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Routing over Large Clouds Working Group                        Dave Katz
INTERNET-DRAFT                                           (cisco Systems)
<draft-ietf-rolc-nhrp-04.txt>                           David Piscitello
                                                 (Core Competence, Inc.)
                                                               May, 1995


                NBMA Next Hop Resolution Protocol (NHRP)


Status of this Memo

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   To learn the current status of any Internet-Draft, please check the
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   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 IP 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.  Although
   this document focuses on NHRP in the context of IP, the technique is
   applicable to other internetwork layer protocols (e.g., IPX, CLNP,
   Appletalk) as well.

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

   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.



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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 IP and NBMA
   addresses of the "NBMA next hop" 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 logically
   independent IP subnets (LISs), defined in [3] and [4] as having 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.

   IP routing 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 IP routing 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 routing method that relaxes the forwarding
   restrictions of the LIS model.  With NHRP, once the NBMA next hop has
   been resolved, the source may either start sending IP packets to the
   destination (in a connectionless NBMA subnetwork such as SMDS) or may
   first establish a connection to the destination with the desired
   bandwidth and QOS characteristics (in a connection-oriented NBMA
   subnetwork such as ATM).

   NHRP in its most basic form provides a simple IP-to-NBMA-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



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   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.

   NHRP supports both a server-based style of deployment and a
   ubiquitous "fabric", consisting of NHRP-capable routers.  The
   server-based approach requires a smaller number of machines (possibly
   one) to support NHRP, but requires significantly more manual
   configuration.

   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
   the "NBMA next hop" to destination D.




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   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, otherwise 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 (IP-to-NBMA address).  This table can be constructed
   from information gleaned from NHRP Register packets (see Section
   5.4), extracted from NHRP requests or 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 6.3 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.]

   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 resource reservation request, 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



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   destination IP address).  If the next hop is reachable through its
   NBMA interface, S constructs an NHRP request packet (see Section 5.2)
   containing station D's IP address as the (target) destination
   address, S's own IP address as the source address (NHRP request
   initiator), and station S's NBMA addressing information.  Station S
   may also indicate that it prefers an authoritative reply (i.e.,
   station S only wishes to receive a reply from the NHS-speaker that
   maintains the NBMA-to-IP address mapping for this destination).
   Station S encapsulates the NHRP request packet in an IP packet
   containing as its destination address the IP address of its
   configured NHS.  This IP packet is emitted across the NBMA interface
   to the NBMA address of the NHS.

   If the NHRP request is triggered by a data packet, station S may
   choose to dispose of the data packet while awaiting an NHRP reply in
   one of the following ways:

     (a)  Drop the packet
     (b)  Retain the packet until the 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 NHRP request for a given destination MUST NOT be
   triggered on every packet, though periodically retrying a request is
   permitted.

   When the NHS receives an NHRP request, a check is made to see if it
   "serves" station D, i.e., the NHS checks to see if there is a "next
   hop" entry for D in its next-hop resolution cache.  If the NHS does
   not serve D, the NHS forwards the NHRP request to another NHS.
   (Mechanisms for determining how to forward the NHRP request are
   discussed in Section 3, Modes of Deployment.)

   If this NHS serves D, the NHS resolves station D's NBMA address, and
   generates a positive NHRP reply on D's behalf.  (NHRP replies in this
   scenario are always marked as "authoritative".)  The NHRP reply
   packet contains the next hop IP and 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 IP address will be that of the egress router
   through which packets for station D are forwarded.)

   An NHS receiving an NHRP reply may cache the NBMA next hop
   information contained therein.  To a subsequent NHRP request, this
   NHS may respond with the cached, non-authoritative, NBMA next hop
   information or with cached negative information, or may not be



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   allowed to respond with the cached information (see section 6.3).
   Non-authoritative NHRP replies are distinguished from authoritative
   replies so that if a communication attempt based on non-authoritative
   information fails, a source station can choose to send an
   authoritative NHRP request.  NHSs MUST NOT respond to authoritative
   NHRP requests with cached information.

     [Note: An NHRP reply can be returned directly to the NHRP request
     initiator, i.e., without traversing the list of NHSs that forwarded
     the 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 NHRP request initiator or is permitted
           to open one.
       (b) The NHRP request initiator has not included the NHRP
           Reverse NHS record Extension (see Section 5.7.5).
       (c) The authentication policy in force permits direct
           communication between the NHS and the NHRP request
           initiator.

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

   The process of forwarding the NHRP request is repeated until the
   request is satisfied, or an error occurs (e.g., no NHS in the NBMA
   subnetwork can resolve the request.) If the determination is made
   that station D's next hop cannot be resolved, a negative reply 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 NHRP request (e.g., connectivity is lost).

   NHRP requests and replies MUST NOT cross the borders of a logical
   NBMA subnetwork (an explicit NBMA subnetwork identifier may be
   included as an extension in the NHRP request, see section 5.7.2).
   Thus, IP traffic out of and into a logical NBMA subnetwork always
   traverses an IP router at its border.  Internetwork layer filtering
   can then be implemented at these border routers.

