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Network Working Group                                          S. CorsonRequest for Comments: 2501                        University of MarylandCategory: Informational                                        J. Macker                                               Naval Research Laboratory                                                            January 1999Mobile Ad hoc Networking (MANET):Routing Protocol Performance Issues and Evaluation ConsiderationsStatus 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 beCorson & 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 19993. 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 sameCorson & 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 canCorson & 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 1999Authors' 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.milCorson & Macker              Informational                     [Page 11]

RFC 2501                MANET Performance Issues            January 1999Full Copyright Statement   Copyright (C) The Internet Society (1999).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Corson & Macker              Informational                     [Page 12]