Multicast over Vehicle Ad Hoc Networks

1 Multicast over Vehicle Ad Hoc Networks Alberto Gordillo Muñoz Abstract Vehicular networks may improve the safety and efficiency of road travel but there are many challenges that still need to be overcome. Group communication is one of them; it has many potential applications for Vehicle Ad Hoc Network (VANET) however providing an efficient and scalable VANET multicast routing has multiple issues due to its highly dynamic topology. There exist many different protocols that try to solve these issues using different approaches. In this paper, these approaches are classified and compared remarking its advantages and drawbacks in order to provide the best solution for multicast over VANET. I. INTRODUCTION Transportation systems are a fundamental piece in modern societies. Most people use a car to go to work, travel or at their free time. But there are more vehicles on the roads every year that increase traffic delays and traffic jams and result in lost productivity and wasted energy. Car accidents are another important problem that causes a high number of deaths every year. Although there are a great number of initiatives in the public and private sectors in order to solve these problems still more are required. Vehicular networks may be the key of the solution in a near future. The idea is that vehicles and roadside equipment should collect and distribute information in order to improve safety, early response against complications, get better traffic efficiency and even improve driving experience to driver and passengers. Although nowadays all of these are still in a very early phase is expected that in a near future vehicular networks become a reality. In this scenario, every vehicle acts as a mobile ad hoc. Thus, vehicles can create or join to an ad hoc network with other vehicles which is called Vehicle Ad Hoc Network (VANET). As an ad hoc network, VANET has to be selfconfigured with decentralized medium access control and variable network topology but also has to face the high speed of the mobile s. However, vehicles do not move randomly, they are restricted to follow predefined ways which makes its movement relatively organized. VANET can be used to exchange vehicle sensors information but also to transmit information between non adjacent vehicles in order to improve safety or to early inform of road congestion. Multicasting is the transmission of data to an address that is shared by multiple s. Due to the characteristics of VANETs, classic wired multicasting protocols do not offer efficient service. There exists a large amount of protocols proposed to take VANET issues into consideration. This article provides a classification and an analysis of the principal multicast protocols for VANETs and the open issues that need further investigation. The rest of the paper is organized as follows. In Section II a classification of the different approaches for providing multicast for VANET is given and some routing protocols are described as example. Section III presents a comparison of the different approaches and remarks which are the most convenient for VANETs. Finally, Section IV is devoted to conclusions and open issues. II. VANET MULTICAST PROTOCOLS Conventional multicast protocols were designed for wired networks which have a stable network topology. VANETs are very different from these networks and due to that these protocols do not offer good performance for vehicular environments. VANET multicast protocols need to adapt to the characteristics of this kind of networks. They need to take into consideration high mobility, the high speed of this movement and frequent topology changes and due to that constant delivery path updates. They also need to keep as little state information as possible owing to short route duration. However, VANETs also have benefits for multicast, due to their wireless nature when a sends a message it is broadcasted to all s in range and VANET s also do not need to save power consumption, vehicles provide a powerful power supply of long duration, and due to that they can also perform complex computational costs. Several multicast protocols have been proposed for Mobile Ad Hoc Network (MANET), and the great majority of them are also valid for VANETs. Based on the classification of [1] where protocols are grouped based on route creation form we present the following classification: Flooding Tree-based Mesh-based Overlay-based Backbone-based Stateless A. Flooding Using flooding for multicasting is the simplest way to provide multicast in VANETs. Each that receives a

