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Internet Draft S. Corson
Expiration: March 1998 University of Maryland
File: draft-ietf-manet-issues-00.txt J. Macker
Naval Research Laboratory
September 1997
Mobile Ad hoc Networking (MANET):
Routing Protocol Performance Issues and Evaluation Considerations
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference
material or to cite them other than as "work in progress."
To view the entire list of current Internet-Drafts, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), ftp.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).
Distribution of this memo is unlimited.
Summary
This memo describes the concept of mobile ad hoc networking--giving a
rationale for its existence and outlining the unique issues and
challenges found in this form of purely wireless, mobile networking.
1. Introduction
With recent performance advancements in computer and wireless
communications technologies, advanced mobile wireless computing is
expected to see increasingly widespread use and application, much of
which will involve the use of the Internet Protocol (IP) suite. The
vision of mobile ad hoc networking is to support robust and efficient
operation in mobile wireless networks by incorporating routing
functionality into mobile nodes. Such networks are envisioned to
have dynamic, sometimes rapidly-changing, random, multihop topologies
which are likely composed of relatively bandwidth-constrained
wireless links.
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Within the Internet community, routing support for mobile hosts is
presently being formulated as "mobile IP" technology. This is a
technology to support nomadic host "roaming", where a roaming host
may be connected through various means to the Internet other than its
well known fixed-address domain space. The host may be directly
physically connected to the fixed network on a foreign subnet, or be
connected via a wireless link, dial-up line, etc. Supporting this
form of host mobility (or nomadicity) requires address management,
protocol interoperability enhancements and the like, but core network
functions such as hop-by-hop routing still presently rely upon pre-
existing routing protocols operating within the fixed network. In
contrast, the goal of mobile ad hoc networking is to extend mobility
into the realm of autonomous, mobile, wireless domains, where the
nodes themselves form the network routing infrastructure in an ad hoc
fashion.
This memo first describes the characteristics of Mobile Ad hoc
Networks (MANETs), and their idiosyncrasies with respect to
traditional, hardwired packet networks. It then discusses the effect
these differences have on the design and evaluation of network
control protocols with an emphasis on routing.
2. Applications
The technology of Mobile Ad hoc Networking is somewhat synonomous
with Mobile Packet Radio Networking (a term coined via during early
military research in the 70's and 80's), Mobile Mesh Networking (a
term that appeared in an article in The Economist regarding the
structure of future military networks) and Mobile, Multihop, Wireless
Networking (perhaps the most accurate term, although a bit
cumbersome).
There is current and future need for dynamic ad hoc networking
technology. The emerging field of mobile and nomadic computing, with
its current emphasis on mobile IP operation, should gradually broaden
and require highly adaptive mobile networking technology to
effectively manage multihop, ad hoc network clusters which can
operate autonomously or, more than likely, be attached at some
point(s) to the fixed Internet.
Some applications of MANET technology could include industrial and
commercial applications involving cooperative mobile data exchange.
In addition, mesh-based mobile networks can be operated as robust,
inexpensive alternatives or enhancements to cell-based mobile network
infrastructures. There are also existing and future military
networking requirements for robust, IP-compliant data services within
mobile wireless communication networks [1]--many of these networks
consist of highly-dynamic autonomous topology segments. Also, the
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developing technologies of "wearable" computing and communications
may provide applications for MANET technology. When properly combined
with satellite-based information delivery, MANET technology can
provide an extremely flexible method for establishing communications
for fire/safety/rescue operations or other scenarios requiring
rapidly-deployable communications with survivable, efficient dynamic
networking. There are likely other applications for MANET technology
which are not presently realized or envisioned by the authors. It
is, simply put, efficient IP-based routing technology for highly
dynamic, autonomous wireless networks.
3. Characteristics of MANETs
A MANET consists of mobile platforms (combined router, host and
wireless communications platforms)--herein simply referred to as
"nodes"--which are free to move about arbitrarily. The nodes may be
located in or on airplanes, ships, trucks, cars, perhaps even on
people, and there may be multiple hosts per router. A MANET is an
autonomous system of mobile nodes. The system may operate in
isolation, or may have gateways to and interface with a fixed
network--typically envisioned to operate as a stub network connecting
to a fixed internetwork.
The nodes are equipped with wireless transmitters and receivers using
antennas which may be omnidirectional (broadcast), highly-directional
(point-to-point) or some combination thereof. At a given point in
time, depending on the nodes' positions and their transmitter and
receiver coverage patterns, transmission power levels and cochannel
interference levels, a wireless connectivity in the form of a random,
multihop graph or "ad hoc" network exists between the nodes. This ad
hoc topology may change with time as the nodes move or adjust their
transmission and reception parameters.
MANETs have several salient characteristics:
1) Dynamic topologies: Nodes are free to move arbitrarily; thus,
the network topology--which is typically multihop--may change
randomly and rapidly at unpredictable times, and may consist of
both bidirectional and unidirectional links.
