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NHRP Protocol Applicability Statement

The information below is for an old version of the document that is already published as an RFC.
Document Type This is an older version of an Internet-Draft that was ultimately published as an RFC.
Author Dr. Derya Cansever
Last updated 2013-03-02 (Latest revision 1997-03-24)
Stream Internet Engineering Task Force (IETF)
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ION Working Group                                      Derya H. Cansever
INTERNET DRAFT                                    GTE Laboratories, Inc.
                                                           March    1997
Expiration Date September 1997

                 NHRP Protocol Applicability Statement

Status of this Memo

   This document is an Internet  Draft.   Internet  Drafts  are  working
   documents  of  the Internet Engineering Task Force (IETF), its Areas,
   and its Working Groups.  Note that other groups may  also  distribute
   working documents as Internet Drafts.

   Internet Drafts are draft  documents  valid  for  a  maximum  of  six
   months.   Internet  Drafts  may be updated, replaced, or obsoleted by
   other documents at any time.  It is not appropriate to  use  Internet
   Drafts as reference material or to cite them other than as a "working
   draft" or "work in progress."

   Please  check  the  1id-abstracts.txt  listing   contained   in   the
   internet-drafts  Shadow  Directories  on,,,,  or  to  learn   the
   current status of any Internet Draft.


   As required by the Routing Protocol Criteria [RFC 1264],  this  draft
   report  discusses  the  applicability  of  the  Next  Hop  Resolution
   Protocol  (NHRP)  in  routing  of  IP  datagrams  over  Non-Broadcast
   Multiple  Access  (NBMA)  networks,  such  as ATM, SMDS and X.25. The
   final form of this draft report is a prerequisite to  advancing  NHRP
   on the standards track.

1. Protocol Documents

   The NHRP protocol description is defined in [1] in  its  draft  form.
   The NHRP MIB description is defined in [2] in its draft form.

2. Introduction

   This document summarizes the key features of NHRP and  discusses  the

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                 NHRP Protocol Applicability Statement     November 1996

   environments  for which the protocol is well suited. For the purposes
   of description, NHRP can be considered a generalization of  Classical
   IP  and  ARP over ATM which is defined in [3] and of the Transmission
   of  IP  Datagrams  over  the  SMDS  Service,  defined  in  [4].  This
   generalization occurs in 2 distinct directions.

   Firstly, NHRP avoids the need to go through  extra  hops  of  routers
   when  the Source and Destination belong to different Logical Internet
   Subnets (LIS). Of course, [3] and [4] specify that  when  the  source
   and  destination  belong  to  different LISs, the source station must
   forward data packets to a router that is a member of  multiple  LISs,
   even  though  the  source and destination stations may be on the same
   logical NBMA network. If the source and destination  stations  belong
   to  the  same  logical NBMA network, NHRP provides the source station
   with an inter-LIS address resolution mechanism at the  end  of  which
   both stations can exchange packets without having to use the services
   of intermediate routers. This feature is also referred to as  "short-
   cut"  routing.  If the destination station is not part of the logical
   NBMA network, NHRP provides the source with the NBMA address  of  the
   egress router towards the destination.

   The  second  generalization  is  that  NHRP  is  not  specific  to  a
   particular NBMA technology. Of course, [3] assumes an ATM network and
   [4] assumes an SMDS network at their respective subnetwork layers.

   NHRP is specified for resolving the destination NBMA addresses of  IP
   datagrams  over  IP  subnets within a large NBMA cloud. In principle,
   NHRP should also be extensible to network layer protocols other  than
   IP,  possibly  subject  to  other  network  layer  protocol  specific

   As an important application  of  NHRP,  the  MultiProtocol  Over  ATM
   (MPOA)  Working  Group  of  the ATM Forum has decided to adopt and to
   integrate NHRP into its MPOA Protocol  specification  [5].  As  such,
   NHRP  will  be  used  in  resolving the ATM addresses of MPOA packets
   destined outside the originating Internetwork Address Sub-Group.

