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Routing over Large Clouds Working Group                 James V. Luciani
INTERNET-DRAFT                                            (Bay Networks)
<draft-ietf-rolc-nhrp-08.txt>                                  Dave Katz
                                                         (cisco Systems)
                                                        David Piscitello
                                                 (Core Competence, Inc.)
                                                              Bruce Cole
                                                         (cisco Systems)
                                                  Expires December 1996


                NBMA Next Hop Resolution Protocol (NHRP)


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
   and may be updated, replaced, or obsoleted by other documents at any
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   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
   Directories on ds.internic.net (US East Coast), nic.nordu.net
   (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
   Rim).

Abstract

   This document describes the NBMA Next Hop Resolution Protocol (NHRP).
   NHRP can be used by a source station (host or router) connected to a
   Non-Broadcast, Multi-Access (NBMA) subnetwork to determine the
   internetworking layer address and NBMA subnetwork addresses of the
   "NBMA next hop" towards a destination station.  If the destination is
   connected to the NBMA subnetwork, 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.  NHRP is intended for use in a multiprotocol
   internetworking layer environment over NBMA subnetworks.

   This document is intended to be a functional superset of the NBMA
   Address Resolution Protocol (NARP) documented in [1].




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   Operation of NHRP as a means of establishing a transit path across an
   NBMA subnetwork between two routers will be addressed in a separate
   document.


1. Introduction

   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.  A subnetwork can be non-broadcast
   either because it technically doesn't support broadcasting (e.g., an
   X.25 subnetwork) or because broadcasting is not feasible for one
   reason or another (e.g., an SMDS multicast group or an extended
   Ethernet would be too large).  If the destination is connected to the
   NBMA subnetwork, 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.

   One way to model an NBMA network is by using the notion of logically
   independent IP subnets (LISs). LISs, as defined in [3] and [4], have
   the following properties:

      1)  All members of a LIS have the same IP network/subnet number
          and address mask.

      2)  All members within a LIS are directly connected to the same
          NBMA subnetwork.

      3)  All members outside of the LIS are accessed via a router.

      4)  All members within the LIS access each other directly
          (without routers)

   Address resolution as described in [3] and [4] only resolves the next
   hop address if the destination station is a member of the same LIS as
   the source station; otherwise, the source station must forward
   packets to a router that is a member of multiple LIS's.  In multi-LIS
   configurations, hop-by-hop address resolution may not be sufficient
   to resolve the "NBMA next hop" toward the destination station, and IP
   packets may have multiple IP hops through the NBMA subnetwork.

   Another way to model NBMA is by using the notion of Local Address
   Groups (LAGs) [10]. The essential difference between the LIS and the
   LAG models is that while with the LIS model the outcome of the
   "local/remote" forwarding decision is driven purely by addressing
   information, with the LAG model the outcome of this decision is



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   decoupled from the addressing information and is coupled with the
   Quality of Service and/or traffic characteristics.  With the LAG
   model any two entities on a common NBMA network could establish a
   direct communication with each other, irrespective of the entities'
   addresses.

   Support for the LAG model assumes the existence of a mechanism that
   allows any entity (i.e., host or router) connected to an NBMA network
   to resolve an internetworking layer address to an NBMA address for
   any other entity connected to the same NBMA network.  This resolution
   would take place regardless of the address assignments to these
   entities. NHRP describes such a mechanism.  For example, when the
   internetworking layer address is of type IP, once the NBMA next hop
   has been resolved, the source may either start sending IP packets to
   the destination (in a connectionless NBMA subnetwork such as SMDS) or
   may first establish a connection to the destination with the desired
   bandwidth and QOS characteristics (in a connection-oriented NBMA
   subnetwork such as ATM).

   Use of NHRP may be sufficient for hosts doing address resolution when
   those hosts are directly connected to an NBMA subnetwork, allowing
   for straightforward implementations in NBMA stations. NHRP also has
   the capability of determining the egress point from an NBMA
   subnetwork when the destination is not directly connected to the NBMA
   subnetwork and the identity of the egress router is not learned by
   other methods (such as routing protocols).  Optional extensions to
   NHRP provide additional robustness and diagnosability.

   Address resolution techniques such as those described in [3] and [4]
   may be in use when NHRP is deployed.  ARP servers and services over
   NBMA subnetworks may be required to support hosts that are not
   capable of dealing with any model for communication other than the
   LIS model, and deployed hosts may not implement NHRP but may continue
   to support ARP variants such as those described in [3] and [4].  NHRP
   is intended to reduce or eliminate the extra router hops required by
   the LIS model, and can be deployed in a non-interfering manner
   alongside existing ARP services.

   The operation of NHRP to establish transit paths across NBMA
   subnetworks between two routers requires additional mechanisms to
   avoid stable routing loops, and will be described in a separate
   document.









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2. Overview

2.1 Terminology

   The term "network" is highly overloaded, and is especially confusing
   in the context of NHRP.  We use the following terms:

     Internetwork layer--the media-independent layer (IP in the case of
     TCP/IP networks).

     Subnetwork layer--the media-dependent layer underlying the
     internetwork layer, including the NBMA technology (ATM, X.25, SMDS,
     etc.)

     The term "server", unless explicitly stated to the contrary, refers
     to an Next Hop Server (NHS).  An NHS is an entity performing the
     Next Hop Resolution Protocol service within the NBMA cloud.  An NHS
     is always tightly coupled with a routing entity (router, route
     server or edge device) although the converse is not yet guaranteed
     until ubiquitous deployment of this functionality occurs.

     The term "client", unless explicitly stated to the contrary, refers
     to an Next Hop Resolution Protocol client (NHC).  An NHC is an
     entity which initiates NHRP requests of various types in order to
     obtain access to the NHRP service.

     The term "station" generally refers to a host or router which
     contains an NHRP entity.  Occasionally, the term station will
     describe a "user" of the NHRP client or service functionality; the
     difference in usage is largely semantic.

2.2 Protocol Overview

   In this section, we briefly describe how a source S (which
   potentially can be either a router or a host) uses NHRP to determine
   the "NBMA next hop" to destination D.

   For administrative and policy reasons, a physical NBMA subnetwork may
   be partitioned into several, disjoint "Logical NBMA subnetworks".  A
   Logical NBMA subnetwork is defined as a collection of hosts and
   routers that share unfiltered subnetwork connectivity over an NBMA
   subnetwork.  "Unfiltered subnetwork connectivity" refers to the
   absence of closed user groups, address screening or similar features
   that may be used to prevent direct communication between stations
   connected to the same NBMA subnetwork.  (Hereafter, unless otherwise
   specified, we use the term "NBMA subnetwork" to mean *logical* NBMA
   subnetwork.)




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   Placed within the NBMA subnetwork are one or more entities that
   implement the NHRP protocol.  Such stations which are capable of
   answering Next Hop Resolution Requests are known as "Next Hop
   Servers" (NHSs).  Each NHS serves a set of destination hosts, which
   may or may not be directly connected to the NBMA subnetwork.  NHSs
   cooperatively resolve the NBMA next hop within their logical NBMA
   subnetwork.  In addition to NHRP, NHSs may participate in protocols
   used to disseminate routing information across (and beyond the
   boundaries of) the NBMA subnetwork, and may support "classical" ARP
   service as well.

   An NHS maintains a "next-hop resolution" cache, which is a table of
   address mappings (internetwork layer address to NBMA subnetwork layer
   address).  This table can be constructed from information gleaned
   from NHRP Register packets (see Section 5.2.3 and 5.2.4), extracted
   from Next Hop Resolution Requests/Replies that traverse the NHS as
   they are forwarded, or through mechanisms outside the scope of this
   document (examples of such mechanisms include ARP [2, 3, 4] and pre-
   configured tables).  Section 6.2 further describes cache management
   issues.

   A host or router that is not an NHRP server must be configured with
   the identity of the NHS which serves it (see Configuration, Section
   4).

   [Note: for NBMA subnetworks that offer group or multicast addressing
   features, it may be desirable to configure stations with a group
   identity for NHSs, i.e., addressing information that would solicit a
   response from "all NHSs".  The means whereby a group of NHSs divide
   responsibilities for next hop resolution are not described here.]

   Whether or not a particular station within the NBMA subnetwork which
   is making use of the NHRP protocol needs to be able to act as an NHS
   is a local matter.  For a station to avoid providing NHS
   functionality, there must be one or more NHSs within the NBMA
   subnetwork which are providing authoritative NBMA information on its
   behalf.  If NHRP is to be able to resolve the NBMA address for
   stations that lack NHS functionality, these serving NHSs must exist
   along all routed paths between Next Hop Resolution Requesters and the
   station which cannot answer Next Hop Resolution Requests.

   The protocol proceeds as follows.  An event occurs triggering station
   S to want to resolve the NBMA address of a path to D.  This is most
   likely to be when a data packet addressed to station D is to be
   emitted from station S (either because station S is a host, or
   station S is a transit router), but the address resolution could also
   be triggered by other means (a routing protocol update packet, for
   example).  Station S first determines the next hop to station D



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   through normal routing processes (for a host, the next hop may simply
   be the default router; for routers, this is the "next hop" to the
   destination internetwork layer address).  If the next hop is
   reachable through one of its NBMA interfaces, S constructs an Next
   Hop Resolution Request packet (see Section 5.2.1) containing station
   D's internetwork layer address as the (target) destination address,
   S's own internetwork layer address as the source address (Next Hop
   Resolution Request initiator), and station S's NBMA addressing
   information.  Station S may also indicate that it prefers an
   authoritative Next Hop Resolution Reply (i.e., station S only wishes
   to receive a Next Hop Resolution Reply from the NHS-speaker that
   maintains the NBMA-to-internetwork layer address mapping for this
   destination).  Station S emits the Next Hop Resolution Request packet
   towards the destination.

   If the Next Hop Resolution Request is triggered by a data packet,
   station S may choose to dispose of the data packet while awaiting a
   Next Hop Resolution Reply in one of the following ways:

     (a)  Drop the packet
     (b)  Retain the packet until the Next Hop Resolution Reply arrives
          and a more optimal path is available
     (c)  Forward the packet along the routed path toward D

   The choice of which of the above to perform is a local policy matter,
   though option (c) is the recommended default, since it may allow data
   to flow to the destination while the NBMA address is being resolved.
   Note that an Next Hop Resolution Request for a given destination MUST
   NOT be triggered on every packet, though periodically retrying a Next
   Hop Resolution Request is permitted.

