RFC2333 - NHRP Protocol Applicability Statement

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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 [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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