On Demand Multicast Routing Protocol (ODMR): Algorithm, Illustration, Advantages, Disadvantages

ON DEMAND MULTICAST ROUTING PROTOCOL (ODMR)

1.     On- Demand Multicast routing protocol is a mesh architecture protocol, i.e, it has multiple paths from the sender to the receivers and uses a forwarding group concept.

2.     It applies on-demand procedures to dynamically build route and maintain multicast group membership.

3.     By maintaining a mesh instead of a tree, the drawbacks of multicast trees in ad hoc networks like frequent tree reconfiguration and non-shortest path in a shared tree are avoided.

4.     In ODMRP, group membership and multicast routes are established by the source on demand when a multicast source has packets to send, but no route to the multicast group, it broadcasts a Join-Query control packets to the entire network.

5.     This control packet is periodically broadcast to refresh the membership information and  updates routes.

6.     When the Join-Query packet reaches a multicast receiver, it creates and broadcasts Join-   Reply to its neighbours. When it has been received by the node, it checks if the next hop own id.

7.     If it is does, the node realizes that it is on the path to the source and becomes the part of the forwarding group by setting the FG_FLAG (Forwarding Group flag).

8.     When receiving a multicast  data  packet, a node forwards it  only when it  is  not a duplicate, hence minimizing traffic overhead. Because the nodes maintain soft state, finding the optimal flooding interval is critical to ODMRP performance.

9.     ODMRP uses location and movement information to predict the duration of time that routes will remain valid. With the predicted time of route disconnection, a ―join data‖ packet is flooded when route breaks of ongoing data sessions are imminent.

10.      It reveals that ODMRP is better suited for ad hoc networks in terms of bandwidth utilization

Example

Consider the source node ‗S‘. It will flood the JOIN_DATA packets to all other nodes in the network. When a host node receives first JOIN_DATA packet it will rebroadcast it to form a reverse path with the previous host. Each host in the network acts as multicast receiver. It receives JOIN_DATA packet and replies in turn with a JOIN_TABLE packet to the upstream to establish reverse paths.

The process repeats until source host ‗S‘ is reached. The method of packet forwarding is for Figure (a) is shown in Figure (b). As the JOIN_TABLE is received a host has to build a multicast table so as to facilitate future packet forwarding. For example the host B receives the R1‘s JOIN_TABLE which is shown in the diagram.

It will add R1 as its next hop step. Assume B receives R2 ‘s JOIN_TABLE. Now it will add R2 as its next hop step. A simple final multicast table for each host is shown in the Figure (c) in propagation of data packets.

Advantages

1.  Low channel and storage overhead

3.     Usage of up-to-date shortest routes

4.     Robustness to host mobility

5.     Maintenance and exploitation of multiple redundant paths

6.     Exploitation of the broadcast nature of the wireless environment

7.     Unicast routing capability

Disadvantages

0       The main disadvantage of ODMRP is its excessive overhead, because broadcasting of the reply packets to many nodes.

1       It has a complex topology.