   NHRP optionally provides a mechanism to reply with 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 IP address prefix with station D.
   When an NHRP reply is generated in response to a request, the
   responder may augment the IP address of station D with a bit count
   defining this prefix (see Section 5.7.1).  A subsequent (non-



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   authoritative) NHRP request for some destination that shares an IP
   address prefix with D may be satisfied with this cached information.
   See section 6.3 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, NHRP
   optionally incorporates a "Route Record" in requests and replies (see
   Sections 5.7.4 and 5.7.5).  The Route Record extensions contain the
   internetwork (and subnetwork layer) addresses of all intermediate
   NHSs between source and destination (in the forward direction) and
   between destination and source (in the reverse direction).  When a
   source station is unable to communicate with the responder (e.g., an
   attempt to open an SVC fails), it may attempt to do so successively
   with other subnetwork layer addresses in the Route Record until it
   succeeds (if authentication policy permits such action).  This
   approach can find a suitable egress point in the presence of
   subnetwork-layer filtering (which may be source/destination
   sensitive, for instance, without necessarily creating separate
   logical NBMA subnetworks) or subnetwork-layer congestion (especially
   in connection-oriented media).

   NHRP messages, with the exception of Purge packets, are sent
   unreliably.  NHRP requests should be retransmitted periodically until
   either a Reply or an Error packet is received.



3. Modes of Deployment

   NHRP supports two deployment modes of operation: "server" and
   "fabric" modes.  The two modes differ only in the way NHRP packets
   are propagated, which is driven by differences in configuration.

   It is desirable that hosts attached directly to the NBMA subnetwork
   have no knowledge of whether NHRP is deployed in "server" or "fabric"
   modes, so that a change in deployment strategy can be done within a
   single administration, transparently to hosts.  For this reason, host
   configuration is invariant between the two cases.  Note that
   irrespective of which mode is deployed, NHRP clients must nominally
   be configured with the NBMA (and IP) address of at least one NHS.  In
   practice, a host's default router should also be its NHS.

   Server Mode

     In "server" mode, the expectation is that a small number of NHSs
     will be fielded in an NBMA subnetwork.  This may be appropriate in
     subnetworks containing routers that do not support NHRP, or



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     subnetworks that have large numbers of directly-attached hosts (and
     relatively few routers).  Server mode assumes that NHRP is very
     loosely coupled with IP routing, and that the path taken by NHRP
     requests has little to do with the path taken by IP data packets
     routed to the desired destination.  Note that in server mode the
     NHSs need not be routers, since they will not be required to
     forward transit data packets.

     [Note: This is the likely scenario for initial deployment of NHRP.
     It is also likely that single and Multi-LIS configurations using
     either group-addressed ARP (in the case of SMDS) or ARP servers (in
     the case of ATM or SMDS) may already be in place.]

     Server mode uses static configuration of NHS identity.  The client
     station must be configured with the IP address of one or more NHSs,
     and there must be a path to that NHS (either directly, in which
     case the NHS's NBMA address must be known, or indirectly, through a
     router whose NBMA address is known).  If there are multiple NHSs,
     they must be configured with each others' addresses, the identities
     of the destinations that each of them serves, and optionally a
     logical NBMA subnetwork identifier.  (This static configuration
     requirement, which may involve authentication as well as addressing
     information, tends to limit such deployments to a very small number
     of NHSs.)

     If the NBMA subnetwork offers a 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 NHRP 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 direct replies may apply.

     The servers can also be deployed with the group or multicast
     address of their peers, and an NHS might use this as a means of
     forwarding NHRP 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
     NHRP reply to the NRHP 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 NHRP 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 NHRP request originator to cache next hop
     information associated with the reply (again, within the



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     constraints described in Section 2).

     The NHRP request packet's destination IP address is set by the
     source station to the first-hop NHS's IP address.  If the addressed
     NHS does not serve the destination, the NHRP request is forwarded
     to the IP address of the NHS that serves the destination.

     The responding NHS uses the source address from within the NHRP
     packet (not the source address of the IP packet) as the IP
     destination of the NHRP reply.

     Note that, in many cases, NHSs deployed in Server Mode are unlikely
     to be able to resolve the next hop of destination that lies outside
     of the NBMA subnetwork, since doing so requires routing knowledge
     that is only provided by certain protocols (Link State routing
     protocols, for example);  with many routing protocols, only the
     egress router itself knows that it is the egress router.  The
     identity of the egress router may be provided by a server if such
     information is very static;  in practical terms the egress router
     can only be guaranteed to be fixed if static routing is in use, or
     there is only one egress router.  If the identity of egress routers
     cannot be determined, then the NHSs can only provide information
     about destinations directly attached to the NBMA subnetwork.