1) RREQ ) RREP 3) MACT Member Forwarding Fig. 1. MAODV operation Non-member message broadcasts it to its neighbors and its neighbors broadcast it again to their neighbors to grant that every possible destination receives a message. Although it is a robust solution for reliable multicast in scenarios as VANETs where s are highly mobile, it is complete inefficient, it produces a high number of duplicates and may disrupt other communications due to the collisions produced. case it selects the shortest one (using hop count as metric) and sends an unicast Activation (MACT) message along this path. Thanks to this message exchange the becomes member of the multicast group and all the s along the selected path from this to the that receives the MACT become forwarding s. The tree links status is monitored by the multicast group leader, the first member of the group, by sending Group Hello messages along the tree. If a link breaks, the forwarding s involved are pruned and a new multicast tree is formed by the s disconnected from the tree of the leader. They select a new leader and if they later receive a Group Hello for a different leader a reconnection to the main tree is initiated. Other tree-based protocols are Ad Hoc Multicast Routing Protocol Utilizing Increasing ID Numbers (AMRIS) [3] and Distance Vector Multicast Routing Protocol (DVMRP) [4]. C. Mesh-based Mesh-based multicast protocols try to solve the robustness problem of tree-based protocols. They provide redundancy by using alternative paths in order to mitigate effects of frequent B. Tree-based Traditional multicast protocols use tree topologies for message transmission since they provide an efficient distribution topology and because robustness is not a critical issue. There are two types of tree protocols, source-based and shared-tree-based. The former one builds a tree for each multicast source, usually the shortest-path tree. Although it is more efficient it is also less scalable because it produces higher routing overhead. The latter one builds only one tree for each group, all sources use this tree to transmit. It is less efficient but is more scalable due to the lower overhead and routing state needed. Tree-based protocols work poorly on VANETs because they need to rebuild the distribution tree frequently due to high mobility which produces continuous service disruptions. Nevertheless there are some tree-based protocols which try to provide multicast for mobile networks. Multicast Ad Hoc On-Demand Distance Vector (MAODV) protocol [] is an example of this kind of multicast protocols. It uses broadcast to discover on-demand new routes. As seen in Fig. 1 when a wants to join a multicast group or has data to send but do not has a route to the multicast tree it broadcast a Route Request (RREQ) message. The rest of the s will rebroadcast the message to its neighbors until it reaches a that is part of the multicast group tree. These s will save at their routing tables the address of the that has sent them the message in order to establish a reverse route to the source of the RREQ. When a multicast group member receives the message it sends back a Request Response (RREP) via unicast. The message originator may receive more than one RREP, in this 1) JOIN QUERY ) JOIN REPLY Fig.. ODMRP operation topology changes. On-Demand Multicast Routing Protocol (ODMRP) [5] is an example of this kind of protocols. ODMRP uses a forwarding group concept; the multicast packets are forwarded via scoped flooding by a subset of s. As seen in Fig., when a wants to join a multicast group or has data to send, it periodically broadcast to the network a JOIN QUERY message. This message also refreshes routing and group membership information because ODMRP uses soft state to keep its multicast routing information. When a which is not member of any multicast group receives a JOIN QUERY it checks if it is a duplicate, if not it store the upstream s ID and then rebroadcast the message. When the message reaches a multicast group member, the receiver creates an entry in its MemberTable, or updates the correspondent entry if already exists, and broadcast it to its neighbors in a JOIN REPLY message. Every that receives the message and reads its ID on the table knows that it is on the path so it establishes itself as a forwarding (sets the forwarding group flag) and propagates a new JOIN REPLY message. This process creates a mesh that connects the source with every multicast group member.

3 Movement When a wants to leave the multicast group it only needs to stop sending JOIN QUERYs because as previously said ODMRP uses soft-state approach and if a route is not refreshed it is deleted. Other protocol that uses mesh-based approach is Forwarding Group Multicast Protocol (FGMP) [6]. D. Overlay-based 1) ) 3) VANET link Overlay link Fig. 3. Overlay inefficient paths. The movement of a may produce redundant paths like in 3) where the upper link is crossed twice by the multicast traffic. Tree-based and mesh-based protocols performance decreases when the number of sources increases due to the higher control overhead, they need to maintain updated the routing structure, and it results on higher collisions. In order to reduce overhead, overlay-based protocols keeps state information only in multicast group members, thanks to that the control overhead while the number of sources increases is more scalable. In overlay-based protocols, a virtual network is built over the VANET topology only among multicast group members and the links among the virtual network s are unicast tunnels in the VANET. The virtual topology remains static even if the underlying topology changes. The cost of this kind of solutions is that may produce inefficient paths and higher overhead on packet delivery as VANET link Overlay link Virtual tree Fig. 4. AMRoute virtual topology. seen in Fig. 3. Ad Hoc Multicast Routing (AMRoute) is one of the protocols that use this approach. It first creates, based on the physical links, a mesh among the multicast group members. Then a is designated as logical core and is responsible for discovering new group members and for creating and maintaining a multicast tree that is used for data distribution. The tree topology remains static even though the underlying physical topology changes thanks to the mesh (see Fig. 4). To create a mesh, every multicast group member sends a JOIN_REQ. When receives a JOIN_REQ for the same multicast group a sends a JOIN_ACK and establishes a tunnel with the other. With these tunnels the overlay network is created and the s execute a deterministic algorithm to designate a core of the mesh. Then the core sends TREE_CREATE messages periodically along the tunnels for creating the multicast tree. When a mesh receives the message it forwards it to all the links except the incoming one. If a receives a duplicated TREE_CREATE message it sends back a TREE_NAK and discard the message. To leave a multicast group a has to send a JOIN_NAK to its mesh neighbors. PAST-DM [7] also uses an overlay-based approach. E. Backbone-based Another approach to reduce the control overhead of maintaining the multicast routing infrastructure is the backbone-based approach. In this approach, a virtual backbone with a simple and stable topology is built and the state information of the protocol is constrained to the s that form the backbone. The selection of the core s is done in a distributed fashion among all the s in the network. Within the backbone, multicast routing may be flooding or mesh-based. Then, to the non-core multicast s a treebased distribution is applied. Backbone-based protocols combine the advantages of tree and mesh approaches however they provide a limited horizontal scalability since traffic of all multicast groups must pass through the backbone. In Multicast Core Extraction Distributed Ad Hoc Routing (MCEDAR) [8] the core s are selected applying a distributed minimum dominating set (MDS) algorithm. With this algorithm the non-core s are just one hop away from a core. Then, MCEDAR builds a mesh within the backbone and a subgraph of the backbone called mgraph for each multicast group which is a source based tree for data distribution. The redundancy of the mesh avoids the recomputation of a mgraph if a link fails. When a non-core wants to join to a multicast group it request its dominating core (the core that connects it to the backbone) to join the group. Then this core broadcasts a JOIN request with the joinid of the to the backbone. If a core that is not member of the multicast group receives a JOIN it rebroadcasts the message, if it is member it replies with a JOIN_ACK with its joinid and the distribution path is established. Source 1 3,4,5 X,Y 3,4,5 3 4,5 3 Packet destination X and Y Fig. 5. DDM operation. 5 4 5 4

