RFC2333 - NHRP Protocol Applicability Statement

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.

Abstract

As required by the Routing Protocol Criteria [RFC1264], 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

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.

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.

4. 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.

5. 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.

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 in Section 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 their

use in environments in which spoofing is a concern. Details can be

found in sections 5.3.4 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)", RFC

2332, 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", RFC

2225, April 1998.

[4] Lawrance, J., and D. Piscitello, "The Transmission of IP

datagrams over the SMDS service", RFC1209, 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", RFC1937, May 1996.

[8] Luciani, J., Armitage, G., Halpern, J., and N. Doraswamy, "Server

Cache Synchronization Protocol (SCSP) - NBMA", RFC2334, April

1998.

[9] Cole, R., Shur, D., and C. Villamizar, "IP over ATM: A Framework

Document", RFC1932, 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.

Author's Address

Derya H. Cansever

GTE Laboratories Inc.

40 Sylvan Rd. MS 51

Waltham MA 02254

Phone: +1 617 466 4086

EMail: dcansever@gte.com

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