   Fabric Mode

     In "fabric" mode, it is expected that NHRP-capable routers are
     ubiquitous throughout the NBMA subnetwork, and that NHSs acquire
     knowledge about destinations other NHSs serve as a direct
     consequence of participating in intradomain and interdomain routing
     protocol exchange.  In particular, 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.

     Fabric mode leverages a routed infrastructure that "overlays" the
     NBMA subnetwork.  The source station passes the NHRP request to the
     router which serves as the next hop toward the destination.  Each
     router in turn forwards the NHRP request toward the destination.
     Eventually, the NHRP request arrives at a router that is acting as
     an NHS serving the destination (or the destination itself, if it is
     an NHRP-speaker), which generates the NHRP reply.

     If the source station is a host, it sets the IP destination address
     of the NHRP request to the first-hop NHS/router (so that hosts
     needn't know the mode in which the subnetwork is running).  If the



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     source station is a router, the destination IP address may be set
     either to the next-hop router or to the ultimate destination being
     resolved.  Each NHS/router examines the NHRP request packet on its
     way toward the destination, optionally modifying it on the way
     (such as updating the Forward Record extension).  The Router Alert
     option [5] is added by the first NHS in order to ensure that
     NHS/routers along the path process the packet, even though it may
     be addressed to the ultimate destination.

     If an NHS/router receives an NHRP packet addressed to itself to
     which it cannot reply (because it does not serve the destination
     directly), it will forward the NHRP request with the destination IP
     address set to the ultimate destination (thus allowing invariant
     host behavior).  Eventually, the NHRP packet will arrive at the
     NHS/router that serves the destination (which will return a
     positive NHRP reply) or it will arrive at a NHS/router that has no
     route to the destination (which will return a negative NHRP reply),
     or it may arrive at a NHS/router that cannot reach the NHS that
     serves the destination due to a loss of reachability among the NHSs
     (in which case the router will return a negative NHRP reply).

     The procedural difference between server mode and fabric mode is
     reduced to deciding how to update the destination address in the IP
     packet carrying the NHRP request.

     Note that addressing the NHRP request to the ultimate destination
     allows for subnetworks that do not have NHSs deployed in all
     routers;  typically a very large NBMA subnetwork might only deploy
     NHSs in egress routers, and not in transit routers.


4. Configuration

   Stations

     To participate in NHRP, a station connected to an NBMA subnetwork
     should be configured with the IP and 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) may be
     physically located on the stations's default or peer routers, so
     their addresses may be obtained from the station's IP forwarding
     table.  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 IP networks are reachable
     through which subnetworks.




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   Next Hop Servers

     An NHS is configured with its own identity, a set of IP address
     prefixes that correspond to the IP addresses of the stations it
     serves, a logical NBMA subnetwork identifier (see Section 5.7.2),
     and in the case of "server" mode, the identities of other NHSs in
     the same logical NBMA subnetwork.  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.

     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. 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 8 octets.  The length of the
   Mandatory Part is carried in the Fixed Part.  The length of the
   Extensions Part is implied by the total packet length (Internet
   datagram total length minus IP header length minus NHRP fixed part
   length minus NHRP mandatory part length).

   NHRP packets are carried in IP packets as protocol type 54 (decimal).
   NHSs may increase the size of an NHRP packet as a result of extension
   processing.  IP datagrams containing NHRP packets MUST have the Don't
   Fragment bit set.

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

   Most packet types have both internetwork layer protocol-independent
   fields and protocol-specific fields.  The protocol-independent fields



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   always come first in the packet, and the Protocol ID field qualifies
   the format of the protocol-specific fields.  The protocol-specific
   fields defined in this document are for IPv4 only;  formats of
   protocol-specific fields for other protocols are for further study.

   The protocol ID field in general will contain the Ethertype value for
   the protocol (see [6]).  For protocols that do not have an assigned
   Ethertype, this field will in general contain the Network Layer
   Protocol Identifier (NLPID, [7]) value for the protocol (this is
   guaranteed to not cause collisions since the NLPID cannot be greater
   than 255 decimal, and the Ethertype cannot be less than 1500
   decimal).


5.1 NHRP Fixed Header

   The NHRP Fixed Header is present in all NHRP packets.  It contains
   the basic information needed to parse the rest of the 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Version    |   Hop Count   |          Checksum             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |    Unused     |    Mandatory Part Length      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Version
     The NHRP version number.  Currently this value is 1.

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

   Checksum
     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.

   Type
     The NHRP packet type: Request, Response, Register, Purge, or Error
     Indication (see below).

   Mandatory Part Length
     The length in octets of the Mandatory Part.  This length does not
     include the Fixed Header.



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5.2 NHRP Request

   The NHRP Request packet has a Type code of 1.  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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Q|S|A|P|B|       Unused        |          Protocol ID          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Request ID                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                               (IPv4-Specific)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Destination IP address                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Source IP address                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Holding Time          |         Address Type          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Unused     |  NBMA Length  | NBMA Address (variable length)|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Q
     Set if the Requestor is a router;  clear if the requestor is a
     host.

