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

96 Multicasting in Ad Hoc Networks
Approach 3: Reliance on No Nodes Since all nodes are mobile, the multicast
routes would need maintenance as time goes by. If session states are stored in
packet headers, the protocol does not have to rely on any specific nodes to form
a forwarding path because we do not even need one! Session states carried in
packet header may be a list of node IDs, or a series of location coordinates. In
a protocol using this approach, intermediate nodes check the packet header and
decide where or who to forward the packet.
Examples for this approaches include location guided small group communication
[8] and DDM [9].
Approach 4: Reliance on Stabler Nodes This approach attempts to take
advantage of node mobility and network architecture. If nodes in a network
have different degrees of mobility, for example, fast and slow, we can rely on the
slow (thus stabler) nodes in order to build the multicast route. Note that “fast”
and “slow” could be in the relative sense. For example, a group of nodes may
move fast together toward a common direction, but the relative speed among
group member is “slow”.
An example of this approach is M-LANMAR [10], which is a multicast
protocol exploiting team-based mobility.
Approach 5: Reliance on an Overlay Layer Since all nodes are mobile,
adapting to network dynamics is an extra burden for multicasting protocols. By
inserting a middle layer in between, we can hide dynamics in lower layer and
let multicast protocols concentrate only on multicasting. In this approach, the
protocols normally build an overlay mesh on top of the physical network, and
the multicasting route is built on top of this overlay mesh. Without knowing
the underlying dynamics, it is easier for the multicast protocol to focus on
implementing multicast functionalities.
Protocols using this approach includes AMRoute [11] and PAST-DM [12],
both of which construct a virtual mesh structure on top of the physical network.
The virtual mesh relies on some unicast routing protocols to provide tunneling
route between any two nodes on mesh. Data forwarding tree is extracted on top
of the virtual mesh, and is unaware of underlying topology changes.
Several popular multicasting protocols are classified according to our discussion
and are summarized in Table 4.1.
4.3 Multicasting Protocols
In the previous sections, we reviewed the special properties of mobile ad
hoc networks, and examined how these properties affect the design and implementation
of network protocols. To deliver packets effectively to the multicast
group members, any multicasting protocol should address these properties. In