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
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   Distribution of this memo is unlimited.


   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

   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

   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

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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

      * 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

      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

      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

      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

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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

   Joseph Macker
   Information Technology Division
   Naval Research Laboratory
   Washington, DC 20375
   (202) 767-2001

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