3.3.1 Differential Destination Multicast (DDM) Protocol
DDM [20] is a source control based multicast routing protocol for MANETs. The source
controls the multicast group to achieve the security and admission aspects. DDM
encapsulates the destinations addresses into the data packet. At DDM when an intermediate
node receives data packet including the DDM header, it just looks in the DDM header of the
data packet to decide where to forward this data packet. When changes occur in the
destination list or the underlining unicast routing protocol, an upstream node needs only to
inform its downstream nodes (next hops) about the difference in the destination list for
forwarding since the last successfully transmitted packet, hence the “Differential Destination
Multicast” name comes from.
3.3.1.1 Membership Management
As mentioned above, this protocol is a source based multicast protocol. The source is acting
as an admission controller. When a receiver (Joined node) needs to join the multicast group,
it sends a JOIN message toward the source, this message includes the group ID to join in
addition to the source ID and the receiver (Joining) node ID which already exist in the
unicast IP header as source and destination fields.
When the JOIN message is received and accepted by the source, the source will add this
receiver address to its Multicast List (ML). Then the source sends a unicast acknowledgment
[48] message to the receiver.
The newly joining node, after sending the JOIN message, must wait for a period of time
JOIN-WAITING-PERIOD. If it has not received any ACK or NACK (for secured session)
by the end of this period it will retransmit the JOIN message again and reset the JOIN-
WATING-PERIOD. The value of this period is reduced using exponential backed off
algorithm after each every retransmitting until its value comes to zero. The joining node will
stop sending JOIN message either if the value JOIN-WATING-PERIOD comes to zero or the
joining receives an ACK message.
45
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