   S
     Unused (zero on transmit)

   A
     A response to an NHRP 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 (zero on transmit)

   B
     Unused (zero on transmit)

   Protocol ID
     Specifies the internetwork layer protocol for which we are
     obtaining routing information.  This value also qualifies the
     structure of the remainder of the Mandatory Part.  For IPv4, the
     Protocol ID is hexadecimal 800 (decimal 2048).  Protocol ID values



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     for other internetwork layer protocols are for future study.

   Request ID
     A value which, when coupled with the address of the source,
     provides a unique identifier for the information contained in a
     Request and its associated Reply, and any subsequent Purge.  This
     value can be used by the source to aid in matching requests with
     replies.  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 NHRP request is transmitted.  The same value MUST be
     used when sending another request for the same destination when a
     previous request is still active or pending, i.e., when
     retransmitting a request because a reply was not received, or when
     refreshing an existing entry to avoid holding timer expiration.  A
     new value MUST be used when sending a request when no cache entry
     is present, or a previous cache entry was deleted for any reason.

   Destination and Source IP Addresses
     Respectively, these are the IP addresses of the station for which
     the NBMA next hop is desired, and the NHRP request initiator.

   Source Holding Time, Address Type, NBMA Length, and NBMA Address
     The Holding Time field specifies the number of seconds for which
     the source NBMA information is considered to be valid.  Cached
     information SHALL be discarded when the holding time expires.

     The Address Type field specifies the type of NBMA address
     (qualifying the NBMA address).  Possible address types are listed
     in [6].

     The NBMA length field is the length of the NBMA address of the
     source station in bits.  The NBMA address field itself is zero-
     filled to the nearest 32-bit boundary.


5.3 NHRP Reply

   The NHRP Reply packet has a type code of 2.  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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Q|S|A|P|B|       Unused        |          Protocol ID          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Request ID                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                               (IPv4-Specific)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Destination IP address                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Source IP address                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Next-hop IP address                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Holding Time          |         Address Type          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Preference   |  NBMA Length  | NBMA Address (variable length)|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Next-hop IP address                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Holding Time          |         Address Type          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Preference   |  NBMA Length  | NBMA Address (variable length)|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Q
     Copied from the NHRP Request.  Set if the Requestor is a router;
     clear if the requestor is a host.

   S
     Set if the next hop identified in the reply is a router;  clear if
     the next hop is a host.

   A
     Set if the reply is authoritative;  clear if the reply is non-
     authoritative.

   P
     Set if the reply is positive;  clear if the reply is negative.






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   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
     IP 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 cacheing strategies (see
     section 6.3).


   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.

   Protocol ID
     Specifies the internetwork layer protocol for which we are
     obtaining routing information.  This value also qualifies the
     structure of the remainder of the Mandatory Part.  For IPv4, the
     Protocol ID is hexadecimal 800 (decimal 2048).  Protocol ID values
     for other internetwork layer protocols are for future study.

   Request ID
     Copied from the NHRP Request.

   Destination IP Address
     The address of the target station (copied from the corresponding
     NHRP Request).

   Source IP Address
     The address of the initiator of the request (copied from the
     corresponding NHRP Request).

   Next-hop entry
     A Next-hop entry consists of the following fields: a 32-bit Next-
     hop IP Address, a 16-bit Holding Time, an 8-bit Preference, an 8-
     bit Address Type, an 8-bit NBMA Length, and an NBMA Address whose
     length is the value of the NBMA length field.

     The Next-hop IP Address specifies the IP address of 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.



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     The Holding Time field specifies the number of seconds for which
     the associated Next-hop entry information is considered to be
     valid.  Cached information SHALL be discarded when the holding time
     expires.  (Holding time is to be specified for both positive and
     negative replies).

     The Address Type field specifies the type of NBMA address
     (qualifying the NBMA address).  Possible address types are listed
     in [6].

     The Preference field specifies the preference of the Next-hop
     entry, relative to other Next-hop entries in this NHRP Reply
     packet.  Higher values indicate more preferable Next-hop entries.
     Action taken when multiple next-hop entries have the highest
     preference value is a local matter.

     The NBMA length field specifies the length of the NBMA address of
     the destination station in bits.  The NBMA address field itself is
     zero-filled to the nearest 32-bit boundary.  For negative replies,
     the Holding Time field is relevant; however, the preference,
     Address Type, and NBMA length fields MUST be zero, and the NBMA
     Address SHALL NOT be present.

     There may be multiple Next-hop entries returned in the reply (as
     implied by the Mandatory Part Length).  The preference values are
     used to select the preferred entry.  The same next-hop IP 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).

     If extensions were present in the NHRP Request packet, all of these
     extensions MUST be present in the NHRP Reply.  No additional
     extensions may be added to the reply that were not present in the
     request.


