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Routing over Large Clouds Working Group                        Dave Katz
INTERNET-DRAFT                                           (cisco Systems)
<draft-ietf-rolc-nhrp-07.txt>                           David Piscitello
                                                 (Core Competence, Inc.)
                                                              Bruce Cole
                                                         (cisco Systems)
                                                        James V. Luciani
                                                          (Ascom Nexion)
                                                       Expires June 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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   material or to cite them other than as ``work in progress.''

   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.

   An NBMA subnetwork may, in general, consist of multiple Local Address
   Groups (LAGs).  In the case of IP, a logically independent IP subnet
   (LIS) is an example of a LAG.  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.

   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 traverse the NBMA subnetwork more than once.

   NHRP describes a next hop resolution method that relaxes the
   forwarding restrictions of the LIS model.  With NHRP when the
   internetwork 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



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   subnetwork such as ATM).

   NHRP in its most basic form provides a simple internetworking layer
   to NBMA subnetwork layer address binding service.  This may be
   sufficient for hosts which 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.


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.)


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



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   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.)

   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



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   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
   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 its NBMA interface, 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, using the NBMA address of the next routed
   hop.

   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



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   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 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 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 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.]




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   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 Registration Replies, 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 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.

   Next Hop Resolution Requests and Next Hop Resolution 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 5.3.1).  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), as
   well as 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



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   (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 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 performing a
   routing decision based upon the network layer destination address of
   the Next Hop Resolution Request.  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
   either serves the destination, or is the destination itself if both
   NHRP client and server functionality are co-resident in the same
   station.  The responding station generates a Next Hop Resolution
   Reply using the source address from within the NHRP packet to
   determine where the Next Hop Resolution Reply should be sent.

   The Next Hop Resolution Request packet is carried at the NBMA layer,
   with a destination NBMA address set to that of the locally determined
   NHS.  If the addressed NHS does not serve the destination address
   specified in the Next Hop Resolution Request, the Next Hop Resolution
   Request packet is routed at the network layer based upon the Next Hop
   Resolution Requester's destination address, and forwarded to the
   neighboring NHS determined by the routing decision.  Alternately, the
   NHS may use static configuration information in order to determine to
   which neighboring NHSs to forward the Next Hop Resolution Request
   packet.  Each NHS/router examines the Next Hop Resolution Request
   packet on its way toward the destination, optionally modifying it on
   the way (such as updating the Forward Record extension), and
   continues to forward it until it reaches the NHS that serves the
   destination network layer address.

   In order to forward NHRP packets to a neighboring NHS, NHRP clients
   must nominally be configured with the NBMA address of at least one



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   NHS.  In practice, a client's default router should also be its NHS.
   A client may be able to derive the NBMA address of its NHS from the
   configuration that was already required for the client to be able to
   communicate with its next hop router.

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

   The path taken by Next Hop Resolution Requests will normally be the
   same as the path taken by data packets which are routed at the
   network layer to the desired destination.  (The paths may be
   different in situations where NHSs have been statically configured to
   forward traffic by other means.  For example, an Next Hop Resolution
   Request may be forwarded to a group multicast address.)

   NHSs should acquire knowledge about destinations other NHSs serve as
   a direct consequence of participating in intra-domain and inter-
   domain routing protocol exchange.  In this case, 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 as NHSs.

   NHSs (and end stations) may alternately be statically configured with
   the NBMA addresses of their neighbors, the identities of the
   destinations that each of them serves, and optionally a logical NBMA
   subnetwork identifier.  Such static configurations may be necessary
   in cases where NHSs do not contain network layer routing protocol
   implementations.

   If the NBMA 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.