   When the NHS receives an Next Hop Resolution Request, a check is made
   to see if it "serves" station D, i.e., the NHS checks to see if there
   is a "next hop" entry for D in its next-hop resolution cache.  If the
   NHS does not serve D, the NHS forwards the Next Hop Resolution
   Request to another NHS.  (Mechanisms for determining how to forward
   the Next Hop Resolution Request are discussed in Section 3,
   Deployment.) Note that NHSs must be next hops to one another in order
   for forwarding of NHRP packets to be possible.

   If this NHS serves D, the NHS resolves station D's NBMA address, and
   generates a positive Next Hop Resolution Reply (denoted by a 0 Code
   in the reply) on D's behalf.  (Next Hop Resolution Replies in this
   scenario are always marked as "authoritative".)  The Next Hop
   Resolution Reply packet contains the next hop internetwork layer
   address and the NBMA address for station D and is sent back to S.
   (Note that if station D is not on the NBMA subnetwork, the next hop
   internetwork layer address will be that of the egress router through



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   which packets for station D are forwarded.)

   An NHS receiving a Next Hop Resolution Reply may cache the NBMA next
   hop information contained therein.  To a subsequent Next Hop
   Resolution Request, this NHS may respond with the cached, non-
   authoritative, NBMA next hop information or with cached negative
   information, if the NHS is allowed to do so, see section 5.2.2 and
   6.2.  Non-authoritative Next Hop Resolution Replies are distinguished
   from authoritative Next Hop Resolution Replies so that if a
   communication attempt based on non-authoritative information fails, a
   source station can choose to send an authoritative Next Hop
   Resolution Request.  NHSs MUST NOT respond to authoritative Next Hop
   Resolution Requests with cached information.

     [Note: An Next Hop Resolution Reply can be returned directly to the
     Next Hop Resolution Request initiator, i.e., without traversing the
     list of NHSs that forwarded the Next Hop Resolution Request, if all
     of the following criteria are satisfied:

       (a) Direct communication is available via datagram transfer
           (e.g., SMDS) or the NHS has an existing virtual circuit
           connection to the Next Hop Resolution Request initiator or is
           permitted to open one.
       (b) The Next Hop Resolution Request initiator has not included the
           NHRP Reverse NHS record Extension (see Section 5.3.5).
       (c) The authentication policy in force permits direct
           communication between the NHS and the Next Hop Resolution
           Request initiator.

     The purpose of allowing an NHS to send a Next Hop Resolution Reply
     directly is to reduce response time.  A consequence of allowing a
     direct Next Hop Resolution Reply is that NHSs that would under
     normal circumstances be traversed by the Next Hop Resolution Reply
     would not cache next hop information contained therein.]

   The process of forwarding the Next Hop Resolution Request is repeated
   until the Next Hop Resolution Request is satisfied, or an error
   occurs (e.g., no NHS in the NBMA subnetwork can resolve the Next Hop
   Resolution Request.) If the determination is made that station D's
   next hop cannot be resolved, a negative Next Hop Resolution Reply
   (NAK) is returned.  This occurs when (a) no next-hop resolution
   information is available for station D from any NHS, or (b) an NHS is
   unable to forward the Next Hop Resolution Request (e.g., connectivity
   is lost).

   NHRP Registration Requests, NHRP Purge Requests, NHRP Purge Replies,
   and NHRP Error Indications follow the routed path from sender to
   receiver in the same fashion that Next Hop Resolution Requests and



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   Next Hop Resolution Replies do.  That is, "requests" and
   "indications" follow the routed path from Source Protocol Address
   (which is the address of the station initiating the communication) to
   the Destination Protocol Address.  "Replies", on the other hand,
   follow the routed path from the Destination Protocol Address back to
   the Source Protocol Address with the exceptions mentioned above where
   a direct VC may be created.  In the case of a NHRP Registration
   Reply, the packet is always returned via a direct VC (see Section
   5.2.4).

   NHRP Requests and NHRP Replies MUST NOT cross the borders of a
   logical NBMA subnetwork (an explicit NBMA subnetwork identifier may
   be included as an extension in the Next Hop Resolution Request, see
   section 5.3.2).  Thus, the internetwork layer traffic out of and into
   a logical NBMA subnetwork always traverses an internetwork layer
   router at its border.  Internetwork layer filtering can then be
   implemented at these border routers.

   NHRP optionally provides a mechanism to send a Next Hop Resolution
   Reply which contains aggregated NBMA next hop information.  Suppose
   that router X is the NBMA next hop from station S to station D.
   Suppose further that X is an egress router for all stations sharing
   an internetwork layer address prefix with station D.  When an Next
   Hop Resolution Reply is generated in response to a Next Hop
   Resolution Request, the responder may augment the internetwork layer
   address of station D with a prefix length (see Section 5.2.0.1).  A
   subsequent (non-authoritative) Next Hop Resolution Request for some
   destination that shares an internetwork layer address prefix (for the
   number of bits specified in the prefix length) with D may be
   satisfied with this cached information.  See section 6.2 regarding
   caching issues.

   To dynamically detect subnetwork-layer filtering in NBMA subnetworks
   (e.g., X.25 closed user group facility, or SMDS address screens), to
   trace the routed path that an NHRP packet takes, or to provide loop
   detection and diagnostic capabilities, a "Route Record" may be
   included in NHRP packets (see Sections 5.3.4 and 5.3.5).  The Route
   Record extensions contain the internetwork (and subnetwork layer)
   addresses of all intermediate NHSs between source and destination (in
   the forward direction) and between destination and source (in the
   reverse direction).  When a source station is unable to communicate
   with the responder (e.g., an attempt to open an SVC fails), it may
   attempt to do so successively with other subnetwork layer addresses
   in the Route Record until it succeeds (if authentication policy
   permits such action).  This approach can find a suitable egress point
   in the presence of subnetwork-layer filtering (which may be
   source/destination sensitive, for instance, without necessarily
   creating separate logical NBMA subnetworks) or subnetwork-layer



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   congestion (especially in connection-oriented media).


3. Deployment

   Next Hop Resolution Requests traverse one or more hops within an NBMA
   subnetwork before reaching the station that is expected to generate a
   response.  Each station, including the source station, chooses a
   neighboring NHS to which it will forward the Next Hop Resolution
   Request.  The NHS selection procedure typically involves applying a
   destination protocol layer address to the protocol layer routing
   table which causes a routing decision to be returned.  This routing
   decision is then used to forward the Next Hop Resolution Request to
   the downstream NHS. The destination protocol layer address previously
   mentioned is carried within the Next Hop Resolution Request packet.
   Note that even though a protocol layer address was used to acquire a
   routing decision, NHRP packets are not encapsulated within a protocol
   layer header but rather are carried at the NBMA layer using the
   encapsulation described in Section 5.

   Each NHS/router examines the Next Hop Resolution Request packet on
   its way toward the destination.  Each NHS which the NHRP packet
   traverses on the way to the packet's destination might modify the
   packet (e.g., updating the Forward Record extension).  Ignoring error
   situations, the Next Hop Resolution Request eventually arrives at a
   station that is to generate an Next Hop Resolution Reply.  This
   responding station "serves" the destination.  The responding station
   generates a Next Hop Resolution Reply using the source protocol
   address from within the NHRP packet to determine where the Next Hop
   Resolution Reply should be sent.

   Rather than use routing to determine the next hop for an NHRP packet,
   an NHS may use static configuration information (or other applicable
   means) in order to determine to which neighboring NHSs to forward the
   Next Hop Resolution Request packet.  The use of static configuration
   information for this purpose is beyond the scope of this document.

   In order to forward NHRP packets to a neighboring NHS, NHRP clients
   must nominally be configured with the NBMA address of at least one
   NHS.  In practice, a client's default router should also be its NHS
   in that way a client may be able to know the NBMA address of its NHS
   from the configuration which was already required for the client to
   be able to communicate.

   The NHS serving a particular destination must lie along the routed
   path to that destination.  In practice, this means that all egress
   routers must double as NHSs serving the destinations beyond them, and
   that hosts on the NBMA subnetwork are served by routers that double



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   as NHSs.  Also, this implies that forwarding of NHRP packets within
   an NBMA subnetwork requires a contiguous deployment of NHRP capable
   routers.  During migration to NHRP, it cannot be expected that all
   routers within the NBMA subnetwork are NHRP capable.  Thus, NHRP
   traffic which would otherwise need to be forwarded through such
   routers can be expected to be dropped due to the NHRP packet not
   being recognized.  In this case, NHRP will be unable to establish any
   transit paths whose discovery requires the traversal of the non-NHRP
   speaking routers.  If the client has tried and failed to acquire a
   cut through path then the client should use the network layer routed
   path as a default.

   If a subnetwork offers a link layer group addressing or multicast
   feature, the client (station) may be configured with a group address
   assigned to the group of next-hop servers.  The client might then
   submit Next Hop Resolution Requests to the group address, eliciting a
   response from one or more NHSs, depending on the response strategy
   selected.  Note that the constraints described in Section 2 regarding
   directly sending Next Hop Resolution Reply may apply.


4. Configuration

   Clients

     To participate in NHRP, a client connected to an NBMA subnetwork
     should be configured with the NBMA address(es) of its NHS(s)
     (alternatively, it should be configured with a means of acquiring
     them, i.e., the group address that members of a NHS group use for
     the purpose of address or next-hop resolution.)  The NHS(s) will
     likely also represent the client's default or peer routers, so
     their NBMA addresses may be obtained from the client's existing
     configuration.  If the client is attached to several subnetworks
     (including logical NBMA subnetworks), the client should also be
     configured to receive routing information from its NHS(s) and peer
     routers so that it can determine which internetwork layer networks
     are reachable through which subnetworks.

   Next Hop Servers

     An NHS is configured with knowledge of its own internetwork layer
     and NBMA addresses and a logical NBMA subnetwork identifier (see
     Section 5.3.2).  An NHS MAY also be configured with a set of
     internetwork layer address prefixes that correspond to the
     internetwork layer addresses of the stations it serves. If a served
     client is attached to several subnetworks, the NHS may also need to
     be configured to advertise routing information to such client.




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     If an NHS acts as an egress router for stations connected to other
     subnetworks than the NBMA subnetwork, the NHS must, in addition to
     the above, be configured to exchange routing information between
     the NBMA subnetwork and these other subnetworks.

     In all cases, routing information is exchanged using conventional
     intra-domain and/or inter-domain routing protocols.

     The NBMA addresses of the stations served by the NHS may be learned
     via NHRP Register packets or manual configuration.


5. NHRP Packet Formats
   This section describes the format of NHRP packets.  In the following,
   unless otherwise stated explicitly, the unqualified term "request"
   refers generically to any of the NHRP packet types which are
   "requests".  Further, unless otherwise stated explicitly, the
   unqualified term "reply" refers generically to any of the NHRP packet
   types which are "replies".

