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Routing over Large Clouds Working Group                        Dave Katz
INTERNET-DRAFT                                           (cisco Systems)
<draft-ietf-rolc-nhrp-03.txt>                           David Piscitello
                                                 (Core Competence, Inc.)
                                                       November 29, 1994


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

   Please check the I-D abstract listing contained in each Internet
   Draft directory to learn the current status of this or any Internet
   Draft.

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) network to determine the IP and
   NBMA network addresses of the "NBMA next hop" towards a destination
   station.  If the destination is connected to the NBMA network, then
   the NBMA next hop is the destination station itself.  Otherwise, the
   NBMA next hop is the egress router from the NBMA network that is
   "nearest" to the destination station.  Although this document focuses
   on NHRP in the context of IP, the technique is applicable to other
   network 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].


1. Introduction

   The NBMA Next Hop Resolution Protocol (NHRP) allows a source station
   (a host or router), wishing to communicate over a Non-Broadcast,



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   Multi-Access (NBMA) network, to determine the IP and NBMA addresses
   of the "NBMA next hop" toward a destination station.  A network can
   be non-broadcast either because it technically doesn't support
   broadcasting (e.g., an X.25 network) 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 network, then the NBMA next hop is the
   destination station itself.  Otherwise, the NBMA next hop is the
   egress router from the NBMA network that is "nearest" to the
   destination station.

   An NBMA network 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 network.

      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 network more than once.

   NHRP describes a routing method that obviates the need for LISs.
   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 network such as SMDS) or may first establish a
   connection to the destination with the desired bandwidth and QOS
   characteristics (in a connection-oriented NBMA network 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 network, allowing for straightforward
   implementations in NBMA stations.  Optional services extend this
   functionality to include loop detection, sanity checks, diagnostics,
   security features, and fallback capabilities, providing improved
   robustness and functionality.

   NHRP supports both a server-based style of deployment and a
   ubiquitous "fabric", consisting of NHRP-capable routers.  The



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   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 networks 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
   designed to eliminate the suboptimal routing that results from the
   LIS model, and can be deployed in a non-interfering manner alongside
   existing ARP services.


2. Protocol Overview

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

   For administrative and policy reasons, a physical NBMA network may be
   partitioned into several, disjoint "Logical NBMA networks".  A
   Logical NBMA network is defined as a collection of hosts and routers
   that share ulfiltered data link connectivity over an NBMA network.
   "Unfiltered data link 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 network.  (Hereafter, unless otherwise specified, we use
   NBMA network to mean logical NBMA network.)

   Placed within the NBMA network 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 network.  NHSs cooperatively
   resolve the NBMA next hop within their logical NBMA network.  In
   addition to the NHRP, NHSs participate in protocols used to
   disseminate routing information across (and beyond the boundaries of)
   the NBMA network, 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).




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

   [Note: for NBMA networks 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 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 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 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.

   When the NHS receives an NHRP request, it checks to see if it
   "serves" station D, i.e., the NHS checks to see if it has 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.



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   (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 network,
   the next hop IP address will be that of the egress router through
   which packets for station D are forwarded.)

   NHRP replies usually traverse the same sequence of NHSs as the NHRP
   request (in reverse order).  This is a consequence of having
   symmetric routing, which is typically (but not necessarily) the case.
   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.  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 never 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 Option (see Section 5.6.5).
       (c) The NHRP request initiator has not included the destination
           mask option (see Section 5.6.1).
       (d) 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
   network can resolve the request.) If the determination is made that



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   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 never cross the borders of a logical
   NBMA network (an explicit NBMA network identifier may be included as
   an option in the NHRP request, see section 5.6.2).  Thus, IP traffic
   out of and into a logical NBMA network always traverses an IP router
   at its border.  Network layer filtering can then be implemented at
   these border routers.

   NHRP optionally provides a mechanism to aggregate NBMA next hop
   information in NHS caches.  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 mask
   defining this prefix (see Section 5.6.1).  The prefix to egress
   router mapping in the reply MUST be cached in all NHSs on the path of
   the reply.  A subsequent (non-authoritative) NHRP request for some
   destination that shares an IP address prefix with D can be satisfied
   with this cached information.

   To dynamically detect link-layer filtering in NBMA networks (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 5.6.4 and 5.6.5).  The Route Record options contain the
   network (and link 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, it may attempt
   to do so successively with other link layer addresses in the Route
   Record until it succeeds (if authentication policy permits such
   action).  This approach can find the optimal best hop in the presence
   of link-layer filtering (which may be source/destination sensitive,
   for instance, without necessarily creating separate logical NBMA
   networks) or link-layer congestion (especially in connection-oriented
   media).


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.




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   It is desirable that hosts attached directly to the NBMA network 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 network.  This may be appropriate in
     networks containing routers that do not support NHRP, or networks
     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: 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 network 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 network 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



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     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 network 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
     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.


