NHRP Protocol Applicability Statement
RFC 2333

Document Type RFC - Proposed Standard (April 1998; No errata)
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Network Working Group                                        D. Cansever
Request for Comments: 2333                        GTE Laboratories, Inc.
Category: Standards Track                                     April 1998

                 NHRP Protocol Applicability Statement

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

   As required by the Routing Protocol Criteria [RFC 1264], this memo
   discusses the applicability of the Next Hop Resolution Protocol
   (NHRP) in routing of IP datagrams over Non-Broadcast Multiple Access
   (NBMA) networks, such as ATM, SMDS and X.25.

1. Protocol Documents

   The NHRP protocol description is defined in [1].  The NHRP MIB
   description is defined in [2].

2. Introduction

   This document summarizes the key features of NHRP and discusses the
   environments for which the protocol is well suited.  For the purposes
   of description, NHRP can be considered a generalization of Classical
   IP and ARP over ATM which is defined in [3] and of the Transmission
   of IP Datagrams over the SMDS Service, defined in [4].  This
   generalization occurs in 2 distinct directions.

   Firstly, NHRP avoids the need to go through extra hops of routers
   when the Source and Destination belong to different Logical Internet
   Subnets (LIS).  Of course, [3] and [4] specify that when the source
   and destination belong to different LISs, the source station must
   forward data packets to a router that is a member of multiple LISs,
   even though the source and destination stations may be on the same
   logical NBMA network.  If the source and destination stations belong
   to the same logical NBMA network, NHRP provides the source station

Cansever                    Standards Track                     [Page 1]
RFC 2333              NHRP Protocol Applicability             April 1998

   with an inter-LIS address resolution mechanism at the end of which
   both stations can exchange packets without having to use the services
   of intermediate routers.  This feature is also referred to as
   "short-cut" routing.  If the destination station is not part of the
   logical NBMA network, NHRP provides the source with the NBMA address
   of the current egress router towards the destination.

   The second generalization is that NHRP is not specific to a
   particular NBMA technology.  Of course, [3] assumes an ATM network
   and [4] assumes an SMDS network at their respective subnetwork
   layers.

   NHRP is specified for resolving the destination NBMA addresses of IP
   datagrams over IP subnets within a large NBMA cloud.  NHRP has been
   designed to be extensible to network layer protocols other than IP,
   possibly subject to other network layer protocol specific additions.

   As an important application of NHRP, the Multiprotocol Over ATM
   (MPOA) Working Group of the ATM Forum has decided to adopt and to
   integrate NHRP into its MPOA Protocol specification [5].  As such,
   NHRP will be used in resolving the ATM addresses of MPOA packets
   destined outside the originating subnet.

3. Key Features

   NHRP provides a mechanism to obtain the NBMA network address of the
   destination, or of a router along the path to the destination. NHRP
   is not a routing protocol, but may make use of routing information.
   This is further discussed in Section 5.

   The most prominent feature of NHRP is that it avoids extra router
   hops in an NBMA with multiple LISs.  To this goal, NHRP provides the
   source with the NBMA address of the destination, if the destination
   is directly attached to the NBMA. If the destination station is not
   attached to the NBMA, then NHRP provides the source with the NBMA
   address of an exit router that has connectivity to the destination.
   In general, there may be multiple exit routers that have connectivity
   to the destination.  If NHRP uses the services of a dynamic routing
   algorithm in fulfilling its function, which is necessary for robust
   and scalable operation, then the exit router identified by NHRP
   reflects the selection made by the network layer dynamic routing
   protocol.  In general, the selection made by the routing protocol
   would often reflect a desirable attribute, such as identifying the
   exit router that induces the least number of hops in the original
   routed path.

Cansever                    Standards Track                     [Page 2]
RFC 2333              NHRP Protocol Applicability             April 1998

   NHRP is defined for avoiding extra hops in the delivery of IP packets
   with a single destination.  As such, it is not intended for direct
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