Differential Destination Multicast–A MANETMulticast Routing Protocol for Small GroupsLusheng Ji and M. Scott CorsonInstitute for System ResearchUniversity of MarylandCollege Park, MD 20742e-mail: lji, corson @isr.umd.eduAbstract—In this paper we propose a multicast routing protocol for mo-bile ad hoc networks (MANETs). The protocol—termed Differential Desti-nation Mrrlticast (DDM)-dKfers from common approaches proposed forMANET mrdticast routing in two ways.Firstly, instead of distributingmembership control throughout the network, DDM concentrates this au-thority at the data sources (i.e. senders) thereby giving sources knowledge ofgroup membership. Secondly, dtiercntially-encoded, variable-length desti-nation headers are inserted in data packets which are used in combinationwith unieast routing tables to forward multicast packets towards multicastreceiverx. Instead of requiring that mrdticast forwarding state to be storedin all participating nodes, thm approach also provides the option of statelessmultieasting. Each node independently has the choice of caching forward-ing state or having its upstream neighbor to insert this state into self-routeddata packets, or some combination thereof. The protocol is best suited foruse with small multieast groups operating in dynamic networks of any size.Keywords-MANET, multirast routing, small group multicastI. INTRODUCTIONThere are generally two mainstream approaches used for mul-ticast routing in fixed networks: Group Shared Tree (GST)and Source-Specific Tree (SST). Both construct data forward-ing paths interconnecting all group members, where a memberis understood to be a multicast receiver. Data is firstly forwardedto the tree then along the tree paths to reach all group members.Several protocols include data sources as part of the forwardingtree as well, The GST approach builds one tree for the wholegroup regardless of where the data sources are located. TheSST approach organizes multicast forwarding by data source.A “session” is associate with each source and is identified bythe combination of group ID and source ID. For each sessionthere is one distribution tree formed from the union of all short-est paths between the source and the group members. This formof multicasting usually results in lower end-to-end delay sincethe data is forwarded using the shortest source-receiver paths,but typically consumes a greater amount of network bandwidththan shared-tree approaches.A characteristic shared among these traditional multicast pro-tocols is that the multicast computation is distributed in thenetwork. Not only are the forwarding states constructed andmaintained by the network, group membership control (or lackthereof) is also distributed over the network, While this ap-proach improves scalability with respect to group size, it hascertain drawbacks. Firstly, distributed membership managementmay make aspects of security more difficult due to the lack ofadmission control. Without natural support for end-to-end sig-naling between sources and receivers, such approach needs torely on external mechanisms for security management. At thesame time, billing management becomes more complicated asthe information property owner, typically the source, has no con-trol and knowledge over how and to whom its property (data)is distributed. Secondly, distributed per group forwarding statemaintenance may result in large router resource usage. Thenumber of possible multicast groups formed among n membersgrows combinatorially. While there is no efficient way to aggre-gate multicast routing table entries, the linearly growing multi-cast routing table (with respect to the number of active multicastgroups) may quickly become too much of a storage burden forrouters. The issue is worsen by the number of routers involvedin forwarding since all routers along the multicast forwardingpaths need to participate in multicast routing state maintenance.To address this issue, recently there is an attempt of shiftingtowards stateless multicast routing for small groups. [1] and[2] lift multicasting out of routing layer so that multicasting nolonger requires router support. Multicast data is encapsulated inunicast envelop and transmitted between end receivers. There-fore no multicast routing state is installed on routers. [3] and [4]propose connectionless, small group multicast as a “stateless”approach to multicast routing. In these approaches, variable-length destination lists are placed in packet headers that are self-routed towards the destinations using the underlying unicast for-warding tables.We consider the problem of multicast routing in Mobile Adhoc Networks (MANETs). In MANETs all mobile nodes areequipped with wireless communication interfaces and can moveat will. Here we assume their communications occur usingomni-directional antennas over broadcast media. The combi-nation of node mobility and a wireless environment can result inMANET topologies subject to rapid and unpredictable changes.Because of these dynamics, and the fact that communication iscarried over a bandwidth-constrained broadcast media, the mul-ticast routing problem in MAITETs differs from that in fixed net-works.During recent years several multicast protocols have alsobeen designed specifically for MANETs (e.g. CAMP [5],ODMRP [6], MAODV [7] and LAM [8]). These protocolsall follow the traditional multicast approaches, i.e. distributedgroup membership management and distributed multicast rout-ing state maintenance. In addition to the security and resourceuse issues mentioned before, these approaches, especially whenapplied for use with small and sparsely distributed (potentiallynumerous) groups, may become even less efficient and more ex-pensive to function in MANETs due to bandwidth constraints,0-7803-7018-8/01/$10.00 (C) 2001 IEEE IEEE INFOCOM 2001network topology dynamics, and high channel access cost.II.PROTOCOL DESCRIPTIONA. Overview of Proposed ApproachAiming at the issues briefed in the previous section, we pro-pose the Differential Destination Multicast (DDM) protocol.This approach is motivated, in part, by the approach to unicastrouting of Dynamic Source Routing (DSR) [9] and derived, inpart, from the work of [3] and [4].In DDM, the sources control multicast group membershipto ease certain aspects of security administration. More im-portantly, and a departure from other proposed MANET mul-ticast protocols, DDM encodes the destinations (i.e. the mul-ticast group members to whom the
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