Ad Hoc Networks : Technologies and Protocols - University of ...

Multicasting Protocols 107
Figure 4.8. Concept of virtual topology for overlay multicast.
bandwidth homogeneity among the nodes in a MANET topology. Whereas in
the Internet topology, there is a significant difference in available bandwidth
at the end hosts and the routers. Forwarding and duplicating packets at the
bandwidth limited endhosts are inherently less efficient than at the routers.
Thus, there is a major efficiency problem in overlay multicasting in the Internet,
compared to the network layer multicast. However, this problem does not exist
for overlay multicasting in MANET.
4.3.5.1 Ad-hoc Multicast Routing Protocol(AMRoute). AMRoute
[11] creates a per group multicast distribution tree using unicast tunnels connecting
group members. Each group in the network has at least one logical core that
is responsible for discovering new group members and creating/maintaining the
multicast tree for data distribution. There are two main phases in the protocol
operations: mesh creation and tree creation. Figure 4.8 illustrates the AMRoute
overlay topology and the logical core.
It is much simpler to maintain a mesh than a tree at the member mutual
discovery phase. Initially, each group member declares itself as a core for its
own group of size one. Each core periodically floods JOIN-REQ messages
with increasing TTL to discover other disjoint mesh segments for the group.
When a member node receives a JOIN-REQ from a core of a different mesh
segment for the same group, the node responds back with a JOIN-ACK. A new
bi-directional tunnel is established between the core and the responding node
of the other mesh segment. Due to mesh segment mergers, a mesh segment