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

104 Multicasting in Ad Hoc Networks
order to prevent loop formations during mgraph reconstructions, each member
also maintains a notion of local ordering among each other. Thus, each mgraph
member is assigned a value named as JoinID, which has an initial value as
infinity, and is updated during the course of mgraph construction.
The new joining core broadcasts a join request JOIN(joinID). The joinID
for a fresh joining core is set as infinity. The join request is relayed further by
the non-member nodes in accordance to the core broadcast procedure. When
a group member receives a join request, it sends back a JOIN-ACK(joinID) if
its joinID is less than the joinID field of the arriving join request. When an
intermediate core on the reverse path receives a JOIN-ACK, it relays the packet
only when its number of neighbors in the mgraph does not exceed the robustness
factor, R. Before relaying the JOIN-ACK, the intermediate core updates its own
joinID if the relayed JOIN-ACK has lower joinID value, otherwise, it updates
the joinID field of the JOIN-ACK packet. Figure 4.7 shows an example of the
join procedure. The left-hand side shows how the join request is relayed up to
the nodes in the core graph. The right-hand side shows the reverse paths for
JOIN-ACKs and the update of joinIDs at each relaying nodes.
Figure 4.7. MCEDAR join procedure.
The forwarding protocol of MCEDAR follows the core broadcast procedure.
When a data packet arrives at an mgraph member, the member attempts to
forward the packet only to those nearby cores on the same mgraph. MCEDAR
introduces a core broadcast procedure, which is used for a core node to flood
a message to all the other core nodes in the network. The procedure is more
efficient than the normal hop-by-hop flooding. The core broadcast procedure
implicitly creates a source based tree that represents the fastest delivery structure
for each source of the group.