   NHSs may also be deployed with the group or multicast address of
   their peers, and an NHS might use this as a means of forwarding Next



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   Hop Resolution Requests it cannot satisfy to its peers.  This might
   elicit a response (to the NHS) from one or more NHSs, depending on
   the response strategy.  The NHS would then forward the Next Hop
   Resolution Reply to the Next Hop Resolution Request originator.  The
   purpose of using group addressing or a similar multicast mechanism in
   this scenario would be to eliminate the need to preconfigure each NHS
   in a logical NBMA subnetwork with both the individual identities of
   other NHSs as well as the destinations they serve.  It reduces the
   number of NHSs that might be traversed to process an Next Hop
   Resolution Request (in those configurations where NHSs either respond
   or forward via the multicast, only two NHSs would be traversed), and
   allows the NHS that serves the Next Hop Resolution Request originator
   to cache next hop information associated with the Next Hop Resolution
   Reply (again, within the constraints described in Section 2).

4. Configuration

   Stations

     To participate in NHRP, a station 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 station's default or peer routers, so
     their NBMA addresses may be obtained from the station's existing
     configuration.  If the station is attached to several subnetworks
     (including logical NBMA subnetworks), the station 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, a set of internetwork layer address prefixes
     that correspond to the internetwork layer addresses of the stations
     it serves, and a logical NBMA subnetwork identifier (see Section
     5.3.2).  If a served station is attached to several subnetworks,
     the NHS may also need to be configured to advertise routing
     information to such stations.

     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.




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     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.

   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 minus
   20.  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]).

   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



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   protocol-independent fields always come first in the packet, and 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.
       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



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

   ar$op.version
     This field is set to 0x0001 for NHRP version 1.

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

   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).



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   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.


5.2 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.


5.2.1 NHRP Next Hop Resolution Request

   The NHRP Next Hop Resolution Request packet has a Type code of 1. The
   Mandatory Part has 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
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         | Src Proto Len | Dst Proto Len |Q|A|P|B|       unused          |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                         Request ID                            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |  Maximum Transmission Unit    |        Holding Time           |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                            unused                             |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |            Source NBMA Address (variable length)              |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Source NBMA Subaddress (variable length)             |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Source Protocol Address (variable length)            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |       Destination  Protocol Address (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.

   NHRP Next Hop Resolution Request/Reply Flags

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

     A
       A response to a Next Hop Resolution Request may contain cached
       information. If an authoritative answer is desired, then this bit
       ("Authoritative") should be set.  If non-authoritative (cached)
       information is acceptable, this bit should be clear.

     P
       Unused (clear on transmit)

     B
       Unused (clear on transmit)

   Request ID
     A value which, when coupled with the address of the source,



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     provides a unique identifier for the information contained in a
     Next Hop Resolution Request and its associated Next Hop Resolution
     Reply, and any subsequent Purge.  This  value can be used by the
     source to aid in matching a Next Hop Resolution Request with a Next
     Hop Resolution Reply.  This value could also be sent across a
     virtual circuit (in SVC environments) to aid in matching NHRP
     transactions with virtual circuits (this use is for further study).

     The value is taken from a 32 bit counter that is incremented each
     time a new Next Hop Resolution Request is transmitted.  The same
     value MUST be used when sending another Next Hop Resolution Request
     for the same destination when a previous Next Hop Resolution
     Request is still active or pending, i.e., when retransmitting a
     Next Hop Resolution Request because a Next Hop Resolution Reply was
     not received, or when refreshing an existing entry to avoid holding
     timer expiration.  A new value MUST be used when sending a Next Hop
     Resolution Request when no cache entry is present, or a previous
     cache entry was deleted for any reason.

   Maximum Transmission Unit
     This field gives the maximum transmission unit for the target
     station.  This field is ignored in Next Hop Resolution Requests and
     should be set to 0.  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.  This use is for further study.

   Holding Time
     The Holding Time field specifies the number of seconds for which
     the client NBMA information (the information of the client issuing
     the Next Hop Resolution Request) is considered to be valid. The
     contents of this field along with the source address information
     MAY be cached by transit NHSs.  The holding time should be set to
     the remaining time left in the client's registration with its
     server.  If this field is set to 0 then transit NHSs MUST not cache
     the client's NBMA information.

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




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   Source Protocol Address
     This is the protocol address of the station which is sending the
     Next Hop Resolution Request.