   Fabric Mode

     In "fabric" mode, it is expected that NHRP-capable routers are
     ubiquitous throughout the NBMA network, 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, it is expected that an NHS
     serving a particular destination is guaranteed to 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 network are served by routers that
     double as NHSs.

     Fabric mode leverages a routed infrastructure that "overlays" the
     NBMA network.  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 network 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 option).  The Router Alert
     option [6] is added by the first NHS in order to ensure that
     router/NHSs 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
     router/NHS that serves the destination (which will return a
     positive NHRP reply) or it will arrive at a router/NHS that has no
     route to the destination (which will return a negative NHRP reply),
     or it may arrive at a router/NHS 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 networks that do not have NHSs deployed in all routers;
     typically a very large NBMA network 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 network
     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 link layer networks
     (including logical NBMA networks), 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 link layer networks.




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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 network identifier (see Section 5.6.2), and
     in the case of "server" mode, the identities of other NHSs in the
     same logical NBMA network.  If a served station is attached to
     several link layer networks, 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
     link layer networks than the NBMA network, the NHS must, in
     addition to the above, be configured to exchange routing
     information between the NBMA network and these other link layer
     networks.

     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
   Options Part.  The Fixed Part is common to all NHRP packet types.
   The Mandatory Part must be present, but varies depending on packet
   type.  The Options 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
   Options 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).

   Note that since the lengths of all fields are self-encoding, it is
   permissible to pad the Mandatory and Options parts with arbitrary
   numbers of trailing zero octets to achieve any desired alignment.
   Note however that any padding in the Mandatory Part must be included
   in the 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 option
   processing.  IP datagrams containing NHRP packets must have the Don't



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   Fragment bit set.

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

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


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, 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|       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)

   Protocol ID
     Specifies the network 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
     network layer protocols are for future study.

   Request ID



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     A value chosen 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).

   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 [5].

     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|       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.

   An NHS is not allowed to reply to an NHRP request for authoritative
   information with cached information, but may do so for an NHRP



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   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 network 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
     network 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 network, or the egress router if it is not
     directly attached.

     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 [5].

     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.



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     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 network (such as a failed call attempt in
     connection-oriented NBMA networks).


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:

       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 network 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
     network layer protocols are for future study.

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



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   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 [5].

     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
   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 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 4.  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 Option
       2 - Network 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.


5.6  Options Part

   The Options Part, if present, carries one or more options in {Type,
   Length, Value} triplets.  Options 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
   options need not be able to receive them.  An implementation that is
   incapable of processing options shall return an Error Indication of
   type Unrecognized Option when it receives an NHRP packet containing
   options.



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   Options are typically protocol-specific, as noted.

   Options 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |O|          Type               |        Length                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Value...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   O
     "Optional."  If set, and the NHS does not recognize the type code,
     the option 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 option type code (see below).  The option type is not qualified
     by the Optional bit, but is orthogonal to it.

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

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

   Options may occur in any order, but any particular option type may
   occur only once in an NHRP packet.

   The Optional bit provides for a means to extend the option set.  If
   it is clear, the NHRP request cannot be satisfied if the option is
   unrecognized, so the responder must return an Error Indication of
   type Unrecognized Option.  If set, the option can be safely ignored.
   In this case, the offending option should simply be returned
   unchanged in the NHRP Reply.

   If a transit NHS (one which is not going to generate a reply) detects
   an unrecognized option, it shall ignore the option, and if the
   Optional bit is clear, 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 options).  Effectively, this means
   that a transit NHS that doesn't understand an option with the
   Optional bit clear must not participate in any way in the protocol
   exchange, other than acting as a forwarding agent for the request.



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5.6.1  Destination Mask Option (IPv4-Specific)

   Optional = 0
   Type = 1
   Length = 4

   This option 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 destination IP address in the bit positions
   for which the mask has a one bit are part of the equivalence class.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Destination Mask                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   If an initiator would like to receive this equivalence information,
   it shall add this option to the NHRP Request with a value of
   255.255.255.255.  The responder shall copy the option to the NHRP
   Reply and modify the mask appropriately.


5.6.2  NBMA Network ID Option (Protocol-Independent)

   Optional = 0
   Type = 2
   Length = variable

   This option is used to carry one or more identifiers for the NBMA
   network.  This can be used as a validity check to ensure that the
   request does not leave a particular NBMA network.  The option 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 network.

   Multiple NBMA Network IDs may be used as a transition mechanism while
   NBMA Networks 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 Network ID                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     ...

   Each identifier consists of a 32 bit globally unique value assigned



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   to the NBMA network.  This value should be chosen from the IP address
   space administered by the operators of the NBMA network.  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 Network ID, the NHS shall
   return an Error Indication of type "Network ID Mismatch" and discard
   the NHRP Request.

   When an NHS is building an NHRP Reply and the NBMA Network ID option
   is present in the NHRP Request, the NBMA Network ID option 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 Network ID option, if present.  If none of the values
   matches the NHSs NBMA Network ID, the NHS shall return an Error
   Indication of type "Network ID Mismatch" and discard the NHRP Reply.