   An NHRP packet consists of a Fixed Part, a Mandatory Part, and an
   Extensions Part.  The Fixed Part is common to all NHRP packet types.
   The Mandatory Part MUST be present, but varies depending on packet
   type.  The Extensions Part also varies depending on packet type, and
   need not be present.

   The length of the Fixed Part is fixed at 20 octets.  The length of
   the Mandatory Part is determined by the contents of the extensions
   offset field (ar$extoff).  If ar$extoff=0x0 then the mandatory part
   length is equal to total packet length (ar$pktsz) minus 20 otherwise
   the mandatory part length is equal to ar$extoff minus 20.  The length
   of the Extensions Part is implied by ar$pktsz minus ar$extoff.  NHSs
   may increase the size of an NHRP packet as a result of extension
   processing, but not beyond the offered maximum SDU size of the NBMA
   network.

   NHRP packets are encapsulated using the native formats used on the
   particular NBMA network over which NHRP is carried.  For example,
   SMDS networks always use LLC/SNAP encapsulation at the NBMA layer,
   and an NHRP packet is preceded by the following LLC/SNAP
   encapsulation:

   [0xAA-AA-03] [0x00-00-5E] [0x00-03]

   The first three octets are LLC, indicating that SNAP follows.  The
   SNAP OUI portion is the IANA's OUI, and the SNAP PID portion
   identifies NHRP (see [4]).




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   ATM uses either LLC/SNAP encapsulation of each packet (including
   NHRP), or uses no encapsulation on VCs dedicated to a single protocol
   (see [7]).  Frame Relay and X.25 both use NLPID/SNAP encapsulation or
   identification of NHRP, using a NLPID of 0x0080 and the same SNAP
   contents as above (see [8], [9]).

   Fields marked "unused" MUST be set to zero on transmission, and
   ignored on receipt.

   Most packet types (ar$op.type) have both internetwork layer
   protocol-independent fields and protocol-specific fields. The
   protocol type/snap fields (ar$pro.type/snap) qualify the format of
   the protocol-specific fields.

5.1 NHRP Fixed Header

   The Fixed Part of the NHRP packet contains those elements of the NHRP
   packet which are always present and do not vary in size with the type
   of packet.


          0                   1                   2                   3
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |            ar$afn             |          ar$pro.type          |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                          ar$pro.snap                          |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |  ar$pro.snap  |   ar$hopcnt   |            ar$pktsz           |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |           ar$chksum           |            ar$extoff          |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         | ar$op.version |   ar$op.type  |    ar$shtl    |    ar$sstl    |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   ar$afn
     Defines the type of "link layer" addresses being carried.  This
     number is taken from the 'address family number' list specified in
     [6].  This field has implications to the coding of ar$shtl and
     ar$sstl as described below.

   ar$pro.type
     field is a 16 bit unsigned integer representing the following
     number space:

       0x0000 to 0x00FF  Protocols defined by the equivalent NLPIDs.
       0x0100 to 0x03FF  Reserved for future use by the IETF.



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       0x0400 to 0x04FF  Allocated for use by the ATM Forum.
       0x0500 to 0x05FF  Experimental/Local use.
       0x0600 to 0xFFFF  Protocols defined by the equivalent Ethertypes.

     (based on the observations that valid Ethertypes are never smaller
     than 0x600, and NLPIDs never larger than 0xFF.)

   ar$pro.snap
     When ar$pro.type has a value of 0x0080, a SNAP encoded extension is
     being used to encode the protocol type. This snap extension is
     placed in the ar$pro.snap field.  This is termed the 'long form'
     protocol ID. If ar$pro != 0x0080 then the ar$pro.snap field MUST be
     zero on transmit and ignored on receive. The ar$pro.type field
     itself identifies the protocol being referred to. This is termed
     the 'short form' protocol ID.

     In all cases, where a protocol has an assigned number in the
     ar$pro.type space (excluding 0x0080) the short form MUST be used
     when transmitting NHRP messages. Additionally, where a protocol has
     valid short and long forms of identification, receivers MAY choose
     to recognize the long form.

   ar$hopcnt
     The Hop count indicates the maximum number of NHSs that an NHRP
     packet is allowed to traverse before being discarded.

   ar$pktsz
     The total length of the NHRP packet, in octets (excluding link
     layer encapsulation).

   ar$chksum
     The standard IP checksum over the entire NHRP packet (starting with
     the fixed header).  If only the hop count field is changed, the
     checksum is adjusted without full recomputation.  The checksum is
     completely recomputed when other header fields are changed.

   ar$extoff
     This field identifies the existence and location of NHRP
     extensions.  If this field is 0 then no extensions exist otherwise
     this field represents the offset from the beginning of the NHRP
     packet (i.e., starting from the ar$afn field) of the first
     extension.

   ar$op.version
     This field is set to 0x01 for NHRP version 1.

   ar$op.type
     This is the NHRP packet type: NHRP Next Hop Resolution Request(1),



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     NHRP Next Hop Resolution Reply(2), NHRP Registration Request(3),
     NHRP Registration Reply(4), NHRP Purge Request(5), NHRP Purge
     Reply(6), or NHRP Error Indication(7).  Use of NHRP packet Types in
     the range 128 to 255 are reserved for research or use in other
     protocol development and will be administered by IANA.

   ar$shtl
     Type & length of source NBMA address interpreted in the context of
     the 'address family number'[6] indicated by ar$afn (e.g.,
     ar$afn=0x0003 for NSAP, ar$afn=8 for E.164).  When ar$afn=0x000F
     (E.164 address plus NSAP subaddress) then both ar$shtl and ar$sstl
     must be coded appropriately (see below).

   ar$sstl
     Type & length of source NBMA subaddress interpreted in the context
     of the 'address family number'[6] indicated by ar$afn (e.g.,
     ar$afn=0x000F for NSAP).  When an NBMA technology has no concept of
     a subaddress, the subaddress length is always coded ar$sstl = 0 and
     no storage is allocated for the subaddress in the appropriate
     mandatory part.

   ar$shtl, ar$sstl, subnetwork layer addresses, and subnetwork layer
   subaddresses fields are coded as follows:


                7 6 5 4 3 2 1 0
               +-+-+-+-+-+-+-+-+
               |0|x|  length   |
               +-+-+-+-+-+-+-+-+

   The most significant bit is reserved and MUST be set to zero. The
   second most significant bit (x) is a flag indicating whether the
   address being referred to is in:

      - NSAP format (x = 0).
      - Native E.164 format (x = 1).

   For NBMA technologies that use neither NSAP nor E.164 format
   addresses, x = 0 SHALL be used to indicate the native form for the
   particular NBMA technology.

   In the case where the NBMA is ATM, if a subaddress is to be included
   then ar$afn MUST be set to 0x000F which means that if a subaddress
   exists then it is of type NSAP.

   The bottom 6 bits is an unsigned integer value indicating the length
   of the associated NBMA address in octets. If this value is zero the
   flag x is ignored.



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5.2.0 Mandatory Part

   The Mandatory Part of the NHRP packet contains the operation specific
   information (e.g., Next Hop Resolution Request/Reply, etc.) and
   variable length data which is pertinent to the packet type.

5.2.0.1 Mandatory Part Format

   Sections 5.2.1 through 5.2.6 have a very similar mandatory part.
   This mandatory part includes a common header and zero or more Client
   Information Entries (CIEs). Section 5.2.7 has a different format
   which is specified in that section.

   The common header looks like the following:

          0                   1                   2                   3
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         | Src Proto Len | Dst Proto Len |           Flags               |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                         Request ID                            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |            Source NBMA Address (variable length)              |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Source NBMA Subaddress (variable length)             |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Source Protocol Address (variable length)            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |       Destination  Protocol Address (variable length)         |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   And the CIEs have the following format:



















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          0                   1                   2                   3
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |    Code       | Prefix Length |         unused                |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |  Maximum Transmission Unit    |        Holding Time           |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |  Cli Addr T/L | Cli SAddr T/L | Cli Proto Len |  Preference   |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |            Client NBMA Address (variable length)              |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |           Client NBMA Subaddress (variable length)            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Client Protocol Address (variable length)            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                              .....................
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |    Code       | Prefix Length |         unused                |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |  Maximum Transmission Unit    |        Holding Time           |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |  Cli Addr T/L | Cli SAddr T/L | Cli Proto Len |  Preference   |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |            Client NBMA Address (variable length)              |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |           Client NBMA Subaddress (variable length)            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Client Protocol Address (variable length)            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The meanings of the fields are as follows:

   Src Proto Len
     This field holds the length in octets of the Source Protocol
     Address.

   Dst Proto Len
     This field holds the length in octets of the Destination Protocol
     Address.

   Flags
     These flags are specific to the given message type and they are
     explained in each section.

   Request ID
     A value which, when coupled with the address of the source,
     provides a unique identifier for the information contained in a
     "request" packet.  This value is copied directly from an "request"



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     packet into the associated "reply".  When a sender of a "request"
     receives "reply", it will compare the Request ID and source address
     information in the received "reply" against that found in its
     outstanding "request" list.  When a match is found then the
     "request" is considered to be acknowledged.

     The value is taken from a 32 bit counter that is incremented each
     time a new "request" is transmitted.  The same value MUST be used
     when resending a "request", i.e., when a "reply" has not been
     received for a "request" and a retry is sent after an appropriate
     interval.

   The NBMA address/subaddress form specified below allows combined
   E.164/NSAPA form of NBMA addressing. For NBMA technologies without a
   subaddress concept, the subaddress field is always ZERO length and
   ar$sstl = 0.

   Source NBMA Address
     The Source NBMA address field is the address of the source station
     which is sending the "request". If the field's length as specified
     in ar$shtl is 0 then no storage is allocated for this address at
     all.

   Source NBMA SubAddress
     The Source NBMA subaddress field is the address of the source
     station which is sending the "request".  If the field's length as
     specified in ar$sstl is 0 then no storage is allocated for this
     address at all.

   Source Protocol Address
     This is the protocol address of the station which is sending the
     "request".  This is also the protocol address of the station toward
     which a "reply" packet is sent.

   Destination Protocol Address
     This is the protocol address of the station toward which a
     "request" packet is sent.

   Code
     This field is message specific.  See the relevant message sections
     below.  In general, this field is a NAK code; i.e., when the field
     is 0 in a reply then the packet is acknowledging a request and if
     it contains any other value the packet contains a negative
     acknowledgment.

