Multicast Routing Protocols in Mobile Ad Hoc Networks: A Comparative Survey and Taxonomy <s> Application Layer Multicast (ALM) <s> Many emerging applications (e.g., teleconference, real-time information services, pay per view, distributed interactive simulation, and collaborative work) are based upon a group communications model, i.e., they require packet delivery from one or more authorized senders to a very large number of authorized receivers. As a result, securing group communications (i.e., providing confidentiality, integrity, and authenticity of messages delivered between group members) will become a critical networking issue.In this paper, we present a novel solution to the scalability problem of group/multicast key management. We formalize the notion of a secure group as a triple (U,K,R) where U denotes a set of users, K a set of keys held by the users, and R a user-key relation. We then introduce key graphs to specify secure groups. For a special class of key graphs, we present three strategies for securely distributing rekey messages after a join/leave, and specify protocols for joining and leaving a secure group. The rekeying strategies and join/leave protocols are implemented in a prototype group key server we have built. We present measurement results from experiments and discuss performance comparisons. We show that our group key management service, using any of the three rekeying strategies, is scalable to large groups with frequent joins and leaves. In particular, the average measured processing time per join/leave increases linearly with the logarithm of group size. <s> BIB001 </s> Multicast Routing Protocols in Mobile Ad Hoc Networks: A Comparative Survey and Taxonomy <s> Application Layer Multicast (ALM) <s> Group communication has become increasingly important in mobile ad hoc networks (MANET). Current multicast routing protocols in MANET have a large overhead due to the dynamic network topology. To overcome this problem, there is a recent shift towards stateless multicast in small groups. We introduce a small group multicast scheme, based on packet encapsulation, which uses a novel packet distribution tree construction algorithms for efficient data delivery. The tree is constructed with the goal of minimizing the overall bandwidth cost of the tree. Two construction algorithms, for a location-guided k-ary (LGK) tree and a location-guided Steiner (LGS) tree, utilize the geometric locations of the destination nodes as heuristics to compute the trees. They are accompanied by a hybrid location update mechanism to disseminate location information among a group of nodes. Our simulation results show that LGS tree has lower bandwidth cost than LGK tree when the location information of the nodes is up-to-date, and its cost is similar to that of an optimal Steiner multicast tree. When location information of the nodes is out-dated, LGK tree outperforms LGS tree due to its lower computational complexity. <s> BIB002 </s> Multicast Routing Protocols in Mobile Ad Hoc Networks: A Comparative Survey and Taxonomy <s> Application Layer Multicast (ALM) <s> Overlay multicast protocol builds a virtual mesh spanning all member nodes of a multicast group. It employs standard unicast routing and forwarding to fulfill multicast functionality. The advantages of this approach are robustness and low overhead. However, efficiency is an issue since the generated multicast trees are normally not optimized in terms of total link cost and data delivery delay. In this paper, we propose an efficient overlay multicast protocol to tackle this problem in MANET environment. The virtual topology gradually adapts to the changes in underlying network topology in a fully distributed manner. A novel source-based Steiner tree algorithm is proposed for constructing the multicast tree. The multicast tree is progressively adjusted according to the latest local topology information. Simulations are conducted to evaluate the tree quality. The results show that our approach solves the efficiency problem effectively. <s> BIB003 </s> Multicast Routing Protocols in Mobile Ad Hoc Networks: A Comparative Survey and Taxonomy <s> Application Layer Multicast (ALM) <s> This paper presents the on-demand overlay multicast protocol (ODOMP), a novel approach for multicast data distribution in mobile ad hoc networks. ODOMP is a reactive protocol which creates an overlay among the group members on-demand. The created overlay is a source rooted tree which connects the group members via IP-in-IP tunnels. Routing is done by an arbitrary underlying unicast routing protocol. A novel creation mechanism allows ODOMP to create an efficient overlay with low communication overhead. The performance of ODOMP is compared with the performance of the well known multicast routing protocol ODMRP by means of simulations. <s> BIB004 </s> Multicast Routing Protocols in Mobile Ad Hoc Networks: A Comparative Survey and Taxonomy <s> Application Layer Multicast (ALM) <s> Two nested half cylinders, having a concentric