5.4 NHRP Register

   The NHRP Register packet is sent from a station to an NHS to notify
   the NHS of the station's NBMA address.  It has a Type code of 3.  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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Unused               |          Protocol ID          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                               (IPv4-Specific)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Source IP address                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Holding Time          |         Address Type          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Unused     |  NBMA Length  | NBMA Address (variable length)|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Protocol ID
     Specifies the internetwork layer protocol for which we are
     obtaining routing information.  This value also qualifies the
     structure of the remainder of the Mandatory Part.  For IPv4, the
     Protocol ID is hexadecimal 800 (decimal 2048).  Protocol ID values
     for other internetwork layer protocols are for future study.

   Source IP Address
     The IP address of the station wishing to register its NBMA address
     with an NHS.

   Source Holding Time, Address Type, NBMA Length, and NBMA Address
     The Holding Time field specifies the number of seconds for which
     the source NBMA information is considered to be valid.  Cached
     information SHALL be discarded when the holding time expires.

     The Address Type field specifies the type of NBMA address
     (qualifying the NBMA address).  Possible address types are listed
     in [6].

     The NBMA length field is the length of the NBMA address of the
     source station in bits.  The NBMA address itself is zero-filled to
     the nearest 32-bit boundary.


   This packet is used to register a station's IP and NBMA addresses
   with its configured NHS.  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.

   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



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   reply with an Error Indication 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 an Error Indication of type Registration Overflow.

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




5.5 NHRP Purge

   The NHRP Purge packet has a type code of 4.  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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |A|             Unused          |       Protocol ID             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Request ID                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                               (IPv4-Specific)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Source IP address                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   A
     Clear if this is a purge request, set if this is an
     acknowledgement.

   Protocol ID
     Specifies the internetwork layer protocol for which we are
     obtaining routing information.  This value also qualifies the
     structure of the remainder of the Mandatory Part.  For IPv4, the
     Protocol ID is hexadecimal 800 (decimal 2048).  Protocol ID values
     for other internetwork layer protocols are for future study.

   Request ID
     Copied from the corresponding NHRP Request.  This is used by the
     station receiving the purge to identify which cache entry to purge,
     and by the NHS receiving the acknowledgement to match the



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     acknowledgement with the Purge request.

   Source IP Address
     The address of the initiator of the request (copied from the
     corresponding NHRP Request).  Used by the NHS receiving the
     acknowledgement to match the acknowledgement with the Purge
     request.

   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 destination that is
   not directly connected to the NBMA subnetwork, and the egress point
   to that destination may have changed).

   The IP destination address of the packet containing the Purge request
   is set to the Source IP address from the original Request packet.

   The NHS sending the NHRP Purge request MUST periodically retransmit
   the 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
   previous cached information that matches the Request ID.  It MUST
   then acknowledge the Purge request by setting the Acknowledgement (A)
   bit and returning the Purge request to the sender.  The IP
   destination address of the Purge acknowledgement MUST be set to the
   IP source address of the Purge request.

   An acknowledgement MUST be returned for the Purge request even if the
   station does not have a cache entry with a matching Request ID.

   If the station wishes to reestablish communication with the
   destination shortly after receiving a Purge request, it should make
   an authoritative request in order to avoid any stale cache entries
   that might be present in intermediate NHSs.  (See section 6.3.2.)  It
   is recommended that authoritative requests be made for the duration
   of the holding time of the old information.


5.6  NHRP Error Indication

   The NHRP Error Indication is used to convey error indications to the
   initiator of an NHRP Request packet.  It 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Error Code          |        Error Offset           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +-+-+-+-+-+-+-+  Contents of NHRP Packet in error +-+-+-+-+-+-+-+
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

       1 - Unrecognized Extension
       2 - Subnetwork ID Mismatch
       3 - NHRP Loop Detected
       4 - Can't Serve This Address
       5 - Registration Overflow
       6 - Server Unreachable
       7 - Protocol Error
       8 - NHRP fragmentation failure

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

   The destination IP address of an NHRP Error Indication SHALL be set
   to the IP address of the initiator of the original NHRP Request (as
   extracted from the NHRP Request or NHRP Reply).

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


5.7  Extensions Part

   The Extensions Part, if present, carries one or more extensions in
   {Type, Length, Value} triplets.  Extensions are only present in a
   Reply if they were present in the corresponding Request;  therefore,
   minimal NHRP station implementations that do not act as an NHS and do
   not transmit extensions need not be able to receive them.  An
   implementation that is incapable of processing extensions SHALL
   return an Error Indication of type Unrecognized Extension when it
   receives an NHRP packet containing extensions.



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   Extensions are typically protocol-specific, as noted.

   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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |D|          Type               |        Length                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Value...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   Type
     The extension type code (see below).  The extension type is not
     qualified by the Discretionary 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).

   Each extension is padded with zero octets to a 32 bit boundary.  This
   padding is not included in the Length field.

   Extensions may occur in any order, but any particular extension type
   may occur only once in an NHRP packet.

