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diff --git a/doc/rfc/rfc2501.txt b/doc/rfc/rfc2501.txt new file mode 100644 index 0000000..8957554 --- /dev/null +++ b/doc/rfc/rfc2501.txt @@ -0,0 +1,675 @@ + + + + + + +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 + + +Full 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] + |