4 F. Stateless Stateless multicast protocols reduce the control overhead for non-members of a multicast group by explicitly listing destinations in every packet. Due to that they do not need to keep any multicast routing state at the forwarding s, when a receives a multicast message it reads the destinations from the header and use the underlying unicast protocol to select the next hop. Additionally, thanks to that the source controls the multicast group membership. In Differential Destination Multicast (DDM) [9], when a source sends data it includes a list of destinations in the header as seen in Fig. 5. When an intermediate receives the packet it queries the underlying unicast protocol to select the next hop and forwards the packet. It do not need to maintain multicast session state at the forwarding s, thanks to this DDM is more scalable. However, sending a list of destinations in every packet header is inefficient for a high number of users, due to that, DDM is only suitable for a small group of users. Nevertheless DDM also supports a soft state approach. Using it, forwarding s maintains a forwarding set (FS), a list with the destinations of previous messages, and a direction set (DS), a list of next hops of previous messages, and thanks to that the source do not need to send again a list of destination in next messages. To join to a multicast group in DDM, a needs to send a JOIN message to the source, the source will response with an ACK and periodically sends a POLL flag in a data packet to force multicast members to refresh their membership, i.e., they need to send another JOIN message. To leave the group, a may send a LEAVE message or simply do not respond to a POLL request. [10] also uses stateless approach and [11] introduces a mixed approach, it divides a multicast group into two layers, for the upper layer it uses an overlay multicast and for the lower layer (for s mall groups of s) it uses stateless multicast, by this approach it avoids the main drawback of the stateless approach, its inefficiency for a large group of users. TABLE I MULTICAST APPROACHES COMPARISON Approaches Efficiency Robustness Scalability State Overhead Flooding 1 5 1 1 Tree 5 1 3 4 Mesh 3 4 3 5 Overlay 3 3 4 3 Backbone 3 3 4 3 Stateless 4 3 1 Values: from 1 (lowest) to 5 (highest) III. COMPARISON OF VANET MULTICAST APPROACHES As seen in the previous section, there are six different approaches to provide multicast service to VANETs. Each approach focuses on solving one issue; flooding ensures robustness but lacks of a poor efficiency. Tree-based approach offers efficiency but fails on robustness which is an important issue for VANETs due to the frequent topology changes. Mesh improves robustness at the expense of lower efficiency and higher routing state overhead. Other approaches as overlay and backbone-based try to balance efficiency and robustness by combining the advantages of tree and mesh. And at last, the stateless approach, although lacks of scalability problems, minimizes the overhead by explicitly list destinations in each packet. Table I compares these approaches. In the previous section the requirements of a nice protocol for VANET multicasting are showed. Both flooding and treebased may be discarded because they do not offer good performance for VANETs, the former one since it is complete inefficient, produces a high number of collisions and disturbs other services, the latter one because of its poor robustness against the mobility of VANET s. Nevertheless, choosing among the rest is not as easy and may differ in function of the requirements of the applications, the nature of the multicast traffic and the protocol implementation (e.g. paths may be calculated once and be periodically refreshed or may be created on demand, the implementation may be loop free or not, the number of users may be large or low ). For that reason a further study need to be done analyzing the performance and effectiveness of the protocols. IV. CONCLUSIONS AND OPEN ISSUES Vehicular networks have introduced many new challenges for communications and VANET multicast routing is one of them because conventional multicast protocols were designed for wired networks and do not offer good performance for vehicular environments. Many protocols have been proposed to provide better solutions for multicast over VANETs but in order to choose one for a certain service a further study need to be done because depending on the scenario some protocols may provide considerably better performance than others. As previously said vehicles do not move freely, they can only move along streets and according to traffic models. The previous approaches do not take this into account but using this information may improve the performance of the protocols. Additionally geographic routing could provide additional information that allows opportunistic routing (identifying the shortest path to a destination before sending the packet) which may improve the robustness and the reliability of the multicast networks and reduce the end to end delay. The knowledge of the position and direction of the s also allows building more stable multicast topologies by using the s that moves at the same speed from the same direction and additionally allows recalculating the multicast topology before a link breaks. Furthermore, there are special VANET s, the road side units which are static s that may provide access to the Internet to the VANET. These s may have access to a more complete view of the network topology and to additional information that may also improve not only routing but also the construction of the multicast network topology.

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