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Network Working Group                                        D. CanseverRequest for Comments: 2333                        GTE Laboratories, Inc.Category: Standards Track                                     April 1998NHRP Protocol Applicability StatementStatus 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 stationCansever                    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 inSection 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   use in a point-to-multipoint communication setting.  However,   elements of NHRP may be used in certain multicast scenarios for the   purpose of providing short cut routing. Such an effort is discussed   in [6].  In this case, NHRP would avoid intermediate routers in the   multicast path. The scalability of providing short-cut paths in a   multicast environment is an open issue.   NHRP can be used in host-host, host-router and router-host   communications.  When used in router-router communication, NHRP (as   defined in [1]) can produce persistent routing loops if the   underlying routing protocol looses information critical to loop   suppression. This may occur when there is a change in router metrics   across the autonomous system boundaries.  NHRP for router-router   communication that avoids persistent forwarding loops will be   addressed in a separate document.   A special case of router-router communication where loops will not   occur is when the destination host is directly adjacent to the non-   NBMA interface of the egress router.  If it is believed that the   adjacency of the destination station to the egress router is a stable   topological configuration, then NHRP can safely be used in this   router-router communication scenario.  If the NHRP Request has the Q   bit set, indicating that the requesting party is a router, and if the   destination station is directly adjacent to the egress router as a   stable topological configuration, then the egress router can issue a   corresponding NHRP reply.  If the destination is not adjacent to the   egress router, and if Q bit is set in the Request, then a safe mode   of operation for the egress router would be to issue a negative NHRP   Reply (NAK) for this particular request, thereby enforce data packets   to follow the routed path.   As a result of having inter-LIS address resolution capability, NHRP   allows the communicating parties to exchange packets by fully   utilizing the particular features of the NBMA network.  One such   example is the use of QoS guarantees when the NMBA network is ATM.   Here, due to short-cut routing, ATM provided QoS guarantees can be   implemented without having to deal with the issues of re-assembling   and re-segmenting IP packets at each network layer hop.   NHRP protocol can be viewed as a client-server interaction.  An NHRP   Client is the one who issues an NHRP Request. An NHRP Server is the   one who issues a reply to an NHRP request, or the one who forwards a   received NHRP request to another Server. Of course, an NHRP entity   may act both as a Client and a Server.Cansever                    Standards Track                     [Page 3]

RFC 2333              NHRP Protocol Applicability             April 19984. Use of NHRP   In general, issuing an NHRP request is an application dependent   action [7].  For applications that do not have particular QoS   requirements, and that are executed within a short period of time, an   NBMA short-cut may not be a necessity. In situations where there is a   "cost" associated with NBMA short-cuts, such applications may be   better served by network layer hop-by-hop routing. Here, "cost" may   be understood in a monetary context, or as additional strain on the   equipment that implements short-cuts. Therefore, there is a trade-off   between the "cost" of a short-cut path and its utility to the user.   Reference [7] proposes that this trade-off should be addressed at the   application level. In an environment consisting of LANs and routers   that are interconnected via dedicated links, the basic routing   decision is whether to forward a packet to a router, or to broadcast   it locally.  Such a decision on local vs. remote is based on the   destination address. When routing IP packets over an NBMA network,   where there is potentially a direct Source to Destination   connectivity with QoS options, the decision on local vs. remote is no   longer as fundamentally important as in the case where packets have   to traverse routers that are interconnected via dedicated links.   Thus, in an NBMA network with QoS options, the basic decision becomes   the one of short-cut vs. hop-by-hop network layer routing.  In this   case, the relevant criterion becomes applications' QoS requirements   [7]. NHRP is particularly applicable for environments where the   decision on local vs. remote is superseded by the decision on short-   cut vs. hop-by-hop network layer routing.   Let us assume that the trade-off is in favor of a short-cut NBMA   route.  Generally, an NHRP request can be issued by a variety of NHRP   aware entities, including hosts and routers with NBMA interfaces.  If   an IP packet traverses multiple hops before a short-cut path has been   established, then there is a chance that multiple short-cut paths   could be formed. In order to avoid such an undesirable situation, a   useful operation rule is to authorize only the following entities to   issue an NHRP request and to perform short-cut routing.     i)  The host that originates the IP packet, if the host has an NBMA         interface.     ii) The first router along the routing path of the IP packet such         that the next hop is reachable through the NBMA interface of         that particular router.    iii) A policy router within an NBMA network through which the IP         packet has to traverse.Cansever                    Standards Track                     [Page 4]

RFC 2333              NHRP Protocol Applicability             April 19985. Protocol Scalability   As previously indicated, NHRP is defined for the delivery of IP   packets with a single destination. Thus, this discussion is confined   to a unicast setting.  The scalability of NHRP can be analyzed at   three distinct levels:     o Client level     o LIS level     o Domain level   At the the Client level, the scalability of NHRP is affected by the   processing and memory limitations of the NIC that provides interface   to the NBMA network.  When the NBMA network is connection oriented,   such as ATM, NIC limitations may bound the scalability of NHRP in   certain applications.  For example, a server that handles hundreds of   requests per second using an ATM interface may be bounded by the   performance characteristics of the corresponding NIC.  Similarly,   when the NHRP Client resides at an NBMA interface of a router, memory   and processing limitations of router's NIC may bound the scalability   of NHRP.  This is because routers generally deal with an aggregation   of traffic from multiple sources, which in turn creates a potentially   large number of SVCCs out of the router's NBMA interface.   At the LIS level, the main issue is to maintain and deliver a sizable   number of NBMA to Network layer address mappings within large LISs.   To this goal, NHRP implementations can use the services of the Server   Cache Synchronization Protocol (SCSP) [8] that allows multiple   synchronized NHSs within an LIS, and hence resolve the associated   scalability issue.   At the NHRP Domain level, network layer routing is used in resolving   the NBMA address of a destination outside the LIS.  As such, the   scalability of NHRP is closely tied to the scalability of the network   layer routing protocol used by NHRP.  Dynamic network layer routing   protocols are proven to scale well.  Thus, when used in conjunction   with dynamic routing algorithms, at the NHRP domain level, NHRP   should scale in the same order as the routing algorithm, subject to   the assumption that all the routers along the path are NHRP aware.   If an NHRP Request is processed by a router that does not implement   NHRP, it will be silently discarded.  Then, short-cuts cannot be   implemented and connectivity will be provided on a hop-by-hop basis.   Thus, when NHRP is implemented in conjunction with dynamic network   layer routing, a scaling requirement for NHRP is that virtually all   the routers within a logical NBMA network should be NHRP aware.Cansever                    Standards Track                     [Page 5]