   Prefix Length
     This field is message specific.  See the relevant message sections
     below.  In general, however, this fields is used to indicate that



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     the information carried in an NHRP the message pertains to an
     equivalence class of internetwork layer addresses rather than just
     a single internetwork layer address specified. All internetwork
     layer addresses that match the first "Prefix Length" bit positions
     for the specific internetwork layer address are included in the
     equivalence class.

   Maximum Transmission Unit
     This field gives the maximum transmission unit for the relevant
     client station.  If this value is 0 then either the default MTU is
     used or the MTU negotiated via signaling is used if such
     negotiation is possible for the given NBMA.

   Holding Time
     The Holding Time field specifies the number of seconds for which
     the Next Hop NBMA information specified in the CIE is considered to
     be valid.  Cached information SHALL be discarded when the holding
     time expires.  This field must be set to 0 on a NAK.

   Cli Addr T/L
     Type & length of next hop NBMA address specified in the CIE.  This
     field is interpreted in the context of the 'address family
     number'[6] indicated by ar$afn (e.g., ar$afn=0x0003 for ATM).

   Cli SAddr T/L
     Type & length of next hop NBMA subaddress specified in the CIE.
     This field is interpreted in the context of the 'address family
     number'[6] indicated by ar$afn (e.g., ar$afn=0x0015 for ATM makes
     the address an E.164 and the subaddress an ATM Forum NSAP address).
     When an NBMA technology has no concept of a subaddress, the
     subaddress is always null with a length of 0.  When the address
     length is specified as 0 no storage is allocated for the address.

   Cli Proto Len
     This field holds the length in octets of the Client Protocol
     Address specified in the CIE.

   Preference
     This field specifies the preference for use of the specific CIE
     relative to other CIEs.  Higher values indicate higher preference.
     Action taken when multiple CIEs have equal or highest preference
     value is a local matter.

   Client NBMA Address
     This is the client's NBMA address.

   Client NBMA SubAddress
     This is the client's NBMA subaddress.



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   Client Protocol Address
     This is the client's internetworking layer address specified.

   Note that an NHS SHOULD NOT cache source information which is in an
   NHRP message because this information could be inappropriately used
   to set up a cut-through without doing proper filtering along a routed
   path.  Further, in the case where a distributed router exists in the
   network, incorrect or incomplete information may be included in the
   source information.

5.2.1 NHRP Next Hop Resolution Request

The NHRP Next Hop Resolution Request packet has a Type code of 1. Its
mandatory part is coded as described in Section 5.2.0.1 and the message
specific meanings of the fields are as follows:

   Flags - The flags field is coded as follows:

      0                   1
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Q|A|B|U|         unused        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Q
       Set if the station sending the Next Hop Resolution Request is a
       router;  clear if the it is a host.

     A
       This bit is set in a Next Hop Resolution Request if only
       authoritative next hop information is desrired and is clear
       otherwise.  See the NHRP Next Hop Resolution Reply section below
       for further details on the "A" bit and its usage.

     B
       Unused (clear on transmit)

     U
       This is the Uniqueness bit. This bit aids in duplicate address
       detection.  When this bit is set in an NHRP Resolution Request
       and one or more entries exist in the NHS cache which meet the
       requirements of the NHRP Resolution Request then only the CIE in
       the NHS's cache with this bit set will be returned.  Note that
       even if this bit was set at registration time, there may still be
       multiple CIEs that might fulfill the NHRP Resolution Request
       because an entire subnet can be registered through use of the
       Prefix Length in the CIE and the address of interest might be
       within such a subnet. If the "uniqueness" bit is set and the



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       responding NHS has one or more cache entries which match the
       request but no such cache entry has the "uniqueness" bit set,
       then the NHRP Resolution Reply returns with a NAK code of "13 -
       Binding Exists But Is Not Unique" and no CIE is included.  If a
       client wishes  to  receive  non- unique  Next  Hop Entries, then
       the client must have the "uniqueness" bit set to zero in its NHRP
       Resolution Request. Note that when this bit is set in an NHRP
       Registration Request, only a single CIE may be specified in the
       NHRP Registration Request and that CIE must have the Prefix
       Length field set to 0xFF.

   Zero or one CIEs (see Section 5.2.0.1) may be specified in an NHRP
   Next Hop Resolution Request.  If one is specified then that entry
   carries the pertinent information for the client sourcing the NHRP
   Next Hop Resolution Request.  Usage of the CIE in the NHRP Next Hop
   Resolution Request is described below:

     Prefix Length
       If a CIE is specified in the NHRP Next Hop Resolution Request
       then the Prefix Length field may be used to qualify the widest
       acceptable prefix which may be used to satisfy the NHRP Next Hop
       Resolution Request.  In the case of NHRP Next Hop Resolution
       Request/Reply, the Prefix Length specifies the equivalence class
       of addresses which match the first "Prefix Length" bit positions
       of the Destination Protocol Address.  If this field is set to
       0x00 then this field MUST be ignored.  If the "U" bit is set in
       the common header then this field MUST be set to 0xFF.

     Maximum Transmission Unit
       This field gives the maximum transmission unit for the source
       station.  A possible use of this field in the Next Hop Resolution
       Request packet is for the Next Hop Resolution Requester to ask
       for a target MTU. In lieu of that usage, the CIE must be omitted.

     All other fields in the CIE MUST be ignored and SHOULD be set to 0.

5.2.2 NHRP Next Hop Resolution Reply

The NHRP Next Hop Resolution Reply packet has a Type code of 2. CIEs
correspond to Next Hop Entries in an NHS's cache which match the
criteria in the NHRP Next Hop Resolution Request.  Its mandatory part is
coded as described in Section 5.2.0.1.  The message specific meanings of
the fields are as follows:

   Flags - The flags field is coded as follows:






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      0                   1
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Q|A|B|U|         unused        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Q
       Copied from the Next Hop Resolution Request.  Set if the Next Hop
       Resolution Requester is a router;  clear if it is a host.

     A
       Set if the next hop CIE in the Next Hop Resolution Reply is
       authoritative; clear if the Next Hop Resolution Reply is non-
       authoritative.

       When an NHS receives a Next Hop Resolution Request for
       authoritative information for which it is the authoritative
       source, it MUST respond with a Next Hop Resolution Reply
       containing all and only those next hop CIEs which are contained
       in the NHS's cache which both match the criteria of the Next Hop
       Resolution Request and are authoritative cache entries.  An NHS
       is an authoritative source for a Next Hop Resolution Request if
       the information in the NHS's cache matches the Next Hop
       Resolution Request criteria and that information was obtained
       through a NHRP Registration Request or through synchronization
       with an NHS which obtained this information through a NHRP
       Registration Request.  An authoritative cache entry is one which
       is obtained through a NHRP Registration Request or through
       synchronization with an NHS which obtained this information
       through a NHRP Registration Request.

       An NHS obtains non-authoriative CIEs through promiscuous
       listening to NHRP packets other than NHRP Registrations which are
       directed at it.  A Next Hop Resolution Request which indicates a
       request for non-authoritative information should cause a Next Hop
       Resolution Reply which contains all entries in the replying NHS's
       cache (i.e., both authoritative and non-authoritative) which
       match the criteria specified in the request.

     B
       Set if the association between the destination and the next hop
       information is guaranteed to be stable for the lifetime of the
       information (the holding time).  This is the case if the Next Hop
       protocol address identifies the destination (though it may be
       different in value than the Destination address if the
       destination system has multiple addresses) or if the destination
       is not connected directly to the NBMA subnetwork but the egress
       router to that destination is guaranteed to be stable (such as



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       when the destination is immediately adjacent to the egress router
       through a non-NBMA interface).  This information affects caching
       strategies (see section 6.2).

     U
       This is the Uniqueness bit. See the NHRP Resolution Request
       section above for details.  When this bit is set only, only one
       CIE is included since only one unique binding should exist in an
       NHS's cache.

   One or more CIEs are specified in the NHRP Next Hop Resolution Reply.
   Each CIE contains NHRP next hop information which the responding NHS
   has cached and which matches the parameters specified in the NHRP
   Next Hop Resolution Request.  If no match is found by the NHS issuing
   the NHRP Next Hop Resolution Reply then a single CIE is enclosed with
   the a CIE Code set appropriately (see below) and all other fields
   MUST be ignored and SHOULD be set to 0.  In order to facilitate the
   use of NHRP by minimal client implementations, the first CIE MUST
   contain the next hop with the highest preference value so that such
   an implementation need parse only a single CIE.

     Code
       If this field is set to zero then this packet contains a
       positively acknowledged NHRP Resolution Reply.  If this field
       contains any other value then this message contains an NHRP
       Resolution Reply NAK which means that an appropriate
       internetworking layer to NBMA address binding was not available
       in the responding NHS's cache.  If NHRP Resolution Reply contains
       a Client Information Entry with a NAK Code other than 0 then it
       MUST NOT contain any other CIE.  Currently defined NAK Codes are
       as follows:

       12 - No Internetworking Layer Address to NBMA Address Binding
       Exists

         This code states that there were absolutely no internetworking
         layer address to NBMA address bindings found in the responding
         NHS's cache.

       13 - Binding Exists But Is Not Unique

         This code states that there were one or more internetworking
         layer address to NBMA address bindings found in the responding
         NHS's cache, however none of them had the uniqueness bit set.

     Prefix Length
       In the case of NHRP Next Hop Resolution Reply, the Prefix Length
       specifies the equivalence class of addresses which match the



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       first "Prefix Length" bit positions of the Destination Protocol
       Address.

     Holding Time
       The Holding Time specified in a CIE of an NHRP Resolution Reply
       is the amount of time remaining before the expiration of the
       client information which is cached at the replying NHS.  It is
       not the value which was registered by the client.

     The remainder of the fields for the CIE for each next hop are
     filled out as they were defined when the next hop was registered
     with the responding NHS (or one of the responding NHS's
     synchronized servers) via the NHRP Registration Request.

   Load-splitting may be performed when more than one Client Information
   Entry is returned to a requester when equal preference values are
   specified.  Also, the alternative addresses may be used in case of
   connectivity failure in the NBMA subnetwork (such as a failed call
   attempt in connection-oriented NBMA subnetworks).

   Any extensions present in the Next Hop Resolution Request packet MUST
   be present in the NHRP Next Hop Resolution Reply even if the
   extension is non-Compulsory.

   If an unsolicited NHRP Next Hop Resolution Reply packet is received,
   an Error Indication of type Invalid Next Hop Resolution Reply
   Received SHOULD be sent in response.

