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rfc:rfc2501

Network Working Group S. Corson Request for Comments: 2501 University of Maryland Category: Informational J. Macker

                                             Naval Research Laboratory
                                                          January 1999
                 Mobile Ad hoc Networking (MANET):
 Routing Protocol Performance Issues and Evaluation Considerations

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

 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 performance evaluation
 considerations.

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.
 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

Corson & Macker Informational [Page 1] RFC 2501 MANET Performance Issues January 1999

 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 a set
 of nodes--which may be combined routers and hosts--themselves form
 the network routing infrastructure in an ad hoc fashion.

2. Applications

 The technology of Mobile Ad hoc Networking is somewhat synonymous
 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
 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, improved IP-based networking technology for dynamic,
 autonomous wireless networks.

Corson & Macker Informational [Page 2] RFC 2501 MANET Performance Issues January 1999

3. Characteristics of MANETs

 A MANET consists of mobile platforms (e.g., a router with multiple
 hosts and wireless communications devices)--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 or very small devices, 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. In the latter operational mode, it is typically
 envisioned to operate as a "stub" network connecting to a fixed
 internetwork.  Stub networks carry traffic originating at and/or
 destined for internal nodes, but do not permit exogenous traffic to
 "transit" through the stub network.
 MANET nodes are equipped with wireless transmitters and receivers
 using antennas which may be omnidirectional (broadcast), highly-
 directional (point-to-point), possibly steerable, 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 co-channel 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 much less than a radio's maximum transmission rate.
    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.

Corson & Macker Informational [Page 3] RFC 2501 MANET Performance Issues January 1999

    3) Energy-constrained operation: Some or all of the nodes in a
    MANET may rely on batteries or other exhaustible means for their
    energy. For these nodes, the most important system design criteria
    for optimization may be energy 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 networks to reduce security threats. As a benefit, the
    decentralized nature of network control in MANETs provides
    additional robustness against the 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 one
 (or more) intra-domain unicast routing protocol(s), and related
 network-layer support technology which:
  • provides for effective operation over a wide range of mobile

networking "contexts" (a context is a set of characteristics

    describing a mobile network and its environment);
  • supports traditional, connectionless IP service;
  • reacts efficiently to topological changes and traffic demands

while maintaining effective routing in a mobile networking

    context.

Corson & Macker Informational [Page 4] RFC 2501 MANET Performance Issues January 1999

 The working group will also consider issues pertaining to addressing,
 security, and interaction/interfacing with lower and upper layer
 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. IP-Layer Mobile Routing

 An improved mobile routing capability 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.
 In other words, a real benefit to using IP-level routing in a MANET
 is to provide network-level consistency for multihop networks
 composed of nodes using a *mixture* of physical-layer media; i.e. a
 mixture of what are commonly thought of as subnet technologies.  A
 MANET node principally consists of a router, which may be physically
 attached to multiple IP hosts (or IP-addressable devices), which has
 potentially *multiple* wireless interfaces--each interface using a
 *different* wireless technology.  Thus, a MANET node with interfaces
 using technologies A and B can communicate with any other MANET node
 possessing an interface with technology A or B.  The multihop
 connectivity of technology A forms a physical-layer multihop
 topology, the multihop connectivity of technology B forms *another*
 physical-layer topology (which may differ from that of A's topology),
 and the *union* of these topologies forms another topology (in graph
 theoretic terms--a multigraph), termed the "IP routing fabric", of
 the MANET.  MANET nodes making routing decisions using the IP fabric
 can intercommunicate using either or both physical-layer topologies
 simultaneously.  As new physical-layer technologies are developed,
 new device drivers can be written and another physical-layer multihop
 topology can be seamlessly added to the IP fabric.  Likewise, older
 technologies can easily be dropped.  Such is the functionality and
 architectural flexibility that IP-layer routing can support, which
 brings with it hardware economies of scale.
 The concept of a "node identifier" (separate and apart from the
 concept of an "interface identifier") is crucial to supporting the
 multigraph topology of the routing fabric. It is what *unifies* a set
 of wireless interfaces and identifies them as belonging to the same

Corson & Macker Informational [Page 5] RFC 2501 MANET Performance Issues January 1999

 mobile platform.  This approach permits maximum flexibility in
 address assignment.  Node identifiers are used at the IP layer for
 routing computations.

5.1. Interaction with Standard IP Routing

 In the near term, it is currently envisioned that MANETs will
 function as *stub* networks, meaning that all traffic carried by
 MANET nodes will either be sourced or sinked within the MANET.
 Because of bandwidth and possibly power constraints, MANETs are not
 presently envisioned to function as *transit* networks carrying
 traffic which enters and then leaves the MANET (although this
 restriction may be removed by subsequent technology advances).  This
 substantially reduces the amount of route advertisement required for
 interoperation with the existing fixed Internet. For stub operation,
 routing interoperability in the near term may be achieved using some
 combination of mechanisms such as MANET-based anycast and mobile IP.
 Future interoperability may be achieved using mechanisms other than
 mobile IP.
 Interaction with Standard IP Routing will be greatly facilitated by
 usage of a common MANET addressing approach by all MANET routing
 protocols. Development of such an approach is underway which permits
 routing through a multi-technology fabric, permits multiple hosts per
 router and ensures long-term interoperability through adherence to
 the IP addressing architecture.  Supporting these features appears
 only to require identifying host and router interfaces with IP
 addresses, identifying a router with a separate Router ID, and
 permitting routers to have multiple wired and wireless interfaces.

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 *independent* of any given
 routing protocol.
 The following is a list of desirable qualitative properties of MANET
 routing protocols:
    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 (i.e. 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

Corson & Macker Informational [Page 6] 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.

Corson & Macker Informational [Page 7] 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.

Corson & Macker Informational [Page 8] 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.

Corson & Macker Informational [Page 9] 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.

Corson & Macker Informational [Page 10] RFC 2501 MANET Performance Issues January 1999

Authors' Addresses

 M. Scott Corson
 Institute for Systems Research
 University of Maryland
 College Park, MD 20742
 Phone: (301) 405-6630
 EMail: corson@isr.umd.edu
 Joseph Macker
 Information Technology Division
 Naval Research Laboratory
 Washington, DC 20375
 Phone: (202) 767-2001
 EMail: macker@itd.nrl.navy.mil

Corson & Macker Informational [Page 11] RFC 2501 MANET Performance Issues January 1999

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Corson & Macker Informational [Page 12]

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