   The Discretionary bit provides for a means to add to the extension
   set.  If the bit is clear, the NHRP request cannot be satisfied
   unless the extension is processed, so the responder MUST return an
   Error Indication of type Unrecognized Extension.  If the bit is set,
   the extension can be safely ignored, though unrecognized extensions
   so ignored that were received in an NHRP Request packet MUST be
   returned unchanged in the corresponding NHRP Reply.

   If a transit NHS (one which is not going to generate a reply) detects
   an unrecognized extension, it SHALL ignore the extension.  If the
   Discretionary bit is clear, the transit NHS MUST NOT cache the
   information (in the case of a reply) and MUST NOT identify itself as
   an egress router (in the Forward Record or Reverse Record
   extensions).  Effectively, this means that a transit NHS that
   encounters an extension that it cannot process and determines that



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   the Discretionary bit is clear MUST NOT participate in any way in the
   protocol exchange, other than acting as a forwarding agent for the
   request.


5.7.1  Destination Prefix Extension (IPv4-Specific)

   Discretionary = 1
   Type = 1
   Length = 1

   This extension is used to indicate that the information carried in an
   NHRP Reply pertains to an equivalence class of destinations rather
   than just the destination IP address specified in the request.  All
   addresses that match the IP address prefix defined by the prefix
   length are part of the equivalence class.

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

   If an initiator would like to receive this equivalence information,
   it SHALL add this extension to the NHRP Request with a value of 32.
   The responder SHALL copy the extension to the NHRP Reply and modify
   the prefix length appropriately.


5.7.2  NBMA Subnetwork ID Extension (Protocol-Independent)

   Discretionary = 0
   Type = 2
   Length = variable

   This extension is used to carry one or more identifiers for the NBMA
   subnetwork.  This can be used as a validity check to ensure that the
   request does not leave a particular NBMA subnetwork.  The extension
   is placed in an NHRP Request packet by the initiator with an ID value
   of zero;  the first NHS 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.







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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    NBMA Subnetwork ID                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     ...

   Each identifier consists of a 32 bit globally unique value assigned
   to the NBMA subnetwork.  This value should be chosen from the IP
   address space administered by the operators of the NBMA subnetwork.
   This value is used for identification only, not for routing or any
   other purpose.

   Each NHS processing an NHRP Request SHALL verify these values.  If
   none of the values matches the NHS's NBMA Subnetwork ID, the NHS
   SHALL return an Error Indication of type "Subnetwork ID Mismatch" and
   discard the NHRP Request.

   When an NHS is building an NHRP Reply and the NBMA Subnetwork ID
   extension is present in the NHRP Request, the NBMA Subnetwork ID
   extension SHALL be copied from the Request to the Reply, including
   all values carried therein.

   Each NHS processing an NHRP Reply SHALL verify the values carried in
   the NBMA Subnetwork ID extension, if present.  If none of the values
   matches the NHSs NBMA Subnetwork ID, the NHS SHALL return an Error
   Indication of type "Subnetwork ID Mismatch" and discard the NHRP
   Reply.


5.7.3  Responder Address Extension (IPv4-Specific)

   Discretionary = 0
   Type = 3
   Length = 4

   This extension is used to determine the IP address of the NHRP
   Responder, that is, the entity that generates the NHRP Reply packet.
   The intent is to identify the entity responding to the request, which
   may be different (in the case of cached replies) than the system
   identified in the Next-hop field of the reply, and to 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 Responder's IP Address                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   If a requestor desires this information, it SHALL include this
   extension, with a value of zero, in the NHRP Request packet.

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

   If an NHRP Reply packet being forwarded by an NHS contains an IP
   address of that NHS in the Responder Address Extension, the NHS SHALL
   generate an Error Indication of type "NHRP Loop Detected" and discard
   the Reply.

   If an NHRP 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).


5.7.4  NHRP Forward NHS Record Extension (IPv4-Specific)

   Discretionary = 0
   Type = 4
   Length = variable

   The NHRP forward NHS record is a list of NHSs through which an NHRP
   request traverses.  Each NHS SHALL append a Next-hop element
   containing its IP address to 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.

   Each Next-hop element is formatted as follows:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    IP address                                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Holding Time          |         Address Type          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Unused     |  NBMA Length  | NBMA Address (variable length)|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   IP address
     The IP address of the NHS.

   Holding Time
     The number of seconds for which this information is valid.  If a
     station chooses to use this information as a next-hop entry, it may
     not be used once the holding timer expires.

   Address Type, NBMA Length, and NBMA Address
     The Address Type field specifies the type of NBMA address
     (qualifying the NBMA address).  Possible address types are listed
     in [6].

     The NBMA length field is the length of the NBMA address of the
     destination station in bits.  The NBMA address itself is zero-
     filled to the nearest 32-bit boundary.

     NHSs that are not egress routers SHALL specify an NBMA Length of
     zero and SHALL NOT include an NBMA Address.

   If a requestor wishes to obtain this information, it SHALL include
   this extension with a length of zero.

