Multicast Routing Protocols

1. introduction:
the main problem in mobile ad hoc networks is limited bandwidth and frequent change in the topology. although there are lots of routing protocols that can be used for unicast and multicast communication within manets, it observes that any one protocol cannot fit in all the different scenarios, different topologies, and traffic patterns of mobile ad hoc networks applications. for example, proactive routing protocols are very useful for small-scale manets with high mobility, while reactive routing protocols are very useful for a large-scale manets with moderate or fewer topology changes.
another set of routing protocols, known as hybrid routing protocols, will try to balance between the two—for example, proactive for neighborhoods and reactive for far away. apart from this multicast is another category of routing protocol in manets that efficiently supports the group communication with the high throughput.
the use of multicasting within manets has many benefits. it can decrease the cost of wireless communication and increase the efficiency and throughput of the wireless link between two nodes whenever multiple copies of the same messages are sent by the inherent broadcasting properties of wireless transmission. in place of sending the same data through multiple unicasts, multicasting decreases channel capacity consumption, sender nodes and router processing, energy utilization, and data delivery delay, which are important for manets.
multicasting consists of concurrently sending the same message from one source to multiple destinations it can be used in video conferencing, distance education, cooperative work, video on demand, replicated database updating and querying, etc.

2. issues in design of multicast routing protocols
1. scalability: a multicast protocol is scalable with constraints.
2. address configuration: different multicast groups have different addresses reuse of a multicast group’s address does not happen. node movement causes synchronization of multicast addresses—a difficult task.
3. multicast service support: multicast participants should be able to join or leave the group on their own.
4. security: the intruder should be stopped from joining an ongoing multicast session or receiving packets from other sessions.
5. traffic control: the traffic should be efficiently distributed from central node to other members of the manet.
6. qos (quality of service): multicast routing protocols should be designed in a way that satisfies a set of performance measures in terms of end-to-end delay, jitter, and available bandwidth.
7. power control: routing protocols must use less power as much as possible.
8. multiple accesses: most of the multicast protocols are designed for single source multicasting. in multiple source multicasting, each multicast source will induce its own overhead for multicast routing and waste network resources.

3. classification of multicast routing protocols
depending on the topology used for communication, the multicast protocols can be classified as tree-based and mesh-based protocols.
3.1 . tree-based protocols: only one route exists between a source and a destination and hence these protocols are efficient in terms of the number of link transmissions. there are two major categories of tree-based protocols: source tree based (the tree is rooted at the source) and shared tree based (the tree is rooted at a core node and all communication from the source nodes to the receiver nodes is routed through this core node). the shared tree-based multicast protocols are more scalable with respect to the number of sources these protocols suffer under a single point of failure, the core node. on the other hand, source tree-based protocols are more efficient in terms of traffic distribution.
3.2 . mesh-based multicast routing: multiple routes exist between the source node and each of the receivers of the multicast group. a receiver node receives several copies of the data packets: one copy through each of the multiple paths. mesh-based multicast routing protocols provide robustness in the presence of node mobility— however, at the expense of a larger number of link transmissions, leading to inefficient bandwidth usage. the mesh-based protocols are classified into source-initiated and receiver-initiated protocols depending on the entity (source or receiver) that initiates mesh formation.

4. qos routing:
as the real-world applications need qos support, qos routing is helpful in multicasting. since network topology changes very frequently in manets, qos routing in manets is difficult. another challenge in qos for real-time applications is associated with design of the mac protocol. because the topology changes dynamically, it is difficult to provide reservation, central controller, etc. the requirement for qos in manets is to find a route through the network that is capable of supporting a requested level of qos. when existing network topology changes, new routes can support existing qos and respond to the changes in available resources.
multicast routing in qos a node that has data to send starts a session by broadcasting a session initiation as a quality of service route request (qrreq) with ttl greater than zero. the intermediate node rebroadcasts the qrreq, if it has bandwidth, until ttl is equal to zero. the destination node receives qrreq and sends a qos route reply (qrrep) to the source. forward group and member management. when an intermediate node receives qrreq from a source, it stores the source id and sequence number in the cache to detect any duplicate message. it rebroadcasts the qrreq and the routing table is updatingd.

5. energy-efficient multicast routing protocols
the network lifetime is a key design factor of manets. to prolong the lifetime of manets, one is forced to attain the trade-off of minimizing the energy consumption and load balancing. in manets, energy waste resulting from retransmission due to high frame error rate (fer) of a wireless channel is significant.
5.1. metrics for energy-efficient multicast
minimum energy constraint: in routing protocols, we need to minimize energy consumed by reducing the energy consumption through all intermediate nodes through which the packet passes. for example, from n to nk, the packets are passing through intermediate nodes, where n is the source and n1, n2, n3…are intermediate nodes. then, the energy consumed for all transmissions for packet j is ej = (ni), where i is from 1 to k the goal is to minimize the ej.
maximize time to network partition: as soon as one node in the path dies, the network is said to be partitioned, and the power consumed for each of these networks will be more. therefore, the aim is to maximize the partition time.
minimize the variance in node power levels: the idea is to treat all nodes as important nodes and balance the node energy level equally.
minimize cost per packet: choose the path into nodes as intermediate nodes, which have enough energy to reduce the cost.
energy-efficient multicast routing protocols have the following unique characteristics:
1. energy in wireless nodes is crucial because of the limited capacity of the battery.
2. since nodes can move in a random way, there is frequent path failure.
3. wireless channels have limited and more variable bandwidth compared to wired networks.