5.6.3  Responder Address Option (IPv4-Specific)

   Optional = 0
   Type = 3
   Length = 4

   This option 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                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   If a requestor desires this information, it shall include this
   option, 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 option, it shall include this option, 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



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   options.  The choice of which of several IP addresses to include in
   this option 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 Option, 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 option
   (differentiating it from the address in the Next-hop field).


5.6.4  NHRP Forward NHS Record Option (IPv4-Specific)

   Optional = 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 option.

   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.





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   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 [5].

     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 option with a length of zero.

   Each NHS shall append an appropriate Next-hop element to this option
   when processing an NHRP Request.  The option 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 option (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 options.  The choice of which of
   several IP addresses to include in this option 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 Option, the NHS shall
   generate an Error Indication of type "NHRP Loop Detected" and discard
   the Request.


5.6.5  NHRP Reverse NHS Record Option (IPv4-Specific)

   Optional = 0
   Type = 5
   Length = variable

   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 option.

   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.




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   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 [5].

     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 option with a length of zero.

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

   The NHS generating the NHRP Reply shall not update this option.

   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 options.  The choice of which of
   several IP addresses to include in this option 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 Option, the NHS shall



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   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 option may be used to disambiguate the set
   of NHSs that actually processed the reply.


5.6.6  NHRP QoS Option

   Optional = 0
   Type = 6
   Length = variable

   The NHRP QoS Option 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 option are TBD;  alignment with
   resource reservation may be useful.


5.6.7  NHRP Authentication Option

   Optional = 0
   Type = 7
   Length = variable

   The NHRP Authentication Option is carried in NHRP packets to convey
   authentication information between NHRP speakers.  The Authentication
   Option may be included in any NHRP packet type.

   Authentication is done pairwise on an NHRP hop-by-hop basis;  the
   authentication option 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
   receipt of an Error Indication packet, however.  Note that one
   possible authentication failure is the lack of an Authentication
   Option;  the presence or absence of the Authentication Option is a
   local matter.






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

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

           1 - Cleartext Password
           2 - Keyed MD5

   All other values are reserved.

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

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

   In the case of Keyed MD5 Authentication, the Authentication Data
   contains the 16 byte MD5 digest of the entire NHRP packet, including
   the 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.


5.6.8  NHRP Vendor-Private Option

   Optional = 0
   Type = 8
   Length = variable

   The NHRP Vendor-Private Option is carried in NHRP packets to convey
   vendor-private information or NHRP extensions between NHRP speakers.
   This option may be used at any time; if the receiver does not handle
   this option, or does not match the vendor ID in the option, then the
   option may be completely ignored by the receiver.  The first 24 bits
   of the option's payload (following the length field) contains the 802
   vendor ID as assigned by the IEEE [5].  The remaining octets in the
   payload are vendor-dependent.




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6. 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.


7. Discussion

   The result of an NHRP request depends on how routing is configured
   among the NHSs of an NBMA network.  If the destination station is
   directly connected to the NBMA network and the NHSs always prefer
   NBMA routes over routes via other link layer networks, the NHRP
   replies always return the NBMA address of the destination station
   itself rather than the NBMA address of some egress router.  For
   destinations outside the NBMA network, egress routers and routers in
   the other link layer networks should exchange routing information so
   that the optimal egress router is always 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, and most likely more aggressively
   than that.  Alternatively, the source can be configured to receive
   routing information from its NHSs.  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.)

   The dynamic nature of routing impacts caching strategies as well,
   since cached information may not be up-to-date.  This is especially
   an issue when NHSs are deployed in server mode, since the NHSs may
   not be privy to routing information.  However, stale cached
   information may only cause suboptimal routing (choosing the wrong
   egress point and taking extra hops across the NBMA network) rather



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   than causing black holes.  Cache management strategies are for
   further study.

   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.

   NHRP supports portability of NBMA stations.  A station can be moved
   anywhere within the NBMA network and still keep its original IP
   address as long as its NHS(s) remain the same.  Requests for
   authoritative information will always return the correct link layer
   address.


8. Protocol Operation

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


8.1 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.  This situation can be avoided if there are no
   "back door" paths between the entry and egress router outside of the
   NBMA network, and can be ameliorated by periodically reissuing the
   NHRP request.  If these conditions cannot be satisfied, the use of
   NHRP between routers is not recommended.

   One approach to the router-to-router case that is being considered is
   to run a limited instance of a routing protocol between the two
   routers.  Any routing protocol that provides loop detection may be
   used.  This routing protocol instance will likely only carry a subset
   of the total routing information, and is unlikely to be closely
   integrated into the routing in which each of the routers is otherwise
   participating (due to the abitrary connectivity possible in such
   situations and its impact on the stability and quality of overall
   routing).  This approach is for further study.





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8.2 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).

   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



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     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
       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 [6] so that intermediate NHS/routers can
       examine the NHRP packet.


8.3 Pseudocode

TBD.


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

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







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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.

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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