   Each NHS SHALL append an appropriate Next-hop element to this
   extension when processing an NHRP Request.  The extension length
   field and NHRP checksum SHALL be adjusted as necessary.

   The last-hop NHS (the one that will be generating the NHRP Reply)
   SHALL NOT update this extension (since this information will be in
   the reply).

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

   If an NHRP Request packet being forwarded by an NHS contains the IP
   address of that NHS in the Forward NHS Record Extension, the NHS
   SHALL generate an Error Indication of type "NHRP Loop Detected" and
   discard the Request.


5.7.5  NHRP Reverse NHS Record Extension (IPv4-Specific)

   Discretionary = 0
   Type = 5
   Length = variable




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   The NHRP reverse NHS record is a list of NHSs through which an NHRP
   reply traverses.  Each NHS SHALL append a Next-hop element containing
   its IP address to 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.

   Each Next-hop element is formatted as follows:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    IP address                                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Holding Time          |         Address Type          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Unused     |  NBMA Length  | NBMA Address (variable length)|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IP address
     The IP address of the NHS.

   Holding Time
     The number of seconds for which this information is valid.  If a
     station chooses to use this information as a next-hop entry, it may
     not be used once the holding timer expires.

   Address Type, NBMA Length, and NBMA Address
     The Address Type field specifies the type of NBMA address
     (qualifying the NBMA address).  Possible address types are listed
     in [6].

     The NBMA length field is the length of the NBMA address of the
     destination station in bits.  The NBMA address itself is zero-
     filled to the nearest 32-bit boundary.

     NHSs that are not egress routers SHALL specify an NBMA Length of
     zero and SHALL NOT include an NBMA Address.

   If a requestor wishes to obtain this information, it SHALL include
   this extension with a length of zero.

   Each NHS SHALL append an appropriate Next-hop element to this
   extension when processing an NHRP Reply.  The extension length field
   and NHRP checksum SHALL be adjusted as necessary.

   The NHS generating the NHRP Reply SHALL NOT update this extension.



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

   If an NHRP Reply packet being forwarded by an NHS contains the IP
   address of that NHS in the Reverse NHS Record Extension, the NHS
   SHALL generate an 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.  Note
   that the Responder Address extension may be used to disambiguate the
   set of NHSs that actually processed the reply.


5.7.6  NHRP QoS Extension

   Discretionary = 1
   Type = 6
   Length = variable

   The NHRP QoS Extension is carried in NHRP 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 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.


5.7.7  NHRP Authentication Extension

   Discretionary = 0
   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 packet type.

   Authentication is done pairwise on an NHRP hop-by-hop basis;  the
   authentication extension is regenerated on each hop.  If a received
   packet fails the authentication test, the NHS SHALL generate an Error
   Indication of type "Authentication Failure" and discard the packet.
   In no case SHALL an Error Indication packet be generated on the



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   receipt of an Error Indication packet, however.  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 IP 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.


5.7.8  NHRP Vendor-Private Extension

   Discretionary = 1
   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.  This extension may be used at any time; if the receiver



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   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.  The first 24 bits of the extension's payload (following
   the length field) contains the 802 vendor ID as assigned by the IEEE
   [6].  The remaining octets in the payload are vendor-dependent.


6. Protocol Operation

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


6.1 Router-to-Router Operation

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

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


6.2 Handling of IP Destination Address Field

   NHRP packets are self-contained in terms of the IP addressing
   information needed for protocol operation--the IP source and
   destination addresses in the encapsulating IP header are not used.
   However, the setting of the IP destination address field does impact
   how NHRP requests are forwarded.

   There are essentially three choices in how to set the destination IP
   address field at any particular point in the forwarding of an NHRP
   request: the ultimate destination being resolved, the next-hop IP
   router on the path to the destination, and the next-hop NHS (which
   might not be adjacent to the NHS forming the packet header).



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   The first case, addressing the packet to the destination being
   resolved (in the hopes that an NHS lies along the path) is desirable
   for at least two reasons.  It simplifies configuration (since the
   identity of the next NHS need not be known explicitly), and it
   simplifies deployment (since the packet will pass silently through
   routers that are not NHSs).  However, it assumes that the serving NHS
   lies along the path to the destination, and it requires NHSs along
   the path to examine the packet even though it is not addressed to
   them.

   The second case, addressing the packet to the next-hop router, is
   similar to the first in that it follows the path to the destination,
   thus reducing configuration complexity.  It furthermore only requires
   NHSs to process the packet if they are directly addressed.  It too
   assumes that the responding NHS is on the path to the destination.
   However, it requires that all routers along the path are also NHSs.

   The third case, addressing the packet to the next-hop NHS, allows the
   NHSs to be independent of routing, and requires only addressed NHSs
   to examine the packet.  However, there is no reasonable way, other
   than manual configuration, to determine the identity of the next hop
   NHS if it is not also the next hop IP router (making it option two).