   Destination Protocol Address
     This is the protocol address of the station for which the NBMA next
     hop is desired.

   (The NBMA address/subaddress form 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.)


5.2.2 NHRP Next Hop Resolution Reply

   The NHRP Next Hop Resolution Reply packet has a type code of 2.  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 |Q|A|P|B|       unused          |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                         Request ID                            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |  Maximum Transmission Unit    |        Holding Time           |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |  NH Addr T/L   | NH SAddr T/L |  NH Proto Len |  Preference   |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |            Source NBMA Address (variable length)              |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Source NBMA Subaddress (variable length)             |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Source Protocol Address (variable length)            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |       Destination  Protocol Address (variable length)         |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Next Hop NBMA Address (variable length)              |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |         Next Hop NBMA Subaddress (variable length)            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |         Next Hop Protocol Address (variable length)           |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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



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

   Next Hop Resolution Request/Reply Flags

     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 in the Next Hop Resolution Reply is
       authoritative; clear if the Next Hop Resolution Reply is non-
       authoritative.

     P
       Set if the Next Hop Resolution Reply is positive;  clear if the
       Next Hop Resolution Reply is negative.

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

     An NHS is not allowed to send a Next Hop Resolution Reply to an
     Next Hop Resolution Request for authoritative information with
     cached information, but may do so for an NHRP Next Hop Resolution
     Request which indicates a request for non-authoritative
     information. An NHS may send an Next Hop Resolution Reply to an
     Next Hop Resolution Request for non-authoritative information with
     authoritative information.

   Request ID
     A value which, when coupled with the address of the source,
     provides a unique identifier for the information contained in a
     Next Hop Resolution Request and its associated Next Hop Resolution
     Reply, and any subsequent Purge.  This value can be used by the
     source to aid in matching a Next Hop Resolution Request with a Next
     Hop Resolution Reply.  This value could also be sent across a
     virtual circuit (in SVC environments) to aid in matching NHRP



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     transactions with virtual circuits (this use is for further study).

     The value is taken from a 32 bit counter that is incremented each
     time a new Next Hop Resolution Request is transmitted.  The same
     value MUST be used when sending another Next Hop Resolution Request
     for the same destination when a previous Next Hop Resolution
     Request is still active or pending, i.e., when retransmitting a
     Next Hop Resolution Request because a Next Hop Resolution Reply was
     not received, or when refreshing an existing entry to avoid holding
     timer expiration.  A new value MUST be used when sending a Next Hop
     Resolution Request when no cache entry is present, or a previous
     cache entry was deleted for any reason.

   Maximum Transmission Unit
     This field gives the maximum transmission unit for the Next Hop
     information supplied in the mandatory part of the packet.  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 mandatory part of
     the packet 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.

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

   NH SAddr T/L
     Type & length of next hop NBMA subaddress specified in the
     mandatory part of the packet. 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.

   NH Proto Len
     This field holds the length in octets of the Next Hop Protocol
     Address specified in the mandatory part of the packet (additional
     next hop entries may be specified in the Additional Next Hop
     Entries Extension (see Section 5.2.9)).



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   Preference
     This field specifies the preference of the Next Hop entry specified
     in the mandatory part of the packet.  This preference value is
     relative to other Next Hop entries in this NHRP Next Hop Resolution
     Reply packet which may be by the Additional Next Hop Entries
     Extension (see Section 5.3.9) for the given internetworking
     protocol.  Higher values indicate higher preference.  Action taken
     when multiple next hop entries have the highest preference value is
     a local matter. Set to 0 on a NAK.

   Source NBMA Address
     The Source NBMA address field is the address of the source station
     which sent the Next Hop Resolution 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 sent the Next Hop Resolution 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 sent the Next Hop
     Resolution Request.

   Destination Protocol Address
     This is the protocol address of the station for which the NBMA next
     hop is desired.

     (The NBMA address/subaddress form 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.)

