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RFC 2501                MANET Performance Issues            January 1999


      bound the problem, but a more structured and well-formed approach
      is generally desirable as it usually leads to better overall
      performance.

      3) Demand-based operation:  Instead of assuming an 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 can utilize network energy and bandwidth
      resources more efficiently, at the cost of increased route
      discovery delay.

      4) Proactive operation:  The flip-side of demand-based operation.
      In certain contexts, the additional latency demand-based operation
      incurs may be unacceptable.  If bandwidth and energy resources
      permit, proactive operation is desirable in these contexts.

      5) Security: Without some form of network-level 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
      disruption 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 energy 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
      accommodate such sleep periods without overly adverse
      consequences. This property may require close coupling with the
      link-layer protocol through a standardized interface.

      7) 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. Oftentimes, 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 regions, the ability to make use of them is
      valuable.



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RFC 2501                MANET Performance Issues            January 1999


   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
      effectiveness--how well it does its job--as measured from the
      *external* perspective of other policies that make use of routing.

      2) Route Acquisition Time: A particular form of *external* end-
      to-end delay measurement--of particular concern with "on demand"
      routing algorithms--is the time required to establish route(s)
      when requested.

      3) Percentage Out-of-Order Delivery: An external measure of
      connectionless routing performance of particular interest to
      transport layer protocols such as TCP which prefer in-order
      delivery.

      4) 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 can 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 often impacts data routing performance.

      It is useful to track several 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 bit efficiency of
         delivering data within the network.  Indirectly, it also gives
         the average hop count taken by data packets.

         * Average number of control bits transmitted/data bit
         delivered--this measures the bit efficiency of the protocol in
         expending control overhead to delivery data.  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.






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RFC 2501                MANET Performance Issues            January 1999


         * Average number of control and data packets transmitted/data
         packet delivered--rather than measuring pure algorithmic
         efficiency in terms of bit count, this measure tries to capture
         a protocol's channel access efficiency, as the cost of channel
         access is high in contention-based link layers.

   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?

      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.



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RFC 2501                MANET Performance Issues            January 1999


   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 related efforts mature.

7. Security Considerations

   Mobile wireless networks are generally more prone to physical
   security threats than are fixed, hardwired networks. Existing link-
   level security techniques (e.g. encryption) are often applied within
   wireless networks to reduce these threats.  Absent link-level
   encryption, at the network layer, the most pressing issue is one of
   inter-router authentication prior to the exchange of network control
   information.  Several levels of authentication ranging from no
   security (always an option) and simple shared-key approaches, to full
   public key infrastructure-based authentication mechanisms will be
   explored by the group.  As an adjunct to the working groups efforts,
   several optional authentication modes may be standardized for use in
   MANETs.

8. References

   [1] Adamson, B., "Tactical Radio Frequency Communication Requirements
       for IPng", RFC 1677, August 1994.













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RFC 2501                MANET Performance Issues            January 1999


Authors' Addresses

   M. Scott Corson
   Institute for Systems Research
   University of Maryland
   College Park, MD 20742

   Phone: [phone]
   EMail: [e-mail]


   Joseph Macker
   Information Technology Division
   Naval Research Laboratory
   Washington, DC 20375

   Phone: [phone]
   EMail: [e-mail]

































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RFC 2501                MANET Performance Issues            January 1999


Full Copyright Statement

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