5.2.3 NHRP Registration Request

   The NHRP Registration Request is sent from a station to an NHS to
   notify the NHS of the station's NBMA information.  It has a Type code
   of 3. Each CIE corresponds to Next Hop information which is to be
   cached at an NHS.  The mandatory part of an NHRP Registration Request
   is coded as described in Section 5.2.0.1.  The message specific
   meanings of the fields are as follows:

   Flags - The flags field is coded as follows:

      0                   1
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |U|         unused              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     U
       This is the Uniqueness bit. When set in an NHRP Registration
       Request, this bit indicates that the registration of the protocol



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       address is unique within the confines of the set of synchronized
       NHSs.  This "uniqueness" qualifier MUST be stored in the NHS/NHC
       cache.  Any attempt to register a binding between the protocol
       address and an NBMA address when this bit is set MUST be rejected
       with a Code of "14 - Unique Internetworking Layer Address Already
       Registered" if the replying NHS already has a cache entry for the
       protocol address and the cache entry has the "uniqueness" bit
       set.  A registration of a CIE's information is rejected when the
       CIE is returned with the Code field set to anything other than
       0x00.  See the description of the uniqueness bit in NHRP
       Resolution Request section above for further details.  When this
       bit is set only, only one CIE MAY be included in the NHRP
       Registration Request.


   Request ID
     The request ID has the same meaning as described in Section
     5.2.0.1.  However, the request ID for NHRP Registrations which is
     maintained at each client MUST be kept in non-volatile memory so
     that when a client crashes and reregisters there will be no
     inconsistency in the NHS's database.  In order to reduce the
     overhead associated with updating non-volatile memory, the actual
     updating need not be done with every increment of the Request ID
     but could be done, for example, every 50 or 100 increments.  In
     this scenario, when a client crashes and reregisters it knows to
     add 100 to the value of the Request ID in the non-volatile memory
     before using the Request ID for subsequent registrations.


   One or more CIEs are specified in the NHRP Registration Request.
   Each CIE contains next hop information which a client is attempting
   to register with its servers.  Generally, all fields in CIEs enclosed
   in NHRP Registration Requests are coded as described in Section
   5.2.0.1.  However, if a station is only registering itself with the
   NHRP Registration Request then it MAY code the Cli Addr T/L, Cli
   SAddr T/L, and Cli Proto Len as zero which signifies that the client
   address information is to be taken from the source information in the
   common header (see Section 5.2.0.1).  Below, further clarification is
   given for some fields in a CIE in the context of a NHRP Registration
   Request.

     Code
       This field is set to 0x00 in NHRP Registration Requests.

     Prefix Length

       This field may be used in a NHRP Registration Request to register
       equivalence information for the Client Protocol Address specified



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       in the CIE of an NHRP Registration Request In the case of NHRP
       Registration Request, the Prefix Length specifies the equivalence
       class of addresses which match the first "Prefix Length" bit
       positions of the Client Protocol Address.  If this field is set
       to 0x00 then this field MUST be ignored and no equivalence
       information is assumed (i.e., only Client Protocol Address is
       bound to the NBMA information).  If the "U" bit is set in the
       common header then this field MUST be set to 0xFF.

   This packet is used to register a station's NHRP information with its
   NHSs, as configured or known through conventional routing means.
   NHSs may also be configured with the identities of stations that they
   serve.  If an NHS receives an NHRP Registration Request packet which
   has the Destination Protocol Address field set to an address other
   than the NHS's own protocol address then the NHS MUST forward the
   packet along the routed path toward the Destination Protocol Address.

   It is possible that a misconfigured station will attempt to register
   with the wrong NHS (i.e., one that cannot serve it due to policy
   constraints or routing state).  If this is the case, the NHS MUST
   reply with a NAK-ed Registration Reply of type Can't Serve This
   Address.

   If an NHS cannot serve a station due to a lack of resources, the NHS
   MUST reply with a NAK-ed Registration Reply of type Registration
   Overflow.

   In order to keep the registration entry from being discarded, the
   station MUST re-send the NHRP Registration Request packet often
   enough to refresh the registration, even in the face of occasional
   packet loss. It is recommended that the NHRP Registration Request
   packet be sent at an interval equal to one-third of the Holding Time
   specified therein.

5.2.4 NHRP Registration Reply

   The NHRP Registration Reply is sent by an NHS to a client in response
   to that client's NHRP Registration Request. If the Code field of a
   CIE in the NHRP Registration Reply has anything other than 0 zero in
   it then the NHRP Registration Reply is a NAK otherwise the reply is
   an ACK.  The NHRP Registration Reply has a Type code of 4.

   An NHRP Registration Reply is formed from an NHRP Registration
   Request by changing the type code to 4, updating the CIE Code field,
   and filling in the appropriate extensions if they exist.  The message
   specific meanings of the fields are as follows:

   Attempts to register the information in the CIEs of an NHRP



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   Registration Request may fail for various reasons.  If this is the
   case then each failed attempt to register the information in a CIE of
   an NHRP Registration Request is logged in the associated NHRP
   Registration Reply by setting the CIE Code field to the appropriate
   error code as shown below:

     CIE Code

       0 - Successful Registration

         The information in the CIE was sucessfully registered with the
         NHS.

       4 - Can't Serve This Address

         An NHS may refuse an NHRP Registration Request attempt for
         administrative reasons (due to policy constraints or routing
         state).  If so, the NHS MUST send an NHRP Registration Reply
         which contains a NAK code of 4.

       5 - Registration Overflow

         If an NHS cannot serve a station due to a lack of resources,
         the NHS MUST reply with a NAKed NHRP Registration Reply which
         contains a NAK code of 5.

       14 - Unique Internetworking Layer Address Already Registered
         If a client tries to register a protocol address to NBMA
         address binding with the uniqueness bit on and the protocol
         address already exists in the NHS's cache then if that cache
         entry also has the uniqueness bit on then this NAK Code is
         returned in the CIE in the NHRP Registration Reply.

   Due to the possible existence of asymmetric routing, an NHRP
   Registration Reply may not be able to merely follow the routed path
   back to the source protocol address specified in the common header of
   the NHRP Registration Reply.  As a result, there MUST exist a direct
   NBMA level connection between the client and its NHS on which to send
   the NHRP Registration Reply before NHRP Registration Reply may be
   returned to the client.  If such a connection does not exist then the
   NHS must setup such a connection to he client by using the source
   NBMA information supplied in the common header of the NHRP
   Registration Request.

5.2.5 NHRP Purge Request

   The NHRP Purge Request packet is sent in order to invalidate cached
   information in a station.  The NHRP Purge Request packet has a type



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   code of 5.  The mandatory part of an NHRP Purge Request is coded as
   described in Section 5.2.0.1.  The message specific meanings of the
   fields are as follows:

   Flags - The flags field is coded as follows:

      0                   1
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |N|         unused              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     N
       When set, this bit tells the receiver of the NHRP Purge Request
       that the requester does not expect to receive an NHRP Purge
       Reply.  If an unsolicited NHRP Purge Reply is received by a
       station where that station is identified in the Source Protocol
       Address of the packet then that packet must be ignored.

   One or more CIEs are specified in the NHRP Purge Request.  Each CIE
   contains next hop information which is to be purged from an NHS/NHC
   cache.  Generally, all fields in CIEs enclosed in NHRP Purge Requests
   are coded as described in Section 5.2.0.1.  Below, further
   clarification is given for some fields in a CIE in the context of a
   NHRP Purge Request.

     Code
       This field is set to 0x00 in NHRP Purge Requests.

     Prefix Length

       In the case of NHRP Purge Requests, the Prefix Length specifies
       the equivalence class of addresses which match the first "Prefix
       Length" bit positions of the Client Protocol Address specified in
       the CIE.  All next hop information which contains a protocol
       address which matches an element of this equivalence class is to
       be purged from the receivers cache.  If this field is set to 0x00
       then this field MUST be ignored and no equivalence information is
       assumed.

     The Maximum Transmission Unit and Preference fields of the CIE are
     coded as zero.  The Holding Time should be coded as zero but there
     may be some utility in supplying a "short" holding time to be
     applied to the matching next hop information before that
     information would be purged; this usage is for further study. The
     Client Protocol Address field and the Cli Proto Len field MUST be
     filled in.  The Client Protocol Address is filled in with the
     protocol address to be purged from the receiving station's cache



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     while the Cli Proto Len is set the length of the purged client's
     protocol address.  All remaining fields in the CIE MAY be set to
     zero although the client NBMA information (and associated length
     fields) MAY be specified to narrow the scope of the NHRP Purge
     Request if requester desires.  However, the receiver of an NHRP
     Purge Request may choose to ignore the Client NBMA information if
     it is supplied.

   An NHRP Purge Request packet is sent from an NHS to a station to
   cause it to delete previously cached information.  This is done when
   the information may be no longer valid (typically when the NHS has
   previously provided next hop information for a station that is not
   directly connected to the NBMA subnetwork, and the egress point to
   that station may have changed).

   An NHRP Purge Request packet may also be sent from a client to an NHS
   with which the client had previously registered.  This allows for a
   client to invalidate its registration with NHRP before it would
   otherwise expire via the holding timer.

   The station sending the NHRP Purge Request MAY periodically
   retransmit the NHRP Purge Request until either NHRP Purge Request is
   acknowledged or until the holding time of the information being
   purged has expired.  Retransmission strategies for NHRP Purge
   Requests are a local matter.

   When a station receives an NHRP Purge Request, it MUST discard any
   previously cached information that matches the information in the
   CIEs.

   An NHRP Purge Reply MUST be returned for the NHRP Purge Request even
   if the station does not have a matching cache entry assuming that the
   "N" bit is off in the NHRP Purge Request.

   If the station wishes to reestablish communication with the
   destination shortly after receiving an NHRP Purge Request, it should
   make an authoritative Next Hop Resolution Request in order to avoid
   any stale cache entries that might be present in intermediate NHSs
   (See section 6.2.2.).  It is recommended that authoritative Next Hop
   Resolution Requests be made for the duration of the holding time of
   the old information.

5.2.6 NHRP Purge Reply

   The NHRP Purge Reply packet is sent in order to assure the sender of
   an NHRP Purge Request that all cached information of the specified
   type has been purged from the station sending the reply.  The NHRP
   Purge Reply has a type code of 6.



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   An NHRP Purge Reply is formed from an NHRP Purge Request by merely
   changing the type code in the request to 6.  The packet is then
   returned to the requester after filling in the appropriate extensions
   if they exist.

5.2.7  NHRP Error Indication

   The NHRP Error Indication is used to convey error indications to the
   sender of an NHRP packet.  It has a type code of 7.  The Mandatory
   Part has the following format:


          0                   1                   2                   3
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         | Src Proto Len | Dst Proto Len |            unused             |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |           Error Code          |        Error Offset           |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |            Source NBMA Address (variable length)              |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Source NBMA Subaddress (variable length)             |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Source Protocol Address (variable length)            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |       Destination  Protocol Address (variable length)         |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |       Contents of NHRP Packet in error (variable length)      |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Src Proto Len
     This field holds the length in octets of the Source Protocol
     Address.