2) Bandwidth-constrained, variable capacity links: Wireless links
will continue to have significantly lower capacity than their
hardwired counterparts. In addition, the realized throughput of
wireless communications--after accounting for the effects of
multiple access, fading, noise, and interference conditions,
etc.--is often very much less than a radio's maximum transmission
rate.
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One effect of the relatively low to moderate link capacities is
that congestion is typically the norm rather than the exception,
i.e. aggregate application demand will likely approach or exceed
network capacity frequently. As the mobile network is often simply
an extension of the fixed network infrastructure, mobile ad hoc
users will demand similar services. These demands will continue to
increase as multimedia computing and collaborative networking
applications rise.
3) Power-constrained operation: Some or all of the nodes in a
MANET may rely on batteries for their energy. For these nodes, the
most important system design criteria for optimization may be
power conservation.
4) Limited physical security: Mobile wireless networks are
generally more prone to physical security threats than are fixed-
cable nets. The increased possibility of eavesdropping, spoofing,
and denial of service attacks should be carefully considered.
Existing link security techniques are often applied within
wireless network to reduce security threats. As a benefit, the
decentralized nature of network control in MANETs provides
additional robustness against single points of failure of more
centralized approaches.
In addition, some envisioned networks (e.g. mobile military networks
or highway networks) may be relatively large (e.g. tens or hundreds
of nodes per routing area). The need for scalability is not unique
to MANETS. However, in light of the preceding characteristics, the
mechanisms required to achieve scalability likely are.
These characteristics create a set of underlying assumptions and
performance concerns for protocol design which extend beyond those
guiding the design of routing within the higher-speed, semi-static
topology of the fixed Internet.
4. Goals of IETF Mobile Ad Hoc Network (manet) Working Group
The intent of the newly formed IETF manet working group is to develop
a peer-to-peer mobile routing capability in a purely mobile, wireless
domain. This capability will exist beyond the fixed network (as
supported by traditional IP networking) and beyond the one-hop fringe
of the fixed network.
The near-term goal of the manet working group is to standardize an
intra-domain unicast routing protocol which:
* provides a mode(s) of operation for effective operation in a
mobile networking "context". (a context is a set of defined
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networking characteristics),
* provides a standard "mode discovery" protocol so that newly-
arriving nodes may learn the mode in which a given MANET is
operating,
* supports traditional, connectionless IP service,
* reacts efficiently to topological changes while maintaining
effective routing in a mobile networking context.
The working group will also address issues pertaining to security and
interaction/interface with link-layer protocols and internet security
protocols. In the longer term, the group may look at the issues of
layering more advanced mobility services on top of the initial
unicast routing developed. These longer term issues will likely
include investigating multicast and QoS extensions for a dynamic,
mobile area.
5. Why an IP-Layer Routing Solution?
A solution at the IP layer can provide a benefit similar to the
intention of the original Internet, viz. "an interoperable
internetworking capability over a heterogeneous networking
infrastructure". In this case, the infrastructure is wireless, rather
than hardwired, consisting of multiple wireless technologies, channel
access protocols, etc. Improved IP routing and related networking
services provide the glue to preserve the integrity of the mobile
internetwork segment in this more dynamic environment.
6. MANET Routing Protocol Performance Issues
To judge the merit of a routing protocol, one needs metrics--both
qualitative and quantitative--with which to measure its suitability
and performance. These metrics should be somewhat *independent* of
any given routing protocol.
The following is a list of desirable qualitative properties:
1) Distributed operation: This is an essential property, but it
should be stated nonetheless.
2) Loop-freedom: Not required per se in light of certain
quantitative measures (performance criteria), but generally
desirable to avoid problems such as worst-case phenomena, e.g. a
small fraction of packets spinning around in the network for
arbitrary time periods. Ad hoc solutions such as TTL values can
bound the problem, but a more structured and well-formed approach
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is generally desirable as it oftentimes leads to better overall
performance.
3) Demand-based operation: Instead of assuming uniform traffic
distribution within the network (and maintaining routing between
all nodes at all times), let the routing algorithm adapt to the
traffic pattern on a demand or need basis. If this is done
intelligently, it will utilize network resources more efficiently.
4) Unidirectional link support: Bidirectional links are typically
assumed in the design of routing algorithms, and many algorithms
are incapable of functioning properly over unidirectional links.
Nevertheless, unidirectional links can and do occur in wireless
networks. Often times, a sufficient number of duplex links exist
so that usage of unidirectional links is of limited added value.
However, in situations where a pair of unidirectional links (in
opposite directions) form the *only* bidirectional connection
between two ad hoc clusters, the ability to make use of them is
invaluable.
