This protocol improves the scalability issue in multicast ad hoc networks. In [7] some
drawbacks are mentioned for this protocol:
• All nodes on the path from root to XF should know the total number of receivers
in its downstream network.
• Any node can play the role of XF without doing the real thing.
• This protocol did not effectively take into consideration the node mobility.
3.3.3 An Explicit Multicast protocol for MANETs
An Explicit Multicast protocol for MANETs (EM2NET) [7] has been proposed to support
the scalability issue in MANETs. This protocol constructs a multicast tree consisting of set of
Intercepting Nodes (IN), defined as a node that is part of the multicast group and serves two
disjoint next hops in the group.
3.3.3.1 Messages and Data Structure
Each IN node should maintain a Multicast forwarding IN Table (MFIT). Each entry in this
table consists of a unique pair of (S,G), where S is the session source and G is the group
address, previous IN node address and a list of next INs.
EM2NET implements two types of messages: - control messages and data messages. Control
messages are used to build the multicast tree for the group members and to keep this tree
connected in case of node mobility. When a node need to join multicast group it sends JOIN
message towards the source, which upon receiving this message reply by a BRANCH _ACK
message to the JOIN message sender. During the BRANCH_ACK message the IN discovery
will be started. If an intermediate node is a group member and received this message, it just
needs to forward the message. Otherwise, if the node is an IN node then an entry in its MFIT
table is created to the receiver node. The IN node send a BRANCH_UP message to the
previous node to inform it about its new situation and continue forwarding the
BRANCH_ACK message to the downstream nodes. At the end, the multicast tree will be
constructed, consisting of a set of IN nodes with other group nodes. BRANCH_UP and
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