RFC 2333              NHRP Protocol Applicability             April 1998   One can also use static routing in conjunction with NHRP.  Then, not   all the routers in the NBMA network need to be NHRP aware.  That is,   since the routers that need to process NHRP control messages are   specified by static routing, routers that are not included in the   manually defined static paths do not have to be NHRP aware.  Of   course, static routing does not scale, and if the destination is off   the NBMA network, then the use of static routing could result in   persistently suboptimal routes.  Use of static routing also has   fairly negative failure modes.6. Discussion   NHRP does not replace existing routing protocols. In general, routing   protocols are used to determine the proper path from a source host or   router, or intermediate router, to a particular destination.  If the   routing protocol indicates that the proper path is via an interface   to an NBMA network, then NHRP may be used at the NBMA interface to   resolve the destination IP address into the corresponding NBMA   address.  Of course, the use of NHRP is subject to considerations   discussed inSection 4.   Assuming that NHRP is applicable and the destination address has been   resolved, packets are forwarded using the particular data forwarding   and path determination mechanisms of the underlying NBMA network.   Here, the sequence of events are such that route determination is   performed by IP routing, independent of NHRP. Then, NHRP is used to   create a short-cut track upon the path determined by the IP routing   protocol. Therefore, NHRP "shortens" the routed path.  NHRP (as   defined in [1]) is not sufficient to suppress persistent forwarding   loops when used for router-router communication if the underlying   routing protocol looses information critical to loop suppression [9].   Work is in progress [10] to augment NHRP to enable its use for the   router-router communication without persistent forwarding loops.   When the routed path keeps changing on some relatively short time   scale, such as seconds, this situation will have an effect on the   operation of NHRP. In certain router-router operations, changes in   the routed path could create persistent routing loops. In host-   router, or router-host communications, frequent changes in routed   paths could result in inefficiencies such as frequent creation of   short-cut paths which are short lived.7. Security Considerations   NHRP is an address resolution protocol, and SCSP is a database   synchronization protocol.  As such, they are possibly subject to   server (for NHRP) or peer (for SCSP) spoofing and denial of service   attacks.  They both provide authentication mechanisms to allow theirCansever                    Standards Track                     [Page 6]

RFC 2333              NHRP Protocol Applicability             April 1998   use in environments in which spoofing is a concern.  Details can be   found in sections5.3.4 in [1] and B.3.1 in [8].  There are no   additional security constraints or concerns raised in this document   that are not already discussed in the referenced sections.References   [1] Luciani, J., Katz, D., Piscitello, D., Cole, B., and       N. Doraswamy, "NMBA Next Hop Resolution Protocol (NHRP)",RFC2332, April 1998.   [2] Greene, M., and J. Luciani,"NHRP Management Information Base",       Work in Progress.   [3] Laubach, M., and J. Halpern, "Classical IP and ARP over ATM",RFC2225, April 1998.   [4] Lawrance, J., and D. Piscitello, "The Transmission of IP       datagrams over the SMDS service",RFC 1209, March 1991.   [5] Multiprotocol Over ATM Version 1.0, ATM Forum Document       af-mpoa-0087.000   [6] Rekhter, Y., and D. Farinacci, "Support for Sparse Mode PIM over       ATM", Work in Progress.   [7] Rekhter, Y., and D. Kandlur, "Local/Remote" Forwarding Decision       in Switched Data Link Subnetworks",RFC 1937, May 1996.   [8] Luciani, J., Armitage, G., Halpern, J., and N. Doraswamy, "Server       Cache Synchronization Protocol (SCSP) - NBMA",RFC 2334, April       1998.   [9] Cole, R., Shur, D., and C. Villamizar, "IP over ATM: A Framework       Document",RFC 1932, April 1996.   [10] Rekhter, Y.,"NHRP for Destinations off the NBMA Subnetwork",        Work in Progress.Acknowledgements   The author acknowledges valuable contributions and comments from many   participants of the ION Working Group, in particular from Joel   Halpern of Newbridge Networks, David Horton of Centre for Information   Technology Research, Andy Malis of Nexion, Yakov Rekhter and George   Swallow of Cisco Systems and Curtis Villamizar of ANS.Cansever                    Standards Track                     [Page 7]

RFC 2333              NHRP Protocol Applicability             April 1998Author's Address   Derya H. Cansever   GTE Laboratories Inc.   40 Sylvan Rd. MS 51   Waltham MA 02254   Phone: +1 617 466 4086   EMail: dcansever@gte.comCansever                    Standards Track                     [Page 8]

RFC 2333              NHRP Protocol Applicability             April 1998Full Copyright Statement   Copyright (C) The Internet Society (1998).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Cansever                    Standards Track                     [Page 9]