The Broadcast Storm Problem in a Mobile Ad Hoc Network * Sze-Yao Ni, Yu-Chee Tseng, Yuh-Shyan Chen, and Jang-Ping Sheu Department of Computer Science and Information Engineering National Central University Chung-Li, 32054, Taiwan Tel: 886-3-4227 15 1 ext. 45 12, Fax: 886-3-422268 1 Email: [email protected] URL: http://www.csie.ncu.edu.tw/~yctseng/ Abstract an environment. Due to considerations such as radio power limitation, channel utilization, and power-saving concerns, a Broadcastingis a common operation in a network to resolve mobile host may not be able to communicate directly with many issues. In a mobile ad hoc network (MANET) in par- other hosts in a single-hop fashion. In this case, a multihop ticular, due to host mobility, such operations are expected scenario occurs, where the packets sent by the source host to be executed more frequently (such as finding a route to are relayed by several intermediate hosts before reaching the a particular host, paging a particular host, and sending an destination host. alarm signal). Because radio signals are likely to overlap Applications of MANETs occur in situations like battle- with others in a geographical area, a straightforward broad- fields or major disaster areas where networks need to be de- casting by flooding is usually very costly and will result in ployed immediately but base stations or fixed network in- serious redundancy, contention, and collision, to which we frastructures are not available. Unicast routing in MANET refer as the broadcast storm problem. In this paper, we iden- has been studied in several articles [6,7, 14, 15,231. A work- tify this problem by showing how serious it is through anal- ing group called “manet” has been formed by the Internet yses and simulations. We propose several schemes to reduce Engineering Task Force (IETF) to study the related issues redundant rebroadcasts and differentiate timing of rebroad- and stimulate research in MANET [21]. casts to alleviate this problem. Simulation results are pre- This paper studies the problem of sending a broadcast sented, which show different levels of improvement over the message in a MANET. Broadcasting is a common operation basic flooding approach. in many applications, e.g., graph-related problems and dis- Keywords: broadcast, communication, mobile ad hoc net- tributed computing problems. It is also widely used to re- work (MANET), mobile computing, wireless network. solve many network layer problems. In a MANET in partic- ular, due to host mobility, broadcastings are expected to be 1 Introduction performed more frequently (e.g., for paging a particular host, sending an alarm signal, and finding a route to a particular The advancement in wireless communication and economi- host [6, 14, 15,231). Broadcasting may also be used in LAN cal, portable computing devices have made mobile comput- emulation [2] or serve as a last resort to provide multicast ing possible. One research issue that has attracted a lot of services in networks with rapid changing topologies. attention recently is the design of mobile ad hoc network In this paper, we assume that mobile hosts in the MANET (MANET). A MANET is one consisting of a set of mo- share a single common channel with carrier sense multiple bile hosts that may communicate with one another and roam access (CSMA), but no collision detection (CD), capability. around at their will. No base stations are supported in such Synchronization in such a network with mobility is unlikely, and global network topology information is unavailable to ‘This work is supported by the National Science Council of the Republic of China under Grants #NSC88-2213-E-008-013 and #NSC88-2213-E-008-014. facilitate the scheduling of a broadcast. So one straight- forward and obvious solution is broadcasting by flooding. Permission to make digital or hard copies ol‘ all or part of this work for Unfortunately, in this paper we observe that serious redun- Personal Or classmom use is granted without fee providedthat copies dancy, contention, and collision could exist if flooding is are not made or distributed for profit or commercial advantage at,d that done blindly. First, because the radio propagation is omni- copies bear this notice and the full citation oll the tirst page:e. ~~ copy otherwise, to republish, to post on servers or to redistehute Lolists. directional and a physical location may be covered by the rcqoircs prior spCCifiC permission and/or a fee, transmission ranges of several hosts, many rebroadcasts are Mobicom ‘99 Seattle Washington USA considered to be redundant. Second, heavy contention could ‘%‘yright ACM 1999 1-58113-142-g/gg/o8...$5.oo exist because rebroadcasting hosts are probably close to each 151 other. Third, collisions are more likely to occur because the l The broadcast is unreliable.’ No acknowledgement RTSKTS dialogue is inapplicable and the timing of rebroad- mechanism will be used.2 However, attempt should casts is highly correlated. be made to distribute a broadcast message to as many Collectively, we refer to these problems associated with hosts as possible without paying too much effort. The flooding as the broadcast storm problem. Through analy- motivations to make such an assumption are (i) a host ses and simulations, we demonstrate how serious the storm may miss a broadcast message because it is off-line, is. Two directions to alleviate this problem is to reduce the it is temporarily isolated from the network, or it ex- possibility of redundant rebroadcasts and differentiate the periences repetitive collisions, (ii) acknowledgements timing of rebroadcasts. Following these directions, we de- may cause serious medium contention (and thus an- velop several schemes, called probabilistic, counter-bused, other “storm”) surrounding the sender, and (iii) in many distance-bused, locution-bused, and cluster-bused schemes, applications (e.g., the route discovery in [6, 14, 15, to facilitate MANET broadcasting. Simulation results are 23]), a 100% reliable broadcast is unnecessary. presented to study the effectiveness of these schemes. To the best of our knowledge, the broadcast storm prob- In addition, we assume that a host can detect duplicate lem has not been addressed in depth for MANET before. It is broadcast messages. This is essential to prevent endless flood- however worth of summarizing some results for broadcast- ing of a message. One way to do so is to associate with each ing that are for other environments. Works in [3,4,9, 10, 11, broadcast message a tuple (source ID, sequence number) as 201 assume a packet-radio network environment. Most of that in [6,23]. these results rely on time division multiple access (TDMA, Finally, we comment that we do not confine ourselves to which requires timing synchronization) and certain levels of the broadcasting of the same message.3 In this paper we fo- topology information. Their goal is to find a slot assign- cus on the flooding behavior in MANET - the phenomenon ment. Obtaining an optimal assignment has been shown to where the transmission of a packet will trigger other sur- be NP-hard [9]. The broadcast scheduling problem studied rounding hosts to transmit the same (or modified) packet. in [8, 16,24, 251, although carries a similar name, is not in- We shall show that if flooding is used blindly, many redun- tended to solve the problem addressed in this paper. Its goal dant messages will be sent and serious contention/collision is to assign a contention-free time slot to each radio station. will be incurred. Our goal is to solve broadcast with effi- The rest of this paper is organized as follows. Section 2 ciency in mind. defines and analyzes the broadcast storm problem. Mecha- nisms to alleviate the storm are proposed in Sections 3. Sim- 2.2 Broadcast Storm Caused by Flooding ulation results are in Section 4 and conclusions are drawn in A straight-forward approach to perform broadcast is byfiod- Section 5. ing. A host, on receiving a broadcast message for the first time, has the obligation to rebroadcast the message. Clearly, 2 Preliminaries this costs n transmissions in a network of it hosts. In a CSMAICA network, drawbacks of flooding include: 2.1 Broadcasting in a MANET Redundant rebroadcasts: When a mobile host de- A MANET consists of a set of mobile hosts that may com- cides to rebroadcast a broadcast message to its neigh- municate with one another from time to time. No base sta- bors, all its neighbors already have the message. tions are supported. Each host is equipped with a CSMAICA (carrier sense multiple access with collision avoidance) [ 191 Contention: After a mobile host broadcasts a mes- transceiver. In such environment, a host may communicate sage, if many of its neighbors decide to rebroadcast with another directly or indirectly. In the latter case, a mul- the message, these transmissions (which are all from tihop scenario occurs, where the packets originated from the nearby hosts) may severely contend with each other. source host are relayed by several intermediate hosts before Collision: Because of the deficiency of backoff mech- reaching the destination. anism, the lack of RTSKTS dialogue, and the absence The broadcastproblem refers to the sending of a message of CD, collisions are more likely to occur and cause to other hosts in the network. The problem considered here more damage. has the following characteristics. ‘A more strict one is rdiuhlr hmudcust [I, 221, whose goal is to ensure all hosts re- ceive the message. High-level acknowledgements between hosts are exchanged. Such l The broadcast is spontaneous. Any mobile host can protocols are typically accomplished at the application layer and is out of the scope issue a broadcast operation at any time.