3. Key Features

   NHRP is not a routing protocol, but an inter-LIS  address  resolution
   mechanism that may make use of network layer routing in resolving the
   NBMA address of the destination. This is further discussed in Section

   The most prominent feature of NHRP is that it avoids extra hops in an
   NBMA  with multiple LISs. To this goal, NHRP provides the source with
   the NBMA address of the destination, if the destination  is  directly
   attached  to  the NBMA. If the destination station is not attached to

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                 NHRP Protocol Applicability Statement     November 1996

   the NBMA, then NHRP provides the source with the NBMA address  of  an
   exit  router  that  has  connectivity to the destination. In general,
   there may be multiple exit routers  that  have  connectivity  to  the
   destination. If NHRP uses the services of a dynamic routing algorithm
   in fulfilling  its  function,  which  is  necessary  for  robust  and
   scalable  operation, then the exit router identified by NHRP reflects
   the selection made by the network layer dynamic routing protocol.  In
   general,  the  selection  made  by  the  routing protocol would often
   reflect a desirable attribute, such as identifying  the  exit  router
   that induces the least number of hops in the original routed path.

   NHRP is defined for avoiding extra hops in the delivery of IP packets
   with a single destination. As such, it is not intended for direct use
   in a point-to-multipoint communication setting. However, elements  of
   NHRP  may  be  used in certain multicast scenarios for the purpose of
   providing short cut routing. Such an effort is discussed in  [6].  In
   this  case,  NHRP  would  avoid intermediate routers in the multicast
   path. The scalability of providing short-cut  paths  in  a  multicast
   environment remains an issue.

   NHRP  can  be  used  in  host-host,   host-router   and   router-host
   communications.  When  used  in router-router communication, NHRP (as
   defined in [1])  can  produce  persistent  routing  loops.  NHRP  for
   router-router  communication  that avoids persistent forwarding loops
   will be addressed in separate document.

   A special case of router-router communication where  loops  will  not
   occur  is  when the destination host is directly adjacent to the non-
   NBMA interface of the egress router.  If  it  is  believed  that  the
   adjacency of the destination station to the egress router is a stable
   topological configuration, then NHRP  can  safely  be  used  in  this
   router-router  communication  scenario. If the NHRP Request has the Q
   bit set, indicating that the requesting party is a router, and if the
   destination  station  is  directly adjacent to the egress router as a
   stable topological configuration, then the egress router can issue  a
   corresponding  NHRP reply.  If the destination is not adjacent to the
   egress router, and if Q bit is set in the Request, then a  safe  mode
   of  operation for the egress router would be to issue a negative NHRP
   Reply (NAK) for this particular request, thereby enforce data packets
   to follow the routed path.

   As a result of inter-LIS address resolution capability,  NHRP  allows
   the  communicating parties to exchange packets by fully utilizing the
   particular features of the NBMA network. One such example is the  use
   of  QoS  guarantees when the NMBA network is ATM. Here, due to short-
   cut routing, ATM provided QoS guarantees can be  implemented  without
   having  to deal with the issues of re-assembling and re-segmenting IP
   packets at each network layer hop.

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                 NHRP Protocol Applicability Statement     November 1996

   NHRP protocol can be viewed as a client-server interaction.  An  NHRP
   Client  is  the one who issues an NHRP Request. An NHRP Server is the
   one who issues a reply to an NHRP request, or the one who forwards  a
   received  NHRP  request  to another Server. Of course, an NHRP entity
   may act both as a Client  and  a  Server.  NHRP  uses  network  layer
   routing in resolving the NBMA address of the destination. The related
   routing tables can be populated using the services of  network  layer
   dynamic  routing algorithms, or they may be statically configured. If
   network layer dynamic routing algorithms are used, NHRP Servers would
   be  implemented in a router, or in some device that participates to a
   network layer dynamic routing algorithm. If static routing  is  used,
   then  NHRP  Servers do not necessarily have to participate to network
   layer dynamic routing algorithms. All they are required to do  is  to
   reply  to  NHRP  Requests  using  their IP to NBMA address resolution
   tables, or to  forward  them  to  another  Server,  using  some  pre-
   determined forwarding rules.