   The following is the Next Hop entry as specified in the Mandatory
   Part of the packet:

     Next Hop NBMA Address
       This is the NBMA address of the station that is the next hop for
       packets bound for the internetworking layer address specified.

     Next Hop NBMA SubAddress
       This is the NBMA subaddress of the station that is the next hop
       for packets bound for the internetworking layer address
       specified.

     Next Hop Protocol Address



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       This internetworking layer address specifies the next hop.  This
       will be the address of the destination host if it is directly
       attached to the NBMA subnetwork, or the egress router if it is
       not directly attached.

   There may be multiple Next Hop entries returned in the Next Hop
   Resolution Reply by including the Additional Next Hop Entries
   Extension.  See Section 5.3.9 for use of these entries.  The most
   preferable Next Hop must be specified in the mandatory part of the
   Next Hop Resolution Reply.

   Any extensions present in the Next Hop Resolution Request packet MUST
   be present in the NHRP Next Hop Resolution Reply packet, except for
   the case of unrecognized non-Compulsory extensions.

   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. 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             |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                         Request ID                            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |           unused              |    Register Holding Time      |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |            Source NBMA Address (variable length)              |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Source NBMA Subaddress (variable length)             |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Source Protocol Address (variable length)            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |       Destination  Protocol Address (variable length)         |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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




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

   Request ID
     A value which, when coupled with the address of the source,
     provides a unique identifier for the information contained in an
     NHRP Registration Request packet.  This value is copied directly
     from an NHRP Registration Request packet into the associated
     Registration Reply.  This value could also be sent across a virtual
     circuit (in SVC environments) to aid in matching the NHRP
     transactions with virtual circuits (this use is for further study).

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

   Source NBMA Address
     The Source NBMA address field is the address of the source station
     which is sending the NHRP Registration Request.

   Source NBMA SubAddress
     The Source NBMA subaddress field is the address of the source
     station which is sending the NHRP Registration 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
     NHRP Registration Request.

   Destination Protocol Address
     This is the protocol address of the NHS for which the source NBMA
     next hop information is being registered.

   This packet is used to register a station's Protocol and NBMA
   addresses with its NHSs, as configured or known through conventional
   routing means.  This allows static configuration information to be
   reduced;  the NHSs need not be configured with the identities of all
   of the stations that they serve.  If an NHS receives an NHRP
   Registration Request packet for a station that it does not serve and
   that packet has a Destination Protocol Address which is not the
   protocol address of the NHS that is currently inspecting the packet
   then the NHS inspecting the packet MUST forward the registration
   along the routed path to the Destination Protocol Address.




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   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 NAK Code field 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.  Its 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             |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                         Request ID                            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |   NAK Code    |     unused    |    Register Holding Time      |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |            Source NBMA Address (variable length)              |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Source NBMA Subaddress (variable length)             |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Source Protocol Address (variable length)            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |       Destination  Protocol Address (variable length)         |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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



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

   Request ID
     A value which, when coupled with the address of the source,
     provides a unique identifier for the information contained in an
     NHRP Registration Reply packet.  This value is copied directly from
     an NHRP Registration Request packet into the associated NHRP
     Registration Reply.  This value could also be sent across a virtual
     circuit (in SVC environments) to aid in matching NHRP transactions
     with virtual circuits (this use is for further study).

   NAK Code
     If this field is set to zero then this packet contains a positively
     acknowledged NHRP Registration Reply.  If this field contains any
     other value then this contains an NHRP Registration Reply NAK which
     means that the internetworking layer to NBMA address binding was
     not stored at the client's NHS.  Currently defined NAK Codes are as
     follows:

       4 - Can't Serve This Address

         An NHS may refuse an NHRP Registration Request attempt for
         administrative reasons.  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.

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

   Source NBMA Address
     The Source NBMA address field is the address of the source station
     which sent the Next Hop Registration Request.

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



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   Source Protocol Address
     This is the protocol address of the station which sent the NHRP
     Registration Request.

   Destination Protocol Address
     This is the protocol address of the NHS in which the client is
     attempting to register the client's NBMA information.

