EM2NET improved the scalability in ad hoc environment, but one can notice that the
protocol did not mention the method of constructing the connections between the group
members and there are no details about the joining/ leaving procedure for receivers. These
drawbacks may cause data loss or fragmentation in the multicast tree.
3.4 Discussion
As has been discussed in the previous sections of this chapter, Xcast, Xcast+, REUNITE,
HBH and SEM are examples of explicit multicast protocols. These protocols are sharing in
one thing by trying to improve the scalability issue in multicast networks, and this is done by
trying to remove or reduce the state information stored in the nodes participating in the
multicast tree in order to perform multicast routing.
Xcast can be considered as pure stateless multicast protocol, this protocol encodes all the
receivers’ addresses in an Xcast header, and the intermediate nodes just forward the packet
without any processing. There is no information to be stored in the intermediate nodes. This
protocol works very well for large number of small multicast groups. However, when the
groups become bigger, the efficiency of this protocol decreases because the Xcast header
becomes bigger. Xcast+ protocol came as an improvement to Xcast, at this protocol the Xcast
header encodes the Designated Routers instead of the receivers themselves; hence, this
improves on the scalability issue. However, This protocol faces the problem of real
application in Internet, where end users in general will not work in groups so the number of
designated routers will be large and Xcast + will not take advantage of its improvement and
works as Xcast thus facing the same problem of scalability.
REUNITE and HBH are explicit protocols that are positioned between the state and stateless
protocols. These protocols maintain tables in the group nodes to carry out information about
the network. The main feature in these protocols is that they are using a recursive unicast
method to construct the multicast tree. These protocols improve the scalability issue by
allowing part of the group node to carry information about the group members’ receivers. On
the other hand, these protocols cause high overhead because of the periodical messages
implemented to keep information in the MCT and MFT tables’ updated.
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