   Dst Proto Len
     This field holds the length in octets of the Destination Protocol
     Address.

   Error Code
     An error code indicating the type of error detected, chosen from
     the following list:

       1 - Unrecognized Extension

         When the Compulsory bit of an extension in NHRP packet is set,
         the NHRP packet cannot be processed unless the extension has
         been processed.  The responder MUST return an NHRP Error
         Indication of type Unrecognized Extension if it is incapable of



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         processing the extension.  However, if a transit NHS (one which
         is not going to generate a reply) detects an unrecognized
         extension, it SHALL ignore the extension.

       2 - Subnetwork ID Mismatch

         This error occurs when the current station receives an NHRP
         packet whose NBMA subnetwork identifier matches none of the
         locally known identifiers for the NBMA subnetwork on which the
         packet is received.

       3 - NHRP Loop Detected

         A Loop Detected error is generated when it is determined that
         an NHRP packet is being forwarded in a loop.

       6 - Protocol Address Unreachable

         This error occurs when a packet it moving along the routed path
         and it reaches a point such that the protocol address of
         interest is not reachable.

       7 - Protocol Error

         A generic packet processing error has occurred (e.g., invalid
         version number, invalid protocol type, failed checksum, etc.)

       8 - NHRP SDU Size Exceeded

         If the SDU size of the NHRP packet exceeds the MTU size of the
         NBMA network then this error is returned.

       9 - Invalid Extension

         If an NHS finds an extension in a packet which is inappropriate
         for the packet type, an error is sent back to the sender with
         Invalid Extension as the code.

       10- Invalid Next Hop Resolution Reply Received

         If a client receives a Next Hop Resolution Reply for a Next Hop
         Resolution Request which it believes it did not make then an
         error packet is sent to the station making the reply with an
         error code of Invalid Reply Received.

       11- Authentication Failure

         If a received packet fails an authentication test then this



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         error is returned.


       14- Hop Count Exceeded

         The hop count which was specified in the Fixed Header of an
         NHRP message has been exceeded.

   Error Offset
     The offset in octets into the NHRP packet, starting at the NHRP
     Fixed Header, at which the error was detected.

   Source NBMA Address
     The Source NBMA address field is the address of the station which
     observed the error.

   Source NBMA SubAddress
     The Source NBMA subaddress field is the address of the station
     which observed the error.  If the field's length as specified in
     ar$sstl is 0 then no storage is allocated for this address at all.

   Source Protocol Address
     This is the protocol address of the station which issued the Error
     packet.

   Destination Protocol Address
     This is the protocol address of the station which sent the packet
     which was found to be in error.

   An NHRP Error Indication packet SHALL NEVER be generated in response
   to another NHRP Error Indication packet.  When an NHRP Error
   Indication packet is generated, the offending NHRP packet SHALL be
   discarded.  In no case should more than one NHRP Error Indication
   packet be generated for a single NHRP packet.

   If an NHS sees its own Protocol and NBMA Addresses in the Source NBMA
   and Source Protocol address fields of a transiting NHRP Error
   Indication packet then the NHS will quietly drop the packet and do
   nothing (this scenario would occur when the NHRP Error Indication
   packet was itself in a loop).

   Note that no extensions may be added to an NHRP Error Indication.

5.3  Extensions Part

   The Extensions Part, if present, carries one or more extensions in
   {Type, Length, Value} triplets.  Extensions are only present in a
   "reply" if they were present in the corresponding "request";



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   therefore, minimal NHRP client implementations which do not also act
   as an NHS and do not transmit extensions need not be able to receive
   extensions. The previous statement is not intended to preclude the
   creation of NHS-only extensions which might be added to and removed
   from NHRP packets by the same NHS; such extensions MUST not be
   propagated to clients.  An implementation that is incapable of
   processing extensions SHALL return an NHRP Error Indication of type
   Unrecognized Extension when it receives an NHRP packet containing
   extensions.

   Extensions have the following format:


          0                   1                   2                   3
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |C|u|        Type               |        Length                 |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                         Value...                              |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   C
     "Compulsory."  If clear, and the NHS does not recognize the type
     code, the extension may safely be ignored.  If set, and the NHS
     does not recognize the type code, the NHRP "request" is considered
     to be in error.  (See below for details.)

   u
     Unused and must be set to zero.

   Type
     The extension type code (see below).  The extension type is not
     qualified by the Compulsory bit, but is orthogonal to it.

   Length
     The length in octets of the value (not including the Type and
     Length fields;  a null extension will have only an extension header
     and a length of zero).

   When extensions exist, the extensions list is terminated by the Null
   TLV, having Type = 0 and Length = 0.

   Extensions may occur in any order, but any particular extension type
   (except for the vendor-private extension) may occur only once in an
   NHRP packet.  The vendor-private extension may occur multiple times
   in a packet in order to allow for extensions which do not share the
   same vendor ID to be represented.




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   The Compulsory bit provides for a means to add to the extension set.
   If the bit is set, the NHRP message cannot be properly processed by
   the station responding to the message (e.g., the station that would
   issue a Next Hop Resolution Reply in response to a Next Hop
   Resolution Request) without processing the extension.  As a result,
   the responder MUST return an NHRP Error Indication of type
   Unrecognized Extension.  If the Compulsory bit is clear then the
   extension can be safely ignored; however, if an ignored extension is
   in a "request" then it MUST be returned, unchanged, in the
   corresponding "reply" packet type.

   If a transit NHS (one which is not going to generate a "reply")
   detects an unrecognized extension, it SHALL ignore the extension.  If
   the Compulsory bit is set, the transit NHS MUST NOT cache the
   information contained in the packet and MUST NOT identify itself as
   an egress router (in the Forward Record or Reverse Record
   extensions).  Effectively, this means, if a transit NHS encounters an
   extension which it cannot process and which has the Compulsory bit
   set then that NHS MUST NOT participate in any way in the protocol
   exchange other than acting as a forwarding agent.

   Use of NHRP extension Types in the range 8192 to 16383 are reserved
   for research or use in other protocol development and will be
   administered by IANA.

5.3.0  The End Of Extensions

    Compulsory = 1
    Type = 0
    Length = 0

   When extensions exist, the extensions list is terminated by the End
   Of Extensions/Null TLV.

5.3.1  Extension with Type 1 not assigned.

5.3.2  NBMA Subnetwork ID Extension

    Compulsory = 1
    Type = 2
    Length = variable

   This extension is used to carry one or more identifiers for the NBMA
   subnetwork.  This can be used as a validity check to ensure that an
   NHRP packet does not leave a particular NBMA subnetwork.  The
   extension is placed in a "request" packet with an ID value of zero.
   The first NHS along the routed path fills in the field with the
   identifier(s) for the NBMA subnetwork.



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   Multiple NBMA Subnetwork IDs may be used as a transition mechanism
   while NBMA Subnetworks are being split or merged.

          0                   1                   2                   3
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                    NBMA Subnetwork ID                         |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     ...

   Each identifier consists of a 32 bit globally unique value assigned
   to the NBMA subnetwork.  This value may be chosen from the
   internetworking layer address space administered by the operators of
   the NBMA subnetwork if such an address can fit into a 32 bit field.
   This value is used for identification only, not for routing or any
   other purpose.

   Each NHS processing a "request" or "reply" SHALL verify these values.
   If the value is not zero and none of the values matches the NHS's
   NBMA Subnetwork ID, the NHS SHALL return an NHRP Error Indication to
   the entity identified in Source Protocol Address if the packet type
   is a "request" and to the Destination Protocol Address if the packet
   type is a "reply".  The error indicated in this case is "Subnetwork
   ID Mismatch".  The packet is discarded by the station sending the
   NHRP Error Indication.

   When an NHS is building a "reply" and the NBMA Subnetwork ID
   extension is present in the correspond "request" then the NBMA
   Subnetwork ID extension SHALL be copied from the "request" to the
   "reply".

5.3.3  Responder Address Extension

    Compulsory = 1
    Type = 3
    Length = variable

   This extension is used to determine the address of the NHRP
   responder; i.e., the entity that generates the appropriate "reply"
   packet for a given "request" packet.  In the case of an Next Hop
   Resolution Request, the station responding may be different (in the
   case of cached replies) than the system identified in the Next Hop
   field of the Next Hop Resolution Reply.  Further, this extension may
   aid in detecting loops in the NHRP forwarding path.







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          0                   1                   2                   3
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |           unused              |         Holding Time          |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |  Res Addr T/L  | Res SAddr T/L| Res Proto Len |     unused    |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Responder NBMA Address (variable length)             |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |         Responder NBMA Subaddress (variable length)           |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |       Responder Protocol Address (variable length)            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Holding Time
     The Holding Time field specifies the number of seconds for which
     the NBMA information is considered to be valid.  Cached information
     SHALL be discarded when the holding time expires.

   Res Addr T/L
     Type & length of the responder NHS's NBMA address interpreted in
     the context of the 'address family number'[6] indicated by ar$afn
     (e.g., ar$afn=0x0003 for ATM).  When the address length is
     specified as 0 no storage is allocated for the address.

   Res SAddr T/L
     Type & length of responder NHS's NBMA subaddress interpreted in the
     context of the 'address family number'[6] indicated by ar$afn
     (e.g., ar$afn=0x0015 for ATM makes the address an E.164 and the
     subaddress an ATM Forum NSAP address).  When an NBMA technology has
     no concept of a subaddress, the subaddress is always null with a
     length of 0.  When the address length is specified as 0 no storage
     is allocated for the address.

   Res Proto Len
     This field holds the length in octets of responding NHS's Protocol
     Address.

   Responder NBMA Address
     This is the NBMA address of the responding NHS.

   Responder NBMA SubAddress
     This is the NBMA subaddress of the responding NHS.

   Responder Protocol Address
     This is the Protocol Address of responding NHS.

   If a "requester" desires this information, the "requester" SHALL



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   include this extension with a value of zero.  Note that this implies
   that no storage is allocated for the Holding Time and Type/Length
   fields until the "Value" portion of the extension is filled out.

   If an NHS is generating a "reply" packet in response to a "request"
   containing this extension, the NHS SHALL include this extension,
   containing its protocol address in the "reply".  If an NHS has more
   than one protocol address, it SHALL use the same protocol address
   consistently in all of the Responder Address, Forward NHS Record, and
   Reverse NHS Record extensions.  The choice of which of several
   protocol address to include in this extension is a local matter.