4. Use of NHRP

   In general, issuing an NHRP request would be an application dependent
   action  [7].  For  applications  that  do  not  have  particular  QoS
   requirements, and that are executed within a short period of time, an
   NBMA short-cut may not be a necessity. In situations where there is a
   "cost" associated with NBMA  short-cuts,  such  applications  may  be
   better  served  by network layer hop-by-hop routing. Here, "cost" may
   be understood in a monetary  context, or as additional strain on  the
   equipment that implements short-cuts. Therefore, there is a trade-off
   between the "cost" of a short-cut path and its utility to  the  user.
   Reference [7] proposes that this trade-off should be addressed at the
   application level. In an environment consisting of LANs  and  routers
   that  are  interconnected  via  dedicated  links,  the  basic routing
   decision is whether to forward a packet to a router, or to  broadcast
   it  locally.  Such  a  decision  on  local vs. remote is based on the
   destination address. When routing IP packets over  an  MBMA  network,
   where   there   is   potentially   a  direct  Source  to  Destination
   connectivity with QoS options, the decision on local vs. remote is no
   longer  as  fundamentally important as in the case where packets have
   to traverse routers that  are  interconnected  via  dedicated  links.
   Then, in an NBMA network with QoS options, the basic decision becomes
   the one of short-cut vs. hop-by-hop network layer  routing.  In  this
   case,  the  relevant criterion becomes applications' QoS requirements
   [7]. NHRP is  particularly  applicable  for  environments  where  the
   decision  on local vs. remote is superseded by the decision on short-
   cut vs. hop-by-hop network layer routing.

   Let us assume that the trade-off is in  favor  of  a  short-cut  NBMA
   route.  Generally, an NHRP request can be issued by a variety of NHRP
   aware entities, including hosts and routers with NBMA interfaces.  If

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                 NHRP Protocol Applicability Statement     November 1996

   an IP packet traverses multiple hops before a short-cut path has been
   established, then there is a chance  that  multiple  short-cut  paths
   could  be  formed. In order to avoid such an undesirable situation, a
   useful operation rule is to authorize only the following entities  to
   issue an NHRP request and to perform short-cut routing.
    i) The host that originates the IP packet, if the host has an NBMA
    ii) The first router along the routing path of the IP packet such
        that the next hop is reachable through the NBMA interface of
        that particular router.
    iii) A policy router within an NBMA network through which the IP
         packet has to traverse.

5. Protocol Scalability

   As previously indicated, NHRP is  defined  for  the  delivery  of  IP
   packets  with a single destination. Thus, this discussion is confined
   to a unicast setting. The scalability of  NHRP  can  be  analyzed  at
   three distinct levels:

     o Client level
     o LIS level
     o Domain level

   Like in any other protocol, at the the Client level, the  scalability
   of  NHRP  is  bounded by the processing and memory limitations of the
   NIC that provides interface to the NBMA network. Such limitations  do
   not  generally  constitute an issue in connectionless  NBMA networks.
   When the NBMA network  is  connection  oriented,  such  as  ATM,  NIC
   limitations might bound the scalability of NHRP in certain cases. For
   example, a server that handles hundreds of requests per second  using
   an  ATM interface might be bounded by the performance characteristics
   of the corresponding NIC. Similarly, when the NHRP Client resides  at
   an  NBMA  interface of a router, memory and processing limitations of
   router's NIC might constitute a bound on  the  scalability  of  NHRP.
   This  is  because  routers  deal  with an aggregation of traffic from
   multiple sources, which in turn creates a potentially large number of
   SVCCs out of the router's NBMA interface.

   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

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                 NHRP Protocol Applicability Statement     November 1996

   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.

   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  [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
   timescale, such as seconds, this situation will have an effect on the

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                 NHRP Protocol Applicability Statement     November 1996

   operation of NHRP. In router-router operation, 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.


   [1] NMBA Next Hop Resolution Protocol (NHRP), J. Luciani,
       D. Katz, D. Piscitello and B. Cole. Work in progress.

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

   [3] Classical IP and ARP over ATM, Mark Laubach, RFC 1577.

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

   [5] MPOA Baseline Version 1, ATM Forum Contribution,
       ATM Forum/95-0824

   [6] Support for Sparse Mode PIM over ATM, Yakov Rekhter and Dino
       Farinacci, Work in progress.

   [7] "Local/Remote" Forwarding Decision in Switched Data Link
       Subnetworks, Yakov Rekhter and Dilip Kandlur, RFC 1937.

   [8] Server Cache Synchronization Protocol (SCSP) - NBMA,
       J. Luciani, G. Armitage and J. Halpern,
       Work in progress.

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

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


   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 of Cisco
   Systems and Curtis Villamizar of ANS.

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                 NHRP Protocol Applicability Statement     November 1996

Author's Address

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

   Phone: +1 617 466 4086

   Expiration Date May 1997

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