tied to the scalability of the network
   layer routing protocol used by NHRP.  Dynamic network layer routing
   protocols are proven to scale well.  Thus, when used in conjunction
   with dynamic routing algorithms, at the NHRP domain level, NHRP
   should scale in the same order as the routing algorithm, subject to
   the assumption that all the routers along the path are NHRP aware.
   If an NHRP Request is processed by a router that does not implement
   NHRP, it will be silently discarded.  Then, short-cuts cannot be
   implemented and connectivity will be provided on a hop-by-hop basis.

   Thus, when NHRP is implemented in conjunction with dynamic network
   layer routing, a scaling requirement for NHRP is that virtually all
   the routers within a logical NBMA network should be NHRP aware.





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   One can also use static routing in conjunction with NHRP.  Then, not
   all the routers in the NBMA network need to be NHRP aware.  That is,
   since the routers that need to process NHRP control messages are
   specified by static routing, routers that are not included in the
   manually defined static paths do not have to be NHRP aware.  Of
   course, static routing does not scale, and if the destination is off
   the NBMA network, then the use of static routing could result in
   persistently suboptimal routes.  Use of static routing also has
   fairly negative failure modes.

6. Discussion

   NHRP does not replace existing routing protocols. In general, routing
   protocols are used to determine the proper path from a source host or
   router, or intermediate router, to a particular destination.  If the
   routing protocol indicates that the proper path is via an interface
   to an NBMA network, then NHRP may be used at the NBMA interface to
   resolve the destination IP address into the corresponding NBMA
   address.  Of course, the use of NHRP is subject to considerations
   discussed in Section 4.

   Assuming that NHRP is applicable and the destination address has been
   resolved, packets are forwarded using the particular data forwarding
   and path determination mechanisms of the underlying NBMA network.
   Here, the sequence of events are such that route determination is
   performed by IP routing, independent of NHRP. Then, NHRP is used to
   create a short-cut track upon the path determined by the IP routing
   protocol. Therefore, NHRP "shortens" the routed path.  NHRP (as
   defined in [1]) is not sufficient to suppress persistent forwarding
   loops when used for router-router communication if the underlying
   routing protocol looses information critical to loop suppression [9].
   Work is in progress [10] to augment NHRP to enable its use for the
   router-router communication without persistent forwarding loops.

   When the routed path keeps changing on some relatively short time
   scale, such as seconds, this situation will have an effect on the
   operation of NHRP. In certain router-router operations, changes in
   the routed path could create persistent routing loops. In host-
   router, or router-host communications, frequent changes in routed
   paths could result in inefficiencies such as frequent creation of
   short-cut paths which are short lived.

7. Security Considerations

   NHRP is an address resolution protocol, and SCSP is a database
   synchronization protocol.  As such, they are possibly subject to
   server (for NHRP) or peer (for SCSP) spoofing and denial of service
   attacks.  They both provide authentication mechanisms to allow their



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   use in environments in which spoofing is a concern.  Details can be
   found in sections 5.3.4 in [1] and B.3.1 in [8].  There are no
   additional security constraints or concerns raised in this document
   that are not already discussed in the referenced sections.

References

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

   [2] Greene, M., and J. Luciani, "NHRP Management Information Base",
       Work in Progress.


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

   [4] Lawrance, J., and D. Piscitello, "The Transmission of IP
       datagrams over the SMDS service", RFC 1209, March 1991.

   [5] Multiprotocol Over ATM Version 1.0, ATM Forum Document
       af-mpoa-0087.000

   [6] Rekhter, Y., and D. Farinacci, "Support for Sparse Mode PIM over
       ATM", Work in Progress.

   [7] Rekhter, Y., and D. Kandlur, "Local/Remote" Forwarding Decision
       in Switched Data Link Subnetworks", RFC 1937, May 1996.

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

   [9] Cole, R., Shur, D., and C. Villamizar, "IP over ATM: A Framework
       Document", RFC 1932, April 1996.

   [10] Rekhter, Y., "NHRP for Destinations off the NBMA Subnetwork",
        Work in Progress.

Acknowledgements

   The author acknowledges valuable contributions and comments from many
   participants of the ION Working Group, in particular from Joel
   Halpern of Newbridge Networks, David Horton of Centre for Information
   Technology Research, Andy Malis of Nexion, Yakov Rekhter and George
   Swallow of Cisco Systems and Curtis Villamizar of ANS.




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Author's Address

   Derya H. Cansever
   GTE Laboratories Inc.
   40 Sylvan Rd. MS 51
   Waltham MA 02254

   Phone: [phone]
   EMail: [e-mail]










































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Full Copyright Statement

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