5) Security: Without some form of network-level security or link
layer security, a MANET routing protocol is vulnerable to many
forms of attack. It may be relatively simple to snoop network
traffic, replay transmissions, manipulate packet headers, and
redirect routing messages, within a wireless network without
appropriate security provisions. While these concerns exist within
wired infrastructures and routing protocols as well, maintaining
the "physical" security of of the transmission media is harder in
practice with MANETs. Sufficient security protection to prohibit
distruption of modification of protocol operation is desired.
This may be somewhat orthogonal to any particular routing protocol
approach, e.g. through the application of IP Security techniques.
6) "Sleep" period operation: As a result of power conservation,
or some other need to be inactive, nodes of a MANET may stop
transmitting and/or receiving (even receiving requires power) for
arbitrary time periods. A routing protocol should be able to
accomodate such sleep periods without overly adverse consequences.
This property may require close coupling with the link layer
protocol through a standardized interface.
The following is a list of quantitative metrics that can be used to
assess the performance of any routing protocol.
1) End-to-end data throughput and delay: Statistical measures of
data routing performance (e.g., means, variances, distributions)
are important. These are the measures of a routing policy's
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effectiveness--how well it does its job--as measured from the
*external* perspective of other policies that make use of routing.
2) Efficiency: If data routing effectiveness is the external
measure of a policy's performance, efficiency is the *internal*
measure of its effectiveness. To achieve a given level of data
routing performance, two different policies may expend differing
amounts of overhead, depending on their internal efficiency.
Protocol efficiency may or may not directly affect data routing
performance. If control and data traffic must share the same
channel, and the channel's capacity is limited, then excessive
control traffic may impact data routing performance.
It is useful to track two ratios that illuminate the *internal*
efficiency of a protocol in doing its job (there may be others
that the authors have not considered):
* Average number of data bits transmitted/data bit delivered--
this can be thought of as a measure of the efficiency of
delivering data within the network.
* Average number of control bits transmitted/data bit
delivered--this measures the efficiency of the protocol in
expending control overhead to delivery data packets. Note that
this should include not only the bits in the routing control
packets, but also the bits in the header of the data packets.
In other words, anything that is not data is control overhead,
and should be counted in the control portion of the algorithm.
Also, we must consider the networking *context* in which a protocol's
performance is measured. Essential parameters that should be varied
include:
1) Network size--measured in the number of nodes
2) Network connectivity--the average degree of a node (i.e. the
average number of neighbors of a node)
3) Topological rate of change--the speed with which a network's
topology is changing
4) Link capacity--effective link speed measured in bits/second,
after accounting for losses due to multiple access, coding,
framing, etc.
5) Fraction of unidirectional links--how effectively does a
protocol perform as a function of the presence of unidirectional
links?
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6) Traffic patterns--how effective is a protocol in adapting to
non-uniform or bursty traffic patterns?
7) Mobility--when, and under what circumstances, is temporal and
spatial topological correlation relevant to the performance of a
routing protocol? In these cases, what is the most appropriate
model for simulating node mobility in a MANET?
8) Fraction and frequency of sleeping nodes--how does a protocol
perform in the presence of sleeping and awakening nodes?
A MANET protocol should function effectively over a wide range of
networking contexts--from small, collaborative, ad hoc groups to
larger mobile, multihop networks. The preceding discussion of
characteristics and evaluation metrics somewhat differentiate MANETs
from traditional, hardwired, multihop networks. The wireless
networking environment is one of scarcity rather than abundance,
wherein bandwidth is relatively limited, and energy may be as well.
In summary, the networking opportunities for MANETs are intriguing
and the engineering tradeoffs are many and challenging. A diverse
set of performance issues requires new protocols for network control.
A question which arises is "how should the *goodness* of a policy be
measured?". To help answer that, we proposed here an outline of
protocol evaluation issues that highlight performance metrics that
can help promote meaningful comparisons and assessments of protocol
performance. It should be recognized that a routing protocol tends
to be well-suited for particular network contexts, and less well-
suited for others. In putting forth a description of a protocol, both
its *advantages* and *limitations* should be mentioned so that the
appropriate networking context(s) for its usage can be identified.
These attributes of a protocol can typically be expressed
*qualitatively*, e.g., whether the protocol can or cannot support
shortest-path routing. Qualitative descriptions of this nature
permit broad classification of protocols, and form a basis for more
detailed *quantitative* assessments of protocol performance. In
future documents, the group may put forth candidate recommendations
regarding protocol design for MANETs. The metrics and the philosophy
presented within this document are expected to continue to evolve as
MANET technology and efforts mature.
6. References
[1] B. Adamson, "Tactical Radio Frequency Communication Requirements for
IPng," RFC 1677, Aug. 1994.
Authors' Addresses
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M. Scott Corson
Institute for Systems Research
University of Maryland
College Park, MD 20742
(301) 405-6630
corson@isr.umd.edu
Joseph Macker
Information Technology Division
Naval Research Laboratory
Washington, DC 20375
(202) 767-2001
macker@itd.nrl.navy.mil
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