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draft-ietf-rolc-nhrp-appl
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ROLC Working Group Derya H. Cansever
INTERNET DRAFT GTE Laboratories, Inc.
February 1996
Expiration Date July 1996
NHRP Protocol Applicability Statement
<draft-ietf-rolc-nhrp-appl-02.txt>
Status of this Memo
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Abstract
As required by the Routing Protocol Criteria [RFC 1264], this draft
report 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. The final form of
this draft report is a prerequisite to advancing NHRP on the standards
track.
1. Protocol Documents
The NHRP protocol description is defined in [1] in its draft form.
The NHRP protocol analysis is documented in TBD [2].
The NHRP MIB description is defined in [3] in its draft form.
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 [4] and of the Transmission
of IP Datagrams over the SMDS Service, defined in [5]. 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, [4] and [5] 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 egress router towards the destination.
The second generalization is that NHRP is not specific to a particular
NBMA technology. Of course, [4] assumes an ATM network and [5] assumes
an SMDS network at their respective subnetwork layers.
NHRP is specified for resolving the NBMA addresses IP datagrams over
large clouds of NBMA networks. In principle, NHRP should also be
extensible to network layer protocols other than IP, without major
modifications to the specification provided in [1].
3. Key Features
NHRP is not a routing protocol, but an inter-LIS address resolution
mechanism that may make use of network layer routing in resolving
the NBMA address of the destination. This is further discussed in
Section 5.
The most prominent feature of NHRP is that it avoids extra 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.
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].
NHRP can be used in host-host, host-router and router-host communications.
When used in router-router communication, NHRP can produce
persistent routing loops. NHRP for router-router communication will
be addressed in 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,
possibly with the B bit set, indicating that the information is stable.
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 not to issue an NHRP Reply for this particular request.
As a result of 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. NHRP uses network layer routing
in resolving the NBMA address of the destination. The related routing
tables can be populated using the services of network layer dynamic
routing algorithms, or they may be statically configured. If network
layer dynamic routing algorithms are used, NHRP Servers would be
implemented in a router, or in some device that participates to a
network layer dynamic routing algorithm. If static routing is used,
then NHRP Servers do not necessarily have to participate
to network layer dynamic routing algorithms. All they are required to do
is to reply to NHRP Requests using their IP to NBMA address resolution
tables, or to forward them to another Server, using some pre-determined
forwarding rules.
4. Use of NHRP
In general, issuing an NHRP request would be 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 MBMA 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. Then, 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
NHRP uses network layer routing in resolving the NBMA address of the
destination. 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, NHRP should scale
in the same order as the routing algorithm, subject to two issues. The
first issue is related to the memory size and the required processing
power for the address resolution tables at the NHSs. The routing
algorithm divides the network into moderately sized subnets. Assignment
of the areas of responsibility for each NHS in a way similar to the
operation of the routing algorithm will resolve the issue of address
resolution table size. The second issue is related to the NHRP awareness
of the routers. If a router on the routed path of an NHRP Request
does not implement NHRP, it will silently discard the Request. 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, 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. Of course, static
routing does not scale well. Also, when static routing is used, it may
not be possible to forward the IP packet to be transmitted along with
the path of the NHRP Request.
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 the 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. An advantage of this approach is that it "shortens"
the routed path. A disadvantage is that it may create persistent
routing loops when used in router-to-router communication [8]. As
noted in Section 3, Router-to-Router NHRP will be addressed in a
separate document.
References
[1] NMBA Next Hop Resolution Protocol (NHRP), Dave Katz,
David Piscitello, Bruce Cole and James Luciani,
draft-ietf-rolc-nhrp-07.txt.
[2] TBD
[3] NHRP Management Information Base, M. Patrick, draft-ietf-rolc
-nhrp-mib-01.txt
[4] Classical IP and ARP over ATM, Mark Laubach, RFC 1577.
[5] Transmission of IP datagrams over the SMDS service, J. Lawrance
and D. Piscitello, RFC 1209.
[6] Support for Sparse Mode PIM over ATM, Yakov Rekhter and Dino
Farinacci, draft-rekhter-pim-atm-00.txt
[7] "Local/Remote" Forwarding Decision in Switched Data Link Subnetworks,
Yakov Rekhter and Dilp Kandlur, draft-ietf-rolc-apr-04.txt.
[8] IP over ATM: A Framework Document, R.G. Cole, D.H. Shur and
C. Villamizar, draft-ietf-ipatm-framework-doc-07.ps
Acknowledgements
The author acknowledges valuable contributions and comments from
many participants of the ROLC Working Group, in particular from
Curtis Villamizar, Yakov Rekhter, Joel Halpern and Andy Malis.
Author's Address
Derya H. Cansever
GTE Laboratories Inc.
40 Sylvan Rd. MS 51
Waltham MA 02254
Phone: +1 617 466 4086
Email: dhc2@gte.com
Expiration Date July 1996
