Design and investigation of scalable multicast recursive protocols for wired and wireless ad hoc networks

3.2.5.2 Receivers leave and tree maintenance

Because DRs run IGMP protocol, they automatically discover if any of the receivers
connected or if all have gone. When DR discovers that there are no receivers connected
directly to it, it creates an alive message and sends it towards the previous branching router.
The branching router upon receiving this message eliminates this DR from its TRM table and
creates a leave message and sends it directly to the source. When receiving the leave
message, the source eliminates the corresponding state and sends a new branch message to
all the lists (Li) to reconstruct the tree.

Moreover, if one of the branching routers or DRs breaks down, the upstream branching
router will not receive an alive message from this broken node. Consequently, the upstream
branching router will eliminate the state and sends a leave message to the source. The source
at the same time will eliminate the corresponding state and re-build the tree by sending a new
branch message. The source upon receiving the leave messages will not start directly the tree
reconstruction, rather it will wait for a period of time and then start reconstruction in order to
gather as much as possible of the leave messages and rebuild the tree again.

SEM has the advantage of reducing the state information in the multicast tree nodes. The
drawback of this protocol in the case of node leaving is that it needs to reconstruct the whole
multicast tree which will cause an extra routing overhead and will cause loss of time during
the reconstruction.

3.3 Explicit Multicast in MANETs

In the previous section, a number of explicit multicast protocols for fixed networks were
discussed. These protocols cannot be used for MANETs networks due to the nature of these
kind of networks. Node mobility and wireless environment in combination can result in rapid
and dynamic topology changes in MANETs.

Recently, explicit multicast ad hoc protocols were proposed to achieve the idea of stateless
multicast for MANETs. In the next subsections, the most recently proposed and most popular
protocols will be discussed.

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