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

Multicasting Protocols 105
A member of the mgraph issues a leave message only when the following
two conditions hold. (1) It does not have any local members in its domain, and
(2) its child list is empty, A leaving member needs to issue the message to all
its parents. 1 A parent that receives the leave message from one of its children
deletes the corresponding child’s ID from its child list.
In some cases, an mgraph member can issue a reconstruction request to the
other members. In order to prevent loop formations during this process, the
JoinID field at each node is used in the following manner. A member issues a
reconstruction request only when it looses connectivity with all its neighboring
mgraph members who has smaller join times, i.e., JoinID values. A member
responds to a reconstruction request only if its join time is less than the join
time of the originator of the request.
4.3.4 Stateless Multicasting
In the stateless multicast protocols, the forwarding states are included in
packet header, and no protocol state is maintained at any nodes except for
the source node. From the information included in the packet headers, any
intermediate node knows how to forward or duplicate the packet. Although
packing routing information together with data traffic will enlarge data packet
size, it reduces the total number of control packets generated by the protocol.
Besides, when the group is idle, there is no control overhead.
4.3.4.1 Differential Destination Multicast (DDM). DDM [9] is intended
for small group multicast. It not only adopts the stateless approach, but
may also operate in a soft state mode. In this mode, intermediate nodes cache
the forwarding states read from the packet header. The protocol no longer
needs to list all destinations in every data packet header. When changes occur,
an upstream node only needs to inform its downstream neighbors regarding the
difference in destination forwarding since the last packet.
In DDM, the multicast data packets contain a payload and a DDM header,
which is composed of a list of DDM blocks. Each DDM block is constructed
for a particular downstream neighbor. Each DDM block contains the intended
receiver, the DDM block type, the block sequence number and some other fields
depending on the type. There are three types of the DDM blocks: Empty (E)
blocks, Refresh (R) blocks and Difference (D) blocks. Except for the E block,
both R block and D block have a destination list L. When used in broadcast media
networks, DDM blocks for different downstream neighbors may be aggregated
together into the header of one data packet. When the intended neighbors
1 Since a mgraph is a mesh structure, each member can have multiple parents. The hierarchy is derived from
the JoinID field of each node.