rfc2332.txt

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Network Working Group                                         J. Luciani
Request for Comments: 2332                                  Bay Networks
Category: Standards Track                                        D. Katz
                                                           cisco Systems
                                                           D. Piscitello
                                                   Core Competence, Inc.
                                                                 B. Cole
                                                        Juniper Networks
                                                            N. Doraswamy
                                                            Bay Networks
                                                              April 1998


                NBMA Next Hop Resolution Protocol (NHRP)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

Abstract

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

   Note that while this protocol was developed for use with NBMA
   subnetworks, it is possible, if not likely, that it will be applied
   to BMA subnetworks as well.  However, this usage of NHRP is for
   further study.

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




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RFC 2332                       NBMA NHRP                      April 1998


   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 (see [13]).

1. Introduction

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [15].

   The NBMA Next Hop Resolution Protocol (NHRP) allows a source station
   (a host or router), wishing to communicate over a Non-Broadcast,
   Multi-Access (NBMA) subnetwork, to determine the internetworking
   layer addresses and NBMA addresses of suitable "NBMA next hops"
   toward a destination station.  A subnetwork can be non-broadcast
   either because it technically doesn't support broadcasting (e.g., an
   X.25 subnetwork) or because broadcasting is not feasible for one
   reason or another (e.g., an SMDS multicast group or an extended
   Ethernet would be too large).  If the destination is connected to the
   NBMA subnetwork, then the NBMA next hop is the destination station
   itself.  Otherwise, the NBMA next hop is the egress router from the
   NBMA subnetwork that is "nearest" to the destination station.

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

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

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

      3)  All hosts and routers outside of the LIS are accessed via
          a router.

      4)  All members of a LIS access each other directly (without
          routers).

   Address resolution as described in [3] and [4] only resolves the next
   hop address if the destination station is a member of the same LIS as
   the source station; otherwise, the source station must forward
   packets to a router that is a member of multiple LIS's.  In multi-LIS








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RFC 2332                       NBMA NHRP                      April 1998


   configurations, hop-by-hop address resolution may not be sufficient
   to resolve the "NBMA next hop" toward the destination station, and IP
   packets may have multiple IP hops through the NBMA subnetwork.

   Another way to model NBMA is by using the notion of Local Address
   Groups (LAGs) [10]. The essential difference between the LIS and the
   LAG models is that while with the LIS model the outcome of the
   "local/remote" forwarding decision is driven purely by addressing
   information, with the LAG model the outcome of this decision is
   decoupled from the addressing information and is coupled with the
   Quality of Service and/or traffic characteristics.  With the LAG
   model any two entities on a common NBMA network could establish a
   direct communication with each other, irrespective of the entities'
   addresses.

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

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

   Address resolution techniques such as those described in [3] and [4]
   may be in use when NHRP is deployed.  ARP servers and services over
   NBMA subnetworks may be required to support hosts that are not
   capable of dealing with any model for communication other than the
   LIS model, and deployed hosts may not implement NHRP but may continue
   to support ARP variants such as those described in [3] and [4].  NHRP
   is intended to reduce or eliminate the extra router hops required by
   the LIS model, and can be deployed in a non-interfering manner with
   existing ARP services [14].





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RFC 2332                       NBMA NHRP                      April 1998


   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 (see [13]).

2. Overview

2.1 Terminology

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

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

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

     The term "server", unless explicitly stated to the contrary, refers
     to a Next Hop Server (NHS).  An NHS is an entity performing the
     Next Hop Resolution Protocol service within the NBMA cloud.  An NHS
     is always tightly coupled with a routing entity (router, route
     server or edge device) although the converse is not yet guaranteed
     until ubiquitous deployment of this functionality occurs.  Note
     that the presence of intermediate routers that are not coupled with
     an NHS entity may preclude the use of NHRP when source and
     destination stations on different sides of such routers and thus
     such routers may partition NHRP reachability within an NBMA
     network.

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

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

2.2 Protocol Overview

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





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RFC 2332                       NBMA NHRP                      April 1998


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

   Placed within the NBMA subnetwork are one or more entities that
   implement the NHRP protocol.  Such stations which are capable of
   answering NHRP Resolution Requests are known as "Next Hop Servers"
   (NHSs).  Each NHS serves a set of destination hosts, which may or may
   not be directly connected to the NBMA subnetwork.  NHSs cooperatively
   resolve the NBMA next hop within their logical NBMA subnetwork.  In
   addition to NHRP, NHSs may support "classical" ARP service; however,
   this will be the subject of a separate document [14].

   An NHS maintains a cache which contains protocol layer address to
   NBMA subnetwork layer address resolution information.  This cache can
   be constructed from information obtained from NHRP Register packets
   (see Section 5.2.3 and 5.2.4), from NHRP Resolution Request/Reply
   packets, or through mechanisms outside the scope of this document
   (examples of such mechanisms might include ARP[3] and pre-configured
   tables).  Section 6.2 further describes cache management issues.

   For a station within a given LIS to avoid providing NHS
   functionality, there must be one or more NHSs within the NBMA
   subnetwork which are providing authoritative address resolution
   information on its behalf.  Such an NHS is said to be "serving" the
   station.  A station on a LIS that lacks NHS functionality and is a
   client of the NHRP service is known as NHRP Client or just NHCs.  If
   a serving NHS is to be able to supply the address resolution
   information for an NHC then NHSs must exist at each hop along all
   routed paths between the NHC making the resolution request and the
   destination NHC.  The last NHRP entity along the routed path is the
   serving NHS; that is, NHRP Resolution Requests are not forwarded to
   destination NHCs but rather are processed by the serving NHS.

   An NHC also maintains a cache of protocol address to NBMA address
   resolution information.  This cache is populated through information
   obtained from NHRP Resolution Reply packets, from manual
   configuration, or through mechanisms outside the scope of this
   document.





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RFC 2332                       NBMA NHRP                      April 1998