   In order to balance all of these issues, the following rules SHALL be
   used when constructing IP packets to carry NHRP requests.


     Stations

     Stations SHALL address NHRP packets to the NHS by which they are
     served, regardless of whether NHRP has been deployed in Server mode
     or Fabric mode.

     NHSs

     If an NHS receives an NHRP packet in which the IP destination
     address does not match any of its own IP addresses, it SHALL
     process the NHRP packet as appropriate, and if it MUST forward the
     NHRP packet to another NHS, SHALL transmit the packet with the same
     IP destination address with which it was received.

     If an NHS receives an NHRP packet in which the IP destination
     address matches one of its own IP addresses, it SHALL process the
     NHRP packet as appropriate, and if it MUST forward the NHRP packet
     to another NHS, SHALL set the destination IP address in one of the
     following ways:

       If there is a configured next-hop NHS for the destination being



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       resolved (Server mode), it SHALL transmit the packet with the IP
       destination address set to the next-hop NHS.

       If there is no configured next-hop NHS (Fabric Mode), it SHALL
       transmit the packet with the IP destination address set to the
       address of the destination being resolved, and SHALL include the
       Router Alert option [5] so that intermediate NHS/routers can
       examine the NHRP packet.


6.3 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.


   6.3.1 Cacheing Requirements

     Source Stations

     Source stations must of course cache all received replies that they
     are actively using.  They also must cache "incomplete" entries,
     i.e., those for which a request has been sent but which a 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 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.

     Source stations that are also NHSs may return cached information
     learned in response to its own NHRP Request packets in reply to
     requests it receives, within the rules for Transit NHSs below.


     Serving NHSs

     The NHS serving the destination (the one which responds
     authoritatively to NHRP requests) MUST cache information about all
     requests to which it has responded if the information in the 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 NHRP 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.



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     Transit NHSs

     A Transit NHS (lying along the NHRP path between the source station
     and the responding NHS) may cache information contained in NHRP
     Request packets that it forwards.  A Transit NHS may cache
     information contained in NHRP Reply packets that it forwards only
     if that 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 requests only.


   6.3.2 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 NHRP 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 replies as being
     "stable" (via the "B" bit).

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

     Note that the information carried in the NHRP 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 request is a host and the destination is not
     directly attached to the NBMA subnetwork and 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).

     If the egress router/NHS detects a routing change toward the
     destination, it MUST send an NHRP Purge packet to the source, which



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     will usually cause the source to issue a authoritative request and
     find the new egress point.  If the egress router for some reason
     sees no change in routing toward the destination, then it should
     still be a viable, if suboptimal, hop toward the destination.  The
     consequence of the source making a non-authoritative request after
     a Purge, in the presence of stale cache entries (not removed by a
     Purge), is also suboptimal routing.


6.4 Use of the Destination Prefix Extension

   A certain amount of care needs to be taken when using the Destination
   Prefix 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 can result.  For example, it should not be assumed
   that the proper prefix to advertise is the one provided by the
   routing system (especially if the prefix is determined from the
   default route).

   The approach used to determine the prefix width is likely to vary
   based on the particulars of the situation.  Information could be
   gleaned from local topology, routing protocols, and other sources.

   In general, the width of the prefix should be handled conservatively
   (erring toward a longer prefix).

   If multiple cache entries match the desired destination address (due
   to overlapping prefixes), the longest prefix MUST be used.


7. Security Considerations

   As in any routing protocol, there are a number of potential security
   attacks possible, particularly denial-of-service attacks.  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.






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8. Discussion

   The result of an NHRP request depends on how routing is configured
   among the NHSs of an NBMA subnetwork.  If the destination station is
   directly connected to the NBMA subnetwork and the the routed path to
   it lies entirely within the NBMA subnetwork, the NHRP 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.

   When the NBMA next hop toward a destination is not the destination
   station itself, the optimal NBMA next hop may change dynamically.
   This can happen, for instance, when an egress router nearer to the
   destination becomes available.  This change can be detected in a
   number of ways.  First of all, the source station will need to
   periodically reissue the NHRP Request at a minimum just prior to the
   expiration of the holding timer.  Alternatively, the source can be
   configured to receive routing information from the routing system.
   When it detects an improvement in the route to the destination, the
   source can reissue the NHRP request to obtain the current optimal
   NBMA next hop.  Source stations that are routers may choose to
   establish a routing association with the egress router, allowing the
   egress router to explicitly inform the source about changes in
   routing (and providing additional routing information,
   authentication, etc.)  Such strategies will be discussed in a
   separate document.

   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 IP 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
   IP switching functionality.


References

   [1] NBMA Address Resolution Protocol (NARP), Juha Heinanen and Ramesh
       Govindan, draft-ietf-rolc-nbma-arp-00.txt.




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   [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] IP Router Alert Option, Dave Katz, draft-katz-router-alert-
       00.txt.

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

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


Acknowledgements

   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, and Tony Li and Bruce Cole of cisco 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













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