Mobile Ad hoc Networking (MANET) - Seminar Paper

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.

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

   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.

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 --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.

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.

      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.

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) or mode(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);

      * provides a standard "protocol or mode discovery" algorithm 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 and traffic demands

      while maintaining effective routing in a mobile networking

      context.

   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.

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 "router ID" (separate and apart from IP addressing)

   is crucial to supporting the multigraph topology of the routing

   fabric. It is what *unifies* a set of wireless IP interfaces (each

   with their own IP address) and identifies them as belonging to the

   same mobile platform.  This approach permits maximum flexibility in

   address assignment, and does not require that all IP addresses

   attached to a given router fall under a common CIDR prefix.  Router

   IDs are used at the IP layer for routing computations.  To enable IP

   routing to hosts associated with the router, the subnet mask(s)

   (encompassing the hosts on the mobile platform) should be advertised

   with the router ID to permit routing table construction.

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 must 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.

 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 (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

      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 will utilize network energy and bandwidth

      resources more efficiently.

      4) 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.

      5) "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.

      6) 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 clusters, the ability to make use of them is

      valuable.

   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 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?

      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 related efforts mature.

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.