1Reliable Adaptive Lightweight Multicast ProtocolKen Tang, Katia Obraczka, Sung-Ju Lee, and Mario GerlaAbstract— Typical applications of mobile ad hoc networks(MANET) require group-oriented services. Digital battlefields anddisaster relief operations make data dissemination and teleconfer-ences a key application domain. Network-supported multicast ishence critical for efficient any-to-many communications. How-ever, very little work has been done on “reliable” transport mul-ticast. We propose and evaluate Reliable Adaptive LightweightMulticast (RALM). The design choices of RALM are motivated bylessons we learned from evaluating the performance of traditionalwired reliable multicast transport protocols (in particular, SRM)in ad hoc networks. We argue that two components, reliability andcongestion control, are essential in designing a reliable multicasttransport protocol for MANETs. RALM addresses both reliabilityand congestion control. Itachievesreliability by guaranteeing datadelivery to troubled receivers in a round-robin fashion. RALM’ssend-and-wait congestion control uses NACK feedback to adjustto congestion experienced by receivers. We show through simula-tions that RALM achieves perfect reliability while exhibiting lowend-to-end delay and minimal control overhead compared againstother protocols.I. INTRODUCTIONAN ad hoc network enables wireless communications with-out any fixed infrastructure or central administration.Each node communicates with each other through packet ra-dios. Hence, every host acts as a packet forwarder as well asa source or a destination. Because of its ease of deployment,an ad hoc network is an attractive choice for scenarios wherethe fixed network infrastructure is non-existent (e.g., remotelocations), unusable because it is insecure (e.g., covert mili-tary operations) or unavailable due to some catastrophic events(e.g., major earthquake). The types of scenarios targeted byMANETs make group-oriented services such as data dissem-ination and teleconferences a key application domain. Multi-cast communication is an efficient means of supporting group-oriented applications. This is especially true in mobile wire-less environments where nodes are energy and bandwidth con-strained. Because MANETs are particularly well suited formission-critical applications, ad hoc network protocols mustprovide reliable and timely data delivery even in the presenceof mobility and frequent outages.MANETs have been the subject of extensive research. De-spite the fact that reliable multicasting is vital to the success ofmission critical applications, surprisingly little work has beendone in this area. One of the few exceptions is the AnonymousGossip (AG) protocol [6] that recovers from losses by havingpairs of multicast members exchange information on messagesthey have received or lost. One potential problem with this pro-Ken Tang is with Scalable Networks Technologies, Katia Obraczka is withComputer Engineering Department, University of California, Santa Cruz,Sung-Ju Lee is with Hewlett Packard Laboratories, and Mario Gerla is withComputer Science Department, University of California, Los Angeles.tocol is the delay it takes for nodes to recover from losses. Re-liable multicast for wired networks on the other hand, has beena very active area of research [11]. MAC and transport levelprotocols for wireless cellular networks have also been pro-posed [2], [4], [5], [8], [13]. One may consider applying theseschemes to MANETs. We argue that the design choices under-lying wired reliable multicast transport protocols are not ade-quate for MANETs. Ad hoc networking protocols must handlenode mobility. In addition, MANETs are extremely sensitiveto network load and congestion, even more so than in wiredshared-medium networks because of the hidden terminal prob-lem. Generating additional control message overhead withoutperforming adequate congestion control will considerably de-grade the performance.We propose the Reliable Adaptive Lightweight Multicast(RALM) transport protocol that favors reliability and conges-tion control over throughput. Applications that are willing totrade throughput for reliability include military convert oper-ations and search and rescue missions. For example, an op-eration commander disseminating mission critical data to histroops in a covert operation is more interested in reliably deliv-ering the commands rather than obtaining high throughput (as-suming adequate throughput is obtained). In such a scenario,any data loss can be fatal to the success of the entire operation.RALM is a reliable, rate-based, congestion controlled proto-col that targets small group operation scenarios ranging fromspecial military operations to civilian emergency rescue appli-cations. When there is no packet loss, RALM sends packets atthe specified application sending rate. Once a loss is detected,RALM recovers by initiating a modified send-and-wait proce-dure. Send-and-wait is performed with each multicast receiverthat experiences losses, one at a time in a round-robin fashion.Once all receivers have up-to-date packets, RALM reverts tothe application sending rate. In our previous work [15], we as-sumed that the multicast sources know the receiver informationahead of time and was able to use a window-based congestioncontrol approach. In this paper, we do not make such an as-sumption and hence use a send-and-wait procedure.We start this study by evaluating how a “wired” reliable mul-ticast protocol performs in MANETs. While we acknowledgethat wired protocols were not designed for MANETs, study-ing the behavior of these protocols in various scenarios willgive us insights into designing new protocols for MANETs.To this end, we evaluate the performance of the Scalable Re-liable Multicast (SRM) protocol [7]. SRM is one of the early,wired reliable multicast protocols and can be considered rep-resentative of reliable multicast protocol behavior, as later de-veloped protocols use common error control mechanisms sim-ilar to SRM (e.g., negative acknowledgments, multicasting ofNACKs and retransmitted data, NACK suppression and local2recovery). SRM was selected as we are particularly interestedin protocols that rely exclusively on error recovery to achievereliability. Our hypothesis is that, since MANETs are extremelysensitive to offered load, protocols that fall in this category willnot perform well in MANETs.The remainder of this paper is organized as follows. Theperformance of SRM in
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