axis and closed ends, are pivoted together on aligned pivot pins mounted in the closed ends and on the concentric axis. The nested half cylinders are actuated to pivot 90 degrees oppositely around the concentric axis and aligned pivot pins to close non-compressively around free flowing granular material in a closed cylindrical hoist bucket, thereby saving the additional weight and strength of material and the actuating energy therefor required for compressive loading of hoist buckets having off center or non-concentric axis of pivot. Additionally, the non-compressive loading does not move the granular material nor overflow the closed cylindrical hoist bucket to pollute the environment with dust therefrom. <s> BIB005 </s> Multicast Routing Protocols in Mobile Ad Hoc Networks: A Comparative Survey and Taxonomy <s> Application Layer Multicast (ALM) <s> Application layer multicasting has emerged as an appealing alternative to network layer multicasting in wireline networks. Here, we examine the suitability of application layer multicast in ad hoc networks. To this effect, we propose a flexible receiver-driven overlay multicast protocol which we call Application Layer Multicast Algorithm (ALMA). ALMA constructs an overlay multicast tree in a dynamic, decentralized and incremental way. First, ALMA is receiver-driven: the member nodes find their connections according to their needs. Second, it is flexible, and thus, it can satisfy the performance goals and the needs of a wide range of applications. Third, it is highly adaptive: it reconfigures the tree in response to mobility or congestion. In addition, our protocol has the advantages of an application layer protocol: (a) simplicity of deployment, (b) independence from lower layer protocols, and (c) capability of exploiting features such as reliability and security that may be provided by the lower layers. Through extensive simulations, we show that ALMA performs favorably against the currently best application layer and network layer protocols. In more detail, we find that ALMA performs significantly better than ODMRP, a network layer, for small group sizes. We conclude that the application layer approach and ALMA seem very promising for ad hoc multicasting. <s> BIB006
. ALM, or overlay multicast, has received little attention in the MANET domain  BIB006 BIB003 BIB004 BIB002 . Despite the fact that network layer multicast is known as the most efficient way to support multicast (since the majority of the proposed multicast protocols are implemented at the network layer), the overlay multicast handles several features, such as the following: (1) it is simple to deploy, because it does not require changes at the network layer; (2) intermediate (forwarder) nodes do not have to maintain their pergroup state for each multicast group (maintaining that state has always been a problem in multicasting, even on the Internet); (3) the creation of a virtual (logical) topology hides routing complications, such as link failure instances, which are left to be taken care of at the network layer; and finally (4) overlay multicasting can deploy the capabilities of lower-layer protocols in providing flow control, congestion control, security, or reliability according to the requirements of the application. For example, if the application needs reliability, it can choose, at run time, to use TCP between group members, and UDP, otherwise. Moreover, secure group communications are reduced to secure unicast communications, which makes it possible to avoid the use of complex protocols for the group key BIB006 BIB001 . The main problems with the overlay multicast method are routing efficiency and robustness. The robustness problem we refer to is that the distribution of the multicast tree is dependent on the end nodes. The routing efficiency we refer to is that the use of overlay multicasting can result in the transmission of multiple copies of multicast data packets over each physical link (multiple unicasts), which occurs because nonmulticast group members cannot make copies of multicast data packets. This effect clearly appears when the network load is high and/or if there are a large number of multicast group members. In addition, since all multicast data packets are relayed from one group member to another in the form of a unicast packet, a large number of packet collisions and low resource utilization may result, especially where group member location density is high. Furthermore, the communicating member nodes are not aware of the increases in the physical hop count from the source node. As a result, in the case of mobility, using virtual links may lead to suboptimal paths (in terms of the number of hops). Reconfiguring the virtual connections is possible, but this introduces additional overhead. Overlay multicasting can improve routing efficiency by exploiting the broadcast nature of ad hoc networks. For instance, a one broadcast packet can be received simultaneously by two neighboring group members BIB006 BIB005 .