   If an NHRP Next Hop Resolution Reply packet being forwarded by an NHS
   contains a protocol address of that NHS in the Responder Address
   Extension then that NHS SHALL generate an NHRP Error Indication of
   type "NHRP Loop Detected" and discard the Next Hop Resolution Reply.

   If an NHRP Next Hop Resolution Reply packet is being returned by an
   intermediate NHS based on cached data, it SHALL place its own address
   in this extension (differentiating it from the address in the Next
   Hop field).

5.3.4  NHRP Forward Transit NHS Record Extension

    Compulsory = 1
    Type = 4
    Length = variable

   The NHRP Forward Transit NHS record contains a list of transit NHSs
   through which a "request" has traversed.  Each NHS SHALL append to
   the extension a Forward Transit NHS element (as specified below)
   containing its Protocol address The extension length field and the
   ar$chksum fields SHALL be adjusted appropriately.

   The responding NHS, as described in Section 5.3.3, SHALL NOT update
   this extension.

   In addition, NHSs that are willing to act as egress routers for
   packets from the source to the destination SHALL include information
   about their NBMA Address.

   The Forward Transit NHS element has the following form:









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          0                   1                   2                   3
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |           unused              |         Holding Time          |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |  NHS Addr T/L  | NHS SAddr T/L| NHS Proto Len |     unused    |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |              NHS NBMA Address (variable length)               |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |              NHS NBMA Subaddress (variable length)            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |             NHS Protocol Address (variable length)            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Holding Time
     The Holding Time field specifies the number of seconds for which
     the NBMA information is considered to be valid.  Cached information
     SHALL be discarded when the holding time expires.

   NHS Addr T/L
     Type & length of the transit NHS's NBMA address interpreted in the
     context of the 'address family number'[6] indicated by ar$afn
     (e.g., ar$afn=0x0003 for ATM).  When the address length is
     specified as 0 no storage is allocated for the address.

   NHS SAddr T/L
     Type & length of the transit NHS's NBMA subaddress interpreted in
     the context of the 'address family number'[6] indicated by ar$afn
     (e.g., ar$afn=0x0015 for ATM makes the address an E.164 and the
     subaddress an ATM Forum NSAP address).  When an NBMA technology has
     no concept of a subaddress the subaddress is always null with a
     length of 0.  When the address length is specified as 0 no storage
     is allocated for the address.

   NHS Proto Len
     This field holds the length in octets of the transit NHS's Protocol
     Address.

   NHS NBMA Address
     This is the NBMA address of the transit NHS.

   NHS NBMA SubAddress
     This is the NBMA subaddress of the transit NHS.

   NHS Protocol Address
     This is the Protocol Address of the transit NHS.

   If a "requester" wishes to obtain this information, it SHALL include



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   this extension with a length of zero.  Note that this implies that no
   storage is allocated for the Holding Time and Type/Length fields
   until the "Value" portion of the extension is filled out.

   If an NHS has more than one Protocol address, it SHALL use the same
   Protocol address consistently in all of the Responder Address,
   Forward NHS Record, and Reverse NHS Record extensions.  The choice of
   which of several Protocol addresses to include in this extension is a
   local matter.

   If a "request" that is being forwarded by an NHS contains the
   Protocol Address of that NHS in one of the Forward Transit NHS
   elements then the NHS SHALL generate an NHRP Error Indication of type
   "NHRP Loop Detected" and discard the "request".

5.3.5  NHRP Reverse Transit NHS Record Extension

    Compulsory = 1
    Type = 5
    Length = variable

   The NHRP Reverse Transit NHS record contains a list of transit NHSs
   through which a "reply" has traversed.  Each NHS SHALL append a
   Reverse Transit NHS element (as specified below) containing its
   Protocol address to this extension.  The extension length field and
   ar$chksum SHALL be adjusted appropriately.

   The responding NHS, as described in Section 5.3.3, SHALL NOT update
   this extension.

   In addition, NHSs that are willing to act as egress routers for
   packets from the source to the destination SHALL include information
   about their NBMA Address.

   The Reverse Transit NHS element has the following form:
          0                   1                   2                   3
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |           unused              |         Holding Time          |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |  NHS Addr T/L  | NHS SAddr T/L| NHS Proto Len |     unused    |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |              NHS NBMA Address (variable length)               |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |              NHS NBMA Subaddress (variable length)            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |             NHS Protocol Address (variable length)            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   Holding Time
     The Holding Time field specifies the number of seconds for which
     the NBMA information is considered to be valid.  Cached information
     SHALL be discarded when the holding time expires.

   NHS Addr T/L
     Type & length of the responding NHS's NBMA address interpreted in
     the context of the 'address family number'[6] indicated by ar$afn
     (e.g., ar$afn=0x0003 for ATM).  When the address length is
     specified as 0 no storage is allocated for the address.

   NHS SAddr T/L
     Type & length of the responding NHS's NBMA subaddress interpreted
     in the context of the 'address family number'[6] indicated by
     ar$afn (e.g., ar$afn=0x0015 for ATM makes the address an E.164 and
     the subaddress an ATM Forum NSAP address).  When an NBMA technology
     has no concept of a subaddress the subaddress is always null with a
     length of 0.  When the address length is specified as 0 no storage
     is allocated for the address.

   NHS Proto Len
     This field holds the length in octets of the transit NHS's Protocol
     Address.

   NHS NBMA Address
     This is the NBMA address of the transit NHS.

   NHS NBMA SubAddress
     This is the NBMA subaddress of the transit NHS.

   NHS Protocol Address
     This is the Protocol Address of the transit NHS.

   If a "requester" wishes to obtain this information, it SHALL include
   this extension with a length of zero.  Note that this implies that no
   storage is allocated for the Holding Time and Type/Length fields
   until the "Value" portion of the extension is filled out.

   If an NHS has more than one Protocol address, it SHALL use the same
   Protocol address consistently in all of the Responder Address,
   Forward NHS Record, and Reverse NHS Record extensions.  The choice of
   which of several Protocol addresses to include in this extension is a
   local matter.

   If a "reply" that is being forwarded by an NHS contains the Protocol
   Address of that NHS in one of the Reverse Transit NHS elements then
   the NHS SHALL generate an NHRP Error Indication of type "NHRP Loop
   Detected" and discard the "reply".



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   Note that this information may be cached at intermediate NHSs;  if
   so, the cached value SHALL be used when generating a reply.

5.3.6  NHRP QoS Extension

    Compulsory = 0
    Type = 6
    Length = variable

   The NHRP QoS Extension is carried in Next Hop Resolution Request
   packets to indicate the desired QoS of the path to the indicated
   destination.  This information may be used to help select the
   appropriate NBMA Next Hop.

   It may also be carried in NHRP Register Request packets to indicate
   the QoS to which the registration applies.

   The syntax and semantics of this extension are TBD;  alignment with
   resource reservation may be useful.

5.3.7  NHRP Authentication Extension

    Compulsory = 1
    Type = 7
    Length = variable

   The NHRP Authentication Extension is carried in NHRP packets to
   convey authentication information between NHRP speakers.  The
   Authentication Extension may be included in any NHRP "request" or
   "reply".

   Except in the case of an NHRP Registration Request/Reply
   Authentication is done pairwise on an NHRP hop-by-hop basis;  i.e.,
   the authentication extension is regenerated at each hop. In the case
   of an NHRP Registration Request/Reply, the Authentication is checked
   on an end-to-end basis rather than hop-by-hop. If a received packet
   fails the authentication test, the station SHALL generate an Error
   Indication of type "Authentication Failure" and discard the packet.
   Note that one possible authentication failure is the lack of an
   Authentication Extension;  the presence or absence of the
   Authentication Extension is a local matter.










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          0                   1                   2                   3
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                     Authentication Type                       |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                                                               |
         +-+-+-+-+-+-+-+-+-+-+ Authentication Data... -+-+-+-+-+-+-+-+-+-+
         |                                                               |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Authentication Type field identifies the authentication method in
   use.  Currently assigned values are:

   1 - Cleartext Password
   2 - Keyed MD5

   All other values are reserved.

   The Authentication Data field contains the type-specific
   authentication information.

   In the case of Cleartext Password Authentication, the Authentication
   Data consists of a variable length password.

   In the case of Keyed MD5 Authentication, the Authentication Data
   contains the 16 byte MD5 digest of the entire NHRP packet, including
   the encapsulated protocol's header, with the authentication key
   appended to the end of the packet.  The authentication key is not
   transmitted with the packet.

   Distribution of authentication keys is outside the scope of this
   document.

5.3.8  NHRP Vendor-Private Extension

    Compulsory = 0
    Type = 8
    Length = variable

   The NHRP Vendor-Private Extension is carried in NHRP packets to
   convey vendor-private information or NHRP extensions between NHRP
   speakers.

          0                   1                   2                   3
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                  Vendor ID                    |  Data....     |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   Vendor ID
     802 Vendor ID as assigned by the IEEE [6]

   Data
     The remaining octets after the Vendor ID in the payload are
     vendor-dependent data.

   This extension may be added to any "request" or "reply" packet and it
   is the only extension that may be included multiple times.  If the
   receiver does not handle this extension, or does not match the Vendor
   ID in the extension then the extension may be completely ignored by
   the receiver.  If a Vendor Private Extension is included in a
   "request" then is must be copied in the corresponding "reply".

5.3.9  Extension with Type 9 not assigned.


6. Protocol Operation

   In this section, we discuss certain operational considerations of
   NHRP.

6.1 Router-to-Router Operation

   In practice, the initiating and responding stations may be either
   hosts or routers.  However, there is a possibility under certain
   conditions that a stable routing loop may occur if NHRP is used
   between two routers.  In particular, attempting to establish an NHRP
   path across a boundary where information used in route selection is
   lost may result in a routing loop.  Such situations include the loss
   of BGP path vector information, the interworking of multiple routing
   protocols with dissimilar metrics (e.g, RIP and OSPF), etc.  In such
   circumstances, NHRP should not be used.  This situation can be
   avoided if there are no "back door" paths between the entry and
   egress router outside of the NBMA subnetwork.  Protocol mechanisms to
   relax these restrictions are under investigation.

   In general it is preferable to use mechanisms, if they exist, in
   routing protocols to resolve the egress point when the destination
   lies outside of the NBMA subnetwork, since such mechanisms will be
   more tightly coupled to the state of the routing system and will
   probably be less likely to create loops.

6.2 Cache Management Issues

   The management of NHRP caches in the source station, the NHS serving
   the destination, and any intermediate NHSs is dependent on a number
   of factors.