   This packet is used to register a station's Protocol and NBMA
   addresses with its neighboring NHSs, as configured or known through
   conventional routing means.  This allows static configuration
   information to be reduced;  the NHSs need not be configured with the
   identities of all of the stations that they serve. If an NHS receives
   an NHRP Registration Request packet for a station that it does not
   serve and that packet has a Destination Protocol Address which is not
   the protocol address of the NHS that is currently inspecting the
   packet then the NHS inspecting the packet MUST forward the
   registration along the routed path to the Destination Protocol
   Address.

   It is possible that a misconfigured station will attempt to send a
   Next Hop Registration Request to 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 NHRP 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 NHRP Registration Reply of type Registration
   Overflow.

   In order to keep the client's registration entry in the client's NHS
   from being timed out, the station MUST re-send the NHRP Registration
   Request packet often enough to refresh the registration entry, 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.


5.2.5 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
   code of 5.  The Mandatory Part has 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
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         | Src Proto Len | Dst Proto Len |Trgt Proto Len |    unused     |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |            Source NBMA Address (variable length)              |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Source NBMA Subaddress (variable length)             |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Source Protocol Address (variable length)            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |       Destination  Protocol Address (variable length)         |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |           Target Protocol Address (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.

   Trgt Proto Len
     This field holds the length in octets of the Target Protocol
     Address.

   Source NBMA Address
     The Source NBMA address field is the address of the source station
     which is sending the NHRP Purge Request.

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

   Source Protocol Address
     The address of the station which is sending the NHRP Purge Request.

   Destination Protocol Address
     The address of the station that will be receiving the NHRP Purge
     Request.

   Target Protocol Address
     The address which is to be purged from the receiver's database.




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   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 it's 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 are for further
   investigation.

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

   An NHRP Purge Reply MUST be returned for the NHRP Purge Request even
   if the station does not have a matching cache entry.

   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 packet has a type code of 6.  The Mandatory Part has 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
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         | Src Proto Len | Dst Proto Len |Trgt Proto Len |    unused     |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |            Source NBMA Address (variable length)              |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Source NBMA Subaddress (variable length)             |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Source Protocol Address (variable length)            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |       Destination  Protocol Address (variable length)         |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |           Target Protocol Address (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.

   Trgt Proto Len
     This field holds the length in octets of the Target Protocol
     Address.

   Source NBMA Address
     The Source NBMA address field is the address of the source station
     which sent the NHRP Purge Request.

   Source NBMA SubAddress
     The Source NBMA subaddress field is the address of the source
     station which sent the NHRP Purge Request.  If its length as
     specified in ar$sstl is 0 then no storage is allocated for this
     address at all.

   Source Protocol Address
     The address of the station which sent the NHRP Purge Request.

   Destination Protocol Address
     The address of the station which is sending the NHRP Purge Reply.

   Target Protocol Address
     The address which is to be purged from the receiver's database.





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   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 it's 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 it is acknowledged, or until
   the holding time of the information being purged has expired.
   Retransmission strategies are for further investigation.

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

   An NHRP Purge Reply MUST be returned as a result of receiving an NHRP
   Purge Request even if the station does not have a matching cache
   entry.

   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.


5.2.7  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 6.  The Mandatory
   Part has 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
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         | 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
         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



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


       8 - NHRP SDU Size Exceeded

         If the SDU size of the NHRP packet exceeds the maximum SDU size
         of the NBMA network, 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.

   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.



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   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

   In the following, unless otherwise stated explicitly, the term
   "request" refers generically to any of the NHRP packet types which
   are "requests".  Also, unless otherwise stated explicitly, the term
   "reply" refers generically to any of the NHRP packet types which are
   "replies".

   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";
   therefore, minimal NHRP station implementations that do not act as an
   NHS and do not transmit extensions need not be able to receive
   extensions.  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



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     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.

   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.


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.





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5.3.1  Destination Prefix Length

    Compulsory = 0
    Type = 1
    Length = 1

   This extension is used to indicate that the information carried in an
   NHRP packet 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.

          0                   1
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |         Prefix Length         |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   In the case of an Next Hop Resolution Request, if equivalence
   information is desired from the Next Hop Resolution Reply then the
   Destination Prefix Length extension is included in the Next Hop
   Resolution Request and the Prefix Length value is coded as 0xffff.
   For the Next Hop Resolution Reply, the Prefix Length is set to the
   length of the prefix of the Next Hop Protocol Address present in the
   mandatory part of the packet.

   In the case of an NHRP Registration Request, if equivalence
   information is desired to be registered then the Destination Prefix
   Length extension is included in the NHRP Registration Request with
   the Prefix Length value set to the length of the prefix of the
   equivalence information for the Source Protocol Address.  In Next Hop
   Registration Reply, the Destination Prefix Length extension is merely
   copied unchanged.

   In the case of an Next Hop Purge Request, if equivalence information
   is desired then the Prefix Length value is set to the length of the
   prefix of the Target Protocol Address which represents the
   equivalence information to be purged.  In Next Hop Purge Reply, the
   Destination Prefix Length extension is merely copied unchanged.


5.3.2  NBMA Subnetwork ID Extension

    Compulsory = 1
    Type = 2
    Length = variable




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   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.

   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 = 4

   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



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   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.

          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.



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

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















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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 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



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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 "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".

   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".




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   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.

          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



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    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....     |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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  Additional Next Hop Entries Extension

    Compulsory = 0
    Type = 9
    Length = variable

   This extension may be used to return multiple Next Hop entries in a
   single NHRP Reply packet.  This extension MUST only be used for
   positive replies.  The preference values are used to specify the
   relative preference of the entries contained in the extension.  The
   same next Hop Protocol address may be associated with multiple NBMA
   addresses.  Load-splitting may be performed over the addresses, given
   equal preference values, and 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).

   The following shows the format for additional Next Hop Entries:






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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
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |  Maximum Transmission Unit    |        Holding Time           |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |  NH Addr T/L   | NH SAddr T/L |  NH Proto Len |  Preference   |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Next Hop NBMA Address (variable length)              |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |         Next Hop NBMA Subaddress (variable length)            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |         Next Hop Protocol Address (variable length)           |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                              .....................
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |  Maximum Transmission Unit    |        Holding Time           |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |  NH Addr T/L   | NH SAddr T/L |  NH Proto Len |  Preference   |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Next Hop NBMA Address (variable length)              |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |         Next Hop NBMA Subaddress (variable length)            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |         Next Hop Protocol Address (variable length)           |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   An NHS is not allowed to reply to an NHRP request for authoritative
   information with cached information, but may do so for an NHRP
   Request which indicates a request for non-authoritative information.
   An NHS may reply to an NHRP request for non-authoritative information
   with authoritative information.

   Maximum Transmission Unit
     This field gives the maximum transmission unit for the target
     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 Next Hop Entry 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.

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



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     ATM).

   NH SAddr T/L
     Type & length of next hop NBMA subaddress specified in the Next Hop
     Entry.  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.

   NH Proto Len
     This field holds the length in octets of the Next Hop Protocol
     Address specified in the Next Hop Entry.

   Preference
     This field specifies the preference of the specific Next Hop Entry
     relative to other Next Hop entries in this Next Hop Resolution
     Reply mandatory part or in the Additional Next Hop Entries
     Extension for the given internetworking protocol.  Higher values
     indicate higher preference.  Action taken when multiple next hop
     entries have the highest preference value is a local matter.

   Next Hop NBMA Address
     This is the NBMA address of the station that is the next hop for
     packets bound for the internetworking layer address specified.

   Next Hop NBMA SubAddress
     This is the NBMA subaddress of the station that is the next hop for
     packets bound for the internetworking layer address specified.

   Next Hop Protocol Address
     This internetworking layer address specifies the next hop.  This
     will be the address of the destination host if it is directly
     attached to the NBMA subnetwork, or the egress router if it is not
     directly attached.