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   6.2.1 Caching Requirements

     Source Stations

     Source stations MUST cache all received Next Hop Resolution Replies
     that they are actively using.  They also must cache "incomplete"
     entries, i.e., those for which a Next Hop Resolution Request has
     been sent but which a Next Hop Resolution Reply has not been
     received.  This is necessary in order to preserve the Request ID
     for retries, and provides the state necessary to avoid triggering
     Next Hop Resolution Requests for every data packet sent to the
     destination.

     Source stations MUST purge expired information from their caches.
     Source stations MUST purge the appropriate cached information upon
     receipt of an NHRP Purge Request packet.

     When a station has a co-resident client and NHS, the station may
     reply to Next Hop Resolution Requests with information which the
     station cached as a result of the station making its own Next Hop
     Resolution Requests and receiving its own Next Hop Resolution
     Replies as long as the station follows the rules for Transit NHSs
     as seen below.

     Serving NHSs

     The NHS serving the destination (the one which responds
     authoritatively to Next Hop Resolution Requests) SHOULD cache
     information about all Next Hop Resolution Requests to which it has
     responded if the information in the Next Hop Resolution Reply has
     the possibility of changing during its lifetime (so that an NHRP
     Purge Request packet can be sent).  The NBMA information provided
     by the source station in the Next Hop Resolution Request may be
     cached for the duration of its holding time.  This information is
     considered to be stable, since it identifies a station directly
     attached to the NBMA subnetwork.  An example of unstable
     information is NBMA information derived from a routing table, where
     that routing table information has not been guaranteed to be stable
     through administrative means.

     Transit NHSs

     A Transit NHS (lying along the NHRP path between the source station
     and the responding NHS) may cache information contained in Next Hop
     Resolution Request packets that it forwards.  A Transit NHS may
     cache information contained in Next Hop Resolution Reply packets
     that it forwards only if that Next Hop Resolution Reply has the
     Stable (B) bit set.  It MUST discard any cached information whose



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     holding time has expired.  It may return cached information in
     response to non-authoritative Next Hop Resolution Requests only.

   6.2.2 Dynamics of Cached Information

   NBMA-Connected Destinations

     NHRP's most basic function is that of simple NBMA address
     resolution of stations directly attached to the NBMA subnetwork.
     These mappings are typically very static, and appropriately chosen
     holding times will minimize problems in the event that the NBMA
     address of a station must be changed.  Stale information will cause
     a loss of connectivity, which may be used to trigger an
     authoritative Next Hop Resolution Request and bypass the old data.
     In the worst case, connectivity will fail until the cache entry
     times out.

     This applies equally to information marked in Next Hop Resolution
     Replies as being "stable" (via the "B" bit).

     This also applies equally well to source stations that are routers
     as well as those which are hosts.

     Note that the information carried in the Next Hop Resolution
     Request packet is always considered "stable" because it represents
     a station that is directly connected to the NBMA subnetwork.

   Destinations Off of the NBMA Subnetwork

     If the source of a Next Hop Resolution Request is a host and the
     destination is not directly attached to the NBMA subnetwork, and
     the route to that destination is not considered to be "stable," the
     destination mapping may be very dynamic (except in the case of a
     subnetwork where each destination is only singly homed to the NBMA
     subnetwork).  As such the cached information may very likely become
     stale.  The consequence of stale information in this case will be a
     suboptimal path (unless the internetwork has partitioned or some
     other routing failure has occurred).

6.3 Use of the Prefix Length field of a CIE

   A certain amount of care needs to be taken when using the Prefix
   Length field of a CIE, in particular with regard to the prefix length
   advertised (and thus the size of the equivalence class specified by
   it).  Assuming that the routers on the NBMA subnetwork are exchanging
   routing information, it should not be possible for an NHS to create a
   black hole by advertising too large of a set of destinations, but
   suboptimal routing (e.g., extra internetwork layer hops through the



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   NBMA) can result.  To avoid this situation an NHS that wants to send
   the Prefix Length MUST obey the following rule:

     The NHS examines the Network Layer Reachability Information (NLRI)
     associated with the route that the NHS would use to forward towards
     the destination (as specified by the Destination internetwork layer
     address in the Next Hop Resolution Request), and extracts from this
     NLRI the shortest address prefix such that: (a) the Destination
     internetwork layer address (from the Next Hop Resolution Request)
     is covered by the prefix, (b) the NHS does not have any routes with
     NLRI that forms a subset of what is covered by the prefix. The
     prefix may then be used in the CIE.

   The Prefix Length field of the CIE should be used with restraint, in
   order to avoid NHRP stations choosing suboptimal transit paths when
   overlapping prefixes are available.  This document specifies the use
   of the prefix length only when all the destinations covered by the
   prefix are "stable". That is, either:

     (a) All destinations covered by the prefix are on the NBMA network, or

     (b) All destinations covered by the prefix are directly attached to
         the NHRP responding station.

   Use of the Prefix Length field of the CIE in other circumstances is
   outside the scope of this document.

6.4 Domino Effect

   One could easily imagine a situation where a router, acting as an
   ingress station to the NBMA subnetwork, receives a data packet, such
   that this packet triggers an Next Hop Resolution Request.  If the
   router forwards this data packet without waiting for an NHRP transit
   path to be established, then when the next router along the path
   receives the packet, the next router may do exactly the same -
   originate its own Next Hop Resolution Request (as well as forward the
   packet).  In fact such a data packet may trigger Next Hop Resolution
   Request generation at every router along the path through an NBMA
   subnetwork.  We refer to this phenomena as the NHRP "domino" effect.

   The NHRP domino effect is clearly undesirable.  At best it may result
   in excessive NHRP traffic.  At worst it may result in an excessive
   number of virtual circuits being established unnecessarily.
   Therefore, it is important to take certain measures to avoid or
   suppress this behavior.  NHRP implementations for NHSs MUST provide a
   mechanism to address this problem. One possible strategy to address
   this problem would be to configure a router in such a way that Next
   Hop Resolution Request generation by the router would be driven only



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   by the traffic the router receives over its non-NBMA interfaces
   (interfaces that are not attached to an NBMA subnetwork).  Traffic
   received by the router over its NBMA-attached interfaces would not
   trigger NHRP Next Hop Resolution Requests.  Such a router avoids the
   NHRP domino effect through administrative means.


7. NHRP over Legacy BMA Networks

   There would appear to be no significant impediment to running NHRP
   over legacy broadcast subnetworks.  There may be issues around
   running NHRP across multiple subnetworks. Running NHRP on broadcast
   media has some interesting possibilities; especially when setting up
   a cut-through for inter-ELAN inter-LIS/LAG traffic when one or both
   end stations are legacy attached.  This use for NHRP requires further
   research.


8. Security Considerations

   As in any routing protocol, there are a number of potential security
   attacks possible.  Plausible examples include denial-of-service
   attacks, and masquerade attacks using register and purge packets.
   The use of authentication on all packets is recommended to avoid such
   attacks.

   The authentication schemes described in this document are intended to
   allow the receiver of a packet to validate the identity of the
   sender; they do not provide privacy or protection against replay
   attacks.

   Detailed security analysis of this protocol is for further study.


9. Discussion

   The result of an Next Hop Resolution Request depends on how routing
   is configured among the NHSs of an NBMA subnetwork.  If the
   destination station is directly connected to the NBMA subnetwork and
   the the routed path to it lies entirely within the NBMA subnetwork,
   the Next Hop Resolution Replies always return the NBMA address of the
   destination station itself rather than the NBMA address of some
   egress router.  On the other hand, if the routed path exits the NBMA
   subnetwork, NHRP will be unable to resolve the NBMA address of the
   destination, but rather will return the address of the egress router.
   For destinations outside the NBMA subnetwork, egress routers and
   routers in the other subnetworks should exchange routing information
   so that the optimal egress router may be found.



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   In addition to NHSs, an NBMA station could also be associated with
   one or more regular routers that could act as "connectionless
   servers" for the station.  The station could then choose to resolve
   the NBMA next hop or just send the packets to one of its
   connectionless servers.  The latter option may be desirable if
   communication with the destination is short-lived and/or doesn't
   require much network resources.  The connectionless servers could, of
   course, be physically integrated in the NHSs by augmenting them with
   internetwork layer switching functionality.


References

   [1] NBMA Address Resolution Protocol (NARP), Juha Heinanen and Ramesh
       Govindan, RFC1735.

   [2] Address Resolution Protocol, David C. Plummer, RFC 826.

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

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

   [5] Protocol Identification in the Network Layer, ISO/IEC TR
       9577:1990.

   [6] Assigned Numbers, J. Reynolds and J. Postel, RFC 1700.

   [7] Multiprotocol Encapsulation over ATM Adaptation Layer 5, J. Heinanen,
       RFC1483.

   [8] Multiprotocol Interconnect on X.25 and ISDN in the Packet Mode,
       A. Malis, D. Robinson, and R. Ullmann, RFC1356.

   [9] Multiprotocol Interconnect over Frame Relay, T. Bradley, C. Brown, and
       A. Malis, RFC1490.

   [10] "Local/Remote" Forwarding Decision in Switched Data Link Subnetworks,
        Yakov Rekhter, Dilip Kandlur, RFCxxxx.



Acknowledgments

   We would like to thank Juha Heinenan of Telecom Finland and Ramesh
   Govidan of ISI for their work on NBMA ARP and the original NHRP
   draft, which served as the basis for this work.  Russell Gardo of
   IBM, John Burnett of Adaptive, Dennis Ferguson of ANS, Joel Halpern



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   of Newbridge, Paul Francis of NTT, Tony Li and Yakov Rekhter of
   cisco, and Grenville Armitage of Bellcore should also be acknowledged
   for comments and suggestions that improved this work substantially.
   We would also like to thank the members of the ION working group of
   the IETF, whose review and discussion of this document have been
   invaluable.

Authors' Addresses


   James V. Luciani                    Dave Katz
   Bay Networks                        cisco Systems
   3 Federal Street                    170 W. Tasman Dr.
   Mail Stop: BL3-04                   San Jose, CA 95134 USA
   Billerica, MA 01821                 Phone:  +1 408 526 8284
   Phone:  +1 508 439 4737             Email:  dkatz@cisco.com
   Email:  luciani@baynetworks.com

   David Piscitello                    Bruce Cole
   Core Competence                     cisco Systems
   1620 Tuckerstown Road               170 W. Tasman Dr.
   Dresher, PA 19025 USA               San Jose, CA 95134 USA
   Phone:  +1 215 830 0692Phone:       Phone:  +1 408 526 4000
   Email: dave@corecom.comEmail:       Email:  bcole@cisco.com



























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