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



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   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.


   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.





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     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
     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.




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     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).

     Strategies for maintaining NHRP cache information in the presence
     of dynamic routing changes will be discussed in a separate
     document.

6.3 Use of the Destination Prefix Length Extension

   A certain amount of care needs to be taken when using the Destination
   Prefix Length Extension, 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 NBMA) can result.  To avoid this situation an NHS
   that wants to send the Destination Prefix Length Extension 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 for the Destination Prefix Length
     Extension.

   The NHRP Destination Prefix Length Extension should be used with
   restraint, in order to avoid NHRP stations choosing suboptimal
   transit paths when overlapping prefixes are available.  This



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   extension SHOULD only be used in a Next Hop Resolution Reply when
   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.

   For other cases, there may be no single optimal transit path for
   destinations encompassed by the address prefix, and an NHRP station
   may fail to choose the optimal transit path simply because it is not
   aware of all such paths.  So for cases not covered by (a) and (b), an
   Next Hop Resolution Reply packet should not include the NHRP
   Destination Prefix Length Extension.

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.  It is recommended that
   implementations provide one or more of the following solutions.

   Possibly the most straightforward solution for suppressing the domino
   effect would be to require transit routers to be preconfigured not to
   originate Next Hop Resolution Requests for data traffic which is
   simply being forwarded (not originated).  In this case the routers
   avoid the domino effect through an administrative policy.

   A second possible solution would be to require that when a router
   forwards an Next Hop Resolution Request, the router instantiates a
   (short-lived) state.  This state consists of the route that was used
   to forward the Next Hop Resolution Request.  If the router receives a
   data packet, and the packet triggers an Next Hop Resolution Request
   generation by the router, the router checks whether the route to
   forward the Next Hop Resolution Request was recently used to forward



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   some other Next Hop Resolution Request.  If so, then the router
   suppresses generation of the new Next Hop Resolution Request (but
   still forwards the data packet).  This solution also requires that
   when a station attempts to originate an Next Hop Resolution Request
   the station should send the Next Hop Resolution Request before the
   data packet that triggered the origination of the Next Hop Resolution
   Request.  Otherwise, unnecessary Next Hop Resolution Requests may
   still be generated.

   A third possible strategy would be to configure a router in such a
   way that Next Hop Resolution Request generation by the router would
   be driven only 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.  Just as in the
   first case, such a router avoids the NHRP domino effect through
   administrative means.

   Lastly, rate limiting of Next Hop Resolution Requests may help to
   avoid the NHRP domino effect.  Intermediate routers which would
   otherwise generate unnecessary Next Hop Resolution Requests may
   instead suppress such Next Hop Resolution Requests due to the
   measured Next Hop Resolution Request rate exceeding a certain
   threshold.  Of course, such rate limiting would have to be very
   aggressive in order to completely avoid the domino effect.  Further
   work is needed to analyze this solution.

7. 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.


8. 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



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   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.

   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, draft-ietf-rolc-nbma-arp-00.txt.

   [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.





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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.  John Burnett of
   Adaptive, Dennis Ferguson of ANS, Joel Halpern 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 Routing Over Large Clouds working
   group of the IETF, whose review and discussion of this document have
   been invaluable.

Authors' Addresses


   Dave Katz                           David Piscitello
   cisco Systems                       Core Competence
   170 W. Tasman Dr.                   1620 Tuckerstown Road
   San Jose, CA 95134 USA              Dresher, PA 19025 USA

   Phone:  +1 408 526 8284             Phone:  +1 215 830 0692
   Email:  dkatz@cisco.com             Email: dave@corecom.com

   Bruce Cole                          James V. Luciani
   cisco Systems                       Ascom Nexion
   170 W. Tasman Dr.                   289 Great Road
   San Jose, CA 95134 USA              Acton, MA 01720-4379 USA

   Phone:  +1 408 526 4000             Phone:  +1 508 266 3450
   Email:  bcole@cisco.com             Email: luciani@nexen.com




















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